1 .TH PCREPATTERN 3

     2 .SH NAME

     3 PCRE - Perl-compatible regular expressions

     4 .SH "PCRE REGULAR EXPRESSION DETAILS"

     5 .rs

     6 .sp

     7 The syntax and semantics of the regular expressions that are supported by PCRE

     8 are described in detail below. There is a quick-reference syntax summary in the

     9 .\" HREF

    10 \fBpcresyntax\fP

    11 .\"

    12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE

    13 also supports some alternative regular expression syntax (which does not

    14 conflict with the Perl syntax) in order to provide some compatibility with

    15 regular expressions in Python, .NET, and Oniguruma.

    16 .P

    17 Perl's regular expressions are described in its own documentation, and

    18 regular expressions in general are covered in a number of books, some of which

    19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",

    20 published by O'Reilly, covers regular expressions in great detail. This

    21 description of PCRE's regular expressions is intended as reference material.

    22 .P

    23 The original operation of PCRE was on strings of one-byte characters. However,

    24 there is now also support for UTF-8 character strings. To use this, you must

    25 build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with

    26 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several

    27 places below. There is also a summary of UTF-8 features in the

    28 .\" HTML <a href="pcre.html#utf8support">

    29 .\" </a>

    30 section on UTF-8 support

    31 .\"

    32 in the main

    33 .\" HREF

    34 \fBpcre\fP

    35 .\"

    36 page.

    37 .P

    38 The remainder of this document discusses the patterns that are supported by

    39 PCRE when its main matching function, \fBpcre_exec()\fP, is used.

    40 From release 6.0, PCRE offers a second matching function,

    41 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not

    42 Perl-compatible. Some of the features discussed below are not available when

    43 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the

    44 alternative function, and how it differs from the normal function, are

    45 discussed in the

    46 .\" HREF

    47 \fBpcrematching\fP

    48 .\"

    49 page.

    50 .

    51 .

    52 .SH "NEWLINE CONVENTIONS"

    53 .rs

    54 .sp

    55 PCRE supports five different conventions for indicating line breaks in

    56 strings: a single CR (carriage return) character, a single LF (linefeed)

    57 character, the two-character sequence CRLF, any of the three preceding, or any

    58 Unicode newline sequence. The

    59 .\" HREF

    60 \fBpcreapi\fP

    61 .\"

    62 page has

    63 .\" HTML <a href="pcreapi.html#newlines">

    64 .\" </a>

    65 further discussion

    66 .\"

    67 about newlines, and shows how to set the newline convention in the

    68 \fIoptions\fP arguments for the compiling and matching functions.

    69 .P

    70 It is also possible to specify a newline convention by starting a pattern

    71 string with one of the following five sequences:

    72 .sp

    73   (*CR)        carriage return

    74   (*LF)        linefeed

    75   (*CRLF)      carriage return, followed by linefeed

    76   (*ANYCRLF)   any of the three above

    77   (*ANY)       all Unicode newline sequences

    78 .sp

    79 These override the default and the options given to \fBpcre_compile()\fP. For

    80 example, on a Unix system where LF is the default newline sequence, the pattern

    81 .sp

    82   (*CR)a.b

    83 .sp

    84 changes the convention to CR. That pattern matches "a\enb" because LF is no

    85 longer a newline. Note that these special settings, which are not

    86 Perl-compatible, are recognized only at the very start of a pattern, and that

    87 they must be in upper case. If more than one of them is present, the last one

    88 is used.

    89 .P

    90 The newline convention does not affect what the \eR escape sequence matches. By

    91 default, this is any Unicode newline sequence, for Perl compatibility. However,

    92 this can be changed; see the description of \eR in the section entitled

    93 .\" HTML <a href="#newlineseq">

    94 .\" </a>

    95 "Newline sequences"

    96 .\"

    97 below. A change of \eR setting can be combined with a change of newline

    98 convention.

    99 .

   100 .

   101 .SH "CHARACTERS AND METACHARACTERS"

   102 .rs

   103 .sp

   104 A regular expression is a pattern that is matched against a subject string from

   105 left to right. Most characters stand for themselves in a pattern, and match the

   106 corresponding characters in the subject. As a trivial example, the pattern

   107 .sp

   108   The quick brown fox

   109 .sp

   110 matches a portion of a subject string that is identical to itself. When

   111 caseless matching is specified (the PCRE_CASELESS option), letters are matched

   112 independently of case. In UTF-8 mode, PCRE always understands the concept of

   113 case for characters whose values are less than 128, so caseless matching is

   114 always possible. For characters with higher values, the concept of case is

   115 supported if PCRE is compiled with Unicode property support, but not otherwise.

   116 If you want to use caseless matching for characters 128 and above, you must

   117 ensure that PCRE is compiled with Unicode property support as well as with

   118 UTF-8 support.

   119 .P

   120 The power of regular expressions comes from the ability to include alternatives

   121 and repetitions in the pattern. These are encoded in the pattern by the use of

   122 \fImetacharacters\fP, which do not stand for themselves but instead are

   123 interpreted in some special way.

