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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.
83    .
84    .
85  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
86  .rs  .rs
87  .sp  .sp
# Line 149  represents: Line 191  represents:
191    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
192    \ee        escape (hex 1B)    \ee        escape (hex 1B)
193    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
194    \en        newline (hex 0A)    \en        linefeed (hex 0A)
195    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
196    \et        tab (hex 09)    \et        tab (hex 09)
197    \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 206  Thus \ecz becomes hex 1A, but \ec{ becom
206  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
207  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{
208  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
209  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
210  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
211  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
212  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
213  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
214    there is no terminating }, this form of escape is not recognized. Instead, the
215    initial \ex will be interpreted as a basic hexadecimal escape, with no
216    following digits, giving a character whose value is zero.
217  .P  .P
218  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
219  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 285  meanings
285  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
286  .rs  .rs
287  .sp  .sp
288  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
289  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
290  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
291  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
292  .\" </a>  .\" </a>
293  later,  later,
# Line 531  The special property L& is also supporte Line 576  The special property L& is also supporte
576  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
577  a modifier or "other".  a modifier or "other".
578  .P  .P
579    The Cs (Surrogate) property applies only to characters in the range U+D800 to
580    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
581    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
582    (see the discussion of PCRE_NO_UTF8_CHECK in the
583    .\" HREF
584    \fBpcreapi\fP
585    .\"
586    page).
587    .P
588  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})
589  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
590  properties with "Is".  properties with "Is".
# Line 555  atomic group Line 609  atomic group
609  (see below).  (see below).
610  .\"  .\"
611  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
612  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
613  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
614  .P  .P
615  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 1290  previous example can be rewritten as Line 1344  previous example can be rewritten as
1344  .sp  .sp
1345    \ed++foo    \ed++foo
1346  .sp  .sp
1347    Note that a possessive quantifier can be used with an entire group, for
1348    example:
1349    .sp
1350      (abc|xyz){2,3}+
1351    .sp
1352  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1353  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
1354  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 1423  subpattern is possible using named paren
1423  .P  .P
1424  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
1425  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
1426  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
1427  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1428  .sp  .sp
1429    (ring), \e1    (ring), \e1
1430    (ring), \eg1    (ring), \eg1
1431    (ring), \eg{1}    (ring), \eg{1}
1432  .sp  .sp
1433  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1434  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
1435  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1436    example:
1437  .sp  .sp
1438    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1439  .sp  .sp
# Line 1956  description of the interface to the call Line 2016  description of the interface to the call
2016  documentation.  documentation.
2017  .  .
2018  .  .
2019    .SH "BACTRACKING CONTROL"
2020    .rs
2021    .sp
2022    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2023    are described in the Perl documentation as "experimental and subject to change
2024    or removal in a future version of Perl". It goes on to say: "Their usage in
2025    production code should be noted to avoid problems during upgrades." The same
2026    remarks apply to the PCRE features described in this section.
2027    .P
2028    Since these verbs are specifically related to backtracking, they can be used
2029    only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
2030    backtracking algorithm. They cause an error if encountered by
2031    \fBpcre_dfa_exec()\fP.
2032    .P
2033    The new verbs make use of what was previously invalid syntax: an opening
2034    parenthesis followed by an asterisk. In Perl, they are generally of the form
2035    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2036    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2037    are two kinds:
2038    .
2039    .SS "Verbs that act immediately"
2040    .rs
2041    .sp
2042    The following verbs act as soon as they are encountered:
2043    .sp
2044       (*ACCEPT)
2045    .sp
2046    This verb causes the match to end successfully, skipping the remainder of the
2047    pattern. When inside a recursion, only the innermost pattern is ended
2048    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2049    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2050    captured. For example:
2051    .sp
2052      A(A|B(*ACCEPT)|C)D
2053    .sp
2054    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2055    captured.
2056    .sp
2057      (*FAIL) or (*F)
2058    .sp
2059    This verb causes the match to fail, forcing backtracking to occur. It is
2060    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2061    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2062    Perl features that are not present in PCRE. The nearest equivalent is the
2063    callout feature, as for example in this pattern:
2064    .sp
2065      a+(?C)(*FAIL)
2066    .sp
2067    A match with the string "aaaa" always fails, but the callout is taken before
2068    each backtrack happens (in this example, 10 times).
2069    .
2070    .SS "Verbs that act after backtracking"
2071    .rs
2072    .sp
2073    The following verbs do nothing when they are encountered. Matching continues
2074    with what follows, but if there is no subsequent match, a failure is forced.
2075    The verbs differ in exactly what kind of failure occurs.
2076    .sp
2077      (*COMMIT)
2078    .sp
2079    This verb causes the whole match to fail outright if the rest of the pattern
2080    does not match. Even if the pattern is unanchored, no further attempts to find
2081    a match by advancing the start point take place. Once (*COMMIT) has been
2082    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2083    starting point, or not at all. For example:
2084    .sp
2085      a+(*COMMIT)b
2086    .sp
2087    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2088    dynamic anchor, or "I've started, so I must finish."
2089    .sp
2090      (*PRUNE)
2091    .sp
2092    This verb causes the match to fail at the current position if the rest of the
2093    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2094    advance to the next starting character then happens. Backtracking can occur as
2095    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2096    if there is no match to the right, backtracking cannot cross (*PRUNE).
2097    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2098    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2099    be expressed in any other way.
2100    .sp
2101      (*SKIP)
2102    .sp
2103    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2104    "bumpalong" advance is not to the next character, but to the position in the
2105    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2106    was matched leading up to it cannot be part of a successful match. Consider:
2107    .sp
2108      a+(*SKIP)b
2109    .sp
2110    If the subject is "aaaac...", after the first match attempt fails (starting at
2111    the first character in the string), the starting point skips on to start the
2112    next attempt at "c". Note that a possessive quantifer does not have the same
2113    effect in this example; although it would suppress backtracking during the
2114    first match attempt, the second attempt would start at the second character
2115    instead of skipping on to "c".
2116    .sp
2117      (*THEN)
2118    .sp
2119    This verb causes a skip to the next alternation if the rest of the pattern does
2120    not match. That is, it cancels pending backtracking, but only within the
2121    current alternation. Its name comes from the observation that it can be used
2122    for a pattern-based if-then-else block:
2123    .sp
2124      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2125    .sp
2126    If the COND1 pattern matches, FOO is tried (and possibly further items after
2127    the end of the group if FOO succeeds); on failure the matcher skips to the
2128    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2129    is used outside of any alternation, it acts exactly like (*PRUNE).
2130    .
2131    .
2132  .SH "SEE ALSO"  .SH "SEE ALSO"
2133  .rs  .rs
2134  .sp  .sp
# Line 1976  Cambridge CB2 3QH, England. Line 2149  Cambridge CB2 3QH, England.
2149  .rs  .rs
2150  .sp  .sp
2151  .nf  .nf
2152  Last updated: 19 June 2007  Last updated: 21 August 2007
2153  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
2154  .fi  .fi

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