manpagez: man pages & more
man pcre2unicode(3)
Home | html | info | man
pcre2unicode(3)                                                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 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.


       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


       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-

       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.


       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.


       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

       "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-

   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 "". 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- 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  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:


       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.


       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).


       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.


       A 4-byte character has a value greater than 0x10ffff; these code points
       are excluded by RFC 3629.


       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.


       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.


       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.


       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

         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

         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.

       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

       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.


       Philip Hazel
       Retired from University Computing Service
       Cambridge, England.


       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
© 2000-2023
Individual documents may contain additional copyright information.