path: root/doc/pcre2unicode.3

.TH PCRE2UNICODE 3 "04 February 2023" "PCRE2 10.43"
.SH NAME
PCRE - Perl-compatible regular expressions (revised API)
.SH "UNICODE AND UTF SUPPORT"
.rs
.sp
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.
.P
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
.\" HREF
\fBpcre2_compile()\fP
.\"
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.
.P
Note that the PCRE2_MATCH_INVALID_UTF option (see
.\" HTML <a href="#matchinvalid">
.\" </a>
below)
.\"
forces PCRE2_UTF to be set.
.P
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.
.
.
.SH "UNICODE PROPERTY SUPPORT"
.rs
.sp
When PCRE2 is built with Unicode support, the escape sequences \ep{..},
\eP{..}, and \eX 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.
.P
The full lists are given in the
.\" HREF
\fBpcre2pattern\fP
.\"
and
.\" HREF
\fBpcre2syntax\fP
.\"
documentation. In general, only the short names for properties are supported.
For example, \ep{L} matches a letter. Its longer synonym, \ep{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.
.
.
.SH "WIDE CHARACTERS AND UTF MODES"
.rs
.sp
Code points less than 256 can be specified in patterns by either braced or
unbraced hexadecimal escape sequences (for example, \ex{b3} or \exb3). Larger
values have to use braced sequences. Unbraced octal code points up to \e777 are
also recognized; larger ones can be coded using \eo{...}.
.P
The escape sequence \eN{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.
.P
In UTF mode, repeat quantifiers apply to complete UTF characters, not to
individual code units.
.P
In UTF mode, the dot metacharacter matches one UTF character instead of a
single code unit.
.P
In UTF mode, capture group names are not restricted to ASCII, and may contain
any Unicode letters and decimal digits, as well as underscore.
.P
The escape sequence \eC 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 \eC in the
.\" HREF
\fBpcre2pattern\fP
.\"
documentation). For this reason, there is a build-time option that disables
support for \eC completely. There is also a less draconian compile-time option
for locking out the use of \eC when a pattern is compiled.
.P
The use of \eC is not supported by the alternative matching function
\fBpcre2_dfa_match()\fP when in UTF-8 or UTF-16 mode, that is, when a character
may consist of more than one code unit. The use of \eC in these modes provokes
a match-time error. Also, the JIT optimization does not support \eC in these
modes. If JIT optimization is requested for a UTF-8 or UTF-16 pattern that
contains \eC, it will not succeed, and so when \fBpcre2_match()\fP is called,
the matching will be carried out by the interpretive function.
.P
The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW 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 \eb
and \eB, because they are defined in terms of \ew and \eW. If you want
to test for a wider sense of, say, "digit", you can use explicit Unicode
property tests such as \ep{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
.\" HTML <a href="pcre2pattern.html#genericchartypes">
.\" </a>
generic character types
.\"
in the
.\" HREF
\fBpcre2pattern\fP
.\"
documentation.
.P
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.
.P
In contrast to the character escapes and character classes, the special
horizontal and vertical white space escapes (\eh, \eH, \ev, and \eV) do match
all the appropriate Unicode characters, whether or not PCRE2_UCP is set.
.
.
.SH "UNICODE CASE-EQUIVALENCE"
.rs
.sp
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.
.P
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.
.
.
.\" HTML <a name="scriptruns"></a>
.SH "SCRIPT RUNS"
.rs
.sp
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.
.P
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:
.P
"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.
.P
"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.
.P
"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.
.P
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.
.P
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.
.
.
.SS "Basic script run rules"
.rs
.sp
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.
.P
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.
.P
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.
.P
More interesting examples involve characters with more than one script in their
Script Extension. Consider the following characters:
.sp
  U+060C  Arabic comma
  U+06D4  Arabic full stop
.sp
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.
.
.
.SS "The Chinese Han script"
.rs
.sp
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.
.
.
.SS "Decimal digits"
.rs
.sp
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.
.
.
.SH "VALIDITY OF UTF STRINGS"
.rs
.sp
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 \fBpcre2_get_startchar()\fP, which is used for this purpose after a UTF
error.
.P
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.
.P
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
\fBpcre2_compile()\fP and is discussed below in the next section. The rest of
this section covers the case when PCRE2_MATCH_INVALID_UTF is not set.
.P
Passing PCRE2_NO_UTF_CHECK to \fBpcre2_compile()\fP 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
\fBpcre2_match()\fP or \fBpcre2_dfa_match()\fP.
