pcre2unicode(3) pcre2unicode(3)
NAME
PCRE - Perl-compatible regular expressions (revised API)
UNICODE AND UTF SUPPORT
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 char-
acters 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.
UNICODE PROPERTY SUPPORT
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 set-
ting. 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 num-
ber, the Unicode script names such as Arabic or Han, Bidi_Class,
Bidi_Control, and the derived properties Any and LC (synonym L&). Full
lists are given in the pcre2pattern and pcre2syntax documentation. In
general, only the short names for properties are supported. For exam-
ple, \p{L} matches a letter. Its longer synonym, \p{Letter}, is not
supported. Furthermore, in Perl, many properties may optionally be pre-
fixed by "Is", for compatibility 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 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 dis-
ables support for \C completely. There is also a less draconian com-
pile-time option for locking out the use of \C when a pattern is com-
piled.
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 inter-
pretive 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.
UNICODE CASE-EQUIVALENCE
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 charac-
ters 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.
SCRIPT RUNS
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. How-
ever, 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 cor-
responding 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 sym-
bols, and the ASCII digits 0 to 9.
"Inherited" is used for characters such as diacritical marks that mod-
ify 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 Cop-
tic. 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 con-
tains 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 sec-
tions.
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 prop-
erty, not the basic Script property.
If a character's Script Extension property is the single value "Inher-
ited", 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, Syr-
iac, 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; Tai-
wanese 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 Hira-
gana, 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 Stan-
dard 39 ("Unicode Security Mechanisms", http://uni-
code.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.
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, 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 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.
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 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 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 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
MATCHING IN INVALID UTF STRINGS
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.
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 be-
haviour 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 lookbe-
hind 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 arbi-
trary 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.
AUTHOR
Philip Hazel
Retired from University Computing Service
Cambridge, England.
REVISION
Last updated: 22 December 2021
Copyright (c) 1997-2021 University of Cambridge.
PCRE2 10.40 22 December 2021 pcre2unicode(3)
pcre2 10.40 - Generated Mon May 9 16:37:27 CDT 2022