PCRE2UNICODE(3) Library Functions Manual PCRE2UNICODE(3)
NAME
PCRE - Perl-compatible regular expressions (revised API)
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 sub-
ject 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 docu-
mented below.
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 let-
ter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in
Perl, many properties may optionally be prefixed by "Is", for compati-
bility with Perl 5.6. PCRE2 does not support this.
WIDE CHARACTERS AND UTF MODES
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.
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 charac-
ter 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 char-
acter 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 char-
acters, whether or not PCRE2_UCP is set.
CASE-EQUIVALENCE IN UTF MODES
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.
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 pat-
tern, 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-charac-
ter 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 perfor-
mance, 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 com-
pile 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.
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 \x{d800} (a surrogate code point) you can set the
PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option. However, this is pos-
sible only in UTF-8 and UTF-32 modes, because these values are not rep-
resentable in UTF-16.
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 (origi-
nally 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 cor-
rect 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 sec-
ond is 0). Such a byte can only validly occur as the second or subse-
quent 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: 02 September 2018
Copyright (c) 1997-2018 University of Cambridge.
PCRE2 10.32 02 September 2018 PCRE2UNICODE(3)
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