   124 .P

   125 There are two different sets of metacharacters: those that are recognized

   126 anywhere in the pattern except within square brackets, and those that are

   127 recognized within square brackets. Outside square brackets, the metacharacters

   128 are as follows:

   129 .sp

   130   \e      general escape character with several uses

   131   ^      assert start of string (or line, in multiline mode)

   132   $      assert end of string (or line, in multiline mode)

   133   .      match any character except newline (by default)

   134   [      start character class definition

   135   |      start of alternative branch

   136   (      start subpattern

   137   )      end subpattern

   138   ?      extends the meaning of (

   139          also 0 or 1 quantifier

   140          also quantifier minimizer

   141   *      0 or more quantifier

   142   +      1 or more quantifier

   143          also "possessive quantifier"

   144   {      start min/max quantifier

   145 .sp

   146 Part of a pattern that is in square brackets is called a "character class". In

   147 a character class the only metacharacters are:

   148 .sp

   149   \e      general escape character

   150   ^      negate the class, but only if the first character

   151   -      indicates character range

   152 .\" JOIN

   153   [      POSIX character class (only if followed by POSIX

   154            syntax)

   155   ]      terminates the character class

   156 .sp

   157 The following sections describe the use of each of the metacharacters.

   158 .

   159 .

   160 .SH BACKSLASH

   161 .rs

   162 .sp

   163 The backslash character has several uses. Firstly, if it is followed by a

   164 non-alphanumeric character, it takes away any special meaning that character

   165 may have. This use of backslash as an escape character applies both inside and

   166 outside character classes.

   167 .P

   168 For example, if you want to match a * character, you write \e* in the pattern.

   169 This escaping action applies whether or not the following character would

   170 otherwise be interpreted as a metacharacter, so it is always safe to precede a

   171 non-alphanumeric with backslash to specify that it stands for itself. In

   172 particular, if you want to match a backslash, you write \e\e.

   173 .P

   174 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the

   175 pattern (other than in a character class) and characters between a # outside

   176 a character class and the next newline are ignored. An escaping backslash can

   177 be used to include a whitespace or # character as part of the pattern.

   178 .P

   179 If you want to remove the special meaning from a sequence of characters, you

   180 can do so by putting them between \eQ and \eE. This is different from Perl in

   181 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in

   182 Perl, $ and @ cause variable interpolation. Note the following examples:

   183 .sp

   184   Pattern            PCRE matches   Perl matches

   185 .sp

   186 .\" JOIN

   187   \eQabc$xyz\eE        abc$xyz        abc followed by the

   188                                       contents of $xyz

   189   \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz

   190   \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz

   191 .sp

   192 The \eQ...\eE sequence is recognized both inside and outside character classes.

   193 .

   194 .

   195 .\" HTML <a name="digitsafterbackslash"></a>

   196 .SS "Non-printing characters"

   197 .rs

   198 .sp

   199 A second use of backslash provides a way of encoding non-printing characters

   200 in patterns in a visible manner. There is no restriction on the appearance of

   201 non-printing characters, apart from the binary zero that terminates a pattern,

   202 but when a pattern is being prepared by text editing, it is usually easier to

   203 use one of the following escape sequences than the binary character it

   204 represents:

   205 .sp

   206   \ea        alarm, that is, the BEL character (hex 07)

   207   \ecx       "control-x", where x is any character

   208   \ee        escape (hex 1B)

   209   \ef        formfeed (hex 0C)

   210   \en        linefeed (hex 0A)

   211   \er        carriage return (hex 0D)

   212   \et        tab (hex 09)

   213   \eddd      character with octal code ddd, or backreference

   214   \exhh      character with hex code hh

   215   \ex{hhh..} character with hex code hhh..

   216 .sp

   217 The precise effect of \ecx is as follows: if x is a lower case letter, it

   218 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.

   219 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex

   220 7B.

   221 .P

   222 After \ex, from zero to two hexadecimal digits are read (letters can be in

   223 upper or lower case). Any number of hexadecimal digits may appear between \ex{

   224 and }, but the value of the character code must be less than 256 in non-UTF-8

   225 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in

   226 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code

   227 point, which is 10FFFF.

   228 .P

   229 If characters other than hexadecimal digits appear between \ex{ and }, or if

   230 there is no terminating }, this form of escape is not recognized. Instead, the

   231 initial \ex will be interpreted as a basic hexadecimal escape, with no

   232 following digits, giving a character whose value is zero.

   233 .P

   234 Characters whose value is less than 256 can be defined by either of the two

   235 syntaxes for \ex. There is no difference in the way they are handled. For

   236 example, \exdc is exactly the same as \ex{dc}.

   237 .P

   238 After \e0 up to two further octal digits are read. If there are fewer than two

   239 digits, just those that are present are used. Thus the sequence \e0\ex\e07

   240 specifies two binary zeros followed by a BEL character (code value 7). Make

   241 sure you supply two digits after the initial zero if the pattern character that

   242 follows is itself an octal digit.

   243 .P

   244 The handling of a backslash followed by a digit other than 0 is complicated.

   245 Outside a character class, PCRE reads it and any following digits as a decimal

   246 number. If the number is less than 10, or if there have been at least that many

   247 previous capturing left parentheses in the expression, the entire sequence is

   248 taken as a \fIback reference\fP. A description of how this works is given

   249 .\" HTML <a href="#backreferences">

   250 .\" </a>

   251 later,

   252 .\"

   253 following the discussion of

   254 .\" HTML <a href="#subpattern">

   255 .\" </a>

   256 parenthesized subpatterns.

   257 .\"

   258 .P

   259 Inside a character class, or if the decimal number is greater than 9 and there

   260 have not been that many capturing subpatterns, PCRE re-reads up to three octal

   261 digits following the backslash, and uses them to generate a data character. Any

   262 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a

   263 character specified in octal must be less than \e400. In UTF-8 mode, values up

   264 to \e777 are permitted. For example:

   265 .sp

   266   \e040   is another way of writing a space

   267 .\" JOIN

   268   \e40    is the same, provided there are fewer than 40

   269             previous capturing subpatterns

   270   \e7     is always a back reference

   271 .\" JOIN

   272   \e11    might be a back reference, or another way of

   273             writing a tab

   274   \e011   is always a tab

   275   \e0113  is a tab followed by the character "3"

   276 .\" JOIN

   277   \e113   might be a back reference, otherwise the

   278             character with octal code 113

   279 .\" JOIN

   280   \e377   might be a back reference, otherwise

   281             the byte consisting entirely of 1 bits

   282 .\" JOIN

   283   \e81    is either a back reference, or a binary zero

   284             followed by the two characters "8" and "1"

   285 .sp

   286 Note that octal values of 100 or greater must not be introduced by a leading

   287 zero, because no more than three octal digits are ever read.

