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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. There are also some other changes to the way characters are handled, as documented below.
If you do not need Unicode support you can build PCRE2 without it, in which case the library will be smaller.
When PCRE2 is built with Unicode support, the escape sequences \p{..}, \P{..}, and \X 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 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. PCRE2 does not support this.
Code points 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{...}.
The escape sequence \N{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.
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.
In UTF modes, capture group names are not restricted to ASCII, and may contain any Unicode letters and decimal digits, as well as underscore.
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). For this reason, there is a build-time option that disables support for \C completely. There is also a less draconian compile-time option for locking out the use of \C when a pattern is compiled.
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 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.
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.
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:
"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.
"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.
"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.
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.
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.
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.
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.
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.
More interesting examples involve characters with more than one script in their Script Extension. Consider the following characters:
U+060C Arabic comma U+06D4 Arabic full stopThe 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.
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.
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.
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.
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.
If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is set, the result is usually undefined and your program may crash or loop indefinitely. There is, however, one mode of matching that can handle invalid UTF subject strings. This is matching via the JIT optimization using the PCRE2_JIT_INVALID_UTF option when calling pcre2_jit_compile(). For details, see the pcre2jit documentation.
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 same option to pcre2_match() or pcre2_dfa_match().
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.
Unless PCRE2_NO_UTF_CHECK is set, 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.)
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 \x{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.
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_ERR5The 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_ERR10The 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_ERR12A 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_ERR13A 4-byte character has a value greater than 0x10fff; these code points are excluded by RFC 3629.
PCRE2_ERROR_UTF8_ERR14A 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_ERR19A 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_ERR20The 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_ERR21The first byte of a character has the value 0xfe or 0xff. These values can never occur in a valid UTF-8 string.
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
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
Philip Hazel
University Computing Service
Cambridge, England.
Last updated: 06 March 2019
Copyright © 1997-2019 University of Cambridge.
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