pcre2/doc/pcre2pattern.3

.TH PCRE2PATTERN 3 "02 January 2015" "PCRE2 10.00"
.SH NAME
PCRE2 - Perl-compatible regular expressions (revised API)
.SH "PCRE2 REGULAR EXPRESSION DETAILS"
.rs
.sp
The syntax and semantics of the regular expressions that are supported by PCRE2
are described in detail below. There is a quick-reference syntax summary in the
.\" HREF
\fBpcre2syntax\fP
.\"
page. PCRE2 tries to match Perl syntax and semantics as closely as it can.
PCRE2 also supports some alternative regular expression syntax (which does not
conflict with the Perl syntax) in order to provide some compatibility with
regular expressions in Python, .NET, and Oniguruma.
.P
Perl's regular expressions are described in its own documentation, and regular
expressions in general are covered in a number of books, some of which have
copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published
by O'Reilly, covers regular expressions in great detail. This description of
PCRE2's regular expressions is intended as reference material.
.P
This document discusses the patterns that are supported by PCRE2 when its main
matching function, \fBpcre2_match()\fP, is used. PCRE2 also has an alternative
matching function, \fBpcre2_dfa_match()\fP, which matches using a different
algorithm that is not Perl-compatible. Some of the features discussed below are
not available when DFA matching is used. The advantages and disadvantages of
the alternative function, and how it differs from the normal function, are
discussed in the
.\" HREF
\fBpcre2matching\fP
.\"
page.
.
.
.SH "SPECIAL START-OF-PATTERN ITEMS"
.rs
.sp
A number of options that can be passed to \fBpcre2_compile()\fP can also be set
by special items at the start of a pattern. These are not Perl-compatible, but
are provided to make these options accessible to pattern writers who are not
able to change the program that processes the pattern. Any number of these
items may appear, but they must all be together right at the start of the
pattern string, and the letters must be in upper case.
.
.
.SS "UTF support"
.rs
.sp
In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either as
single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32 can be
specified for the 32-bit library, in which case it constrains the character
values to valid Unicode code points. To process UTF strings, PCRE2 must be
built to include Unicode support (which is the default). When using UTF strings
you must either call the compiling function with the PCRE2_UTF option, or the
pattern must start with the special sequence (*UTF), which is equivalent to
setting the relevant option. How setting a UTF mode affects pattern matching is
mentioned in several places below. There is also a summary of features in the
.\" HREF
\fBpcre2unicode\fP
.\"
page.
.P
Some applications that allow their users to supply patterns may wish to
restrict them to non-UTF data for security reasons. If the PCRE2_NEVER_UTF
option is passed to \fBpcre2_compile()\fP, (*UTF) is not allowed, and its
appearance in a pattern causes an error.
.
.
.SS "Unicode property support"
.rs
.sp
Another special sequence that may appear at the start of a pattern is (*UCP).
This has the same effect as setting the PCRE2_UCP option: it causes sequences
such as \ed and \ew to use Unicode properties to determine character types,
instead of recognizing only characters with codes less than 128 via a lookup
table.
.P
Some applications that allow their users to supply patterns may wish to
restrict them for security reasons. If the PCRE2_NEVER_UCP option is passed to
\fBpcre2_compile()\fP, (*UCP) is not allowed, and its appearance in a pattern
causes an error.
.
.
.SS "Locking out empty string matching"
.rs
.sp
Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same effect
as passing the PCRE2_NOTEMPTY or PCRE2_NOTEMPTY_ATSTART option to whichever
matching function is subsequently called to match the pattern. These options
lock out the matching of empty strings, either entirely, or only at the start
of the subject.
.
.
.SS "Disabling auto-possessification"
.rs
.sp
If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from making quantifiers
possessive when what follows cannot match the repeated item. For example, by
default a+b is treated as a++b. For more details, see the
.\" HREF
\fBpcre2api\fP
.\"
documentation.
.
.
.SS "Disabling start-up optimizations"
.rs
.sp
If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
PCRE2_NO_START_OPTIMIZE option. This disables several optimizations for quickly
reaching "no match" results. For more details, see the
.\" HREF
\fBpcre2api\fP
.\"
documentation.
.
.
.SS "Disabling automatic anchoring"
.rs
.sp
If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the same effect as
setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimizations that
apply to patterns whose top-level branches all start with .* (match any number
of arbitrary characters). For more details, see the
.\" HREF
\fBpcre2api\fP
.\"
documentation.
.
.
.SS "Setting match and recursion limits"
.rs
.sp
The caller of \fBpcre2_match()\fP can set a limit on the number of times the
internal \fBmatch()\fP function is called and on the maximum depth of
recursive calls. These facilities are provided to catch runaway matches that
are provoked by patterns with huge matching trees (a typical example is a
pattern with nested unlimited repeats) and to avoid running out of system stack
by too much recursion. When one of these limits is reached, \fBpcre2_match()\fP
gives an error return. The limits can also be set by items at the start of the
pattern of the form
.sp
  (*LIMIT_MATCH=d)
  (*LIMIT_RECURSION=d)
.sp
where d is any number of decimal digits. However, the value of the setting must
be less than the value set (or defaulted) by the caller of \fBpcre2_match()\fP
for it to have any effect. In other words, the pattern writer can lower the
limits set by the programmer, but not raise them. If there is more than one
setting of one of these limits, the lower value is used.
.
.
.\" HTML <a name="newlines"></a>
.SS "Newline conventions"
.rs
.sp
PCRE2 supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (linefeed)
character, the two-character sequence CRLF, any of the three preceding, or any
Unicode newline sequence. The
.\" HREF
\fBpcre2api\fP
.\"
page has
.\" HTML <a href="pcre2api.html#newlines">
.\" </a>
further discussion
.\"
about newlines, and shows how to set the newline convention when calling
\fBpcre2_compile()\fP.
.P
It is also possible to specify a newline convention by starting a pattern
string with one of the following five sequences:
.sp
  (*CR)        carriage return
  (*LF)        linefeed
  (*CRLF)      carriage return, followed by linefeed
  (*ANYCRLF)   any of the three above
  (*ANY)       all Unicode newline sequences
.sp
These override the default and the options given to the compiling function. For
example, on a Unix system where LF is the default newline sequence, the pattern
.sp
  (*CR)a.b
.sp
changes the convention to CR. That pattern matches "a\enb" because LF is no
longer a newline. If more than one of these settings is present, the last one
is used.
.P
The newline convention affects where the circumflex and dollar assertions are
true. It also affects the interpretation of the dot metacharacter when
PCRE2_DOTALL is not set, and the behaviour of \eN. However, it does not affect
what the \eR escape sequence matches. By default, this is any Unicode newline
sequence, for Perl compatibility. However, this can be changed; see the
description of \eR in the section entitled
.\" HTML <a href="#newlineseq">
.\" </a>
"Newline sequences"
.\"
below. A change of \eR setting can be combined with a change of newline
convention.
.
.
.SS "Specifying what \eR matches"
.rs
.sp
It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF
at compile time. This effect can also be achieved by starting a pattern with
(*BSR_ANYCRLF). For completeness, (*BSR_UNICODE) is also recognized,
corresponding to PCRE2_BSR_UNICODE.
.
.
.SH "EBCDIC CHARACTER CODES"
.rs
.sp
PCRE2 can be compiled to run in an environment that uses EBCDIC as its
character code rather than ASCII or Unicode (typically a mainframe system). In
the sections below, character code values are ASCII or Unicode; in an EBCDIC
environment these characters may have different code values, and there are no
code points greater than 255.
.
.
.SH "CHARACTERS AND METACHARACTERS"
.rs
.sp
A regular expression is a pattern that is matched against a subject string from
left to right. Most characters stand for themselves in a pattern, and match the
corresponding characters in the subject. As a trivial example, the pattern
.sp
  The quick brown fox
.sp
matches a portion of a subject string that is identical to itself. When
caseless matching is specified (the PCRE2_CASELESS option), letters are matched
independently of case.
.P
The power of regular expressions comes from the ability to include alternatives
and repetitions in the pattern. These are encoded in the pattern by the use of
\fImetacharacters\fP, which do not stand for themselves but instead are
interpreted in some special way.
.P
There are two different sets of metacharacters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized within square brackets. Outside square brackets, the metacharacters
are as follows:
.sp
  \e      general escape character with several uses
  ^      assert start of string (or line, in multiline mode)
  $      assert end of string (or line, in multiline mode)
  .      match any character except newline (by default)
  [      start character class definition
  |      start of alternative branch
  (      start subpattern
  )      end subpattern
  ?      extends the meaning of (
         also 0 or 1 quantifier
         also quantifier minimizer
  *      0 or more quantifier
  +      1 or more quantifier
         also "possessive quantifier"
  {      start min/max quantifier
.sp
Part of a pattern that is in square brackets is called a "character class". In
a character class the only metacharacters are:
.sp
  \e      general escape character
  ^      negate the class, but only if the first character
  -      indicates character range
.\" JOIN
  [      POSIX character class (only if followed by POSIX
           syntax)
  ]      terminates the character class
.sp
The following sections describe the use of each of the metacharacters.
.
.
.SH BACKSLASH
.rs
.sp
The backslash character has several uses. Firstly, if it is followed by a
character that is not a number or a letter, it takes away any special meaning
that character may have. This use of backslash as an escape character applies
both inside and outside character classes.
.P
For example, if you want to match a * character, you write \e* in the pattern.
This escaping action applies whether or not the following character would
otherwise be interpreted as a metacharacter, so it is always safe to precede a
non-alphanumeric with backslash to specify that it stands for itself. In
particular, if you want to match a backslash, you write \e\e.
.P
In a UTF mode, only ASCII numbers and letters have any special meaning after a
backslash. All other characters (in particular, those whose codepoints are
greater than 127) are treated as literals.
.P
If a pattern is compiled with the PCRE2_EXTENDED option, most white space in
the pattern (other than in a character class), and characters between a #
outside a character class and the next newline, inclusive, are ignored. An
escaping backslash can be used to include a white space or # character as part
of the pattern.
.P
If you want to remove the special meaning from a sequence of characters, you
can do so by putting them between \eQ and \eE. This is different from Perl in
that $ and @ are handled as literals in \eQ...\eE sequences in PCRE2, whereas
in Perl, $ and @ cause variable interpolation. Note the following examples:
.sp
  Pattern            PCRE2 matches   Perl matches
.sp
.\" JOIN
  \eQabc$xyz\eE        abc$xyz        abc followed by the
                                      contents of $xyz
  \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
  \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
.sp
The \eQ...\eE sequence is recognized both inside and outside character classes.
An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
by \eE later in the pattern, the literal interpretation continues to the end of
the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
a character class, this causes an error, because the character class is not
terminated.
.
.
.\" HTML <a name="digitsafterbackslash"></a>
.SS "Non-printing characters"
.rs
.sp
A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters in a pattern, but when a pattern is being prepared by
text editing, it is often easier to use one of the following escape sequences
than the binary character it represents:
.sp
  \ea        alarm, that is, the BEL character (hex 07)
  \ecx       "control-x", where x is any ASCII character
  \ee        escape (hex 1B)
  \ef        form feed (hex 0C)
  \en        linefeed (hex 0A)
  \er        carriage return (hex 0D)
  \et        tab (hex 09)
  \e0dd      character with octal code 0dd
  \eddd      character with octal code ddd, or back reference
  \eo{ddd..} character with octal code ddd..
  \exhh      character with hex code hh
  \ex{hhh..} character with hex code hhh.. (default mode)
  \euhhhh    character with hex code hhhh (when PCRE2_ALT_BSUX is set)
.sp
The precise effect of \ecx on ASCII characters is as follows: if x is a lower
case letter, it is converted to upper case. Then bit 6 of the character (hex
40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
code unit following \ec has a value greater than 127, a compile-time error
occurs. This locks out non-ASCII characters in all modes.
.P
The \ec facility was designed for use with ASCII characters, but with the
extension to Unicode it is even less useful than it once was. It is, however,
recognized when PCRE2 is compiled in EBCDIC mode, where data items are always
bytes. In this mode, all values are valid after \ec. If the next character is a
lower case letter, it is converted to upper case. Then the 0xc0 bits of the
byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
characters also generate different values.
.P
After \e0 up to two further octal digits are read. If there are fewer than two
digits, just those that are present are used. Thus the sequence \e0\ex\e07
specifies two binary zeros followed by a BEL character (code value 7). Make
sure you supply two digits after the initial zero if the pattern character that
follows is itself an octal digit.
.P
The escape \eo must be followed by a sequence of octal digits, enclosed in
braces. An error occurs if this is not the case. This escape is a recent
addition to Perl; it provides way of specifying character code points as octal
numbers greater than 0777, and it also allows octal numbers and back references
to be unambiguously specified.
.P
For greater clarity and unambiguity, it is best to avoid following \e by a
digit greater than zero. Instead, use \eo{} or \ex{} to specify character
numbers, and \eg{} to specify back references. The following paragraphs
describe the old, ambiguous syntax.
.P
The handling of a backslash followed by a digit other than 0 is complicated,
and Perl has changed in recent releases, causing PCRE2 also to change. Outside
a character class, PCRE2 reads the digit and any following digits as a decimal
number. If the number is less than 8, or if there have been at least that many
previous capturing left parentheses in the expression, the entire sequence is
taken as a \fIback reference\fP. A description of how this works is given
.\" HTML <a href="#backreferences">
.\" </a>
later,
.\"
following the discussion of
.\" HTML <a href="#subpattern">
.\" </a>
parenthesized subpatterns.