   288 .P

   289 All the sequences that define a single character value can be used both inside

   290 and outside character classes. In addition, inside a character class, the

   291 sequence \eb is interpreted as the backspace character (hex 08), and the

   292 sequences \eR and \eX are interpreted as the characters "R" and "X",

   293 respectively. Outside a character class, these sequences have different

   294 meanings

   295 .\" HTML <a href="#uniextseq">

   296 .\" </a>

   297 (see below).

   298 .\"

   299 .

   300 .

   301 .SS "Absolute and relative back references"

   302 .rs

   303 .sp

   304 The sequence \eg followed by an unsigned or a negative number, optionally

   305 enclosed in braces, is an absolute or relative back reference. A named back

   306 reference can be coded as \eg{name}. Back references are discussed

   307 .\" HTML <a href="#backreferences">

   308 .\" </a>

   309 later,

   310 .\"

   311 following the discussion of

   312 .\" HTML <a href="#subpattern">

   313 .\" </a>

   314 parenthesized subpatterns.

   315 .\"

   316 .

   317 .

   318 .SS "Absolute and relative subroutine calls"

   319 .rs

   320 .sp

   321 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or

   322 a number enclosed either in angle brackets or single quotes, is an alternative

   323 syntax for referencing a subpattern as a "subroutine". Details are discussed

   324 .\" HTML <a href="#onigurumasubroutines">

   325 .\" </a>

   326 later.

   327 .\"

   328 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP

   329 synonymous. The former is a back reference; the latter is a subroutine call.

   330 .

   331 .

   332 .SS "Generic character types"

   333 .rs

   334 .sp

   335 Another use of backslash is for specifying generic character types. The

   336 following are always recognized:

   337 .sp

   338   \ed     any decimal digit

   339   \eD     any character that is not a decimal digit

   340   \eh     any horizontal whitespace character

   341   \eH     any character that is not a horizontal whitespace character

   342   \es     any whitespace character

   343   \eS     any character that is not a whitespace character

   344   \ev     any vertical whitespace character

   345   \eV     any character that is not a vertical whitespace character

   346   \ew     any "word" character

   347   \eW     any "non-word" character

   348 .sp

   349 Each pair of escape sequences partitions the complete set of characters into

   350 two disjoint sets. Any given character matches one, and only one, of each pair.

   351 .P

   352 These character type sequences can appear both inside and outside character

   353 classes. They each match one character of the appropriate type. If the current

   354 matching point is at the end of the subject string, all of them fail, since

   355 there is no character to match.

   356 .P

   357 For compatibility with Perl, \es does not match the VT character (code 11).

   358 This makes it different from the the POSIX "space" class. The \es characters

   359 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is

   360 included in a Perl script, \es may match the VT character. In PCRE, it never

   361 does.

   362 .P

   363 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or

   364 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode

   365 character property support is available. These sequences retain their original

   366 meanings from before UTF-8 support was available, mainly for efficiency

   367 reasons.

   368 .P

   369 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the

   370 other sequences, these do match certain high-valued codepoints in UTF-8 mode.

   371 The horizontal space characters are:

   372 .sp

   373   U+0009     Horizontal tab

   374   U+0020     Space

   375   U+00A0     Non-break space

   376   U+1680     Ogham space mark

   377   U+180E     Mongolian vowel separator

   378   U+2000     En quad

   379   U+2001     Em quad

   380   U+2002     En space

   381   U+2003     Em space

   382   U+2004     Three-per-em space

   383   U+2005     Four-per-em space

   384   U+2006     Six-per-em space

   385   U+2007     Figure space

   386   U+2008     Punctuation space

   387   U+2009     Thin space

   388   U+200A     Hair space

   389   U+202F     Narrow no-break space

   390   U+205F     Medium mathematical space

   391   U+3000     Ideographic space

   392 .sp

   393 The vertical space characters are:

   394 .sp

   395   U+000A     Linefeed

   396   U+000B     Vertical tab

   397   U+000C     Formfeed

   398   U+000D     Carriage return

   399   U+0085     Next line

   400   U+2028     Line separator

   401   U+2029     Paragraph separator

   402 .P

   403 A "word" character is an underscore or any character less than 256 that is a

   404 letter or digit. The definition of letters and digits is controlled by PCRE's

   405 low-valued character tables, and may vary if locale-specific matching is taking

   406 place (see

   407 .\" HTML <a href="pcreapi.html#localesupport">

   408 .\" </a>

   409 "Locale support"

   410 .\"

   411 in the

   412 .\" HREF

   413 \fBpcreapi\fP

   414 .\"

   415 page). For example, in a French locale such as "fr_FR" in Unix-like systems,

   416 or "french" in Windows, some character codes greater than 128 are used for

   417 accented letters, and these are matched by \ew. The use of locales with Unicode

   418 is discouraged.

   419 .

   420 .

   421 .\" HTML <a name="newlineseq"></a>

   422 .SS "Newline sequences"

   423 .rs

   424 .sp

   425 Outside a character class, by default, the escape sequence \eR matches any

   426 Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is

   427 equivalent to the following:

   428 .sp

   429   (?>\er\en|\en|\ex0b|\ef|\er|\ex85)

   430 .sp

   431 This is an example of an "atomic group", details of which are given

   432 .\" HTML <a href="#atomicgroup">

   433 .\" </a>

   434 below.

