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PCRE2UNICODE(3) Library Functions Manual PCRE2UNICODE(3)
PCRE - Perl-compatible regular expressions (revised API)
PCRE2 is normally built with Unicode support, though if you do
not need it, you can build it without, in which case the library
will be smaller. With Unicode support, PCRE2 has knowledge of
Unicode character properties and can process strings of text in
UTF-8, UTF-16, and UTF-32 format (depending on the code unit
width), but this is not the default. Unless specifically
requested, PCRE2 treats each code unit in a string as one
character.
There are two ways of telling PCRE2 to switch to UTF mode, where
characters may consist of more than one code unit and the range
of values is constrained. The program can call pcre2_compile()
with the PCRE2_UTF option, or the pattern may start with the
sequence (*UTF). However, the latter facility can be locked out
by the PCRE2_NEVER_UTF option. That is, the programmer can
prevent the supplier of the pattern from switching to UTF mode.
Note that the PCRE2_MATCH_INVALID_UTF option (see below) forces
PCRE2_UTF to be set.
In UTF mode, 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.
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
a subset of those that Perl supports. Currently they are limited
to the general category properties such as Lu for an upper case
letter or Nd for a decimal number, the derived properties Any and
LC (synonym L&), the Unicode script names such as Arabic or Han,
Bidi_Class, Bidi_Control, and a few binary properties.
The full lists are given in the pcre2pattern and pcre2syntax
documentation. In general, only the short names for properties
are supported. For example, \p{L} matches a letter. Its longer
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 mode.
In UTF mode, repeat quantifiers apply to complete UTF characters,
not to individual code units.
In UTF mode, the dot metacharacter matches one UTF character
instead of a single code unit.
In UTF mode, 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
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 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, though there are some options that
suppress this for individual escapes. For details see the section
on generic character types in the pcre2pattern documentation.
Like the escapes, characters that match the POSIX named character
classes are all low-valued characters unless the PCRE2_UCP option
is set, but there is an option to override this.
In contrast to the character escapes and character classes, the
special horizontal and vertical white space escapes (\h, \H, \v,
and \V) do match all the appropriate Unicode characters, whether
or not PCRE2_UCP is set.
If either PCRE2_UTF or PCRE2_UCP is set, upper/lower case
processing 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 specially. Setting PCRE2_UCP without PCRE2_UTF allows
Unicode-style case processing for non-UTF character encodings
such as UCS-2.
There are two ASCII characters (S and K) that, in addition to
their ASCII lower case equivalents, have a non-ASCII one as well
(long S and Kelvin sign). Recognition of these non-ASCII
characters as case-equivalent to their ASCII counterparts can be
disabled by setting the PCRE2_EXTRA_CASELESS_RESTRICT option.
When this is set, all characters in a case equivalence must
either be ASCII or non-ASCII; there can be no mixing.
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.
Basic script run rules
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 stop
The 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
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.
Decimal digits
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, a
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 undefined and your program may crash or loop
indefinitely or give incorrect results. There is, however, one
mode of matching that can handle invalid UTF subject strings.
This is enabled by passing PCRE2_MATCH_INVALID_UTF to
pcre2_compile() and is discussed below in the next section. The
rest of this section covers the case when PCRE2_MATCH_INVALID_UTF
is not set.
Passing PCRE2_NO_UTF_CHECK to pcre2_compile() just disables the
UTF 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.
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 0x10ffff; 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
You can run pattern matches on subject strings that may contain
invalid UTF sequences if you call pcre2_compile() with the
PCRE2_MATCH_INVALID_UTF option. This is supported by
pcre2_match(), including JIT matching, but not by
pcre2_dfa_match(). When PCRE2_MATCH_INVALID_UTF is set, it forces
PCRE2_UTF to be set as well. Note, however, that the pattern
itself must be a valid UTF string.
If you do not set PCRE2_MATCH_INVALID_UTF when calling
pcre2_compile, and you are not certain that your subject strings
are valid UTF sequences, you should not make use of the JIT "fast
path" function pcre2_jit_match() because it bypasses sanity
checks, including the one for UTF validity. An invalid string may
cause undefined behaviour, including looping, crashing, or giving
the wrong answer.
Setting PCRE2_MATCH_INVALID_UTF does not affect what
pcre2_compile() generates, but if pcre2_jit_compile() is
subsequently called, it does generate different code. If JIT is
not used, the option affects the behaviour of the interpretive
code in pcre2_match(). When PCRE2_MATCH_INVALID_UTF is set at
compile time, PCRE2_NO_UTF_CHECK is ignored at match time.
In this mode, an invalid code unit sequence in the subject never
matches any pattern item. It does not match dot, it does not
match \p{Any}, it does not even match negative items such as
[^X]. A lookbehind assertion fails if it encounters an invalid
sequence while moving the current point backwards. In other
words, an invalid UTF code unit sequence acts as a barrier which
no match can cross.
You can also think of this as the subject being split up into
fragments of valid UTF, delimited internally by invalid code unit
sequences. The pattern is matched fragment by fragment. The
result of a successful match, however, is given as code unit
offsets in the entire subject string in the usual way. There are
a few points to consider:
The internal boundaries are not interpreted as the beginnings or
ends of lines and so do not match circumflex or dollar characters
in the pattern.
If pcre2_match() is called with an offset that points to an
invalid UTF-sequence, that sequence is skipped, and the match
starts at the next valid UTF character, or the end of the
subject.
At internal fragment boundaries, \b and \B behave in the same way
as at the beginning and end of the subject. For example, a
sequence such as \bWORD\b would match an instance of WORD that is
surrounded by invalid UTF code units.
Using PCRE2_MATCH_INVALID_UTF, an application can run matches on
arbitrary data, knowing that any matched strings that are
returned are valid UTF. This can be useful when searching for UTF
text in executable or other binary files.
Note, however, that the 16-bit and 32-bit PCRE2 libraries process
strings as sequences of uint16_t or uint32_t code points. They
cannot find valid UTF sequences within an arbitrary string of
bytes unless such sequences are suitably aligned.
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 12 October 2023
Copyright (c) 1997-2023 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2024-06-14. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
PCRE2 10.43 04 February 2023 PCRE2UNICODE(3)
Pages that refer to this page: pcre2grep(1), pcre2api(3), pcre2jit(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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