.\"
.P
Inside a character class, or if the decimal number following \e is greater than
7 and there have not been that many capturing subpatterns, PCRE2 handles \e8
and \e9 as the literal characters "8" and "9", and otherwise re-reads up to
three octal digits following the backslash, using them to generate a data
character. Any subsequent digits stand for themselves. For example:
.sp
  \e040   is another way of writing an ASCII space
.\" JOIN
  \e40    is the same, provided there are fewer than 40
            previous capturing subpatterns
  \e7     is always a back reference
.\" JOIN
  \e11    might be a back reference, or another way of
            writing a tab
  \e011   is always a tab
  \e0113  is a tab followed by the character "3"
.\" JOIN
  \e113   might be a back reference, otherwise the
            character with octal code 113
.\" JOIN
  \e377   might be a back reference, otherwise
            the value 255 (decimal)
.\" JOIN
  \e81    is either a back reference, or the two
            characters "8" and "1"
.sp
Note that octal values of 100 or greater that are specified using this syntax
must not be introduced by a leading zero, because no more than three octal
digits are ever read.
.P
By default, after \ex that is not followed by {, from zero to two hexadecimal
digits are read (letters can be in upper or lower case). Any number of
hexadecimal digits may appear between \ex{ and }. If a character other than
a hexadecimal digit appears between \ex{ and }, or if there is no terminating
}, an error occurs.
.P
If the PCRE2_ALT_BSUX option is set, the interpretation of \ex is as just
described only when it is followed by two hexadecimal digits. Otherwise, it
matches a literal "x" character. In this mode mode, support for code points
greater than 256 is provided by \eu, which must be followed by four hexadecimal
digits; otherwise it matches a literal "u" character.
.P
Characters whose value is less than 256 can be defined by either of the two
syntaxes for \ex (or by \eu in PCRE2_ALT_BSUX mode). There is no difference in
the way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
\eu00dc in PCRE2_ALT_BSUX mode).
.
.
.SS "Constraints on character values"
.rs
.sp
Characters that are specified using octal or hexadecimal numbers are
limited to certain values, as follows:
.sp
  8-bit non-UTF mode    less than 0x100
  8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
  16-bit non-UTF mode   less than 0x10000
  16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
  32-bit non-UTF mode   less than 0x100000000
  32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
.sp
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
"surrogate" codepoints), and 0xffef.
.
.
.SS "Escape sequences in character classes"
.rs
.sp
All the sequences that define a single character value can be used both inside
and outside character classes. In addition, inside a character class, \eb is
interpreted as the backspace character (hex 08).
.P
\eN is not allowed in a character class. \eB, \eR, and \eX are not special
inside a character class. Like other unrecognized alphabetic escape sequences,
they cause an error. Outside a character class, these sequences have different
meanings.
.
.
.SS "Unsupported escape sequences"
.rs
.sp
In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
handler and used to modify the case of following characters. By default, PCRE2
does not support these escape sequences. However, if the PCRE2_ALT_BSUX option
is set, \eU matches a "U" character, and \eu can be used to define a character
by code point, as described in the previous section.
.
.
.SS "Absolute and relative back references"
.rs
.sp
The sequence \eg followed by an unsigned or a negative number, optionally
enclosed in braces, is an absolute or relative back reference. A named back
reference can be coded as \eg{name}. Back references are discussed
.\" HTML <a href="#backreferences">
.\" </a>
later,
.\"
following the discussion of
.\" HTML <a href="#subpattern">
.\" </a>
parenthesized subpatterns.
.\"
.
.
.SS "Absolute and relative subroutine calls"
.rs
.sp
For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for referencing a subpattern as a "subroutine". Details are discussed
.\" HTML <a href="#onigurumasubroutines">
.\" </a>
later.
.\"
Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
synonymous. The former is a back reference; the latter is a
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
subroutine
.\"
call.
.
.
.\" HTML <a name="genericchartypes"></a>
.SS "Generic character types"
.rs
.sp
Another use of backslash is for specifying generic character types:
.sp
  \ed     any decimal digit
  \eD     any character that is not a decimal digit
  \eh     any horizontal white space character
  \eH     any character that is not a horizontal white space character
  \es     any white space character
  \eS     any character that is not a white space character
  \ev     any vertical white space character
  \eV     any character that is not a vertical white space character
  \ew     any "word" character
  \eW     any "non-word" character
.sp
There is also the single sequence \eN, which matches a non-newline character.
This is the same as
.\" HTML <a href="#fullstopdot">
.\" </a>
the "." metacharacter
.\"
when PCRE2_DOTALL is not set. Perl also uses \eN to match characters by name;
PCRE2 does not support this.
.P
Each pair of lower and upper case escape sequences partitions the complete set
of characters into two disjoint sets. Any given character matches one, and only
one, of each pair. The sequences can appear both inside and outside character
classes. They each match one character of the appropriate type. If the current
matching point is at the end of the subject string, all of them fail, because
there is no character to match.
.P
The default \es characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
space (32), which are defined as white space in the "C" locale. This list may
vary if locale-specific matching is taking place. For example, in some locales
the "non-breaking space" character (\exA0) is recognized as white space, and in
others the VT character is not.
.P
A "word" character is an underscore or any character that is a letter or digit.
By default, the definition of letters and digits is controlled by PCRE2's
low-valued character tables, and may vary if locale-specific matching is taking
place (see
.\" HTML <a href="pcre2api.html#localesupport">
.\" </a>
"Locale support"
.\"
in the
.\" HREF
\fBpcre2api\fP
.\"
page). For example, in a French locale such as "fr_FR" in Unix-like systems,
or "french" in Windows, some character codes greater than 127 are used for
accented letters, and these are then matched by \ew. The use of locales with
Unicode is discouraged.
.P
By default, characters whose code points are greater than 127 never match \ed,
\es, or \ew, and always match \eD, \eS, and \eW, although this may vary for
characters in the range 128-255 when locale-specific matching is happening.
These escape sequences retain their original meanings from before Unicode
support was available, mainly for efficiency reasons. If the PCRE2_UCP option
is set, the behaviour is changed so that Unicode properties are used to
determine character types, as follows:
.sp
  \ed  any character that matches \ep{Nd} (decimal digit)
  \es  any character that matches \ep{Z} or \eh or \ev
  \ew  any character that matches \ep{L} or \ep{N}, plus underscore
.sp
The upper case escapes match the inverse sets of characters. Note that \ed
matches only decimal digits, whereas \ew matches any Unicode digit, as well as
any Unicode letter, and underscore. Note also that PCRE2_UCP affects \eb, and
\eB because they are defined in terms of \ew and \eW. Matching these sequences
is noticeably slower when PCRE2_UCP is set.
.P
The sequences \eh, \eH, \ev, and \eV, in contrast to the other sequences, which
match only ASCII characters by default, always match a specific list of code
points, whether or not PCRE2_UCP is set. The horizontal space characters are:
.sp
  U+0009     Horizontal tab (HT)
  U+0020     Space
  U+00A0     Non-break space
  U+1680     Ogham space mark
  U+180E     Mongolian vowel separator
  U+2000     En quad
  U+2001     Em quad
  U+2002     En space
  U+2003     Em space
  U+2004     Three-per-em space
  U+2005     Four-per-em space
  U+2006     Six-per-em space
  U+2007     Figure space
  U+2008     Punctuation space
  U+2009     Thin space
  U+200A     Hair space
  U+202F     Narrow no-break space
  U+205F     Medium mathematical space
  U+3000     Ideographic space
.sp
The vertical space characters are:
.sp
  U+000A     Linefeed (LF)
  U+000B     Vertical tab (VT)
  U+000C     Form feed (FF)
  U+000D     Carriage return (CR)
  U+0085     Next line (NEL)
  U+2028     Line separator
  U+2029     Paragraph separator
.sp
In 8-bit, non-UTF-8 mode, only the characters with code points less than 256
are relevant.
.
.
.\" HTML <a name="newlineseq"></a>
.SS "Newline sequences"
.rs
.sp
Outside a character class, by default, the escape sequence \eR matches any
Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
following:
.sp
  (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
.sp
This is an example of an "atomic group", details of which are given
.\" HTML <a href="#atomicgroup">
.\" </a>
below.
.\"
This particular group matches either the two-character sequence CR followed by
LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
line, U+0085). The two-character sequence is treated as a single unit that
cannot be split.
.P
In other modes, two additional characters whose codepoints are greater than 255
are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
Unicode support is not needed for these characters to be recognized.
.P
It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
complete set of Unicode line endings) by setting the option PCRE2_BSR_ANYCRLF
at compile time. (BSR is an abbrevation for "backslash R".) This can be made
the default when PCRE2 is built; if this is the case, the other behaviour can
be requested via the PCRE2_BSR_UNICODE option. It is also possible to specify
these settings by starting a pattern string with one of the following
sequences:
.sp
  (*BSR_ANYCRLF)   CR, LF, or CRLF only
  (*BSR_UNICODE)   any Unicode newline sequence
.sp
These override the default and the options given to the compiling function.
Note that these special settings, which are not Perl-compatible, are recognized
only at the very start of a pattern, and that they must be in upper case. If
more than one of them is present, the last one is used. They can be combined
with a change of newline convention; for example, a pattern can start with:
.sp
  (*ANY)(*BSR_ANYCRLF)
.sp
They can also be combined with the (*UTF) or (*UCP) special sequences. Inside a
character class, \eR is treated as an unrecognized escape sequence, and causes
an error.
.
.
.\" HTML <a name="uniextseq"></a>
.SS Unicode character properties
.rs
.sp
When PCRE2 is built with Unicode support (the default), three additional escape
sequences that match characters with specific properties are available. In
8-bit non-UTF-8 mode, these sequences are of course limited to testing
characters whose codepoints are less than 256, but they do work in this mode.
The extra escape sequences are:
.sp
  \ep{\fIxx\fP}   a character with the \fIxx\fP property
  \eP{\fIxx\fP}   a character without the \fIxx\fP property
  \eX       a Unicode extended grapheme cluster
.sp
The property names represented by \fIxx\fP above are limited to the Unicode
script names, the general category properties, "Any", which matches any
character (including newline), and some special PCRE2 properties (described
in the
.\" HTML <a href="#extraprops">
.\" </a>
next section).
.\"
Other Perl properties such as "InMusicalSymbols" are not supported by PCRE2.
Note that \eP{Any} does not match any characters, so always causes a match
failure.
.P
Sets of Unicode characters are defined as belonging to certain scripts. A
character from one of these sets can be matched using a script name. For
example:
.sp
  \ep{Greek}
  \eP{Han}
.sp
Those that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
.P
Arabic,
Armenian,
Avestan,
Balinese,
Bamum,
Bassa_Vah,
Batak,
Bengali,
Bopomofo,
Brahmi,
Braille,
Buginese,
Buhid,
Canadian_Aboriginal,
Carian,
Caucasian_Albanian,
Chakma,
Cham,
Cherokee,
Common,
Coptic,
Cuneiform,
Cypriot,
Cyrillic,
Deseret,
Devanagari,
Duployan,
Egyptian_Hieroglyphs,
Elbasan,
Ethiopic,
Georgian,
Glagolitic,
Gothic,
Grantha,
Greek,
Gujarati,
Gurmukhi,
Han,
Hangul,
Hanunoo,
Hebrew,
Hiragana,
Imperial_Aramaic,
Inherited,
Inscriptional_Pahlavi,
Inscriptional_Parthian,
Javanese,
Kaithi,
Kannada,
Katakana,
Kayah_Li,
Kharoshthi,
Khmer,
Khojki,
Khudawadi,
Lao,
Latin,
Lepcha,
Limbu,
Linear_A,
Linear_B,
Lisu,
Lycian,
Lydian,
Mahajani,
Malayalam,
Mandaic,
Manichaean,
Meetei_Mayek,
Mende_Kikakui,
Meroitic_Cursive,
Meroitic_Hieroglyphs,
Miao,
Modi,
Mongolian,
Mro,
Myanmar,
Nabataean,
New_Tai_Lue,
Nko,
Ogham,
Ol_Chiki,
Old_Italic,
Old_North_Arabian,
Old_Permic,
Old_Persian,
Old_South_Arabian,
Old_Turkic,
Oriya,
Osmanya,
Pahawh_Hmong,
Palmyrene,
Pau_Cin_Hau,
Phags_Pa,
Phoenician,
Psalter_Pahlavi,
Rejang,
Runic,
Samaritan,
Saurashtra,
Sharada,
Shavian,
Siddham,
Sinhala,
Sora_Sompeng,
Sundanese,
Syloti_Nagri,
Syriac,
Tagalog,
Tagbanwa,
Tai_Le,
Tai_Tham,
Tai_Viet,
Takri,
Tamil,
Telugu,
Thaana,
Thai,
Tibetan,
Tifinagh,
Tirhuta,
Ugaritic,
Vai,
Warang_Citi,
Yi.
.P
Each character has exactly one Unicode general category property, specified by
a two-letter abbreviation. For compatibility with Perl, negation can be
specified by including a circumflex between the opening brace and the property
name. For example, \ep{^Lu} is the same as \eP{Lu}.
.P
If only one letter is specified with \ep or \eP, it includes all the general
category properties that start with that letter. In this case, in the absence
of negation, the curly brackets in the escape sequence are optional; these two
examples have the same effect:
.sp
  \ep{L}
  \epL
.sp
The following general category property codes are supported:
.sp
  C     Other
  Cc    Control
  Cf    Format
  Cn    Unassigned
  Co    Private use
  Cs    Surrogate
.sp
  L     Letter
  Ll    Lower case letter
  Lm    Modifier letter
  Lo    Other letter
  Lt    Title case letter
  Lu    Upper case letter
.sp
  M     Mark
  Mc    Spacing mark
  Me    Enclosing mark
  Mn    Non-spacing mark
.sp
  N     Number
  Nd    Decimal number
  Nl    Letter number
  No    Other number
.sp
  P     Punctuation
  Pc    Connector punctuation
  Pd    Dash punctuation
  Pe    Close punctuation
  Pf    Final punctuation
  Pi    Initial punctuation
  Po    Other punctuation
  Ps    Open punctuation
.sp
  S     Symbol
  Sc    Currency symbol
  Sk    Modifier symbol
  Sm    Mathematical symbol
  So    Other symbol
.sp
  Z     Separator
  Zl    Line separator
  Zp    Paragraph separator
  Zs    Space separator
.sp
The special property L& is also supported: it matches a character that has
the Lu, Ll, or Lt property, in other words, a letter that is not classified as
a modifier or "other".
