pcre2/doc/pcre2unicode.3

.TH PCRE2UNICODE 3 "03 February 2019" "PCRE2 10.33"
.SH NAME
PCRE - Perl-compatible regular expressions (revised API)
.SH "UNICODE AND UTF SUPPORT"
.rs
.sp
When PCRE2 is built with Unicode support (which is the default), it has
knowledge of Unicode character properties and can process text strings in
UTF-8, UTF-16, or UTF-32 format (depending on the code unit width). However, by
default, PCRE2 assumes that one code unit is one character. To process a
pattern as a UTF string, where a character may require more than one code unit,
you must call
.\" HREF
\fBpcre2_compile()\fP
.\"
with the PCRE2_UTF option flag, or the pattern must start with the sequence
(*UTF). When either of these is the case, both the pattern and any subject
strings that are matched against it are treated as UTF strings instead of
strings of individual one-code-unit characters. There are also some other
changes to the way characters are handled, as documented below.
.P
If you do not need Unicode support you can build PCRE2 without it, in which
case the library will be smaller.
.
.
.SH "UNICODE PROPERTY SUPPORT"
.rs
.sp
When PCRE2 is built with Unicode support, the escape sequences \ep{..},
\eP{..}, and \eX can be used. This is not dependent on the PCRE2_UTF setting.
The Unicode properties that can be tested are limited to the general category
properties such as Lu for an upper case letter or Nd for a decimal number, the
Unicode script names such as Arabic or Han, and the derived properties Any and
L&. Full lists are given in the
.\" HREF
\fBpcre2pattern\fP
.\"
and
.\" HREF
\fBpcre2syntax\fP
.\"
documentation. Only the short names for properties are supported. For example,
\ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
compatibility with Perl 5.6. PCRE2 does not support this.
.
.
.SH "WIDE CHARACTERS AND UTF MODES"
.rs
.sp
Code points less than 256 can be specified in patterns by either braced or
unbraced hexadecimal escape sequences (for example, \ex{b3} or \exb3). Larger
values have to use braced sequences. Unbraced octal code points up to \e777 are
also recognized; larger ones can be coded using \eo{...}.
.P
The escape sequence \eN{U+<hex digits>} is recognized as another way of
specifying a Unicode character by code point in a UTF mode. It is not allowed
in non-UTF modes.
.P
In UTF modes, repeat quantifiers apply to complete UTF characters, not to
individual code units.
.P
In UTF modes, the dot metacharacter matches one UTF character instead of a
single code unit.
.P
In UTF modes, capture group names are not restricted to ASCII, and may contain
any Unicode letters and decimal digits, as well as underscore.
.P
The escape sequence \eC can be used to match a single code unit in a UTF mode,
but its use can lead to some strange effects because it breaks up multi-unit
characters (see the description of \eC in the
.\" HREF
\fBpcre2pattern\fP
.\"
documentation). For this reason, there is a build-time option that disables 
support for \eC completely. There is also a less draconian compile-time option
for locking out the use of \eC when a pattern is compiled.
.P
The use of \eC is not supported by the alternative matching function
\fBpcre2_dfa_match()\fP when in UTF-8 or UTF-16 mode, that is, when a character
may consist of more than one code unit. The use of \eC in these modes provokes
a match-time error. Also, the JIT optimization does not support \eC in these
modes. If JIT optimization is requested for a UTF-8 or UTF-16 pattern that
contains \eC, it will not succeed, and so when \fBpcre2_match()\fP is called,
the matching will be carried out by the normal interpretive function.
.P
The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly test
characters of any code value, but, by default, the characters that PCRE2
recognizes as digits, spaces, or word characters remain the same set as in
non-UTF mode, all with code points less than 256. This remains true even when
PCRE2 is built to include Unicode support, because to do otherwise would slow
down matching in many common cases. Note that this also applies to \eb
and \eB, because they are defined in terms of \ew and \eW. If you want
to test for a wider sense of, say, "digit", you can use explicit Unicode
property tests such as \ep{Nd}. Alternatively, if you set the PCRE2_UCP option,
the way that the character escapes work is changed so that Unicode properties
are used to determine which characters match. There are more details in the
section on
.\" HTML <a href="pcre2pattern.html#genericchartypes">
.\" </a>
generic character types
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in the
.\" HREF
\fBpcre2pattern\fP
.\"
documentation.