   435 .\"

   436 This particular group matches either the two-character sequence CR followed by

   437 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,

   438 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next

   439 line, U+0085). The two-character sequence is treated as a single unit that

   440 cannot be split.

   441 .P

   442 In UTF-8 mode, two additional characters whose codepoints are greater than 255

   443 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).

   444 Unicode character property support is not needed for these characters to be

   445 recognized.

   446 .P

   447 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the

   448 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF

   449 either at compile time or when the pattern is matched. (BSR is an abbrevation

   450 for "backslash R".) This can be made the default when PCRE is built; if this is

   451 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.

   452 It is also possible to specify these settings by starting a pattern string with

   453 one of the following sequences:

   454 .sp

   455   (*BSR_ANYCRLF)   CR, LF, or CRLF only

   456   (*BSR_UNICODE)   any Unicode newline sequence

   457 .sp

   458 These override the default and the options given to \fBpcre_compile()\fP, but

   459 they can be overridden by options given to \fBpcre_exec()\fP. Note that these

   460 special settings, which are not Perl-compatible, are recognized only at the

   461 very start of a pattern, and that they must be in upper case. If more than one

   462 of them is present, the last one is used. They can be combined with a change of

   463 newline convention, for example, a pattern can start with:

   464 .sp

   465   (*ANY)(*BSR_ANYCRLF)

   466 .sp

   467 Inside a character class, \eR matches the letter "R".

   468 .

   469 .

   470 .\" HTML <a name="uniextseq"></a>

   471 .SS Unicode character properties

   472 .rs

   473 .sp

   474 When PCRE is built with Unicode character property support, three additional

   475 escape sequences that match characters with specific properties are available.

   476 When not in UTF-8 mode, these sequences are of course limited to testing

   477 characters whose codepoints are less than 256, but they do work in this mode.

   478 The extra escape sequences are:

   479 .sp

   480   \ep{\fIxx\fP}   a character with the \fIxx\fP property

   481   \eP{\fIxx\fP}   a character without the \fIxx\fP property

   482   \eX       an extended Unicode sequence

   483 .sp

   484 The property names represented by \fIxx\fP above are limited to the Unicode

   485 script names, the general category properties, and "Any", which matches any

   486 character (including newline). Other properties such as "InMusicalSymbols" are

   487 not currently supported by PCRE. Note that \eP{Any} does not match any

   488 characters, so always causes a match failure.

   489 .P

   490 Sets of Unicode characters are defined as belonging to certain scripts. A

   491 character from one of these sets can be matched using a script name. For

   492 example:

   493 .sp

   494   \ep{Greek}

   495   \eP{Han}

   496 .sp

   497 Those that are not part of an identified script are lumped together as

   498 "Common". The current list of scripts is:

   499 .P

   500 Arabic,

   501 Armenian,

   502 Balinese,

   503 Bengali,

   504 Bopomofo,

   505 Braille,

   506 Buginese,

   507 Buhid,

   508 Canadian_Aboriginal,

   509 Cherokee,

   510 Common,

   511 Coptic,

   512 Cuneiform,

   513 Cypriot,

   514 Cyrillic,

   515 Deseret,

   516 Devanagari,

   517 Ethiopic,

   518 Georgian,

   519 Glagolitic,

   520 Gothic,

   521 Greek,

   522 Gujarati,

   523 Gurmukhi,

   524 Han,

   525 Hangul,

   526 Hanunoo,

   527 Hebrew,

   528 Hiragana,

   529 Inherited,

   530 Kannada,

   531 Katakana,

   532 Kharoshthi,

   533 Khmer,

   534 Lao,

   535 Latin,

   536 Limbu,

   537 Linear_B,

   538 Malayalam,

   539 Mongolian,

   540 Myanmar,

   541 New_Tai_Lue,

   542 Nko,

   543 Ogham,

   544 Old_Italic,

   545 Old_Persian,

   546 Oriya,

   547 Osmanya,

   548 Phags_Pa,

   549 Phoenician,

   550 Runic,

   551 Shavian,

   552 Sinhala,

   553 Syloti_Nagri,

   554 Syriac,

   555 Tagalog,

   556 Tagbanwa,

   557 Tai_Le,

   558 Tamil,

   559 Telugu,

   560 Thaana,

   561 Thai,

   562 Tibetan,

   563 Tifinagh,

   564 Ugaritic,

   565 Yi.

   566 .P

   567 Each character has exactly one general category property, specified by a

   568 two-letter abbreviation. For compatibility with Perl, negation can be specified

   569 by including a circumflex between the opening brace and the property name. For

   570 example, \ep{^Lu} is the same as \eP{Lu}.

