432 lines
18 KiB
HTML
432 lines
18 KiB
HTML
<html>
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<head>
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<title>pcre2unicode specification</title>
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</head>
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<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
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<h1>pcre2unicode man page</h1>
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<p>
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Return to the <a href="index.html">PCRE2 index page</a>.
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</p>
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<p>
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This page is part of the PCRE2 HTML documentation. It was generated
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automatically from the original man page. If there is any nonsense in it,
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please consult the man page, in case the conversion went wrong.
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<br>
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<br><b>
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UNICODE AND UTF SUPPORT
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</b><br>
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<P>
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When PCRE2 is built with Unicode support (which is the default), it has
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knowledge of Unicode character properties and can process text strings in
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UTF-8, UTF-16, or UTF-32 format (depending on the code unit width). However, by
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default, PCRE2 assumes that one code unit is one character. To process a
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pattern as a UTF string, where a character may require more than one code unit,
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you must call
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<a href="pcre2_compile.html"><b>pcre2_compile()</b></a>
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with the PCRE2_UTF option flag, or the pattern must start with the sequence
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(*UTF). When either of these is the case, both the pattern and any subject
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strings that are matched against it are treated as UTF strings instead of
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strings of individual one-code-unit characters. There are also some other
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changes to the way characters are handled, as documented below.
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</P>
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<P>
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If you do not need Unicode support you can build PCRE2 without it, in which
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case the library will be smaller.
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</P>
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<br><b>
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UNICODE PROPERTY SUPPORT
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</b><br>
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<P>
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When PCRE2 is built with Unicode support, the escape sequences \p{..},
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\P{..}, and \X can be used. This is not dependent on the PCRE2_UTF setting.
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The Unicode properties that can be tested are limited to the general category
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properties such as Lu for an upper case letter or Nd for a decimal number, the
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Unicode script names such as Arabic or Han, and the derived properties Any and
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L&. Full lists are given in the
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<a href="pcre2pattern.html"><b>pcre2pattern</b></a>
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and
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<a href="pcre2syntax.html"><b>pcre2syntax</b></a>
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documentation. Only the short names for properties are supported. For example,
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\p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
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Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
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compatibility with Perl 5.6. PCRE2 does not support this.
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</P>
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<br><b>
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WIDE CHARACTERS AND UTF MODES
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</b><br>
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<P>
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Code points less than 256 can be specified in patterns by either braced or
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unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). Larger
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values have to use braced sequences. Unbraced octal code points up to \777 are
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also recognized; larger ones can be coded using \o{...}.
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</P>
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<P>
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The escape sequence \N{U+<hex digits>} is recognized as another way of
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specifying a Unicode character by code point in a UTF mode. It is not allowed
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in non-UTF modes.
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</P>
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<P>
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In UTF modes, repeat quantifiers apply to complete UTF characters, not to
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individual code units.
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</P>
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<P>
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In UTF modes, the dot metacharacter matches one UTF character instead of a
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single code unit.
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</P>
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<P>
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In UTF modes, capture group names are not restricted to ASCII, and may contain
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any Unicode letters and decimal digits, as well as underscore.
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</P>
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<P>
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The escape sequence \C can be used to match a single code unit in a UTF mode,
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but its use can lead to some strange effects because it breaks up multi-unit
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characters (see the description of \C in the
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<a href="pcre2pattern.html"><b>pcre2pattern</b></a>
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documentation). For this reason, there is a build-time option that disables
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support for \C completely. There is also a less draconian compile-time option
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for locking out the use of \C when a pattern is compiled.
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</P>
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<P>
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The use of \C is not supported by the alternative matching function
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<b>pcre2_dfa_match()</b> when in UTF-8 or UTF-16 mode, that is, when a character
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may consist of more than one code unit. The use of \C in these modes provokes
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a match-time error. Also, the JIT optimization does not support \C in these
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modes. If JIT optimization is requested for a UTF-8 or UTF-16 pattern that
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contains \C, it will not succeed, and so when <b>pcre2_match()</b> is called,
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the matching will be carried out by the normal interpretive function.
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</P>
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<P>
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The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test
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characters of any code value, but, by default, the characters that PCRE2
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recognizes as digits, spaces, or word characters remain the same set as in
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non-UTF mode, all with code points less than 256. This remains true even when
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PCRE2 is built to include Unicode support, because to do otherwise would slow
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down matching in many common cases. Note that this also applies to \b
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and \B, because they are defined in terms of \w and \W. If you want
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to test for a wider sense of, say, "digit", you can use explicit Unicode
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property tests such as \p{Nd}. Alternatively, if you set the PCRE2_UCP option,
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the way that the character escapes work is changed so that Unicode properties
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are used to determine which characters match. There are more details in the
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section on
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<a href="pcre2pattern.html#genericchartypes">generic character types</a>
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in the
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<a href="pcre2pattern.html"><b>pcre2pattern</b></a>
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documentation.