.P
The Cs (Surrogate) property applies only to characters in the range U+D800 to
U+DFFF. Such characters are not valid in Unicode strings and so
cannot be tested by PCRE2, unless UTF validity checking has been turned off
(see the discussion of PCRE2_NO_UTF_CHECK in the
.\" HREF
\fBpcre2api\fP
.\"
page). Perl does not support the Cs property.
.P
The long synonyms for property names that Perl supports (such as \ep{Letter})
are not supported by PCRE2, nor is it permitted to prefix any of these
properties with "Is".
.P
No character that is in the Unicode table has the Cn (unassigned) property.
Instead, this property is assumed for any code point that is not in the
Unicode table.
.P
Specifying caseless matching does not affect these escape sequences. For
example, \ep{Lu} always matches only upper case letters. This is different from
the behaviour of current versions of Perl.
.P
Matching characters by Unicode property is not fast, because PCRE2 has to do a
multistage table lookup in order to find a character's property. That is why
the traditional escape sequences such as \ed and \ew do not use Unicode
properties in PCRE2 by default, though you can make them do so by setting the
PCRE2_UCP option or by starting the pattern with (*UCP).
.
.
.SS Extended grapheme clusters
.rs
.sp
The \eX escape matches any number of Unicode characters that form an "extended
grapheme cluster", and treats the sequence as an atomic group
.\" HTML <a href="#atomicgroup">
.\" </a>
(see below).
.\"
Unicode supports various kinds of composite character by giving each character
a grapheme breaking property, and having rules that use these properties to
define the boundaries of extended grapheme clusters. \eX always matches at
least one character. Then it decides whether to add additional characters
according to the following rules for ending a cluster:
.P
1. End at the end of the subject string.
.P
2. Do not end between CR and LF; otherwise end after any control character.
.P
3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
are of five types: L, V, T, LV, and LVT. An L character may be followed by an
L, V, LV, or LVT character; an LV or V character may be followed by a V or T
character; an LVT or T character may be follwed only by a T character.
.P
4. Do not end before extending characters or spacing marks. Characters with
the "mark" property always have the "extend" grapheme breaking property.
.P
5. Do not end after prepend characters.
.P
6. Otherwise, end the cluster.
.
.
.\" HTML <a name="extraprops"></a>
.SS PCRE2's additional properties
.rs
.sp
As well as the standard Unicode properties described above, PCRE2 supports four
more that make it possible to convert traditional escape sequences such as \ew
and \es to use Unicode properties. PCRE2 uses these non-standard, non-Perl
properties internally when PCRE2_UCP is set. However, they may also be used
explicitly. These properties are:
.sp
  Xan   Any alphanumeric character
  Xps   Any POSIX space character
  Xsp   Any Perl space character
  Xwd   Any Perl "word" character
.sp
Xan matches characters that have either the L (letter) or the N (number)
property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
carriage return, and any other character that has the Z (separator) property.
Xsp is the same as Xps; in PCRE1 it used to exclude vertical tab, for Perl
compatibility, but Perl changed. Xwd matches the same characters as Xan, plus
underscore.
.P
There is another non-standard property, Xuc, which matches any character that
can be represented by a Universal Character Name in C++ and other programming
languages. These are the characters $, @, ` (grave accent), and all characters
with Unicode code points greater than or equal to U+00A0, except for the
surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
where H is a hexadecimal digit. Note that the Xuc property does not match these
sequences but the characters that they represent.)
.
.
.\" HTML <a name="resetmatchstart"></a>
.SS "Resetting the match start"
.rs
.sp
The escape sequence \eK causes any previously matched characters not to be
included in the final matched sequence. For example, the pattern:
.sp
  foo\eKbar
.sp
matches "foobar", but reports that it has matched "bar". This feature is
similar to a lookbehind assertion
.\" HTML <a href="#lookbehind">
.\" </a>
(described below).
.\"
However, in this case, the part of the subject before the real match does not
have to be of fixed length, as lookbehind assertions do. The use of \eK does
not interfere with the setting of
.\" HTML <a href="#subpattern">
.\" </a>
captured substrings.
.\"
For example, when the pattern
.sp
  (foo)\eKbar
.sp
matches "foobar", the first substring is still set to "foo".
.P
Perl documents that the use of \eK within assertions is "not well defined". In
PCRE2, \eK is acted upon when it occurs inside positive assertions, but is
ignored in negative assertions. Note that when a pattern such as (?=ab\eK)
matches, the reported start of the match can be greater than the end of the
match.
.
.
.\" HTML <a name="smallassertions"></a>
.SS "Simple assertions"
.rs
.sp
The final use of backslash is for certain simple assertions. An assertion
specifies a condition that has to be met at a particular point in a match,
without consuming any characters from the subject string. The use of
subpatterns for more complicated assertions is described
.\" HTML <a href="#bigassertions">
.\" </a>
below.
.\"
The backslashed assertions are:
.sp
  \eb     matches at a word boundary
  \eB     matches when not at a word boundary
  \eA     matches at the start of the subject
  \eZ     matches at the end of the subject
          also matches before a newline at the end of the subject
  \ez     matches only at the end of the subject
  \eG     matches at the first matching position in the subject
.sp
Inside a character class, \eb has a different meaning; it matches the backspace
character. If any other of these assertions appears in a character class, an
"invalid escape sequence" error is generated.
.P
A word boundary is a position in the subject string where the current character
and the previous character do not both match \ew or \eW (i.e. one matches
\ew and the other matches \eW), or the start or end of the string if the
first or last character matches \ew, respectively. In a UTF mode, the meanings
of \ew and \eW can be changed by setting the PCRE2_UCP option. When this is
done, it also affects \eb and \eB. Neither PCRE2 nor Perl has a separate "start
of word" or "end of word" metasequence. However, whatever follows \eb normally
determines which it is. For example, the fragment \eba matches "a" at the start
of a word.
.P
The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
dollar (described in the next section) in that they only ever match at the very
start and end of the subject string, whatever options are set. Thus, they are
independent of multiline mode. These three assertions are not affected by the
PCRE2_NOTBOL or PCRE2_NOTEOL options, which affect only the behaviour of the
circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
argument of \fBpcre2_match()\fP is non-zero, indicating that matching is to
start at a point other than the beginning of the subject, \eA can never match.
The difference between \eZ and \ez is that \eZ matches before a newline at the
end of the string as well as at the very end, whereas \ez matches only at the
end.
.P
The \eG assertion is true only when the current matching position is at the
start point of the match, as specified by the \fIstartoffset\fP argument of
\fBpcre2_match()\fP. It differs from \eA when the value of \fIstartoffset\fP is
non-zero. By calling \fBpcre2_match()\fP multiple times with appropriate
arguments, you can mimic Perl's /g option, and it is in this kind of
implementation where \eG can be useful.
.P
Note, however, that PCRE2's interpretation of \eG, as the start of the current
match, is subtly different from Perl's, which defines it as the end of the
previous match. In Perl, these can be different when the previously matched
string was empty. Because PCRE2 does just one match at a time, it cannot
reproduce this behaviour.
.P
If all the alternatives of a pattern begin with \eG, the expression is anchored
to the starting match position, and the "anchored" flag is set in the compiled
regular expression.
.
.
.SH "CIRCUMFLEX AND DOLLAR"
.rs
.sp
The circumflex and dollar metacharacters are zero-width assertions. That is,
they test for a particular condition being true without consuming any
characters from the subject string.
.P
Outside a character class, in the default matching mode, the circumflex
character is an assertion that is true only if the current matching point is at
the start of the subject string. If the \fIstartoffset\fP argument of
\fBpcre2_match()\fP is non-zero, circumflex can never match if the
PCRE2_MULTILINE option is unset. Inside a character class, circumflex has an
entirely different meaning
.\" HTML <a href="#characterclass">
.\" </a>
(see below).
.\"
.P
Circumflex need not be the first character of the pattern if a number of
alternatives are involved, but it should be the first thing in each alternative
in which it appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)
.P
The dollar character is an assertion that is true only if the current matching
point is at the end of the subject string, or immediately before a newline at
the end of the string (by default). Note, however, that it does not actually
match the newline. Dollar need not be the last character of the pattern if a
number of alternatives are involved, but it should be the last item in any
branch in which it appears. Dollar has no special meaning in a character class.
.P
The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE2_DOLLAR_ENDONLY option at compile time. This
does not affect the \eZ assertion.
.P
The meanings of the circumflex and dollar characters are changed if the
PCRE2_MULTILINE option is set. When this is the case, a circumflex matches
immediately after internal newlines as well as at the start of the subject
string. It does not match after a newline that ends the string. A dollar
matches before any newlines in the string, as well as at the very end, when
PCRE2_MULTILINE is set. When newline is specified as the two-character
sequence CRLF, isolated CR and LF characters do not indicate newlines.
.P
For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
\en represents a newline) in multiline mode, but not otherwise. Consequently,
patterns that are anchored in single line mode because all branches start with
^ are not anchored in multiline mode, and a match for circumflex is possible
when the \fIstartoffset\fP argument of \fBpcre2_match()\fP is non-zero. The
PCRE2_DOLLAR_ENDONLY option is ignored if PCRE2_MULTILINE is set.
.P
Note that the sequences \eA, \eZ, and \ez can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
\eA it is always anchored, whether or not PCRE2_MULTILINE is set.
.
.
.\" HTML <a name="fullstopdot"></a>
.SH "FULL STOP (PERIOD, DOT) AND \eN"
.rs
.sp
Outside a character class, a dot in the pattern matches any one character in
the subject string except (by default) a character that signifies the end of a
line.
.P
When a line ending is defined as a single character, dot never matches that
character; when the two-character sequence CRLF is used, dot does not match CR
if it is immediately followed by LF, but otherwise it matches all characters
(including isolated CRs and LFs). When any Unicode line endings are being
recognized, dot does not match CR or LF or any of the other line ending
characters.
.P
The behaviour of dot with regard to newlines can be changed. If the
PCRE2_DOTALL option is set, a dot matches any one character, without exception.
If the two-character sequence CRLF is present in the subject string, it takes
two dots to match it.
.P
The handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newlines. Dot has no
special meaning in a character class.
.P
The escape sequence \eN behaves like a dot, except that it is not affected by
the PCRE2_DOTALL option. In other words, it matches any character except one
that signifies the end of a line. Perl also uses \eN to match characters by
name; PCRE2 does not support this.
.
.
.SH "MATCHING A SINGLE CODE UNIT"
.rs
.sp
Outside a character class, the escape sequence \eC matches any one code unit,
whether or not a UTF mode is set. In the 8-bit library, one code unit is one
byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is a
32-bit unit. Unlike a dot, \eC always matches line-ending characters. The
feature is provided in Perl in order to match individual bytes in UTF-8 mode,
but it is unclear how it can usefully be used. Because \eC breaks up characters
into individual code units, matching one unit with \eC in a UTF mode means that
the rest of the string may start with a malformed UTF character. This has
undefined results, because PCRE2 assumes that it is dealing with valid UTF
strings (and by default it checks this at the start of processing unless the
PCRE2_NO_UTF_CHECK option is used).
.P
PCRE2 does not allow \eC to appear in lookbehind assertions
.\" HTML <a href="#lookbehind">
.\" </a>
(described below)
.\"
in a UTF mode, because this would make it impossible to calculate the length of
the lookbehind.
.P
In general, the \eC escape sequence is best avoided. However, one way of using
it that avoids the problem of malformed UTF characters is to use a lookahead to
check the length of the next character, as in this pattern, which could be used
with a UTF-8 string (ignore white space and line breaks):
.sp
  (?| (?=[\ex00-\ex7f])(\eC) |
      (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
      (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
      (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
.sp
In this example, a group that starts with (?| resets the capturing parentheses
numbers in each alternative (see
.\" HTML <a href="#dupsubpatternnumber">
.\" </a>
"Duplicate Subpattern Numbers"
.\"
below). The assertions at the start of each branch check the next UTF-8
character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
character's individual bytes are then captured by the appropriate number of
\eC groups.
.
.
.\" HTML <a name="characterclass"></a>
.SH "SQUARE BRACKETS AND CHARACTER CLASSES"
.rs
.sp
An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special by default.
If a closing square bracket is required as a member of the class, it should be
the first data character in the class (after an initial circumflex, if present)
or escaped with a backslash. This means that, by default, an empty class cannot
be defined. However, if the PCRE2_ALLOW_EMPTY_CLASS option is set, a closing
square bracket at the start does end the (empty) class.
.P
A character class matches a single character in the subject. A matched
character must be in the set of characters defined by the class, unless the
first character in the class definition is a circumflex, in which case the
subject character must not be in the set defined by the class. If a circumflex
is actually required as a member of the class, ensure it is not the first
character, or escape it with a backslash.
.P
For example, the character class [aeiou] matches any lower case vowel, while
[^aeiou] matches any character that is not a lower case vowel. Note that a
circumflex is just a convenient notation for specifying the characters that
are in the class by enumerating those that are not. A class that starts with a
circumflex is not an assertion; it still consumes a character from the subject
string, and therefore it fails if the current pointer is at the end of the
string.
.P
When caseless matching is set, any letters in a class represent both their
upper case and lower case versions, so for example, a caseless [aeiou] matches
"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
caseful version would.