   571 .P

   572 If only one letter is specified with \ep or \eP, it includes all the general

   573 category properties that start with that letter. In this case, in the absence

   574 of negation, the curly brackets in the escape sequence are optional; these two

   575 examples have the same effect:

   576 .sp

   577   \ep{L}

   578   \epL

   579 .sp

   580 The following general category property codes are supported:

   581 .sp

   582   C     Other

   583   Cc    Control

   584   Cf    Format

   585   Cn    Unassigned

   586   Co    Private use

   587   Cs    Surrogate

   588 .sp

   589   L     Letter

   590   Ll    Lower case letter

   591   Lm    Modifier letter

   592   Lo    Other letter

   593   Lt    Title case letter

   594   Lu    Upper case letter

   595 .sp

   596   M     Mark

   597   Mc    Spacing mark

   598   Me    Enclosing mark

   599   Mn    Non-spacing mark

   600 .sp

   601   N     Number

   602   Nd    Decimal number

   603   Nl    Letter number

   604   No    Other number

   605 .sp

   606   P     Punctuation

   607   Pc    Connector punctuation

   608   Pd    Dash punctuation

   609   Pe    Close punctuation

   610   Pf    Final punctuation

   611   Pi    Initial punctuation

   612   Po    Other punctuation

   613   Ps    Open punctuation

   614 .sp

   615   S     Symbol

   616   Sc    Currency symbol

   617   Sk    Modifier symbol

   618   Sm    Mathematical symbol

   619   So    Other symbol

   620 .sp

   621   Z     Separator

   622   Zl    Line separator

   623   Zp    Paragraph separator

   624   Zs    Space separator

   625 .sp

   626 The special property L& is also supported: it matches a character that has

   627 the Lu, Ll, or Lt property, in other words, a letter that is not classified as

   628 a modifier or "other".

   629 .P

   630 The Cs (Surrogate) property applies only to characters in the range U+D800 to

   631 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so

   632 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off

   633 (see the discussion of PCRE_NO_UTF8_CHECK in the

   634 .\" HREF

   635 \fBpcreapi\fP

   636 .\"

   637 page).

   638 .P

   639 The long synonyms for these properties that Perl supports (such as \ep{Letter})

   640 are not supported by PCRE, nor is it permitted to prefix any of these

   641 properties with "Is".

   642 .P

   643 No character that is in the Unicode table has the Cn (unassigned) property.

   644 Instead, this property is assumed for any code point that is not in the

   645 Unicode table.

   646 .P

   647 Specifying caseless matching does not affect these escape sequences. For

   648 example, \ep{Lu} always matches only upper case letters.

   649 .P

   650 The \eX escape matches any number of Unicode characters that form an extended

   651 Unicode sequence. \eX is equivalent to

   652 .sp

   653   (?>\ePM\epM*)

   654 .sp

   655 That is, it matches a character without the "mark" property, followed by zero

   656 or more characters with the "mark" property, and treats the sequence as an

   657 atomic group

   658 .\" HTML <a href="#atomicgroup">

   659 .\" </a>

   660 (see below).

   661 .\"

   662 Characters with the "mark" property are typically accents that affect the

   663 preceding character. None of them have codepoints less than 256, so in

   664 non-UTF-8 mode \eX matches any one character.

   665 .P

   666 Matching characters by Unicode property is not fast, because PCRE has to search

   667 a structure that contains data for over fifteen thousand characters. That is

   668 why the traditional escape sequences such as \ed and \ew do not use Unicode

   669 properties in PCRE.

   670 .

   671 .

   672 .\" HTML <a name="resetmatchstart"></a>

   673 .SS "Resetting the match start"

   674 .rs

   675 .sp

   676 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously

   677 matched characters not to be included in the final matched sequence. For

   678 example, the pattern:

   679 .sp

   680   foo\eKbar

   681 .sp

   682 matches "foobar", but reports that it has matched "bar". This feature is

   683 similar to a lookbehind assertion

   684 .\" HTML <a href="#lookbehind">

   685 .\" </a>

   686 (described below).

   687 .\"

   688 However, in this case, the part of the subject before the real match does not

   689 have to be of fixed length, as lookbehind assertions do. The use of \eK does

   690 not interfere with the setting of

   691 .\" HTML <a href="#subpattern">

   692 .\" </a>

   693 captured substrings.

   694 .\"

   695 For example, when the pattern

   696 .sp

   697   (foo)\eKbar

   698 .sp

   699 matches "foobar", the first substring is still set to "foo".

   700 .

   701 .

   702 .\" HTML <a name="smallassertions"></a>

   703 .SS "Simple assertions"

   704 .rs

   705 .sp

   706 The final use of backslash is for certain simple assertions. An assertion

   707 specifies a condition that has to be met at a particular point in a match,

   708 without consuming any characters from the subject string. The use of

   709 subpatterns for more complicated assertions is described

   710 .\" HTML <a href="#bigassertions">

   711 .\" </a>

   712 below.

   713 .\"

   714 The backslashed assertions are:

   715 .sp

   716   \eb     matches at a word boundary

   717   \eB     matches when not at a word boundary

   718   \eA     matches at the start of the subject

   719   \eZ     matches at the end of the subject

   720           also matches before a newline at the end of the subject

   721   \ez     matches only at the end of the subject

   722   \eG     matches at the first matching position in the subject

   723 .sp

   724 These assertions may not appear in character classes (but note that \eb has a

   725 different meaning, namely the backspace character, inside a character class).

   726 .P

   727 A word boundary is a position in the subject string where the current character

   728 and the previous character do not both match \ew or \eW (i.e. one matches

   729 \ew and the other matches \eW), or the start or end of the string if the

   730 first or last character matches \ew, respectively.

   731 .P

   732 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and

   733 dollar (described in the next section) in that they only ever match at the very

   734 start and end of the subject string, whatever options are set. Thus, they are

   735 independent of multiline mode. These three assertions are not affected by the

   736 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the

   737 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP

   738 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start

   739 at a point other than the beginning of the subject, \eA can never match. The

   740 difference between \eZ and \ez is that \eZ matches before a newline at the end

   741 of the string as well as at the very end, whereas \ez matches only at the end.

   742 .P

   743 The \eG assertion is true only when the current matching position is at the

   744 start point of the match, as specified by the \fIstartoffset\fP argument of

   745 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is

   746 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate

   747 arguments, you can mimic Perl's /g option, and it is in this kind of

   748 implementation where \eG can be useful.