.P
Similarly, characters that match the POSIX named character classes are all
low-valued characters, unless the PCRE2_UCP option is set.
.P
However, the special horizontal and vertical white space matching escapes (\eh,
\eH, \ev, and \eV) do match all the appropriate Unicode characters, whether or
not PCRE2_UCP is set.
.
.
.SH "CASE-EQUIVALENCE IN UTF MODES"
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.sp
Case-insensitive matching in a UTF mode makes use of Unicode properties except
for characters whose code points are less than 128 and that have at most two
case-equivalent values. For these, a direct table lookup is used for speed. A
few Unicode characters such as Greek sigma have more than two code points that
are case-equivalent, and these are treated as such.
.
.
.\" HTML <a name="scriptruns"></a>
.SH "SCRIPT RUNS"
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.sp
The pattern constructs (*script_run:...) and (*atomic_script_run:...), with
synonyms (*sr:...) and (*asr:...), verify that the string matched within the
parentheses is a script run. In concept, a script run is a sequence of
characters that are all from the same Unicode script. However, because some
scripts are commonly used together, and because some diacritical and other
marks are used with multiple scripts, it is not that simple.
.P
Every Unicode character has a Script property, mostly with a value 
corresponding to the name of a script, such as Latin, Greek, or Cyrillic. There
are also three special values:
.P
"Unknown" is used for code points that have not been assigned, and also for the
surrogate code points. In the PCRE2 32-bit library, characters whose code
points are greater than the Unicode maximum (U+10FFFF), which are accessible 
only in non-UTF mode, are assigned the Unknown script.
.P
"Common" is used for characters that are used with many scripts. These include
punctuation, emoji, mathematical, musical, and currency symbols, and the ASCII
digits 0 to 9.
.P
"Inherited" is used for characters such as diacritical marks that modify a
previous character. These are considered to take on the script of the character
that they modify.
.P
Some Inherited characters are used with many scripts, but many of them are only 
normally used with a small number of scripts. For example, U+102E0 (Coptic 
Epact thousands mark) is used only with Arabic and Coptic. In order to make it 
possible to check this, a Unicode property called Script Extension exists. Its 
value is a list of scripts that apply to the character. For the majority of 
characters, the list contains just one script, the same one as the Script
property. However, for characters such as U+102E0 more than one Script is
listed. There are also some Common characters that have a single, non-Common
script in their Script Extension list.
.P
The next section describes the basic rules for deciding whether a given string 
of characters is a script run. Note, however, that there are some special cases 
involving the Chinese Han script, and an additional constraint for decimal 
digits. These are covered in subsequent sections.
.
.
.SS "Basic script run rules"
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.sp
A string that is less than two characters long is a script run. This is the
only case in which an Unknown character can be part of a script run. Longer
strings are checked using only the Script Extensions property, not the basic
Script property.
.P
If a character's Script Extension property is the single value "Inherited", it
is always accepted as part of a script run. This is also true for the property
"Common", subject to the checking of decimal digits described below. All the
remaining characters in a script run must have at least one script in common in
their Script Extension lists. In set-theoretic terminology, the intersection of
all the sets of scripts must not be empty.
.P
A simple example is an Internet name such as "google.com". The letters are all
in the Latin script, and the dot is Common, so this string is a script run.
However, the Cyrillic letter "o" looks exactly the same as the Latin "o"; a 
string that looks the same, but with Cyrillic "o"s is not a script run.