.P
Characters that might indicate line breaks are never treated in any special way
when matching character classes, whatever line-ending sequence is in use, and
whatever setting of the PCRE2_DOTALL and PCRE2_MULTILINE options is used. A
class such as [^a] always matches one of these characters.
.P
The minus (hyphen) character can be used to specify a range of characters in a
character class. For example, [d-m] matches any letter between d and m,
inclusive. If a minus character is required in a class, it must be escaped with
a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class, or
immediately after a range. For example, [b-d-z] matches letters in the range b
to d, a hyphen character, or z.
.P
It is not possible to have the literal character "]" as the end character of a
range. A pattern such as [W-]46] is interpreted as a class of two characters
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
the end of range, so [W-\e]46] is interpreted as a class containing a range
followed by two other characters. The octal or hexadecimal representation of
"]" can also be used to end a range.
.P
An error is generated if a POSIX character class (see below) or an escape
sequence other than one that defines a single character appears at a point
where a range ending character is expected. For example, [z-\exff] is valid,
but [A-\ed] and [A-[:digit:]] are not.
.P
Ranges operate in the collating sequence of character values. They can also be
used for characters specified numerically, for example [\e000-\e037]. Ranges
can include any characters that are valid for the current mode.
.P
If a range that includes letters is used when caseless matching is set, it
matches the letters in either case. For example, [W-c] is equivalent to
[][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
tables for a French locale are in use, [\exc8-\excb] matches accented E
characters in both cases.
.P
The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
\eV, \ew, and \eW may appear in a character class, and add the characters that
they match to the class. For example, [\edABCDEF] matches any hexadecimal
digit. In UTF modes, the PCRE2_UCP option affects the meanings of \ed, \es, \ew
and their upper case partners, just as it does when they appear outside a
character class, as described in the section entitled
.\" HTML <a href="#genericchartypes">
.\" </a>
"Generic character types"
.\"
above. The escape sequence \eb has a different meaning inside a character
class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
are not special inside a character class. Like any other unrecognized escape
sequences, they cause an error.
.P
A circumflex can conveniently be used with the upper case character types to
specify a more restricted set of characters than the matching lower case type.
For example, the class [^\eW_] matches any letter or digit, but not underscore,
whereas [\ew] includes underscore. A positive character class should be read as
"something OR something OR ..." and a negative class as "NOT something AND NOT
something AND NOT ...".
.P
The only metacharacters that are recognized in character classes are backslash,
hyphen (only where it can be interpreted as specifying a range), circumflex
(only at the start), opening square bracket (only when it can be interpreted as
introducing a POSIX class name, or for a special compatibility feature - see
the next two sections), and the terminating closing square bracket. However,
escaping other non-alphanumeric characters does no harm.
.
.
.SH "POSIX CHARACTER CLASSES"
.rs
.sp
Perl supports the POSIX notation for character classes. This uses names
enclosed by [: and :] within the enclosing square brackets. PCRE2 also supports
this notation. For example,
.sp
  [01[:alpha:]%]
.sp
matches "0", "1", any alphabetic character, or "%". The supported class names
are:
.sp
  alnum    letters and digits
  alpha    letters
  ascii    character codes 0 - 127
  blank    space or tab only
  cntrl    control characters
  digit    decimal digits (same as \ed)
  graph    printing characters, excluding space
  lower    lower case letters
  print    printing characters, including space
  punct    printing characters, excluding letters and digits and space
  space    white space (the same as \es from PCRE2 8.34)
  upper    upper case letters
  word     "word" characters (same as \ew)
  xdigit   hexadecimal digits
.sp
The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
and space (32). If locale-specific matching is taking place, the list of space
characters may be different; there may be fewer or more of them. "Space" and
\es match the same set of characters.
.P
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
5.8. Another Perl extension is negation, which is indicated by a ^ character
after the colon. For example,
.sp
  [12[:^digit:]]
.sp
matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the POSIX
syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
supported, and an error is given if they are encountered.
.P
By default, characters with values greater than 128 do not match any of the
POSIX character classes. However, if the PCRE2_UCP option is passed to
\fBpcre2_compile()\fP, some of the classes are changed so that Unicode
character properties are used. This is achieved by replacing certain POSIX
classes by other sequences, as follows:
.sp
  [:alnum:]  becomes  \ep{Xan}
  [:alpha:]  becomes  \ep{L}
  [:blank:]  becomes  \eh
  [:cntrl:]  becomes  \ep{Cc}
  [:digit:]  becomes  \ep{Nd}
  [:lower:]  becomes  \ep{Ll}
  [:space:]  becomes  \ep{Xps}
  [:upper:]  becomes  \ep{Lu}
  [:word:]   becomes  \ep{Xwd}
.sp
Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
classes are handled specially in UCP mode:
.TP 10
[:graph:]
This matches characters that have glyphs that mark the page when printed. In
Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
properties, except for:
.sp
  U+061C           Arabic Letter Mark
  U+180E           Mongolian Vowel Separator
  U+2066 - U+2069  Various "isolate"s
.sp
.TP 10
[:print:]
This matches the same characters as [:graph:] plus space characters that are
not controls, that is, characters with the Zs property.
.TP 10
[:punct:]
This matches all characters that have the Unicode P (punctuation) property,
plus those characters with code points less than 128 that have the S (Symbol)
property.
.P
The other POSIX classes are unchanged, and match only characters with code
points less than 128.
.
.
.SH "COMPATIBILITY FEATURE FOR WORD BOUNDARIES"
.rs
.sp
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
word". PCRE2 treats these items as follows:
.sp
  [[:<:]]  is converted to  \eb(?=\ew)
  [[:>:]]  is converted to  \eb(?<=\ew)
.sp
Only these exact character sequences are recognized. A sequence such as
[a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
not compatible with Perl. It is provided to help migrations from other
environments, and is best not used in any new patterns. Note that \eb matches
at the start and the end of a word (see
.\" HTML <a href="#smallassertions">
.\" </a>
"Simple assertions"
.\"
above), and in a Perl-style pattern the preceding or following character
normally shows which is wanted, without the need for the assertions that are
used above in order to give exactly the POSIX behaviour.
.
.
.SH "VERTICAL BAR"
.rs
.sp
Vertical bar characters are used to separate alternative patterns. For example,
the pattern
.sp
  gilbert|sullivan
.sp
matches either "gilbert" or "sullivan". Any number of alternatives may appear,
and an empty alternative is permitted (matching the empty string). The matching
process tries each alternative in turn, from left to right, and the first one
that succeeds is used. If the alternatives are within a subpattern
.\" HTML <a href="#subpattern">
.\" </a>
(defined below),
.\"
"succeeds" means matching the rest of the main pattern as well as the
alternative in the subpattern.
.
.
.SH "INTERNAL OPTION SETTING"
.rs
.sp
The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, and
PCRE2_EXTENDED options (which are Perl-compatible) can be changed from within
the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
The option letters are
.sp
  i  for PCRE2_CASELESS
  m  for PCRE2_MULTILINE
  s  for PCRE2_DOTALL
  x  for PCRE2_EXTENDED
.sp
For example, (?im) sets caseless, multiline matching. It is also possible to
unset these options by preceding the letter with a hyphen, and a combined
setting and unsetting such as (?im-sx), which sets PCRE2_CASELESS and
PCRE2_MULTILINE while unsetting PCRE2_DOTALL and PCRE2_EXTENDED, is also
permitted. If a letter appears both before and after the hyphen, the option is
unset.
.P
The PCRE2-specific options PCRE2_DUPNAMES and PCRE2_UNGREEDY can be changed in
the same way as the Perl-compatible options by using the characters J and U
respectively.
.P
When one of these option changes occurs at top level (that is, not inside
subpattern parentheses), the change applies to the remainder of the pattern
that follows. If the change is placed right at the start of a pattern, PCRE2
extracts it into the global options (and it will therefore show up in data
extracted by the \fBpcre2_pattern_info()\fP function).
.P
An option change within a subpattern (see below for a description of
subpatterns) affects only that part of the subpattern that follows it, so
.sp
  (a(?i)b)c
.sp
matches abc and aBc and no other strings (assuming PCRE2_CASELESS is not used).
By this means, options can be made to have different settings in different
parts of the pattern. Any changes made in one alternative do carry on
into subsequent branches within the same subpattern. For example,
.sp
  (a(?i)b|c)
.sp
matches "ab", "aB", "c", and "C", even though when matching "C" the first
branch is abandoned before the option setting. This is because the effects of
option settings happen at compile time. There would be some very weird
behaviour otherwise.
.P
\fBNote:\fP There are other PCRE2-specific options that can be set by the
application when the compiling function is called.
The pattern can contain special leading sequences such as (*CRLF) to override
what the application has set or what has been defaulted. Details are given in
the section entitled
.\" HTML <a href="#newlineseq">
.\" </a>
"Newline sequences"
.\"
above. There are also the (*UTF) and (*UCP) leading sequences that can be used
to set UTF and Unicode property modes; they are equivalent to setting the
PCRE2_UTF and PCRE2_UCP options, respectively. However, the application can set
the PCRE2_NEVER_UTF and PCRE2_NEVER_UCP options, which lock out the use of the
(*UTF) and (*UCP) sequences.
.
.
.\" HTML <a name="subpattern"></a>
.SH SUBPATTERNS
.rs
.sp
Subpatterns are delimited by parentheses (round brackets), which can be nested.
Turning part of a pattern into a subpattern does two things:
.sp
1. It localizes a set of alternatives. For example, the pattern
.sp
  cat(aract|erpillar|)
.sp
matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
match "cataract", "erpillar" or an empty string.
.sp
2. It sets up the subpattern as a capturing subpattern. This means that, when
the whole pattern matches, the portion of the subject string that matched the
subpattern is passed back to the caller, separately from the portion that
matched the whole pattern. (This applies only to the traditional matching
function; the DFA matching function does not support capturing.)
.P
Opening parentheses are counted from left to right (starting from 1) to obtain
numbers for the capturing subpatterns. For example, if the string "the red
king" is matched against the pattern
.sp
  the ((red|white) (king|queen))
.sp
the captured substrings are "red king", "red", and "king", and are numbered 1,
2, and 3, respectively.
.P
The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when a grouping subpattern is required without a
capturing requirement. If an opening parenthesis is followed by a question mark
and a colon, the subpattern does not do any capturing, and is not counted when
computing the number of any subsequent capturing subpatterns. For example, if
the string "the white queen" is matched against the pattern
.sp
  the ((?:red|white) (king|queen))
.sp
the captured substrings are "white queen" and "queen", and are numbered 1 and
2. The maximum number of capturing subpatterns is 65535.
.P
As a convenient shorthand, if any option settings are required at the start of
a non-capturing subpattern, the option letters may appear between the "?" and
the ":". Thus the two patterns
.sp
  (?i:saturday|sunday)
  (?:(?i)saturday|sunday)
.sp
match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the subpattern
is reached, an option setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".
.
.
.\" HTML <a name="dupsubpatternnumber"></a>
.SH "DUPLICATE SUBPATTERN NUMBERS"
.rs
.sp
Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
the same numbers for its capturing parentheses. Such a subpattern starts with
(?| and is itself a non-capturing subpattern. For example, consider this
pattern:
.sp
  (?|(Sat)ur|(Sun))day
.sp
Because the two alternatives are inside a (?| group, both sets of capturing
parentheses are numbered one. Thus, when the pattern matches, you can look
at captured substring number one, whichever alternative matched. This construct
is useful when you want to capture part, but not all, of one of a number of
alternatives. Inside a (?| group, parentheses are numbered as usual, but the
number is reset at the start of each branch. The numbers of any capturing
parentheses that follow the subpattern start after the highest number used in
any branch. The following example is taken from the Perl documentation. The
numbers underneath show in which buffer the captured content will be stored.
.sp
  # before  ---------------branch-reset----------- after
  / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
  # 1            2         2  3        2     3     4
.sp
A back reference to a numbered subpattern uses the most recent value that is
set for that number by any subpattern. The following pattern matches "abcabc"
or "defdef":
.sp
  /(?|(abc)|(def))\e1/
.sp
In contrast, a subroutine call to a numbered subpattern always refers to the
first one in the pattern with the given number. The following pattern matches
"abcabc" or "defabc":
.sp
  /(?|(abc)|(def))(?1)/
.sp
If a
.\" HTML <a href="#conditions">
.\" </a>
condition test
.\"
for a subpattern's having matched refers to a non-unique number, the test is
true if any of the subpatterns of that number have matched.
.P
An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
.
.
.SH "NAMED SUBPATTERNS"
.rs
.sp
Identifying capturing parentheses by number is simple, but it can be very hard
to keep track of the numbers in complicated regular expressions. Furthermore,
if an expression is modified, the numbers may change. To help with this
difficulty, PCRE2 supports the naming of subpatterns. This feature was not
added to Perl until release 5.10. Python had the feature earlier, and PCRE1
introduced it at release 4.0, using the Python syntax. PCRE2 supports both the
Perl and the Python syntax. Perl allows identically numbered subpatterns to
have different names, but PCRE2 does not.
.P
In PCRE2, a subpattern can be named in one of three ways: (?<name>...) or
(?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
parentheses from other parts of the pattern, such as
.\" HTML <a href="#backreferences">
.\" </a>
back references,
.\"
.\" HTML <a href="#recursion">
.\" </a>
recursion,
.\"
and
.\" HTML <a href="#conditions">
.\" </a>
conditions,
.\"
can be made by name as well as by number.
.P
Names consist of up to 32 alphanumeric characters and underscores, but must
start with a non-digit. Named capturing parentheses are still allocated numbers
as well as names, exactly as if the names were not present. The PCRE2 API
provides function calls for extracting the name-to-number translation table
from a compiled pattern. There are also convenience functions for extracting a
captured substring by name.
.P
By default, a name must be unique within a pattern, but it is possible to relax
this constraint by setting the PCRE2_DUPNAMES option at compile time.