   749 .P

   750 Note, however, that PCRE's interpretation of \eG, as the start of the current

   751 match, is subtly different from Perl's, which defines it as the end of the

   752 previous match. In Perl, these can be different when the previously matched

   753 string was empty. Because PCRE does just one match at a time, it cannot

   754 reproduce this behaviour.

   755 .P

   756 If all the alternatives of a pattern begin with \eG, the expression is anchored

   757 to the starting match position, and the "anchored" flag is set in the compiled

   758 regular expression.

   759 .

   760 .

   761 .SH "CIRCUMFLEX AND DOLLAR"

   762 .rs

   763 .sp

   764 Outside a character class, in the default matching mode, the circumflex

   765 character is an assertion that is true only if the current matching point is

   766 at the start of the subject string. If the \fIstartoffset\fP argument of

   767 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE

   768 option is unset. Inside a character class, circumflex has an entirely different

   769 meaning

   770 .\" HTML <a href="#characterclass">

   771 .\" </a>

   772 (see below).

   773 .\"

   774 .P

   775 Circumflex need not be the first character of the pattern if a number of

   776 alternatives are involved, but it should be the first thing in each alternative

   777 in which it appears if the pattern is ever to match that branch. If all

   778 possible alternatives start with a circumflex, that is, if the pattern is

   779 constrained to match only at the start of the subject, it is said to be an

   780 "anchored" pattern. (There are also other constructs that can cause a pattern

   781 to be anchored.)

   782 .P

   783 A dollar character is an assertion that is true only if the current matching

   784 point is at the end of the subject string, or immediately before a newline

   785 at the end of the string (by default). Dollar need not be the last character of

   786 the pattern if a number of alternatives are involved, but it should be the last

   787 item in any branch in which it appears. Dollar has no special meaning in a

   788 character class.

   789 .P

   790 The meaning of dollar can be changed so that it matches only at the very end of

   791 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This

   792 does not affect the \eZ assertion.

   793 .P

   794 The meanings of the circumflex and dollar characters are changed if the

   795 PCRE_MULTILINE option is set. When this is the case, a circumflex matches

   796 immediately after internal newlines as well as at the start of the subject

   797 string. It does not match after a newline that ends the string. A dollar

   798 matches before any newlines in the string, as well as at the very end, when

   799 PCRE_MULTILINE is set. When newline is specified as the two-character

   800 sequence CRLF, isolated CR and LF characters do not indicate newlines.

   801 .P

   802 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where

   803 \en represents a newline) in multiline mode, but not otherwise. Consequently,

   804 patterns that are anchored in single line mode because all branches start with

   805 ^ are not anchored in multiline mode, and a match for circumflex is possible

   806 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The

   807 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.

   808 .P

   809 Note that the sequences \eA, \eZ, and \ez can be used to match the start and

   810 end of the subject in both modes, and if all branches of a pattern start with

   811 \eA it is always anchored, whether or not PCRE_MULTILINE is set.

   812 .

   813 .

   814 .SH "FULL STOP (PERIOD, DOT)"

   815 .rs

   816 .sp

   817 Outside a character class, a dot in the pattern matches any one character in

   818 the subject string except (by default) a character that signifies the end of a

   819 line. In UTF-8 mode, the matched character may be more than one byte long.

   820 .P

   821 When a line ending is defined as a single character, dot never matches that

   822 character; when the two-character sequence CRLF is used, dot does not match CR

   823 if it is immediately followed by LF, but otherwise it matches all characters

   824 (including isolated CRs and LFs). When any Unicode line endings are being

   825 recognized, dot does not match CR or LF or any of the other line ending

   826 characters.

   827 .P

   828 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL

   829 option is set, a dot matches any one character, without exception. If the

   830 two-character sequence CRLF is present in the subject string, it takes two dots

   831 to match it.

   832 .P

   833 The handling of dot is entirely independent of the handling of circumflex and

   834 dollar, the only relationship being that they both involve newlines. Dot has no

   835 special meaning in a character class.

   836 .

   837 .

   838 .SH "MATCHING A SINGLE BYTE"

   839 .rs

   840 .sp

   841 Outside a character class, the escape sequence \eC matches any one byte, both

   842 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending

   843 characters. The feature is provided in Perl in order to match individual bytes

   844 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,

   845 what remains in the string may be a malformed UTF-8 string. For this reason,

   846 the \eC escape sequence is best avoided.

   847 .P

   848 PCRE does not allow \eC to appear in lookbehind assertions

   849 .\" HTML <a href="#lookbehind">

   850 .\" </a>

   851 (described below),

   852 .\"

   853 because in UTF-8 mode this would make it impossible to calculate the length of

   854 the lookbehind.

   855 .

   856 .

   857 .\" HTML <a name="characterclass"></a>

   858 .SH "SQUARE BRACKETS AND CHARACTER CLASSES"

   859 .rs

   860 .sp

   861 An opening square bracket introduces a character class, terminated by a closing

   862 square bracket. A closing square bracket on its own is not special. If a

   863 closing square bracket is required as a member of the class, it should be the

   864 first data character in the class (after an initial circumflex, if present) or

   865 escaped with a backslash.

   866 .P

   867 A character class matches a single character in the subject. In UTF-8 mode, the

   868 character may occupy more than one byte. A matched character must be in the set

   869 of characters defined by the class, unless the first character in the class

   870 definition is a circumflex, in which case the subject character must not be in

   871 the set defined by the class. If a circumflex is actually required as a member

   872 of the class, ensure it is not the first character, or escape it with a

   873 backslash.