.P
More interesting examples involve characters with more than one script in their 
Script Extension. Consider the following characters:
.sp
  U+060C  Arabic comma
  U+06D4  Arabic full stop
.sp
The first has the Script Extension list Arabic, Hanifi Rohingya, Syriac, and 
Thaana; the second has just Arabic and Hanifi Rohingya. Both of them could
appear in script runs of either Arabic or Hanifi Rohingya. The first could also
appear in Syriac or Thaana script runs, but the second could not.
.
.
.SS "The Chinese Han script"
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The Chinese Han script is commonly used in conjunction with other scripts for 
writing certain languages. Japanese uses the Hiragana and Katakana scripts 
together with Han; Korean uses Hangul and Han; Taiwanese Mandarin uses Bopomofo
and Han. These three combinations are treated as special cases when checking
script runs and are, in effect, "virtual scripts". Thus, a script run may
contain a mixture of Hiragana, Katakana, and Han, or a mixture of Hangul and
Han, or a mixture of Bopomofo and Han, but not, for example, a mixture of
Hangul and Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical
Standard 39 ("Unicode Security Mechanisms", http://unicode.org/reports/tr39/)
in allowing such mixtures.
.
.
.SS "Decimal digits"
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Unicode contains many sets of 10 decimal digits in different scripts, and some
scripts (including the Common script) contain more than one set. Some of these
decimal digits them are visually indistinguishable from the common ASCII
digits. In addition to the script checking described above, if a script run
contains any decimal digits, they must all come from the same set of 10
adjacent characters.
.
.
.SH "VALIDITY OF UTF STRINGS"
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.sp
When the PCRE2_UTF option is set, the strings passed as patterns and subjects
are (by default) checked for validity on entry to the relevant functions.
If an invalid UTF string is passed, an negative error code is returned. The
code unit offset to the offending character can be extracted from the match
data block by calling \fBpcre2_get_startchar()\fP, which is used for this
purpose after a UTF error.
.P
UTF-16 and UTF-32 strings can indicate their endianness by special code knows
as a byte-order mark (BOM). The PCRE2 functions do not handle this, expecting
strings to be in host byte order.
.P
A UTF string is checked before any other processing takes place. In the case of
\fBpcre2_match()\fP and \fBpcre2_dfa_match()\fP calls with a non-zero starting
offset, the check is applied only to that part of the subject that could be
inspected during matching, and there is a check that the starting offset points
to the first code unit of a character or to the end of the subject. If there
are no lookbehind assertions in the pattern, the check starts at the starting
offset. Otherwise, it starts at the length of the longest lookbehind before the
starting offset, or at the start of the subject if there are not that many
characters before the starting offset. Note that the sequences \eb and \eB are
one-character lookbehinds.
.P
In addition to checking the format of the string, there is a check to ensure
that all code points lie in the range U+0 to U+10FFFF, excluding the surrogate
area. The so-called "non-character" code points are not excluded because
Unicode corrigendum #9 makes it clear that they should not be.
.P
Characters in the "Surrogate Area" of Unicode are reserved for use by UTF-16,
where they are used in pairs to encode code points with values greater than
0xFFFF. The code points that are encoded by UTF-16 pairs are available
independently in the UTF-8 and UTF-32 encodings. (In other words, the whole
surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8 and
UTF-32.)
.P
In some situations, you may already know that your strings are valid, and
therefore want to skip these checks in order to improve performance, for
example in the case of a long subject string that is being scanned repeatedly.
If you set the PCRE2_NO_UTF_CHECK option at compile time or at match time,
PCRE2 assumes that the pattern or subject it is given (respectively) contains
only valid UTF code unit sequences.
.P
Passing PCRE2_NO_UTF_CHECK to \fBpcre2_compile()\fP just disables the check for
the pattern; it does not also apply to subject strings. If you want to disable
the check for a subject string you must pass this option to \fBpcre2_match()\fP
or \fBpcre2_dfa_match()\fP.
.P
If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the result
is undefined and your program may crash or loop indefinitely.
.P
Note that setting PCRE2_NO_UTF_CHECK at compile time does not disable the error
that is given if an escape sequence for an invalid Unicode code point is
encountered in the pattern. If you want to allow escape sequences such as
\ex{d800} (a surrogate code point) you can set the
PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option. However, this is possible
only in UTF-8 and UTF-32 modes, because these values are not representable in
UTF-16.
