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# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. Perl's regular expressions are described in its own documentation, and
13    regular expressions in general are covered in a number of books, some of which
14    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15    published by O'Reilly, covers regular expressions in great detail. This
16    description of PCRE's regular expressions is intended as reference material.
17  .P  .P
18  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
19  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this, you must
# Line 40  discussed in the Line 44  discussed in the
44  page.  page.
45  .  .
46  .  .
47    .SH "NEWLINE CONVENTIONS"
48    .rs
49    .sp
50    PCRE supports five different conventions for indicating line breaks in
51    strings: a single CR (carriage return) character, a single LF (linefeed)
52    character, the two-character sequence CRLF, any of the three preceding, or any
53    Unicode newline sequence. The
54    .\" HREF
55    \fBpcreapi\fP
56    .\"
57    page has
58    .\" HTML <a href="pcreapi.html#newlines">
59    .\" </a>
60    further discussion
61    .\"
62    about newlines, and shows how to set the newline convention in the
63    \fIoptions\fP arguments for the compiling and matching functions.
64    .P
65    It is also possible to specify a newline convention by starting a pattern
66    string with one of the following five sequences:
67    .sp
68      (*CR)        carriage return
69      (*LF)        linefeed
70      (*CRLF)      carriage return, followed by linefeed
71      (*ANYCRLF)   any of the three above
72      (*ANY)       all Unicode newline sequences
73    .sp
74    These override the default and the options given to \fBpcre_compile()\fP. For
75    example, on a Unix system where LF is the default newline sequence, the pattern
76    .sp
77      (*CR)a.b
78    .sp
79    changes the convention to CR. That pattern matches "a\enb" because LF is no
80    longer a newline. Note that these special settings, which are not
81    Perl-compatible, are recognized only at the very start of a pattern, and that
82    they must be in upper case. If more than one of them is present, the last one
83    is used.
84    .P
85    The newline convention does not affect what the \eR escape sequence matches. By
86    default, this is any Unicode newline sequence, for Perl compatibility. However,
87    this can be changed; see the description of \eR in the section entitled
88    .\" HTML <a href="#newlineseq">
89    .\" </a>
90    "Newline sequences"
91    .\"
92    below. A change of \eR setting can be combined with a change of newline
93    convention.
94    .
95    .
96  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
97  .rs  .rs
98  .sp  .sp
# Line 149  represents: Line 202  represents:
202    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
203    \ee        escape (hex 1B)    \ee        escape (hex 1B)
204    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
205    \en        newline (hex 0A)    \en        linefeed (hex 0A)
206    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
207    \et        tab (hex 09)    \et        tab (hex 09)
208    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
# Line 164  Thus \ecz becomes hex 1A, but \ec{ becom Line 217  Thus \ecz becomes hex 1A, but \ec{ becom
217  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
218  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
219  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
220  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
221  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
222  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
223  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
224  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
225    there is no terminating }, this form of escape is not recognized. Instead, the
226    initial \ex will be interpreted as a basic hexadecimal escape, with no
227    following digits, giving a character whose value is zero.
228  .P  .P
229  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
230  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 240  meanings Line 296  meanings
296  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
297  .rs  .rs
298  .sp  .sp
299  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
300  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
301  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
302  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
303  .\" </a>  .\" </a>
304  later,  later,
# Line 343  accented letters, and these are matched Line 399  accented letters, and these are matched
399  is discouraged.  is discouraged.
400  .  .
401  .  .
402    .\" HTML <a name="newlineseq"></a>
403  .SS "Newline sequences"  .SS "Newline sequences"
404  .rs  .rs
405  .sp  .sp
406  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
407  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
408  the following:  equivalent to the following:
409  .sp  .sp
410    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
411  .sp  .sp
# Line 368  are added: LS (line separator, U+2028) a Line 425  are added: LS (line separator, U+2028) a
425  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
426  recognized.  recognized.