   874 .P

   875 For example, the character class [aeiou] matches any lower case vowel, while

   876 [^aeiou] matches any character that is not a lower case vowel. Note that a

   877 circumflex is just a convenient notation for specifying the characters that

   878 are in the class by enumerating those that are not. A class that starts with a

   879 circumflex is not an assertion: it still consumes a character from the subject

   880 string, and therefore it fails if the current pointer is at the end of the

   881 string.

   882 .P

   883 In UTF-8 mode, characters with values greater than 255 can be included in a

   884 class as a literal string of bytes, or by using the \ex{ escaping mechanism.

   885 .P

   886 When caseless matching is set, any letters in a class represent both their

   887 upper case and lower case versions, so for example, a caseless [aeiou] matches

   888 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a

   889 caseful version would. In UTF-8 mode, PCRE always understands the concept of

   890 case for characters whose values are less than 128, so caseless matching is

   891 always possible. For characters with higher values, the concept of case is

   892 supported if PCRE is compiled with Unicode property support, but not otherwise.

   893 If you want to use caseless matching for characters 128 and above, you must

   894 ensure that PCRE is compiled with Unicode property support as well as with

   895 UTF-8 support.

   896 .P

   897 Characters that might indicate line breaks are never treated in any special way

   898 when matching character classes, whatever line-ending sequence is in use, and

   899 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class

   900 such as [^a] always matches one of these characters.

   901 .P

   902 The minus (hyphen) character can be used to specify a range of characters in a

   903 character class. For example, [d-m] matches any letter between d and m,

   904 inclusive. If a minus character is required in a class, it must be escaped with

   905 a backslash or appear in a position where it cannot be interpreted as

   906 indicating a range, typically as the first or last character in the class.

   907 .P

   908 It is not possible to have the literal character "]" as the end character of a

   909 range. A pattern such as [W-]46] is interpreted as a class of two characters

   910 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or

   911 "-46]". However, if the "]" is escaped with a backslash it is interpreted as

   912 the end of range, so [W-\e]46] is interpreted as a class containing a range

   913 followed by two other characters. The octal or hexadecimal representation of

   914 "]" can also be used to end a range.

   915 .P

   916 Ranges operate in the collating sequence of character values. They can also be

   917 used for characters specified numerically, for example [\e000-\e037]. In UTF-8

   918 mode, ranges can include characters whose values are greater than 255, for

   919 example [\ex{100}-\ex{2ff}].

   920 .P

   921 If a range that includes letters is used when caseless matching is set, it

   922 matches the letters in either case. For example, [W-c] is equivalent to

   923 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character

   924 tables for a French locale are in use, [\exc8-\excb] matches accented E

   925 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for

   926 characters with values greater than 128 only when it is compiled with Unicode

   927 property support.

   928 .P

   929 The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear

   930 in a character class, and add the characters that they match to the class. For

   931 example, [\edABCDEF] matches any hexadecimal digit. A circumflex can

   932 conveniently be used with the upper case character types to specify a more

   933 restricted set of characters than the matching lower case type. For example,

   934 the class [^\eW_] matches any letter or digit, but not underscore.

   935 .P

   936 The only metacharacters that are recognized in character classes are backslash,

   937 hyphen (only where it can be interpreted as specifying a range), circumflex

   938 (only at the start), opening square bracket (only when it can be interpreted as

   939 introducing a POSIX class name - see the next section), and the terminating

   940 closing square bracket. However, escaping other non-alphanumeric characters

   941 does no harm.

   942 .

   943 .

   944 .SH "POSIX CHARACTER CLASSES"

   945 .rs

   946 .sp

   947 Perl supports the POSIX notation for character classes. This uses names

   948 enclosed by [: and :] within the enclosing square brackets. PCRE also supports

   949 this notation. For example,

   950 .sp

   951   [01[:alpha:]%]

   952 .sp

   953 matches "0", "1", any alphabetic character, or "%". The supported class names

   954 are

   955 .sp

   956   alnum    letters and digits

   957   alpha    letters

   958   ascii    character codes 0 - 127

   959   blank    space or tab only

   960   cntrl    control characters

   961   digit    decimal digits (same as \ed)

   962   graph    printing characters, excluding space

   963   lower    lower case letters

   964   print    printing characters, including space

   965   punct    printing characters, excluding letters and digits

   966   space    white space (not quite the same as \es)

   967   upper    upper case letters

   968   word     "word" characters (same as \ew)

   969   xdigit   hexadecimal digits

   970 .sp

   971 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and

   972 space (32). Notice that this list includes the VT character (code 11). This

   973 makes "space" different to \es, which does not include VT (for Perl

   974 compatibility).

   975 .P

   976 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl

   977 5.8. Another Perl extension is negation, which is indicated by a ^ character

   978 after the colon. For example,

   979 .sp

   980   [12[:^digit:]]

   981 .sp

   982 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX

   983 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not

   984 supported, and an error is given if they are encountered.

   985 .P

   986 In UTF-8 mode, characters with values greater than 128 do not match any of

   987 the POSIX character classes.

   988 .

   989 .

   990 .SH "VERTICAL BAR"

   991 .rs

   992 .sp

   993 Vertical bar characters are used to separate alternative patterns. For example,

   994 the pattern

   995 .sp

   996   gilbert|sullivan

   997 .sp

   998 matches either "gilbert" or "sullivan". Any number of alternatives may appear,

   999 and an empty alternative is permitted (matching the empty string). The matching

  1000 process tries each alternative in turn, from left to right, and the first one

  1001 that succeeds is used. If the alternatives are within a subpattern

  1002 .\" HTML <a href="#subpattern">

  1003 .\" </a>

  1004 (defined below),

  1005 .\"

  1006 "succeeds" means matching the rest of the main pattern as well as the

  1007 alternative in the subpattern.