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</P>
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<P>
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Similarly, characters that match the POSIX named character classes are all
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low-valued characters, unless the PCRE2_UCP option is set.
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</P>
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<P>
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However, the special horizontal and vertical white space matching escapes (\h,
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\H, \v, and \V) do match all the appropriate Unicode characters, whether or
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not PCRE2_UCP is set.
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</P>
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<br><b>
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CASE-EQUIVALENCE IN UTF MODES
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</b><br>
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<P>
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Case-insensitive matching in a UTF mode makes use of Unicode properties except
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for characters whose code points are less than 128 and that have at most two
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case-equivalent values. For these, a direct table lookup is used for speed. A
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few Unicode characters such as Greek sigma have more than two code points that
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are case-equivalent, and these are treated as such.
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<a name="scriptruns"></a></P>
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<br><b>
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SCRIPT RUNS
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</b><br>
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<P>
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The pattern constructs (*script_run:...) and (*atomic_script_run:...), with
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synonyms (*sr:...) and (*asr:...), verify that the string matched within the
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parentheses is a script run. In concept, a script run is a sequence of
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characters that are all from the same Unicode script. However, because some
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scripts are commonly used together, and because some diacritical and other
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marks are used with multiple scripts, it is not that simple.
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</P>
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<P>
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Every Unicode character has a Script property, mostly with a value
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corresponding to the name of a script, such as Latin, Greek, or Cyrillic. There
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are also three special values:
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</P>
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<P>
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"Unknown" is used for code points that have not been assigned, and also for the
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surrogate code points. In the PCRE2 32-bit library, characters whose code
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points are greater than the Unicode maximum (U+10FFFF), which are accessible
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only in non-UTF mode, are assigned the Unknown script.
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</P>
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<P>
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"Common" is used for characters that are used with many scripts. These include
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punctuation, emoji, mathematical, musical, and currency symbols, and the ASCII
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digits 0 to 9.
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</P>
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<P>
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"Inherited" is used for characters such as diacritical marks that modify a
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previous character. These are considered to take on the script of the character
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that they modify.
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</P>
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<P>
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Some Inherited characters are used with many scripts, but many of them are only
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normally used with a small number of scripts. For example, U+102E0 (Coptic
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Epact thousands mark) is used only with Arabic and Coptic. In order to make it
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possible to check this, a Unicode property called Script Extension exists. Its
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value is a list of scripts that apply to the character. For the majority of
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characters, the list contains just one script, the same one as the Script
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property. However, for characters such as U+102E0 more than one Script is
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listed. There are also some Common characters that have a single, non-Common
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script in their Script Extension list.
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</P>
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<P>
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The next section describes the basic rules for deciding whether a given string
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of characters is a script run. Note, however, that there are some special cases
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involving the Chinese Han script, and an additional constraint for decimal
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digits. These are covered in subsequent sections.
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</P>
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<br><b>
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Basic script run rules
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</b><br>
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<P>
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A string that is less than two characters long is a script run. This is the
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only case in which an Unknown character can be part of a script run. Longer
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strings are checked using only the Script Extensions property, not the basic
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Script property.
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</P>
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<P>
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If a character's Script Extension property is the single value "Inherited", it
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is always accepted as part of a script run. This is also true for the property
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"Common", subject to the checking of decimal digits described below. All the
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remaining characters in a script run must have at least one script in common in
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their Script Extension lists. In set-theoretic terminology, the intersection of
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all the sets of scripts must not be empty.
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</P>
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<P>
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A simple example is an Internet name such as "google.com". The letters are all
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in the Latin script, and the dot is Common, so this string is a script run.
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However, the Cyrillic letter "o" looks exactly the same as the Latin "o"; a
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string that looks the same, but with Cyrillic "o"s is not a script run.
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</P>
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<P>
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More interesting examples involve characters with more than one script in their
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Script Extension. Consider the following characters:
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<pre>
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U+060C Arabic comma
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U+06D4 Arabic full stop
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</pre>
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The first has the Script Extension list Arabic, Hanifi Rohingya, Syriac, and
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Thaana; the second has just Arabic and Hanifi Rohingya. Both of them could
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appear in script runs of either Arabic or Hanifi Rohingya. The first could also
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appear in Syriac or Thaana script runs, but the second could not.