(Duplicate names are also always permitted for subpatterns with the same
number, set up as described in the previous section.) Duplicate names can be
useful for patterns where only one instance of the named parentheses can match.
Suppose you want to match the name of a weekday, either as a 3-letter
abbreviation or as the full name, and in both cases you want to extract the
abbreviation. This pattern (ignoring the line breaks) does the job:
.sp
  (?<DN>Mon|Fri|Sun)(?:day)?|
  (?<DN>Tue)(?:sday)?|
  (?<DN>Wed)(?:nesday)?|
  (?<DN>Thu)(?:rsday)?|
  (?<DN>Sat)(?:urday)?
.sp
There are five capturing substrings, but only one is ever set after a match.
(An alternative way of solving this problem is to use a "branch reset"
subpattern, as described in the previous section.)
.P
The convenience functions for extracting the data by name returns the substring
for the first (and in this example, the only) subpattern of that name that
matched. This saves searching to find which numbered subpattern it was.
.P
If you make a back reference to a non-unique named subpattern from elsewhere in
the pattern, the subpatterns to which the name refers are checked in the order
in which they appear in the overall pattern. The first one that is set is used
for the reference. For example, this pattern matches both "foofoo" and
"barbar" but not "foobar" or "barfoo":
.sp
  (?:(?<n>foo)|(?<n>bar))\ek<n>
.sp
.P
If you make a subroutine call to a non-unique named subpattern, the one that
corresponds to the first occurrence of the name is used. In the absence of
duplicate numbers (see the previous section) this is the one with the lowest
number.
.P
If you use a named reference in a condition
test (see the
.\"
.\" HTML <a href="#conditions">
.\" </a>
section about conditions
.\"
below), either to check whether a subpattern has matched, or to check for
recursion, all subpatterns with the same name are tested. If the condition is
true for any one of them, the overall condition is true. This is the same
behaviour as testing by number. For further details of the interfaces for
handling named subpatterns, see the
.\" HREF
\fBpcre2api\fP
.\"
documentation.
.P
\fBWarning:\fP You cannot use different names to distinguish between two
subpatterns with the same number because PCRE2 uses only the numbers when
matching. For this reason, an error is given at compile time if different names
are given to subpatterns with the same number. However, you can always give the
same name to subpatterns with the same number, even when PCRE2_DUPNAMES is not
set.
.
.
.SH REPETITION
.rs
.sp
Repetition is specified by quantifiers, which can follow any of the following
items:
.sp
  a literal data character
  the dot metacharacter
  the \eC escape sequence
  the \eX escape sequence
  the \eR escape sequence
  an escape such as \ed or \epL that matches a single character
  a character class
  a back reference (see next section)
  a parenthesized subpattern (including assertions)
  a subroutine call to a subpattern (recursive or otherwise)
.sp
The general repetition quantifier specifies a minimum and maximum number of
permitted matches, by giving the two numbers in curly brackets (braces),
separated by a comma. The numbers must be less than 65536, and the first must
be less than or equal to the second. For example:
.sp
  z{2,4}
.sp
matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
character. If the second number is omitted, but the comma is present, there is
no upper limit; if the second number and the comma are both omitted, the
quantifier specifies an exact number of required matches. Thus
.sp
  [aeiou]{3,}
.sp
matches at least 3 successive vowels, but may match many more, whereas
.sp
  \ed{8}
.sp
matches exactly 8 digits. An opening curly bracket that appears in a position
where a quantifier is not allowed, or one that does not match the syntax of a
quantifier, is taken as a literal character. For example, {,6} is not a
quantifier, but a literal string of four characters.
.P
In UTF modes, quantifiers apply to characters rather than to individual code
units. Thus, for example, \ex{100}{2} matches two characters, each of
which is represented by a two-byte sequence in a UTF-8 string. Similarly,
\eX{3} matches three Unicode extended grapheme clusters, each of which may be
several code units long (and they may be of different lengths).
.P
The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present. This may be useful for
subpatterns that are referenced as
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
subroutines
.\"
from elsewhere in the pattern (but see also the section entitled
.\" HTML <a href="#subdefine">
.\" </a>
"Defining subpatterns for use by reference only"
.\"
below). Items other than subpatterns that have a {0} quantifier are omitted
from the compiled pattern.
.P
For convenience, the three most common quantifiers have single-character
abbreviations:
.sp
  *    is equivalent to {0,}
  +    is equivalent to {1,}
  ?    is equivalent to {0,1}
.sp
It is possible to construct infinite loops by following a subpattern that can
match no characters with a quantifier that has no upper limit, for example:
.sp
  (a?)*
.sp
Earlier versions of Perl and PCRE1 used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
patterns are now accepted, but if any repetition of the subpattern does in fact
match no characters, the loop is forcibly broken.
.P
By default, the quantifiers are "greedy", that is, they match as much as
possible (up to the maximum number of permitted times), without causing the
rest of the pattern to fail. The classic example of where this gives problems
is in trying to match comments in C programs. These appear between /* and */
and within the comment, individual * and / characters may appear. An attempt to
match C comments by applying the pattern
.sp
  /\e*.*\e*/
.sp
to the string
.sp
  /* first comment */  not comment  /* second comment */
.sp
fails, because it matches the entire string owing to the greediness of the .*
item.
.P
If a quantifier is followed by a question mark, it ceases to be greedy, and
instead matches the minimum number of times possible, so the pattern
.sp
  /\e*.*?\e*/
.sp
does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of matches.
Do not confuse this use of question mark with its use as a quantifier in its
own right. Because it has two uses, it can sometimes appear doubled, as in
.sp
  \ed??\ed
.sp
which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.
.P
If the PCRE2_UNGREEDY option is set (an option that is not available in Perl),
the quantifiers are not greedy by default, but individual ones can be made
greedy by following them with a question mark. In other words, it inverts the
default behaviour.
.P
When a parenthesized subpattern is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more memory is required for the
compiled pattern, in proportion to the size of the minimum or maximum.
.P
If a pattern starts with .* or .{0,} and the PCRE2_DOTALL option (equivalent
to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
implicitly anchored, because whatever follows will be tried against every
character position in the subject string, so there is no point in retrying the
overall match at any position after the first. PCRE2 normally treats such a
pattern as though it were preceded by \eA.
.P
In cases where it is known that the subject string contains no newlines, it is
worth setting PCRE2_DOTALL in order to obtain this optimization, or
alternatively, using ^ to indicate anchoring explicitly.
.P
However, there are some cases where the optimization cannot be used. When .*
is inside capturing parentheses that are the subject of a back reference
elsewhere in the pattern, a match at the start may fail where a later one
succeeds. Consider, for example:
.sp
  (.*)abc\e1
.sp
If the subject is "xyz123abc123" the match point is the fourth character. For
this reason, such a pattern is not implicitly anchored.
.P
Another case where implicit anchoring is not applied is when the leading .* is
inside an atomic group. Once again, a match at the start may fail where a later
one succeeds. Consider this pattern:
.sp
  (?>.*?a)b
.sp
It matches "ab" in the subject "aab". The use of the backtracking control verbs
(*PRUNE) and (*SKIP) also disable this optimization, and there is an option,
PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.
.P
When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after
.sp
  (tweedle[dume]{3}\es*)+
.sp
has matched "tweedledum tweedledee" the value of the captured substring is
"tweedledee". However, if there are nested capturing subpatterns, the
corresponding captured values may have been set in previous iterations. For
example, after
.sp
  (a|(b))+
.sp
matches "aba" the value of the second captured substring is "b".
.
.
.\" HTML <a name="atomicgroup"></a>
.SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
.rs
.sp
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
repetition, failure of what follows normally causes the repeated item to be
re-evaluated to see if a different number of repeats allows the rest of the
pattern to match. Sometimes it is useful to prevent this, either to change the
nature of the match, or to cause it fail earlier than it otherwise might, when
the author of the pattern knows there is no point in carrying on.
.P
Consider, for example, the pattern \ed+foo when applied to the subject line
.sp
  123456bar
.sp
After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the \ed+
item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
(a term taken from Jeffrey Friedl's book) provides the means for specifying
that once a subpattern has matched, it is not to be re-evaluated in this way.
.P
If we use atomic grouping for the previous example, the matcher gives up
immediately on failing to match "foo" the first time. The notation is a kind of
special parenthesis, starting with (?> as in this example:
.sp
  (?>\ed+)foo
.sp
This kind of parenthesis "locks up" the  part of the pattern it contains once
it has matched, and a failure further into the pattern is prevented from
backtracking into it. Backtracking past it to previous items, however, works as
normal.
.P
An alternative description is that a subpattern of this type matches exactly
the string of characters that an identical standalone pattern would match, if
anchored at the current point in the subject string.
.P
Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
the above example can be thought of as a maximizing repeat that must swallow
everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?>\ed+) can only match an entire sequence of digits.
.P
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an atomic
group is just a single repeated item, as in the example above, a simpler
notation, called a "possessive quantifier" can be used. This consists of an
additional + character following a quantifier. Using this notation, the
previous example can be rewritten as
.sp
  \ed++foo
.sp
Note that a possessive quantifier can be used with an entire group, for
example:
.sp
  (abc|xyz){2,3}+
.sp
Possessive quantifiers are always greedy; the setting of the PCRE2_UNGREEDY
option is ignored. They are a convenient notation for the simpler forms of
atomic group. However, there is no difference in the meaning of a possessive
quantifier and the equivalent atomic group, though there may be a performance
difference; possessive quantifiers should be slightly faster.
.P
The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
Jeffrey Friedl originated the idea (and the name) in the first edition of his
book. Mike McCloskey liked it, so implemented it when he built Sun's Java
package, and PCRE1 copied it from there. It ultimately found its way into Perl
at release 5.10.
.P
PCRE2 has an optimization that automatically "possessifies" certain simple
pattern constructs. For example, the sequence A+B is treated as A++B because
there is no point in backtracking into a sequence of A's when B must follow.
This feature can be disabled by the PCRE2_NO_AUTOPOSSESS option, or starting
the pattern with (*NO_AUTO_POSSESS).
.P
When a pattern contains an unlimited repeat inside a subpattern that can itself
be repeated an unlimited number of times, the use of an atomic group is the
only way to avoid some failing matches taking a very long time indeed. The
pattern
.sp
  (\eD+|<\ed+>)*[!?]
.sp
matches an unlimited number of substrings that either consist of non-digits, or
digits enclosed in <>, followed by either ! or ?. When it matches, it runs
quickly. However, if it is applied to
.sp
  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
.sp
it takes a long time before reporting failure. This is because the string can
be divided between the internal \eD+ repeat and the external * repeat in a
large number of ways, and all have to be tried. (The example uses [!?] rather
than a single character at the end, because both PCRE2 and Perl have an
optimization that allows for fast failure when a single character is used. They
remember the last single character that is required for a match, and fail early
if it is not present in the string.) If the pattern is changed so that it uses
an atomic group, like this:
.sp
  ((?>\eD+)|<\ed+>)*[!?]
.sp
sequences of non-digits cannot be broken, and failure happens quickly.
.
.
.\" HTML <a name="backreferences"></a>
.SH "BACK REFERENCES"
.rs
.sp
Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a back reference to a capturing subpattern earlier
(that is, to its left) in the pattern, provided there have been that many
previous capturing left parentheses.
.P
However, if the decimal number following the backslash is less than 8, it is
always taken as a back reference, and causes an error only if there are not
that many capturing left parentheses in the entire pattern. In other words, the
parentheses that are referenced need not be to the left of the reference for
numbers less than 8. A "forward back reference" of this type can make sense
when a repetition is involved and the subpattern to the right has participated
in an earlier iteration.
.P
It is not possible to have a numerical "forward back reference" to a subpattern
whose number is 8 or more using this syntax because a sequence such as \e50 is
interpreted as a character defined in octal. See the subsection entitled
"Non-printing characters"
.\" HTML <a href="#digitsafterbackslash">
.\" </a>
above
.\"
for further details of the handling of digits following a backslash. There is
no such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (see below).
.P
Another way of avoiding the ambiguity inherent in the use of digits following a
backslash is to use the \eg escape sequence. This escape must be followed by an
unsigned number or a negative number, optionally enclosed in braces. These
examples are all identical:
.sp
  (ring), \e1
  (ring), \eg1
  (ring), \eg{1}
.sp
An unsigned number specifies an absolute reference without the ambiguity that
is present in the older syntax. It is also useful when literal digits follow
the reference. A negative number is a relative reference. Consider this
example:
.sp
  (abc(def)ghi)\eg{-1}
.sp
The sequence \eg{-1} is a reference to the most recently started capturing
subpattern before \eg, that is, is it equivalent to \e2 in this example.
Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
can be helpful in long patterns, and also in patterns that are created by
joining together fragments that contain references within themselves.
.P
A back reference matches whatever actually matched the capturing subpattern in
the current subject string, rather than anything matching the subpattern
itself (see
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"Subpatterns as subroutines"
.\"
below for a way of doing that). So the pattern
.sp
  (sens|respons)e and \e1ibility
.sp
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If caseful matching is in force at the time of the
back reference, the case of letters is relevant. For example,
.sp
  ((?i)rah)\es+\e1
.sp
matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capturing subpattern is matched caselessly.
.P
There are several different ways of writing back references to named
subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
\ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
back reference syntax, in which \eg can be used for both numeric and named
references, is also supported. We could rewrite the above example in any of
the following ways:
.sp
  (?<p1>(?i)rah)\es+\ek<p1>
  (?'p1'(?i)rah)\es+\ek{p1}
  (?P<p1>(?i)rah)\es+(?P=p1)
  (?<p1>(?i)rah)\es+\eg{p1}
.sp
A subpattern that is referenced by name may appear in the pattern before or
after the reference.