.P
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.
.P
Unless PCRE2_NO_UTF_CHECK is set, a UTF string is checked before any other
processing takes place. In the case of \fBpcre2_match()\fP and
\fBpcre2_dfa_match()\fP 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 \eb and \eB are
one-character lookbehinds.
.P
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.
.P
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.)
.P
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 \ex{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.
.
.
.\" HTML <a name="utf8strings"></a>
.SS "Errors in UTF-8 strings"
.rs
.sp
The following negative error codes are given for invalid UTF-8 strings:
.sp
  PCRE2_ERROR_UTF8_ERR1
  PCRE2_ERROR_UTF8_ERR2
  PCRE2_ERROR_UTF8_ERR3
  PCRE2_ERROR_UTF8_ERR4
  PCRE2_ERROR_UTF8_ERR5
.sp
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.
.sp
  PCRE2_ERROR_UTF8_ERR6
  PCRE2_ERROR_UTF8_ERR7
  PCRE2_ERROR_UTF8_ERR8
  PCRE2_ERROR_UTF8_ERR9
  PCRE2_ERROR_UTF8_ERR10
.sp
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).
.sp
  PCRE2_ERROR_UTF8_ERR11
  PCRE2_ERROR_UTF8_ERR12
.sp
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.
.sp
  PCRE2_ERROR_UTF8_ERR13
.sp
A 4-byte character has a value greater than 0x10ffff; these code points are
excluded by RFC 3629.
.sp
  PCRE2_ERROR_UTF8_ERR14
.sp
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.
.sp
  PCRE2_ERROR_UTF8_ERR15
  PCRE2_ERROR_UTF8_ERR16
  PCRE2_ERROR_UTF8_ERR17
  PCRE2_ERROR_UTF8_ERR18
  PCRE2_ERROR_UTF8_ERR19
.sp
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.
.sp
  PCRE2_ERROR_UTF8_ERR20
.sp
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.
.sp
  PCRE2_ERROR_UTF8_ERR21
.sp
The first byte of a character has the value 0xfe or 0xff. These values can
never occur in a valid UTF-8 string.
.
.
.\" HTML <a name="utf16strings"></a>
.SS "Errors in UTF-16 strings"
.rs
.sp
The following negative error codes are given for invalid UTF-16 strings:
.sp
  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
.sp
.
.
.\" HTML <a name="utf32strings"></a>
.SS "Errors in UTF-32 strings"
.rs
.sp
The following negative error codes are given for invalid UTF-32 strings:
.sp
  PCRE2_ERROR_UTF32_ERR1  Surrogate character (0xd800 to 0xdfff)
  PCRE2_ERROR_UTF32_ERR2  Code point is greater than 0x10ffff
.sp
.
.
.\" HTML <a name="matchinvalid"></a>
.SH "MATCHING IN INVALID UTF STRINGS"
.rs
.sp
You can run pattern matches on subject strings that may contain invalid UTF
sequences if you call \fBpcre2_compile()\fP with the PCRE2_MATCH_INVALID_UTF
option. This is supported by \fBpcre2_match()\fP, including JIT matching, but
not by \fBpcre2_dfa_match()\fP. 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.
.P
If you do not set PCRE2_MATCH_INVALID_UTF when calling \fBpcre2_compile\fP, and
you are not certain that your subject strings are valid UTF sequences, you
should not make use of the JIT "fast path" function \fBpcre2_jit_match()\fP
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.
.P
Setting PCRE2_MATCH_INVALID_UTF does not affect what \fBpcre2_compile()\fP
generates, but if \fBpcre2_jit_compile()\fP is subsequently called, it does
generate different code. If JIT is not used, the option affects the behaviour
of the interpretive code in \fBpcre2_match()\fP. When PCRE2_MATCH_INVALID_UTF
is set at compile time, PCRE2_NO_UTF_CHECK is ignored at match time.
.P
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 \ep{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.
.P
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:
.P
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.
.P
If \fBpcre2_match()\fP 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.
.P
At internal fragment boundaries, \eb and \eB behave in the same way as at the
beginning and end of the subject. For example, a sequence such as \ebWORD\eb
would match an instance of WORD that is surrounded by invalid UTF code units.
.P
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.
.P
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.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
Retired from University Computing Service
Cambridge, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 12 October 2023
Copyright (c) 1997-2023 University of Cambridge.
.fi