427  .P  .P
428    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
429    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
430    either at compile time or when the pattern is matched. (BSR is an abbrevation
431    for "backslash R".) This can be made the default when PCRE is built; if this is
432    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
433    It is also possible to specify these settings by starting a pattern string with
434    one of the following sequences:
435    .sp
436      (*BSR_ANYCRLF)   CR, LF, or CRLF only
437      (*BSR_UNICODE)   any Unicode newline sequence
438    .sp
439    These override the default and the options given to \fBpcre_compile()\fP, but
440    they can be overridden by options given to \fBpcre_exec()\fP. Note that these
441    special settings, which are not Perl-compatible, are recognized only at the
442    very start of a pattern, and that they must be in upper case. If more than one
443    of them is present, the last one is used. They can be combined with a change of
444    newline convention, for example, a pattern can start with:
445    .sp
446      (*ANY)(*BSR_ANYCRLF)
447    .sp
448  Inside a character class, \eR matches the letter "R".  Inside a character class, \eR matches the letter "R".
449  .  .
450  .  .
# Line 531  The special property L& is also supporte Line 608  The special property L& is also supporte
608  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
609  a modifier or "other".  a modifier or "other".
610  .P  .P
611    The Cs (Surrogate) property applies only to characters in the range U+D800 to
612    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
613    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
614    (see the discussion of PCRE_NO_UTF8_CHECK in the
615    .\" HREF
616    \fBpcreapi\fP
617    .\"
618    page).
619    .P
620  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The long synonyms for these properties that Perl supports (such as \ep{Letter})
621  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
622  properties with "Is".  properties with "Is".
# Line 555  atomic group Line 641  atomic group
641  (see below).  (see below).
642  .\"  .\"
643  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
644  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
645  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
646  .P  .P
647  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
# Line 906  alternative in the subpattern. Line 992  alternative in the subpattern.
992  .rs  .rs
993  .sp  .sp
994  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
995  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
996  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
997    The option letters are
998  .sp  .sp
999    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1000    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 921  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1008  PCRE_MULTILINE while unsetting PCRE_DOTA
1008  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1009  unset.  unset.
1010  .P  .P
1011    The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1012    changed in the same way as the Perl-compatible options by using the characters
1013    J, U and X respectively.
1014    .P
1015  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
1016  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
1017  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
# Line 944  branch is abandoned before the option se Line 1035  branch is abandoned before the option se
1035  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1036  behaviour otherwise.  behaviour otherwise.
1037  .P  .P
1038  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1039  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1040  J, U and X respectively.  pattern can contain special leading sequences to override what the application
1041    has set or what has been defaulted. Details are given in the section entitled
1042    .\" HTML <a href="#newlineseq">
1043    .\" </a>
1044    "Newline sequences"
1045    .\"
1046    above.
1047  .  .
1048  .  .
1049  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1290  previous example can be rewritten as Line 1387  previous example can be rewritten as
1387  .sp  .sp
1388    \ed++foo    \ed++foo
1389  .sp  .sp
1390    Note that a possessive quantifier can be used with an entire group, for
1391    example:
1392    .sp
1393      (abc|xyz){2,3}+
1394    .sp
1395  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1396  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1397  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1364  subpattern is possible using named paren Line 1466  subpattern is possible using named paren
1466  .P  .P
1467  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1468  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence, which is a feature introduced in
1469  Perl 5.10. This escape must be followed by a positive or a negative number,  Perl 5.10. This escape must be followed by an unsigned number or a negative
1470  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1471  .sp  .sp
1472    (ring), \e1    (ring), \e1
1473    (ring), \eg1    (ring), \eg1
1474    (ring), \eg{1}    (ring), \eg{1}
1475  .sp  .sp
1476  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1477  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1478  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1479    example:
1480  .sp  .sp
1481    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1482  .sp  .sp
# Line 1956  description of the interface to the call Line 2059  description of the interface to the call
2059  documentation.  documentation.
2060  .  .
2061  .  .
2062    .SH "BACKTRACKING CONTROL"
2063    .rs
2064    .sp
2065    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2066    are described in the Perl documentation as "experimental and subject to change
2067    or removal in a future version of Perl". It goes on to say: "Their usage in
2068    production code should be noted to avoid problems during upgrades." The same
2069    remarks apply to the PCRE features described in this section.
