pcre2unicode specification

# pcre2unicode man page

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UNICODE AND UTF SUPPORT

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 pcre2_compile() 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.

If you do not need Unicode support you can build PCRE2 without it, in which case the library will be smaller.

UNICODE PROPERTY SUPPORT

When PCRE2 is built with Unicode support, the escape sequences \p{..}, \P{..}, and \X can be used. 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 pcre2pattern and pcre2syntax documentation. Only the short names for properties are supported. For example, \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in Perl, many properties may optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE does not support this.

WIDE CHARACTERS AND UTF MODES

Codepoints less than 256 can be specified in patterns by either braced or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3). Larger values have to use braced sequences. Unbraced octal code points up to \777 are also recognized; larger ones can be coded using \o{...}.

In UTF modes, repeat quantifiers apply to complete UTF characters, not to individual code units.

In UTF modes, the dot metacharacter matches one UTF character instead of a single code unit.

The escape sequence \C 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 \C in the pcre2pattern documentation).

The use of \C is not supported by the alternative matching function pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a character may consist of more than one code unit. The use of \C in these modes provokes a match-time error. Also, the JIT optimization does not support \C in these modes. If JIT optimization is requested for a UTF-8 or UTF-16 pattern that contains \C, it will not succeed, and so when pcre2_match() is called, the matching will be carried out by the normal interpretive function.

The character escapes \b, \B, \d, \D, \s, \S, \w, and \W 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 \b and \B, because they are defined in terms of \w and \W. If you want to test for a wider sense of, say, "digit", you can use explicit Unicode property tests such as \p{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 generic character types in the pcre2pattern documentation.

Similarly, characters that match the POSIX named character classes are all low-valued characters, unless the PCRE2_UCP option is set.

However, the special horizontal and vertical white space matching escapes (\h, \H, \v, and \V) do match all the appropriate Unicode characters, whether or not PCRE2_UCP is set.

Case-insensitive matching in UTF mode makes use of Unicode properties. A few Unicode characters such as Greek sigma have more than two codepoints that are case-equivalent, and these are treated as such.

VALIDITY OF UTF STRINGS

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 pcre2_get_startchar(), which is used for this purpose after a UTF error.

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.

A UTF string is checked before any other processing takes place. In the case of pcre2_match() and pcre2_dfa_match() 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 \b and \B are one-character lookbehinds.

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.

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.)

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.

Passing PCRE2_NO_UTF_CHECK to pcre2_compile() 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 pcre2_match() or pcre2_dfa_match().

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.

Errors in UTF-8 strings

The following negative error codes are given for invalid UTF-8 strings:

      PCRE2_ERROR_UTF8_ERR1
      PCRE2_ERROR_UTF8_ERR2
      PCRE2_ERROR_UTF8_ERR3
      PCRE2_ERROR_UTF8_ERR4
      PCRE2_ERROR_UTF8_ERR5

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.

      PCRE2_ERROR_UTF8_ERR6
      PCRE2_ERROR_UTF8_ERR7
      PCRE2_ERROR_UTF8_ERR8
      PCRE2_ERROR_UTF8_ERR9
      PCRE2_ERROR_UTF8_ERR10

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).

      PCRE2_ERROR_UTF8_ERR11
      PCRE2_ERROR_UTF8_ERR12

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.

      PCRE2_ERROR_UTF8_ERR13

A 4-byte character has a value greater than 0x10fff; these code points are excluded by RFC 3629.

      PCRE2_ERROR_UTF8_ERR14

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.

      PCRE2_ERROR_UTF8_ERR15
      PCRE2_ERROR_UTF8_ERR16
      PCRE2_ERROR_UTF8_ERR17
      PCRE2_ERROR_UTF8_ERR18
      PCRE2_ERROR_UTF8_ERR19

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.

      PCRE2_ERROR_UTF8_ERR20

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.

      PCRE2_ERROR_UTF8_ERR21

The first byte of a character has the value 0xfe or 0xff. These values can never occur in a valid UTF-8 string.
Errors in UTF-16 strings

The following negative error codes are given for invalid UTF-16 strings:

      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

Errors in UTF-32 strings

The following negative error codes are given for invalid UTF-32 strings:

      PCRE2_ERROR_UTF32_ERR1  Surrogate character (0xd800 to 0xdfff)
      PCRE2_ERROR_UTF32_ERR2  Code point is greater than 0x10ffff

AUTHOR

Philip Hazel
University Computing Service
Cambridge, England.

REVISION

Last updated: 03 July 2016
Copyright © 1997-2016 University of Cambridge.

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