.P
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references to it always fail by default. For example, the pattern
.sp
  (a|(bc))\e2
.sp
always fails if it starts to match "a" rather than "bc". However, if the
PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a back reference to an
unset value matches an empty string.
.P
Because there may be many capturing parentheses in a pattern, all digits
following a backslash are taken as part of a potential back reference number.
If the pattern continues with a digit character, some delimiter must be used to
terminate the back reference. If the PCRE2_EXTENDED option is set, this can be
white space. Otherwise, the \eg{ syntax or an empty comment (see
.\" HTML <a href="#comments">
.\" </a>
"Comments"
.\"
below) can be used.
.
.
.SS "Recursive back references"
.rs
.sp
A back reference that occurs inside the parentheses to which it refers fails
when the subpattern is first used, so, for example, (a\e1) never matches.
However, such references can be useful inside repeated subpatterns. For
example, the pattern
.sp
  (a|b\e1)+
.sp
matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
the subpattern, the back reference matches the character string corresponding
to the previous iteration. In order for this to work, the pattern must be such
that the first iteration does not need to match the back reference. This can be
done using alternation, as in the example above, or by a quantifier with a
minimum of zero.
.P
Back references of this type cause the group that they reference to be treated
as an
.\" HTML <a href="#atomicgroup">
.\" </a>
atomic group.
.\"
Once the whole group has been matched, a subsequent matching failure cannot
cause backtracking into the middle of the group.
.
.
.\" HTML <a name="bigassertions"></a>
.SH ASSERTIONS
.rs
.sp
An assertion is a test on the characters following or preceding the current
matching point that does not consume any characters. The simple assertions
coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
.\" HTML <a href="#smallassertions">
.\" </a>
above.
.\"
.P
More complicated assertions are coded as subpatterns. There are two kinds:
those that look ahead of the current position in the subject string, and those
that look behind it. An assertion subpattern is matched in the normal way,
except that it does not cause the current matching position to be changed.
.P
Assertion subpatterns are not capturing subpatterns. If such an assertion
contains capturing subpatterns within it, these are counted for the purposes of
numbering the capturing subpatterns in the whole pattern. However, substring
capturing is carried out only for positive assertions. (Perl sometimes, but not
always, does do capturing in negative assertions.)
.P
For compatibility with Perl, assertion subpatterns may be repeated; though
it makes no sense to assert the same thing several times, the side effect of
capturing parentheses may occasionally be useful. In practice, there only three
cases:
.sp
(1) If the quantifier is {0}, the assertion is never obeyed during matching.
However, it may contain internal capturing parenthesized groups that are called
from elsewhere via the
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
subroutine mechanism.
.\"
.sp
(2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
were {0,1}. At run time, the rest of the pattern match is tried with and
without the assertion, the order depending on the greediness of the quantifier.
.sp
(3) If the minimum repetition is greater than zero, the quantifier is ignored.
The assertion is obeyed just once when encountered during matching.
.
.
.SS "Lookahead assertions"
.rs
.sp
Lookahead assertions start with (?= for positive assertions and (?! for
negative assertions. For example,
.sp
  \ew+(?=;)
.sp
matches a word followed by a semicolon, but does not include the semicolon in
the match, and
.sp
  foo(?!bar)
.sp
matches any occurrence of "foo" that is not followed by "bar". Note that the
apparently similar pattern
.sp
  (?!foo)bar
.sp
does not find an occurrence of "bar" that is preceded by something other than
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
(?!foo) is always true when the next three characters are "bar". A
lookbehind assertion is needed to achieve the other effect.
.P
If you want to force a matching failure at some point in a pattern, the most
convenient way to do it is with (?!) because an empty string always matches, so
an assertion that requires there not to be an empty string must always fail.
The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
.
.
.\" HTML <a name="lookbehind"></a>
.SS "Lookbehind assertions"
.rs
.sp
Lookbehind assertions start with (?<= for positive assertions and (?<! for
negative assertions. For example,
.sp
  (?<!foo)bar
.sp
does find an occurrence of "bar" that is not preceded by "foo". The contents of
a lookbehind assertion are restricted such that all the strings it matches must
have a fixed length. However, if there are several top-level alternatives, they
do not all have to have the same fixed length. Thus
.sp
  (?<=bullock|donkey)
.sp
is permitted, but
.sp
  (?<!dogs?|cats?)
.sp
causes an error at compile time. Branches that match different length strings
are permitted only at the top level of a lookbehind assertion. This is an
extension compared with Perl, which requires all branches to match the same
length of string. An assertion such as
.sp
  (?<=ab(c|de))
.sp
is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable to PCRE2 if rewritten to use two top-level
branches:
.sp
  (?<=abc|abde)
.sp
In some cases, the escape sequence \eK
.\" HTML <a href="#resetmatchstart">
.\" </a>
(see above)
.\"
can be used instead of a lookbehind assertion to get round the fixed-length
restriction.
.P
The implementation of lookbehind assertions is, for each alternative, to
temporarily move the current position back by the fixed length and then try to
match. If there are insufficient characters before the current position, the
assertion fails.
.P
In a UTF mode, PCRE2 does not allow the \eC escape (which matches a single code
unit even in a UTF mode) to appear in lookbehind assertions, because it makes
it impossible to calculate the length of the lookbehind. The \eX and \eR
escapes, which can match different numbers of code units, are also not
permitted.
.P
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
"Subroutine"
.\"
calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
as the subpattern matches a fixed-length string.
.\" HTML <a href="#recursion">
.\" </a>
Recursion,
.\"
however, is not supported.
.P
Possessive quantifiers can be used in conjunction with lookbehind assertions to
specify efficient matching of fixed-length strings at the end of subject
strings. Consider a simple pattern such as
.sp
  abcd$
.sp
when applied to a long string that does not match. Because matching proceeds
from left to right, PCRE2 will look for each "a" in the subject and then see if
what follows matches the rest of the pattern. If the pattern is specified as
.sp
  ^.*abcd$
.sp
the initial .* matches the entire string at first, but when this fails (because
there is no following "a"), it backtracks to match all but the last character,
then all but the last two characters, and so on. Once again the search for "a"
covers the entire string, from right to left, so we are no better off. However,
if the pattern is written as
.sp
  ^.*+(?<=abcd)
.sp
there can be no backtracking for the .*+ item because of the possessive
quantifier; it can match only the entire string. The subsequent lookbehind
assertion does a single test on the last four characters. If it fails, the
match fails immediately. For long strings, this approach makes a significant
difference to the processing time.
.
.
.SS "Using multiple assertions"
.rs
.sp
Several assertions (of any sort) may occur in succession. For example,
.sp
  (?<=\ed{3})(?<!999)foo
.sp
matches "foo" preceded by three digits that are not "999". Notice that each of
the assertions is applied independently at the same point in the subject
string. First there is a check that the previous three characters are all
digits, and then there is a check that the same three characters are not "999".
This pattern does \fInot\fP match "foo" preceded by six characters, the first
of which are digits and the last three of which are not "999". For example, it
doesn't match "123abcfoo". A pattern to do that is
.sp
  (?<=\ed{3}...)(?<!999)foo
.sp
This time the first assertion looks at the preceding six characters, checking
that the first three are digits, and then the second assertion checks that the
preceding three characters are not "999".
.P
Assertions can be nested in any combination. For example,
.sp
  (?<=(?<!foo)bar)baz
.sp
matches an occurrence of "baz" that is preceded by "bar" which in turn is not
preceded by "foo", while
.sp
  (?<=\ed{3}(?!999)...)foo
.sp
is another pattern that matches "foo" preceded by three digits and any three
characters that are not "999".
.
.
.\" HTML <a name="conditions"></a>
.SH "CONDITIONAL SUBPATTERNS"
.rs
.sp
It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns, depending on
the result of an assertion, or whether a specific capturing subpattern has
already been matched. The two possible forms of conditional subpattern are:
.sp
  (?(condition)yes-pattern)
  (?(condition)yes-pattern|no-pattern)
.sp
If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alternatives in the
subpattern, a compile-time error occurs. Each of the two alternatives may
itself contain nested subpatterns of any form, including conditional
subpatterns; the restriction to two alternatives applies only at the level of
the condition. This pattern fragment is an example where the alternatives are
complex:
.sp
  (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
.sp
.P
There are five kinds of condition: references to subpatterns, references to
recursion, two pseudo-conditions called DEFINE and VERSION, and assertions.
.
.
.SS "Checking for a used subpattern by number"
.rs
.sp
If the text between the parentheses consists of a sequence of digits, the
condition is true if a capturing subpattern of that number has previously
matched. If there is more than one capturing subpattern with the same number
(see the earlier
.\"
.\" HTML <a href="#recursion">
.\" </a>
section about duplicate subpattern numbers),
.\"
the condition is true if any of them have matched. An alternative notation is
to precede the digits with a plus or minus sign. In this case, the subpattern
number is relative rather than absolute. The most recently opened parentheses
can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
loops it can also make sense to refer to subsequent groups. The next
parentheses to be opened can be referenced as (?(+1), and so on. (The value
zero in any of these forms is not used; it provokes a compile-time error.)
.P
Consider the following pattern, which contains non-significant white space to
make it more readable (assume the PCRE2_EXTENDED option) and to divide it into
three parts for ease of discussion:
.sp
  ( \e( )?    [^()]+    (?(1) \e) )
.sp
The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The second part
matches one or more characters that are not parentheses. The third part is a
conditional subpattern that tests whether or not the first set of parentheses
matched. If they did, that is, if subject started with an opening parenthesis,
the condition is true, and so the yes-pattern is executed and a closing
parenthesis is required. Otherwise, since no-pattern is not present, the
subpattern matches nothing. In other words, this pattern matches a sequence of
non-parentheses, optionally enclosed in parentheses.
.P
If you were embedding this pattern in a larger one, you could use a relative
reference:
.sp
  ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
.sp
This makes the fragment independent of the parentheses in the larger pattern.
.
.
.SS "Checking for a used subpattern by name"
.rs
.sp
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
subpattern by name. For compatibility with earlier versions of PCRE1, which had
this facility before Perl, the syntax (?(name)...) is also recognized.
.P
Rewriting the above example to use a named subpattern gives this:
.sp
  (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
.sp
If the name used in a condition of this kind is a duplicate, the test is
applied to all subpatterns of the same name, and is true if any one of them has
matched.
.
.
.SS "Checking for pattern recursion"
.rs
.sp
If the condition is the string (R), and there is no subpattern with the name R,
the condition is true if a recursive call to the whole pattern or any
subpattern has been made. If digits or a name preceded by ampersand follow the
letter R, for example:
.sp
  (?(R3)...) or (?(R&name)...)
.sp
the condition is true if the most recent recursion is into a subpattern whose
number or name is given. This condition does not check the entire recursion
stack. If the name used in a condition of this kind is a duplicate, the test is
applied to all subpatterns of the same name, and is true if any one of them is
the most recent recursion.
.P
At "top level", all these recursion test conditions are false.
.\" HTML <a href="#recursion">
.\" </a>
The syntax for recursive patterns
.\"
is described below.
.
.
.\" HTML <a name="subdefine"></a>
.SS "Defining subpatterns for use by reference only"
.rs
.sp
If the condition is the string (DEFINE), and there is no subpattern with the
name DEFINE, the condition is always false. In this case, there may be only one
alternative in the subpattern. It is always skipped if control reaches this
point in the pattern; the idea of DEFINE is that it can be used to define
subroutines that can be referenced from elsewhere. (The use of
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
subroutines
.\"
is described below.) For example, a pattern to match an IPv4 address such as
"192.168.23.245" could be written like this (ignore white space and line
breaks):
.sp
  (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
  \eb (?&byte) (\e.(?&byte)){3} \eb
.sp
The first part of the pattern is a DEFINE group inside which a another group
named "byte" is defined. This matches an individual component of an IPv4
address (a number less than 256). When matching takes place, this part of the
pattern is skipped because DEFINE acts like a false condition. The rest of the
pattern uses references to the named group to match the four dot-separated
components of an IPv4 address, insisting on a word boundary at each end.
.
.
.SS "Checking the PCRE2 version"
.rs
.sp
Programs that link with a PCRE2 library can check the version by calling
\fBpcre2_config()\fP with appropriate arguments. Users of applications that do
not have access to the underlying code cannot do this. A special "condition"
called VERSION exists to allow such users to discover which version of PCRE2
they are dealing with by using this condition to match a string such as
"yesno". VERSION must be followed either by "=" or ">=" and a version number.
For example:
.sp
  (?(VERSION>=10.4)yes|no)
.sp
This pattern matches "yes" if the PCRE2 version is greater or equal to 10.4, or
"no" otherwise.
.
.
.SS "Assertion conditions"
.rs
.sp
If the condition is not in any of the above formats, it must be an assertion.
This may be a positive or negative lookahead or lookbehind assertion. Consider
this pattern, again containing non-significant white space, and with the two
alternatives on the second line:
.sp
  (?(?=[^a-z]*[a-z])
  \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
.sp
The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests for the
presence of at least one letter in the subject. If a letter is found, the
subject is matched against the first alternative; otherwise it is matched
against the second. This pattern matches strings in one of the two forms
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
.
.
.\" HTML <a name="comments"></a>
.SH COMMENTS
.rs
.sp
There are two ways of including comments in patterns that are processed by
PCRE2. In both cases, the start of the comment must not be in a character
class, nor in the middle of any other sequence of related characters such as
(?: or a subpattern name or number. The characters that make up a comment play
no part in the pattern matching.