2070    .P
2071    Since these verbs are specifically related to backtracking, they can be used
2072    only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
2073    backtracking algorithm. They cause an error if encountered by
2074    \fBpcre_dfa_exec()\fP.
2075    .P
2076    The new verbs make use of what was previously invalid syntax: an opening
2077    parenthesis followed by an asterisk. In Perl, they are generally of the form
2078    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2079    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2080    are two kinds:
2081    .
2082    .SS "Verbs that act immediately"
2083    .rs
2084    .sp
2085    The following verbs act as soon as they are encountered:
2086    .sp
2087       (*ACCEPT)
2088    .sp
2089    This verb causes the match to end successfully, skipping the remainder of the
2090    pattern. When inside a recursion, only the innermost pattern is ended
2091    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2092    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2093    captured. For example:
2094    .sp
2095      A(A|B(*ACCEPT)|C)D
2096    .sp
2097    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2098    captured.
2099    .sp
2100      (*FAIL) or (*F)
2101    .sp
2102    This verb causes the match to fail, forcing backtracking to occur. It is
2103    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2104    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2105    Perl features that are not present in PCRE. The nearest equivalent is the
2106    callout feature, as for example in this pattern:
2107    .sp
2108      a+(?C)(*FAIL)
2109    .sp
2110    A match with the string "aaaa" always fails, but the callout is taken before
2111    each backtrack happens (in this example, 10 times).
2112    .
2113    .SS "Verbs that act after backtracking"
2114    .rs
2115    .sp
2116    The following verbs do nothing when they are encountered. Matching continues
2117    with what follows, but if there is no subsequent match, a failure is forced.
2118    The verbs differ in exactly what kind of failure occurs.
2119    .sp
2120      (*COMMIT)
2121    .sp
2122    This verb causes the whole match to fail outright if the rest of the pattern
2123    does not match. Even if the pattern is unanchored, no further attempts to find
2124    a match by advancing the start point take place. Once (*COMMIT) has been
2125    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2126    starting point, or not at all. For example:
2127    .sp
2128      a+(*COMMIT)b
2129    .sp
2130    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2131    dynamic anchor, or "I've started, so I must finish."
2132    .sp
2133      (*PRUNE)
2134    .sp
2135    This verb causes the match to fail at the current position if the rest of the
2136    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2137    advance to the next starting character then happens. Backtracking can occur as
2138    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2139    if there is no match to the right, backtracking cannot cross (*PRUNE).
2140    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2141    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2142    be expressed in any other way.
2143    .sp
2144      (*SKIP)
2145    .sp
2146    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2147    "bumpalong" advance is not to the next character, but to the position in the
2148    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2149    was matched leading up to it cannot be part of a successful match. Consider:
2150    .sp
2151      a+(*SKIP)b
2152    .sp
2153    If the subject is "aaaac...", after the first match attempt fails (starting at
2154    the first character in the string), the starting point skips on to start the
2155    next attempt at "c". Note that a possessive quantifer does not have the same
2156    effect in this example; although it would suppress backtracking during the
2157    first match attempt, the second attempt would start at the second character
2158    instead of skipping on to "c".
2159    .sp
2160      (*THEN)
2161    .sp
2162    This verb causes a skip to the next alternation if the rest of the pattern does
2163    not match. That is, it cancels pending backtracking, but only within the
2164    current alternation. Its name comes from the observation that it can be used
2165    for a pattern-based if-then-else block:
2166    .sp
2167      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2168    .sp
2169    If the COND1 pattern matches, FOO is tried (and possibly further items after
2170    the end of the group if FOO succeeds); on failure the matcher skips to the
2171    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2172    is used outside of any alternation, it acts exactly like (*PRUNE).
2173    .
2174    .
2175  .SH "SEE ALSO"  .SH "SEE ALSO"
2176  .rs  .rs
2177  .sp  .sp
# Line 1976  Cambridge CB2 3QH, England. Line 2192  Cambridge CB2 3QH, England.
2192  .rs  .rs
2193  .sp  .sp
2194  .nf  .nf
2195  Last updated: 19 June 2007  Last updated: 17 September 2007
2196  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
2197  .fi  .fi

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