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</P>
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<br><b>
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The Chinese Han script
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</b><br>
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<P>
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The Chinese Han script is commonly used in conjunction with other scripts for
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writing certain languages. Japanese uses the Hiragana and Katakana scripts
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together with Han; Korean uses Hangul and Han; Taiwanese Mandarin uses Bopomofo
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and Han. These three combinations are treated as special cases when checking
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script runs and are, in effect, "virtual scripts". Thus, a script run may
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contain a mixture of Hiragana, Katakana, and Han, or a mixture of Hangul and
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Han, or a mixture of Bopomofo and Han, but not, for example, a mixture of
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Hangul and Bopomofo and Han. PCRE2 (like Perl) follows Unicode's Technical
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Standard 39 ("Unicode Security Mechanisms", http://unicode.org/reports/tr39/)
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in allowing such mixtures.
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</P>
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<br><b>
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Decimal digits
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</b><br>
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<P>
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Unicode contains many sets of 10 decimal digits in different scripts, and some
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scripts (including the Common script) contain more than one set. Some of these
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decimal digits them are visually indistinguishable from the common ASCII
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digits. In addition to the script checking described above, if a script run
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contains any decimal digits, they must all come from the same set of 10
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adjacent characters.
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</P>
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<br><b>
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VALIDITY OF UTF STRINGS
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</b><br>
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<P>
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When the PCRE2_UTF option is set, the strings passed as patterns and subjects
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are (by default) checked for validity on entry to the relevant functions. If an
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invalid UTF string is passed, an negative error code is returned. The code unit
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offset to the offending character can be extracted from the match data block by
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calling <b>pcre2_get_startchar()</b>, which is used for this purpose after a UTF
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error.
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</P>
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<P>
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In some situations, you may already know that your strings are valid, and
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therefore want to skip these checks in order to improve performance, for
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example in the case of a long subject string that is being scanned repeatedly.
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If you set the PCRE2_NO_UTF_CHECK option at compile time or at match time,
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PCRE2 assumes that the pattern or subject it is given (respectively) contains
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only valid UTF code unit sequences.
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</P>
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<P>
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If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the result
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is usually undefined and your program may crash or loop indefinitely. There is,
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however, one mode of matching that can handle invalid UTF subject strings. This
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is matching via the JIT optimization using the PCRE2_JIT_INVALID_UTF option
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when calling <b>pcre2_jit_compile()</b>. For details, see the
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<a href="pcre2jit.html"><b>pcre2jit</b></a>
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documentation.
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</P>
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<P>
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Passing PCRE2_NO_UTF_CHECK to <b>pcre2_compile()</b> just disables the check for
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the pattern; it does not also apply to subject strings. If you want to disable
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the check for a subject string you must pass this same option to
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<b>pcre2_match()</b> or <b>pcre2_dfa_match()</b>.
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</P>
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<P>
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UTF-16 and UTF-32 strings can indicate their endianness by special code knows
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as a byte-order mark (BOM). The PCRE2 functions do not handle this, expecting
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strings to be in host byte order.
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</P>
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<P>
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Unless PCRE2_NO_UTF_CHECK is set, a UTF string is checked before any other
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processing takes place. In the case of <b>pcre2_match()</b> and
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<b>pcre2_dfa_match()</b> calls with a non-zero starting offset, the check is
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applied only to that part of the subject that could be inspected during
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matching, and there is a check that the starting offset points to the first
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code unit of a character or to the end of the subject. If there are no
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lookbehind assertions in the pattern, the check starts at the starting offset.
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Otherwise, it starts at the length of the longest lookbehind before the
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starting offset, or at the start of the subject if there are not that many
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characters before the starting offset. Note that the sequences \b and \B are
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one-character lookbehinds.
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</P>
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<P>
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In addition to checking the format of the string, there is a check to ensure
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that all code points lie in the range U+0 to U+10FFFF, excluding the surrogate
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area. The so-called "non-character" code points are not excluded because
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Unicode corrigendum #9 makes it clear that they should not be.
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</P>
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<P>
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Characters in the "Surrogate Area" of Unicode are reserved for use by UTF-16,
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where they are used in pairs to encode code points with values greater than
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0xFFFF. The code points that are encoded by UTF-16 pairs are available
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independently in the UTF-8 and UTF-32 encodings. (In other words, the whole
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surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8 and
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UTF-32.)