.P
The sequence (?# marks the start of a comment that continues up to the next
closing parenthesis. Nested parentheses are not permitted. If the
PCRE2_EXTENDED option is set, an unescaped # character also introduces a
comment, which in this case continues to immediately after the next newline
character or character sequence in the pattern. Which characters are
interpreted as newlines is controlled by an option passed to the compiling
function or by a special sequence at the start of the pattern, as described in
the section entitled
.\" HTML <a href="#newlines">
.\" </a>
"Newline conventions"
.\"
above. Note that the end of this type of comment is a literal newline sequence
in the pattern; escape sequences that happen to represent a newline do not
count. For example, consider this pattern when PCRE2_EXTENDED is set, and the
default newline convention (a single linefeed character) is in force:
.sp
  abc #comment \en still comment
.sp
On encountering the # character, \fBpcre2_compile()\fP skips along, looking for
a newline in the pattern. The sequence \en is still literal at this stage, so
it does not terminate the comment. Only an actual character with the code value
0x0a (the default newline) does so.
.
.
.\" HTML <a name="recursion"></a>
.SH "RECURSIVE PATTERNS"
.rs
.sp
Consider the problem of matching a string in parentheses, allowing for
unlimited nested parentheses. Without the use of recursion, the best that can
be done is to use a pattern that matches up to some fixed depth of nesting. It
is not possible to handle an arbitrary nesting depth.
.P
For some time, Perl has provided a facility that allows regular expressions to
recurse (amongst other things). It does this by interpolating Perl code in the
expression at run time, and the code can refer to the expression itself. A Perl
pattern using code interpolation to solve the parentheses problem can be
created like this:
.sp
  $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
.sp
The (?p{...}) item interpolates Perl code at run time, and in this case refers
recursively to the pattern in which it appears.
.P
Obviously, PCRE2 cannot support the interpolation of Perl code. Instead, it
supports special syntax for recursion of the entire pattern, and also for
individual subpattern recursion. After its introduction in PCRE1 and Python,
this kind of recursion was subsequently introduced into Perl at release 5.10.
.P
A special item that consists of (? followed by a number greater than zero and a
closing parenthesis is a recursive subroutine call of the subpattern of the
given number, provided that it occurs inside that subpattern. (If not, it is a
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
non-recursive subroutine
.\"
call, which is described in the next section.) The special item (?R) or (?0) is
a recursive call of the entire regular expression.
.P
This PCRE2 pattern solves the nested parentheses problem (assume the
PCRE2_EXTENDED option is set so that white space is ignored):
.sp
  \e( ( [^()]++ | (?R) )* \e)
.sp
First it matches an opening parenthesis. Then it matches any number of
substrings which can either be a sequence of non-parentheses, or a recursive
match of the pattern itself (that is, a correctly parenthesized substring).
Finally there is a closing parenthesis. Note the use of a possessive quantifier
to avoid backtracking into sequences of non-parentheses.
.P
If this were part of a larger pattern, you would not want to recurse the entire
pattern, so instead you could use this:
.sp
  ( \e( ( [^()]++ | (?1) )* \e) )
.sp
We have put the pattern into parentheses, and caused the recursion to refer to
them instead of the whole pattern.
.P
In a larger pattern, keeping track of parenthesis numbers can be tricky. This
is made easier by the use of relative references. Instead of (?1) in the
pattern above you can write (?-2) to refer to the second most recently opened
parentheses preceding the recursion. In other words, a negative number counts
capturing parentheses leftwards from the point at which it is encountered.
.P
It is also possible to refer to subsequently opened parentheses, by writing
references such as (?+2). However, these cannot be recursive because the
reference is not inside the parentheses that are referenced. They are always
.\" HTML <a href="#subpatternsassubroutines">
.\" </a>
non-recursive subroutine
.\"
calls, as described in the next section.
.P
An alternative approach is to use named parentheses. The Perl syntax for this
is (?&name); PCRE1's earlier syntax (?P>name) is also supported. We could
rewrite the above example as follows:
.sp
  (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
.sp
If there is more than one subpattern with the same name, the earliest one is
used.
.P
The example pattern that we have been looking at contains nested unlimited
repeats, and so the use of a possessive quantifier for matching strings of
non-parentheses is important when applying the pattern to strings that do not
match. For example, when this pattern is applied to
.sp
  (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
.sp
it yields "no match" quickly. However, if a possessive quantifier is not used,
the match runs for a very long time indeed because there are so many different
ways the + and * repeats can carve up the subject, and all have to be tested
before failure can be reported.
.P
At the end of a match, the values of capturing parentheses are those from
the outermost level. If you want to obtain intermediate values, a callout
function can be used (see below and the
.\" HREF
\fBpcre2callout\fP
.\"
documentation). If the pattern above is matched against
.sp
  (ab(cd)ef)
.sp
the value for the inner capturing parentheses (numbered 2) is "ef", which is
the last value taken on at the top level. If a capturing subpattern is not
matched at the top level, its final captured value is unset, even if it was
(temporarily) set at a deeper level during the matching process.
.P
If there are more than 15 capturing parentheses in a pattern, PCRE2 has to
obtain extra memory from the heap to store data during a recursion. If no
memory can be obtained, the match fails with the PCRE2_ERROR_NOMEMORY error.
.P
Do not confuse the (?R) item with the condition (R), which tests for recursion.
Consider this pattern, which matches text in angle brackets, allowing for
arbitrary nesting. Only digits are allowed in nested brackets (that is, when
recursing), whereas any characters are permitted at the outer level.
.sp
  < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
.sp
In this pattern, (?(R) is the start of a conditional subpattern, with two
different alternatives for the recursive and non-recursive cases. The (?R) item
is the actual recursive call.
.
.
.\" HTML <a name="recursiondifference"></a>
.SS "Differences in recursion processing between PCRE2 and Perl"
.rs
.sp
Recursion processing in PCRE2 differs from Perl in two important ways. In PCRE2
(like Python, but unlike Perl), a recursive subpattern call is always treated
as an atomic group. That is, once it has matched some of the subject string, it
is never re-entered, even if it contains untried alternatives and there is a
subsequent matching failure. This can be illustrated by the following pattern,
which purports to match a palindromic string that contains an odd number of
characters (for example, "a", "aba", "abcba", "abcdcba"):
.sp
  ^(.|(.)(?1)\e2)$
.sp
The idea is that it either matches a single character, or two identical
characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE2
it does not if the pattern is longer than three characters. Consider the
subject string "abcba":
.P
At the top level, the first character is matched, but as it is not at the end
of the string, the first alternative fails; the second alternative is taken
and the recursion kicks in. The recursive call to subpattern 1 successfully
matches the next character ("b"). (Note that the beginning and end of line
tests are not part of the recursion).
.P
Back at the top level, the next character ("c") is compared with what
subpattern 2 matched, which was "a". This fails. Because the recursion is
treated as an atomic group, there are now no backtracking points, and so the
entire match fails. (Perl is able, at this point, to re-enter the recursion and
try the second alternative.) However, if the pattern is written with the
alternatives in the other order, things are different:
.sp
  ^((.)(?1)\e2|.)$
.sp
This time, the recursing alternative is tried first, and continues to recurse
until it runs out of characters, at which point the recursion fails. But this
time we do have another alternative to try at the higher level. That is the big
difference: in the previous case the remaining alternative is at a deeper
recursion level, which PCRE2 cannot use.
.P
To change the pattern so that it matches all palindromic strings, not just
those with an odd number of characters, it is tempting to change the pattern to
this:
.sp
  ^((.)(?1)\e2|.?)$
.sp
Again, this works in Perl, but not in PCRE2, and for the same reason. When a
deeper recursion has matched a single character, it cannot be entered again in
order to match an empty string. The solution is to separate the two cases, and
write out the odd and even cases as alternatives at the higher level:
.sp
  ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
.sp
If you want to match typical palindromic phrases, the pattern has to ignore all
non-word characters, which can be done like this:
.sp
  ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
.sp
If run with the PCRE2_CASELESS option, this pattern matches phrases such as "A
man, a plan, a canal: Panama!" and it works in both PCRE2 and Perl. Note the
use of the possessive quantifier *+ to avoid backtracking into sequences of
non-word characters. Without this, PCRE2 takes a great deal longer (ten times
or more) to match typical phrases, and Perl takes so long that you think it has
gone into a loop.
.P
\fBWARNING\fP: The palindrome-matching patterns above work only if the subject
string does not start with a palindrome that is shorter than the entire string.
For example, although "abcba" is correctly matched, if the subject is "ababa",
PCRE2 finds the palindrome "aba" at the start, then fails at top level because
the end of the string does not follow. Once again, it cannot jump back into the
recursion to try other alternatives, so the entire match fails.
.P
The second way in which PCRE2 and Perl differ in their recursion processing is
in the handling of captured values. In Perl, when a subpattern is called
recursively or as a subpattern (see the next section), it has no access to any
values that were captured outside the recursion, whereas in PCRE2 these values
can be referenced. Consider this pattern:
.sp
  ^(.)(\e1|a(?2))
.sp
In PCRE2, this pattern matches "bab". The first capturing parentheses match "b",
then in the second group, when the back reference \e1 fails to match "b", the
second alternative matches "a" and then recurses. In the recursion, \e1 does
now match "b" and so the whole match succeeds. In Perl, the pattern fails to
match because inside the recursive call \e1 cannot access the externally set
value.
.
.
.\" HTML <a name="subpatternsassubroutines"></a>
.SH "SUBPATTERNS AS SUBROUTINES"
.rs
.sp
If the syntax for a recursive subpattern call (either by number or by
name) is used outside the parentheses to which it refers, it operates like a
subroutine in a programming language. The called subpattern may be defined
before or after the reference. A numbered reference can be absolute or
relative, as in these examples:
.sp
  (...(absolute)...)...(?2)...
  (...(relative)...)...(?-1)...
  (...(?+1)...(relative)...
.sp
An earlier example pointed out that the pattern
.sp
  (sens|respons)e and \e1ibility
.sp
matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If instead the pattern
.sp
  (sens|respons)e and (?1)ibility
.sp
is used, it does match "sense and responsibility" as well as the other two
strings. Another example is given in the discussion of DEFINE above.
.P
All subroutine calls, whether recursive or not, are always treated as atomic
groups. That is, once a subroutine has matched some of the subject string, it
is never re-entered, even if it contains untried alternatives and there is a
subsequent matching failure. Any capturing parentheses that are set during the
subroutine call revert to their previous values afterwards.
.P
Processing options such as case-independence are fixed when a subpattern is
defined, so if it is used as a subroutine, such options cannot be changed for
different calls. For example, consider this pattern:
.sp
  (abc)(?i:(?-1))
.sp
It matches "abcabc". It does not match "abcABC" because the change of
processing option does not affect the called subpattern.
.
.
.\" HTML <a name="onigurumasubroutines"></a>
.SH "ONIGURUMA SUBROUTINE SYNTAX"
.rs
.sp
For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
a number enclosed either in angle brackets or single quotes, is an alternative
syntax for referencing a subpattern as a subroutine, possibly recursively. Here
are two of the examples used above, rewritten using this syntax:
.sp
  (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
  (sens|respons)e and \eg'1'ibility
.sp
PCRE2 supports an extension to Oniguruma: if a number is preceded by a
plus or a minus sign it is taken as a relative reference. For example:
.sp
  (abc)(?i:\eg<-1>)
.sp
Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
synonymous. The former is a back reference; the latter is a subroutine call.
.
.
.SH CALLOUTS
.rs
.sp
Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
code to be obeyed in the middle of matching a regular expression. This makes it
possible, amongst other things, to extract different substrings that match the
same pair of parentheses when there is a repetition.
.P
PCRE2 provides a similar feature, but of course it cannot obey arbitrary Perl
code. The feature is called "callout". The caller of PCRE2 provides an external
function by putting its entry point in a match context using the function
\fBpcre2_set_callout()\fP and passing the context to \fBpcre2_match()\fP or
\fBpcre2_dfa_match()\fP. If no match context is passed, or if the callout entry
point is set to NULL, callouts are disabled.
.P
Within a regular expression, (?C) indicates the points at which the external
function is to be called. If you want to identify different callout points, you
can put a number less than 256 after the letter C. The default value is zero.
For example, this pattern has two callout points:
.sp
  (?C1)abc(?C2)def
.sp
If the PCRE2_AUTO_CALLOUT flag is passed to \fBpcre2_compile()\fP, callouts are
automatically installed before each item in the pattern. They are all numbered
255. If there is a conditional group in the pattern whose condition is an
assertion, an additional callout is inserted just before the condition. An
explicit callout may also be set at this position, as in this example:
.sp
  (?(?C9)(?=a)abc|def)
.sp
Note that this applies only to assertion conditions, not to other types of
condition.
.P
During matching, when PCRE2 reaches a callout point, the external function is
called. It is provided with the number of the callout, the position in the
pattern, and one item of data that is also set in the match block. The callout
function may cause matching to proceed, to backtrack, or to fail.
.P
By default, PCRE2 implements a number of optimizations at matching time, and
one side-effect is that sometimes callouts are skipped. If you need all
possible callouts to happen, you need to set options that disable the relevant
optimizations. More details, and a complete description of the interface to the
callout function, are given in the
.\" HREF
\fBpcre2callout\fP
.\"
documentation.
.
.
.\" HTML <a name="backtrackcontrol"></a>
.SH "BACKTRACKING CONTROL"
.rs
.sp
Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
are still described in the Perl documentation as "experimental and subject to
change or removal in a future version of Perl". It goes on to say: "Their usage
in production code should be noted to avoid problems during upgrades." The same
remarks apply to the PCRE2 features described in this section.
.P
The new verbs make use of what was previously invalid syntax: an opening
parenthesis followed by an asterisk. They are generally of the form
(*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
differently depending on whether or not a name is present. A name is any
sequence of characters that does not include a closing parenthesis. The maximum
length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
libraries. If the name is empty, that is, if the closing parenthesis
immediately follows the colon, the effect is as if the colon were not there.
Any number of these verbs may occur in a pattern.
