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PCRE - Perl-compatible regular expressions
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.SH "UTF-8, UTF-16, AND UNICODE PROPERTY SUPPORT"
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From Release 8.30, in addition to its previous UTF-8 support, PCRE also
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supports UTF-16 by means of a separate 16-bit library. This can be built as
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well as, or instead of, the 8-bit library.
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In order process UTF-8 strings, you must build PCRE's 8-bit library with UTF
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support, and, in addition, you must call
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with the PCRE_UTF8 option flag, or the pattern must start with the sequence
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(*UTF8). 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-8 strings instead of
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strings of 1-byte characters.
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In order process UTF-16 strings, you must build PCRE's 16-bit library with UTF
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support, and, in addition, you must call
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.\" HTML <a href="pcre_compile.html">
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\fBpcre16_compile()\fP
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with the PCRE_UTF16 option flag, or the pattern must start with the sequence
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(*UTF16). 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-16 strings instead of
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strings of 16-bit characters.
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.SH "UTF SUPPORT OVERHEAD"
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If you compile PCRE with UTF support, but do not use it at run time, the
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library will be a bit bigger, but the additional run time overhead is limited
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to testing the PCRE_UTF8/16 flag occasionally, so should not be very big.
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.SH "UNICODE PROPERTY SUPPORT"
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If PCRE is built with Unicode character property support (which implies UTF
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support), the escape sequences \ep{..}, \eP{..}, and \eX can be used.
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The available properties that can be tested are limited to the general
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category properties such as Lu for an upper case letter or Nd for a decimal
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number, the Unicode script names such as Arabic or Han, and the derived
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properties Any and L&. A full list is given in the
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documentation. Only the short names for properties are supported. For example,
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\ep{L} matches a letter. Its Perl synonym, \ep{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. PCRE does not support this.
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.\" HTML <a name="utf8strings"></a>
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.SS "Validity of UTF-8 strings"
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When you set the PCRE_UTF8 flag, the byte strings passed as patterns and
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subjects are (by default) checked for validity on entry to the relevant
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functions. From release 7.3 of PCRE, the check is according the rules of RFC
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3629, which are themselves derived from the Unicode specification. Earlier
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releases of PCRE followed the rules of RFC 2279, which allows the full range of
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31-bit values (0 to 0x7FFFFFFF). The current check allows only values in the
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range U+0 to U+10FFFF, excluding U+D800 to U+DFFF.
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The excluded code points are the "Surrogate Area" of Unicode. They are reserved
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for use by UTF-16, where they are used in pairs to encode codepoints with
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values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs
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are available independently in the UTF-8 encoding. (In other words, the whole
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surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
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If an invalid UTF-8 string is passed to PCRE, an error return is given. At
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compile time, the only additional information is the offset to the first byte
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of the failing character. The runtime functions \fBpcre_exec()\fP and
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\fBpcre_dfa_exec()\fP also pass back this information, as well as a more
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detailed reason code if the caller has provided memory in which to do this.
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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. If you set
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the PCRE_NO_UTF8_CHECK flag at compile time or at run time, PCRE assumes that
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the pattern or subject it is given (respectively) contains only valid UTF-8
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codes. In this case, it does not diagnose an invalid UTF-8 string.
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If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
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happens depends on why the string is invalid. If the string conforms to the
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"old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
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in the range 0 to 0x7FFFFFFF by \fBpcre_dfa_exec()\fP and the interpreted
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version of \fBpcre_exec()\fP. In other words, apart from the initial validity
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test, these functions (when in UTF-8 mode) handle strings according to the more
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liberal rules of RFC 2279. However, the just-in-time (JIT) optimization for
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\fBpcre_exec()\fP supports only RFC 3629. If you are using JIT optimization, or
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if the string does not even conform to RFC 2279, the result is undefined. Your
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If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
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encoded in a UTF-8-like manner as per the old RFC, you can set
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PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
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situation, you will have to apply your own validity check, and avoid the use of
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.\" HTML <a name="utf16strings"></a>
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.SS "Validity of UTF-16 strings"
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When you set the PCRE_UTF16 flag, the strings of 16-bit data units that are
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passed as patterns and subjects are (by default) checked for validity on entry
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to the relevant functions. Values other than those in the surrogate range
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U+D800 to U+DFFF are independent code points. Values in the surrogate range
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must be used in pairs in the correct manner.
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If an invalid UTF-16 string is passed to PCRE, an error return is given. At
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compile time, the only additional information is the offset to the first data
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unit of the failing character. The runtime functions \fBpcre16_exec()\fP and
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\fBpcre16_dfa_exec()\fP also pass back this information, as well as a more
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detailed reason code if the caller has provided memory in which to do this.
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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. If you set
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the PCRE_NO_UTF16_CHECK flag at compile time or at run time, PCRE assumes that
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the pattern or subject it is given (respectively) contains only valid UTF-16
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sequences. In this case, it does not diagnose an invalid UTF-16 string.
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.SS "General comments about UTF modes"
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1. Codepoints less than 256 can be specified by either braced or unbraced
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hexadecimal escape sequences (for example, \ex{b3} or \exb3). Larger values
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have to use braced sequences.
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2. Octal numbers up to \e777 are recognized, and in UTF-8 mode, they match
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two-byte characters for values greater than \e177.
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3. Repeat quantifiers apply to complete UTF characters, not to individual
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data units, for example: \ex{100}{3}.
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4. The dot metacharacter matches one UTF character instead of a single data
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5. The escape sequence \eC can be used to match a single byte in UTF-8 mode, or
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a single 16-bit data unit in UTF-16 mode, but its use can lead to some strange
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effects because it breaks up multi-unit characters (see the description of \eC
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documentation). The use of \eC is not supported in the alternative matching
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function \fBpcre[16]_dfa_exec()\fP, nor is it supported in UTF mode by the JIT
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optimization of \fBpcre[16]_exec()\fP. If JIT optimization is requested for a
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UTF pattern that contains \eC, it will not succeed, and so the matching will
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be carried out by the normal interpretive function.
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6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
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test characters of any code value, but, by default, the characters that PCRE
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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 values less than 256. This remains true even when PCRE
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is built to include Unicode property support, because to do otherwise would
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slow down PCRE in many common cases. Note in particular that this applies to
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\eb and \eB, because they are defined in terms of \ew and \eW. If you really
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want to test for a wider sense of, say, "digit", you can use explicit Unicode
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property tests such as \ep{Nd}. Alternatively, if you set the PCRE_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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.\" HTML <a href="pcrepattern.html#genericchartypes">
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generic character types
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7. Similarly, characters that match the POSIX named character classes are all
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low-valued characters, unless the PCRE_UCP option is set.
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8. However, the horizontal and vertical whitespace matching escapes (\eh, \eH,
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\ev, and \eV) do match all the appropriate Unicode characters, whether or not
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9. Case-insensitive matching applies only to characters whose values are less
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than 128, unless PCRE is built with Unicode property support. Even when Unicode
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property support is available, PCRE still uses its own character tables when
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checking the case of low-valued characters, so as not to degrade performance.
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The Unicode property information is used only for characters with higher
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values. Furthermore, PCRE supports case-insensitive matching only when there is
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a one-to-one mapping between a letter's cases. There are a small number of
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many-to-one mappings in Unicode; these are not supported by PCRE.
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University Computing Service
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Cambridge CB2 3QH, England.
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Last updated: 13 January 2012
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Copyright (c) 1997-2012 University of Cambridge.
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doc/pcreunicode.3