.
.
.\" HTML <a name="utf8strings"></a>
.SS "Errors in UTF-8 strings"
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.sp
The following negative error codes are given for invalid UTF-8 strings:
.sp
  PCRE2_ERROR_UTF8_ERR1
  PCRE2_ERROR_UTF8_ERR2
  PCRE2_ERROR_UTF8_ERR3
  PCRE2_ERROR_UTF8_ERR4
  PCRE2_ERROR_UTF8_ERR5
.sp
The string ends with a truncated UTF-8 character; the code specifies how many
bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 characters to be
no longer than 4 bytes, the encoding scheme (originally defined by RFC 2279)
allows for up to 6 bytes, and this is checked first; hence the possibility of
4 or 5 missing bytes.
.sp
  PCRE2_ERROR_UTF8_ERR6
  PCRE2_ERROR_UTF8_ERR7
  PCRE2_ERROR_UTF8_ERR8
  PCRE2_ERROR_UTF8_ERR9
  PCRE2_ERROR_UTF8_ERR10
.sp
The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the
character do not have the binary value 0b10 (that is, either the most
significant bit is 0, or the next bit is 1).
.sp
  PCRE2_ERROR_UTF8_ERR11
  PCRE2_ERROR_UTF8_ERR12
.sp
A character that is valid by the RFC 2279 rules is either 5 or 6 bytes long;
these code points are excluded by RFC 3629.
.sp
  PCRE2_ERROR_UTF8_ERR13
.sp
A 4-byte character has a value greater than 0x10fff; these code points are
excluded by RFC 3629.
.sp
  PCRE2_ERROR_UTF8_ERR14
.sp
A 3-byte character has a value in the range 0xd800 to 0xdfff; this range of
code points are reserved by RFC 3629 for use with UTF-16, and so are excluded
from UTF-8.
.sp
  PCRE2_ERROR_UTF8_ERR15
  PCRE2_ERROR_UTF8_ERR16
  PCRE2_ERROR_UTF8_ERR17
  PCRE2_ERROR_UTF8_ERR18
  PCRE2_ERROR_UTF8_ERR19
.sp
A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes for a
value that can be represented by fewer bytes, which is invalid. For example,
the two bytes 0xc0, 0xae give the value 0x2e, whose correct coding uses just
one byte.
.sp
  PCRE2_ERROR_UTF8_ERR20
.sp
The two most significant bits of the first byte of a character have the binary
value 0b10 (that is, the most significant bit is 1 and the second is 0). Such a
byte can only validly occur as the second or subsequent byte of a multi-byte
character.
.sp
  PCRE2_ERROR_UTF8_ERR21
.sp
The first byte of a character has the value 0xfe or 0xff. These values can
never occur in a valid UTF-8 string.
.
.
.\" HTML <a name="utf16strings"></a>
.SS "Errors in UTF-16 strings"
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.sp
The following negative error codes are given for invalid UTF-16 strings:
.sp
  PCRE2_ERROR_UTF16_ERR1  Missing low surrogate at end of string
  PCRE2_ERROR_UTF16_ERR2  Invalid low surrogate follows high surrogate
  PCRE2_ERROR_UTF16_ERR3  Isolated low surrogate
.sp
.
.
.\" HTML <a name="utf32strings"></a>
.SS "Errors in UTF-32 strings"
.rs
.sp
The following negative error codes are given for invalid UTF-32 strings:
.sp
  PCRE2_ERROR_UTF32_ERR1  Surrogate character (0xd800 to 0xdfff)
  PCRE2_ERROR_UTF32_ERR2  Code point is greater than 0x10ffff
.sp
.
.
.SH AUTHOR
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.nf
Philip Hazel
University Computing Service
Cambridge, England.
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.
.
.SH REVISION
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.nf
Last updated: 03 February 2019
Copyright (c) 1997-2019 University of Cambridge.
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