.P
Since these verbs are specifically related to backtracking, most of them can be
used only when the pattern is to be matched using the traditional matching
function, because these use a backtracking algorithm. With the exception of
(*FAIL), which behaves like a failing negative assertion, the backtracking
control verbs cause an error if encountered by the DFA matching function.
.P
The behaviour of these verbs in
.\" HTML <a href="#btrepeat">
.\" </a>
repeated groups,
.\"
.\" HTML <a href="#btassert">
.\" </a>
assertions,
.\"
and in
.\" HTML <a href="#btsub">
.\" </a>
subpatterns called as subroutines
.\"
(whether or not recursively) is documented below.
.
.
.\" HTML <a name="nooptimize"></a>
.SS "Optimizations that affect backtracking verbs"
.rs
.sp
PCRE2 contains some optimizations that are used to speed up matching by running
some checks at the start of each match attempt. For example, it may know the
minimum length of matching subject, or that a particular character must be
present. When one of these optimizations bypasses the running of a match, any
included backtracking verbs will not, of course, be processed. You can suppress
the start-of-match optimizations by setting the PCRE2_NO_START_OPTIMIZE option
when calling \fBpcre2_compile()\fP, or by starting the pattern with
(*NO_START_OPT). There is more discussion of this option in the section
entitled
.\" HTML <a href="pcre2api.html#compiling">
.\" </a>
"Compiling a pattern"
.\"
in the
.\" HREF
\fBpcre2api\fP
.\"
documentation.
.P
Experiments with Perl suggest that it too has similar optimizations, sometimes
leading to anomalous results.
.
.
.SS "Verbs that act immediately"
.rs
.sp
The following verbs act as soon as they are encountered. They may not be
followed by a name.
.sp
   (*ACCEPT)
.sp
This verb causes the match to end successfully, skipping the remainder of the
pattern. However, when it is inside a subpattern that is called as a
subroutine, only that subpattern is ended successfully. Matching then continues
at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
assertion succeeds; in a negative assertion, the assertion fails.
.P
If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
example:
.sp
  A((?:A|B(*ACCEPT)|C)D)
.sp
This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
the outer parentheses.
.sp
  (*FAIL) or (*F)
.sp
This verb causes a matching failure, forcing backtracking to occur. It is
equivalent to (?!) but easier to read. The Perl documentation notes that it is
probably useful only when combined with (?{}) or (??{}). Those are, of course,
Perl features that are not present in PCRE2. The nearest equivalent is the
callout feature, as for example in this pattern:
.sp
  a+(?C)(*FAIL)
.sp
A match with the string "aaaa" always fails, but the callout is taken before
each backtrack happens (in this example, 10 times).
.
.
.SS "Recording which path was taken"
.rs
.sp
There is one verb whose main purpose is to track how a match was arrived at,
though it also has a secondary use in conjunction with advancing the match
starting point (see (*SKIP) below).
.sp
  (*MARK:NAME) or (*:NAME)
.sp
A name is always required with this verb. There may be as many instances of
(*MARK) as you like in a pattern, and their names do not have to be unique.
.P
When a match succeeds, the name of the last-encountered (*MARK:NAME),
(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
caller as described in the section entitled
.\" HTML <a href="pcre2api.html#matchotherdata">
.\" </a>
"Other information about the match"
.\"
in the
.\" HREF
\fBpcre2api\fP
.\"
documentation. Here is an example of \fBpcre2test\fP output, where the "mark"
modifier requests the retrieval and outputting of (*MARK) data:
.sp
    re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
  data> XY
   0: XY
  MK: A
  XZ
   0: XZ
  MK: B
.sp
The (*MARK) name is tagged with "MK:" in this output, and in this example it
indicates which of the two alternatives matched. This is a more efficient way
of obtaining this information than putting each alternative in its own
capturing parentheses.
.P
If a verb with a name is encountered in a positive assertion that is true, the
name is recorded and passed back if it is the last-encountered. This does not
happen for negative assertions or failing positive assertions.
.P
After a partial match or a failed match, the last encountered name in the
entire match process is returned. For example:
.sp
    re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
  data> XP
  No match, mark = B
.sp
Note that in this unanchored example the mark is retained from the match
attempt that started at the letter "X" in the subject. Subsequent match
attempts starting at "P" and then with an empty string do not get as far as the
(*MARK) item, but nevertheless do not reset it.
.P
If you are interested in (*MARK) values after failed matches, you should
probably set the PCRE2_NO_START_OPTIMIZE option
.\" HTML <a href="#nooptimize">
.\" </a>
(see above)
.\"
to ensure that the match is always attempted.
.
.
.SS "Verbs that act after backtracking"
.rs
.sp
The following verbs do nothing when they are encountered. Matching continues
with what follows, but if there is no subsequent match, causing a backtrack to
the verb, a failure is forced. That is, backtracking cannot pass to the left of
the verb. However, when one of these verbs appears inside an atomic group
(which includes any group that is called as a subroutine) or in an assertion
that is true, its effect is confined to that group, because once the group has
been matched, there is never any backtracking into it. In this situation,
backtracking has to jump to the left of the entire atomic group or assertion.
.P
These verbs differ in exactly what kind of failure occurs when backtracking
reaches them. The behaviour described below is what happens when the verb is
not in a subroutine or an assertion. Subsequent sections cover these special
cases.
.sp
  (*COMMIT)
.sp
This verb, which may not be followed by a name, causes the whole match to fail
outright if there is a later matching failure that causes backtracking to reach
it. Even if the pattern is unanchored, no further attempts to find a match by
advancing the starting point take place. If (*COMMIT) is the only backtracking
verb that is encountered, once it has been passed \fBpcre2_match()\fP is
committed to finding a match at the current starting point, or not at all. For
example:
.sp
  a+(*COMMIT)b
.sp
This matches "xxaab" but not "aacaab". It can be thought of as a kind of
dynamic anchor, or "I've started, so I must finish." The name of the most
recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
match failure.
.P
If there is more than one backtracking verb in a pattern, a different one that
follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
match does not always guarantee that a match must be at this starting point.
.P
Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
unless PCRE2's start-of-match optimizations are turned off, as shown in this
output from \fBpcre2test\fP:
.sp
    re> /(*COMMIT)abc/
  data> xyzabc
   0: abc
  data>
  re> /(*COMMIT)abc/no_start_optimize
  data> xyzabc
  No match
.sp
For the first pattern, PCRE2 knows that any match must start with "a", so the
optimization skips along the subject to "a" before applying the pattern to the
first set of data. The match attempt then succeeds. The second pattern disables
the optimization that skips along to the first character. The pattern is now
applied starting at "x", and so the (*COMMIT) causes the match to fail without
trying any other starting points.
.sp
  (*PRUNE) or (*PRUNE:NAME)
.sp
This verb causes the match to fail at the current starting position in the
subject if there is a later matching failure that causes backtracking to reach
it. If the pattern is unanchored, the normal "bumpalong" advance to the next
starting character then happens. Backtracking can occur as usual to the left of
(*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
if there is no match to the right, backtracking cannot cross (*PRUNE). In
simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
possessive quantifier, but there are some uses of (*PRUNE) that cannot be
expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
as (*COMMIT).
.P
The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
It is like (*MARK:NAME) in that the name is remembered for passing back to the
caller. However, (*SKIP:NAME) searches only for names set with (*MARK),
ignoring those set by (*PRUNE) or (*THEN).
.sp
  (*SKIP)
.sp
This verb, when given without a name, is like (*PRUNE), except that if the
pattern is unanchored, the "bumpalong" advance is not to the next character,
but to the position in the subject where (*SKIP) was encountered. (*SKIP)
signifies that whatever text was matched leading up to it cannot be part of a
successful match. Consider:
.sp
  a+(*SKIP)b
.sp
If the subject is "aaaac...", after the first match attempt fails (starting at
the first character in the string), the starting point skips on to start the
next attempt at "c". Note that a possessive quantifer does not have the same
effect as this example; although it would suppress backtracking during the
first match attempt, the second attempt would start at the second character
instead of skipping on to "c".
.sp
  (*SKIP:NAME)
.sp
When (*SKIP) has an associated name, its behaviour is modified. When it is
triggered, the previous path through the pattern is searched for the most
recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
is to the subject position that corresponds to that (*MARK) instead of to where
(*SKIP) was encountered. If no (*MARK) with a matching name is found, the
(*SKIP) is ignored.
.P
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
names that are set by (*PRUNE:NAME) or (*THEN:NAME).
.sp
  (*THEN) or (*THEN:NAME)
.sp
This verb causes a skip to the next innermost alternative when backtracking
reaches it. That is, it cancels any further backtracking within the current
alternative. Its name comes from the observation that it can be used for a
pattern-based if-then-else block:
.sp
  ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
.sp
If the COND1 pattern matches, FOO is tried (and possibly further items after
the end of the group if FOO succeeds); on failure, the matcher skips to the
second alternative and tries COND2, without backtracking into COND1. If that
succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
more alternatives, so there is a backtrack to whatever came before the entire
group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
.P
The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
It is like (*MARK:NAME) in that the name is remembered for passing back to the
caller. However, (*SKIP:NAME) searches only for names set with (*MARK),
ignoring those set by (*PRUNE) and (*THEN).
.P
A subpattern that does not contain a | character is just a part of the
enclosing alternative; it is not a nested alternation with only one
alternative. The effect of (*THEN) extends beyond such a subpattern to the
enclosing alternative. Consider this pattern, where A, B, etc. are complex
pattern fragments that do not contain any | characters at this level:
.sp
  A (B(*THEN)C) | D
.sp
If A and B are matched, but there is a failure in C, matching does not
backtrack into A; instead it moves to the next alternative, that is, D.
However, if the subpattern containing (*THEN) is given an alternative, it
behaves differently:
.sp
  A (B(*THEN)C | (*FAIL)) | D
.sp
The effect of (*THEN) is now confined to the inner subpattern. After a failure
in C, matching moves to (*FAIL), which causes the whole subpattern to fail
because there are no more alternatives to try. In this case, matching does now
backtrack into A.
.P
Note that a conditional subpattern is not considered as having two
alternatives, because only one is ever used. In other words, the | character in
a conditional subpattern has a different meaning. Ignoring white space,
consider:
.sp
  ^.*? (?(?=a) a | b(*THEN)c )
.sp
If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
it initially matches zero characters. The condition (?=a) then fails, the
character "b" is matched, but "c" is not. At this point, matching does not
backtrack to .*? as might perhaps be expected from the presence of the |
character. The conditional subpattern is part of the single alternative that
comprises the whole pattern, and so the match fails. (If there was a backtrack
into .*?, allowing it to match "b", the match would succeed.)
.P
The verbs just described provide four different "strengths" of control when
subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
next alternative. (*PRUNE) comes next, failing the match at the current
starting position, but allowing an advance to the next character (for an
unanchored pattern). (*SKIP) is similar, except that the advance may be more
than one character. (*COMMIT) is the strongest, causing the entire match to
fail.
.
.
.SS "More than one backtracking verb"
.rs
.sp
If more than one backtracking verb is present in a pattern, the one that is
backtracked onto first acts. For example, consider this pattern, where A, B,
etc. are complex pattern fragments:
.sp
  (A(*COMMIT)B(*THEN)C|ABD)
.sp
If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
the next alternative (ABD) to be tried. This behaviour is consistent, but is
not always the same as Perl's. It means that if two or more backtracking verbs
appear in succession, all the the last of them has no effect. Consider this
example:
.sp
  ...(*COMMIT)(*PRUNE)...
.sp
If there is a matching failure to the right, backtracking onto (*PRUNE) causes
it to be triggered, and its action is taken. There can never be a backtrack
onto (*COMMIT).
.
.
.\" HTML <a name="btrepeat"></a>
.SS "Backtracking verbs in repeated groups"
.rs
.sp
PCRE2 differs from Perl in its handling of backtracking verbs in repeated
groups. For example, consider:
.sp
  /(a(*COMMIT)b)+ac/
.sp
If the subject is "abac", Perl matches, but PCRE2 fails because the (*COMMIT)
in the second repeat of the group acts.
.
.
.\" HTML <a name="btassert"></a>
.SS "Backtracking verbs in assertions"
.rs
.sp
(*FAIL) in an assertion has its normal effect: it forces an immediate
backtrack.
.P
(*ACCEPT) in a positive assertion causes the assertion to succeed without any
further processing. In a negative assertion, (*ACCEPT) causes the assertion to
fail without any further processing.
.P
The other backtracking verbs are not treated specially if they appear in a
positive assertion. In particular, (*THEN) skips to the next alternative in the
innermost enclosing group that has alternations, whether or not this is within
the assertion.
.P
Negative assertions are, however, different, in order to ensure that changing a
positive assertion into a negative assertion changes its result. Backtracking
into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
without considering any further alternative branches in the assertion.
Backtracking into (*THEN) causes it to skip to the next enclosing alternative
within the assertion (the normal behaviour), but if the assertion does not have
such an alternative, (*THEN) behaves like (*PRUNE).
.
.
.\" HTML <a name="btsub"></a>
.SS "Backtracking verbs in subroutines"
.rs
.sp
These behaviours occur whether or not the subpattern is called recursively.
Perl's treatment of subroutines is different in some cases.
.P
(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
an immediate backtrack.
.P
(*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
succeed without any further processing. Matching then continues after the
subroutine call.
.P
(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
the subroutine match to fail.
.P
(*THEN) skips to the next alternative in the innermost enclosing group within
the subpattern that has alternatives. If there is no such group within the
subpattern, (*THEN) causes the subroutine match to fail.
.
.
.SH "SEE ALSO"
.rs
.sp
\fBpcre2api\fP(3), \fBpcre2callout\fP(3), \fBpcre2matching\fP(3),
\fBpcre2syntax\fP(3), \fBpcre2\fP(3).
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 02 January 2015
Copyright (c) 1997-2015 University of Cambridge.
.fi