  1008 .

  1009 .

  1010 .SH "INTERNAL OPTION SETTING"

  1011 .rs

  1012 .sp

  1013 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and

  1014 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within

  1015 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".

  1016 The option letters are

  1017 .sp

  1018   i  for PCRE_CASELESS

  1019   m  for PCRE_MULTILINE

  1020   s  for PCRE_DOTALL

  1021   x  for PCRE_EXTENDED

  1022 .sp

  1023 For example, (?im) sets caseless, multiline matching. It is also possible to

  1024 unset these options by preceding the letter with a hyphen, and a combined

  1025 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and

  1026 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also

  1027 permitted. If a letter appears both before and after the hyphen, the option is

  1028 unset.

  1029 .P

  1030 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be

  1031 changed in the same way as the Perl-compatible options by using the characters

  1032 J, U and X respectively.

  1033 .P

  1034 When an option change occurs at top level (that is, not inside subpattern

  1035 parentheses), the change applies to the remainder of the pattern that follows.

  1036 If the change is placed right at the start of a pattern, PCRE extracts it into

  1037 the global options (and it will therefore show up in data extracted by the

  1038 \fBpcre_fullinfo()\fP function).

  1039 .P

  1040 An option change within a subpattern (see below for a description of

  1041 subpatterns) affects only that part of the current pattern that follows it, so

  1042 .sp

  1043   (a(?i)b)c

  1044 .sp

  1045 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).

  1046 By this means, options can be made to have different settings in different

  1047 parts of the pattern. Any changes made in one alternative do carry on

  1048 into subsequent branches within the same subpattern. For example,

  1049 .sp

  1050   (a(?i)b|c)

  1051 .sp

  1052 matches "ab", "aB", "c", and "C", even though when matching "C" the first

  1053 branch is abandoned before the option setting. This is because the effects of

  1054 option settings happen at compile time. There would be some very weird

  1055 behaviour otherwise.

  1056 .P

  1057 \fBNote:\fP There are other PCRE-specific options that can be set by the

  1058 application when the compile or match functions are called. In some cases the

  1059 pattern can contain special leading sequences to override what the application

  1060 has set or what has been defaulted. Details are given in the section entitled

  1061 .\" HTML <a href="#newlineseq">

  1062 .\" </a>

  1063 "Newline sequences"

  1064 .\"

  1065 above.

  1066 .

  1067 .

  1068 .\" HTML <a name="subpattern"></a>

  1069 .SH SUBPATTERNS

  1070 .rs

  1071 .sp

  1072 Subpatterns are delimited by parentheses (round brackets), which can be nested.

  1073 Turning part of a pattern into a subpattern does two things:

  1074 .sp

  1075 1. It localizes a set of alternatives. For example, the pattern

  1076 .sp

  1077   cat(aract|erpillar|)

  1078 .sp

  1079 matches one of the words "cat", "cataract", or "caterpillar". Without the

  1080 parentheses, it would match "cataract", "erpillar" or an empty string.

  1081 .sp

  1082 2. It sets up the subpattern as a capturing subpattern. This means that, when

  1083 the whole pattern matches, that portion of the subject string that matched the

  1084 subpattern is passed back to the caller via the \fIovector\fP argument of

  1085 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting

  1086 from 1) to obtain numbers for the capturing subpatterns.

  1087 .P

  1088 For example, if the string "the red king" is matched against the pattern

  1089 .sp

  1090   the ((red|white) (king|queen))

  1091 .sp

  1092 the captured substrings are "red king", "red", and "king", and are numbered 1,

  1093 2, and 3, respectively.

  1094 .P

  1095 The fact that plain parentheses fulfil two functions is not always helpful.

  1096 There are often times when a grouping subpattern is required without a

  1097 capturing requirement. If an opening parenthesis is followed by a question mark

  1098 and a colon, the subpattern does not do any capturing, and is not counted when

  1099 computing the number of any subsequent capturing subpatterns. For example, if

  1100 the string "the white queen" is matched against the pattern

  1101 .sp

  1102   the ((?:red|white) (king|queen))

  1103 .sp

  1104 the captured substrings are "white queen" and "queen", and are numbered 1 and

  1105 2. The maximum number of capturing subpatterns is 65535.

  1106 .P

  1107 As a convenient shorthand, if any option settings are required at the start of

  1108 a non-capturing subpattern, the option letters may appear between the "?" and

  1109 the ":". Thus the two patterns

  1110 .sp

  1111   (?i:saturday|sunday)

  1112   (?:(?i)saturday|sunday)

  1113 .sp

  1114 match exactly the same set of strings. Because alternative branches are tried

  1115 from left to right, and options are not reset until the end of the subpattern

  1116 is reached, an option setting in one branch does affect subsequent branches, so

  1117 the above patterns match "SUNDAY" as well as "Saturday".

  1118 .

  1119 .

  1120 .SH "DUPLICATE SUBPATTERN NUMBERS"

  1121 .rs

  1122 .sp

  1123 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses

  1124 the same numbers for its capturing parentheses. Such a subpattern starts with

  1125 (?| and is itself a non-capturing subpattern. For example, consider this

  1126 pattern:

  1127 .sp

  1128   (?|(Sat)ur|(Sun))day

  1129 .sp

  1130 Because the two alternatives are inside a (?| group, both sets of capturing

  1131 parentheses are numbered one. Thus, when the pattern matches, you can look

  1132 at captured substring number one, whichever alternative matched. This construct

  1133 is useful when you want to capture part, but not all, of one of a number of

  1134 alternatives. Inside a (?| group, parentheses are numbered as usual, but the

  1135 number is reset at the start of each branch. The numbers of any capturing