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</P>
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<P>
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Setting PCRE2_NO_UTF_CHECK at compile time does not disable the error that is
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given if an escape sequence for an invalid Unicode code point is encountered in
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the pattern. If you want to allow escape sequences such as \x{d800} (a
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surrogate code point) you can set the PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra
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option. However, this is possible only in UTF-8 and UTF-32 modes, because these
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values are not representable in UTF-16.
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<a name="utf8strings"></a></P>
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<br><b>
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Errors in UTF-8 strings
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</b><br>
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<P>
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The following negative error codes are given for invalid UTF-8 strings:
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<pre>
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PCRE2_ERROR_UTF8_ERR1
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PCRE2_ERROR_UTF8_ERR2
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PCRE2_ERROR_UTF8_ERR3
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PCRE2_ERROR_UTF8_ERR4
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PCRE2_ERROR_UTF8_ERR5
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</pre>
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The string ends with a truncated UTF-8 character; the code specifies how many
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bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 characters to be
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no longer than 4 bytes, the encoding scheme (originally defined by RFC 2279)
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allows for up to 6 bytes, and this is checked first; hence the possibility of
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4 or 5 missing bytes.
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<pre>
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PCRE2_ERROR_UTF8_ERR6
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PCRE2_ERROR_UTF8_ERR7
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PCRE2_ERROR_UTF8_ERR8
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PCRE2_ERROR_UTF8_ERR9
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PCRE2_ERROR_UTF8_ERR10
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</pre>
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The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of the
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character do not have the binary value 0b10 (that is, either the most
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significant bit is 0, or the next bit is 1).
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<pre>
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PCRE2_ERROR_UTF8_ERR11
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PCRE2_ERROR_UTF8_ERR12
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</pre>
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A character that is valid by the RFC 2279 rules is either 5 or 6 bytes long;
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these code points are excluded by RFC 3629.
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<pre>
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PCRE2_ERROR_UTF8_ERR13
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</pre>
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A 4-byte character has a value greater than 0x10fff; these code points are
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excluded by RFC 3629.
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<pre>
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PCRE2_ERROR_UTF8_ERR14
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</pre>
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A 3-byte character has a value in the range 0xd800 to 0xdfff; this range of
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code points are reserved by RFC 3629 for use with UTF-16, and so are excluded
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from UTF-8.
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<pre>
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PCRE2_ERROR_UTF8_ERR15
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PCRE2_ERROR_UTF8_ERR16
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PCRE2_ERROR_UTF8_ERR17
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PCRE2_ERROR_UTF8_ERR18
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PCRE2_ERROR_UTF8_ERR19
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</pre>
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A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes for a
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value that can be represented by fewer bytes, which is invalid. For example,
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the two bytes 0xc0, 0xae give the value 0x2e, whose correct coding uses just
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one byte.
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<pre>
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PCRE2_ERROR_UTF8_ERR20
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</pre>
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The two most significant bits of the first byte of a character have the binary
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value 0b10 (that is, the most significant bit is 1 and the second is 0). Such a
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byte can only validly occur as the second or subsequent byte of a multi-byte
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character.
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<pre>
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PCRE2_ERROR_UTF8_ERR21
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</pre>
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The first byte of a character has the value 0xfe or 0xff. These values can
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never occur in a valid UTF-8 string.
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<a name="utf16strings"></a></P>
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<br><b>
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Errors in UTF-16 strings
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</b><br>
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<P>
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The following negative error codes are given for invalid UTF-16 strings:
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<pre>
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PCRE2_ERROR_UTF16_ERR1 Missing low surrogate at end of string
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PCRE2_ERROR_UTF16_ERR2 Invalid low surrogate follows high surrogate
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PCRE2_ERROR_UTF16_ERR3 Isolated low surrogate
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<a name="utf32strings"></a></PRE>
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</P>
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<br><b>
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Errors in UTF-32 strings
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</b><br>
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<P>
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The following negative error codes are given for invalid UTF-32 strings:
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<pre>
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PCRE2_ERROR_UTF32_ERR1 Surrogate character (0xd800 to 0xdfff)
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PCRE2_ERROR_UTF32_ERR2 Code point is greater than 0x10ffff
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</PRE>
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</P>
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<br><b>
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AUTHOR
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</b><br>
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<P>
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Philip Hazel
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<br>
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University Computing Service
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<br>
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Cambridge, England.
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<br>
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</P>
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<br><b>
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REVISION
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</b><br>
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<P>
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Last updated: 06 March 2019
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<br>
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Copyright © 1997-2019 University of Cambridge.
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<br>
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<p>
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