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revision 92 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 93 by nigel, Sat Feb 24 21:41:42 2007 UTC
# Line 36  and how it differs from the normal funct Line 36  and how it differs from the normal funct
36  \fBpcrematching\fP  \fBpcrematching\fP
37  .\"  .\"
38  page.  page.
39  .P  .
40    .
41    .SH "CHARACTERS AND METACHARACTERS"
42    .rs
43    .sp
44  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
45  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
46  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 64  interpreted in some special way.
64  .P  .P
65  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
66  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
67  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
68  as follows:  are as follows:
69  .sp  .sp
70    \e      general escape character with several uses    \e      general escape character with several uses
71    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 96  a character class the only metacharacter
96  .sp  .sp
97  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
98  .  .
99    .
100  .SH BACKSLASH  .SH BACKSLASH
101  .rs  .rs
102  .sp  .sp
# Line 190  parenthesized subpatterns. Line 195  parenthesized subpatterns.
195  .P  .P
196  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
197  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
198  digits following the backslash, ane uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
199  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
200  character specified in octal must be less than \e400. In UTF-8 mode, values up  character specified in octal must be less than \e400. In UTF-8 mode, values up
201  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 221  zero, because no more than three octal d Line 226  zero, because no more than three octal d
226  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
227  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
228  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08), and the
229  sequence \eX is interpreted as the character "X". Outside a character class,  sequences \eR and \eX are interpreted as the characters "R" and "X",
230  these sequences have different meanings  respectively. Outside a character class, these sequences have different
231    meanings
232  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
233  .\" </a>  .\" </a>
234  (see below).  (see below).
235  .\"  .\"
236  .  .
237  .  .
238    .SS "Absolute and relative back references"
239    .rs
240    .sp
241    The sequence \eg followed by a positive or negative number, optionally enclosed
242    in braces, is an absolute or relative back reference. Back references are
243    discussed
244    .\" HTML <a href="#backreferences">
245    .\" </a>
246    later,
247    .\"
248    following the discussion of
249    .\" HTML <a href="#subpattern">
250    .\" </a>
251    parenthesized subpatterns.
252    .\"
253    .
254    .
255  .SS "Generic character types"  .SS "Generic character types"
256  .rs  .rs
257  .sp  .sp
258  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
259  following are always recognized:  following are always recognized:
260  .sp  .sp
261    \ed     any decimal digit    \ed     any decimal digit
# Line 277  character property support is available. Line 300  character property support is available.
300  discouraged.  discouraged.
301  .  .
302  .  .
303    .SS "Newline sequences"
304    .rs
305    .sp
306    Outside a character class, the escape sequence \eR matches any Unicode newline
307    sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
308    the following:
309    .sp
310      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
311    .sp
312    This is an example of an "atomic group", details of which are given
313    .\" HTML <a href="#atomicgroup">
314    .\" </a>
315    below.
316    .\"
317    This particular group matches either the two-character sequence CR followed by
318    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
319    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
320    line, U+0085). The two-character sequence is treated as a single unit that
321    cannot be split.
322    .P
323    In UTF-8 mode, two additional characters whose codepoints are greater than 255
324    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
325    Unicode character property support is not needed for these characters to be
326    recognized.
327    .P
328    Inside a character class, \eR matches the letter "R".
329    .
330    .
331  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
332  .SS Unicode character properties  .SS Unicode character properties
333  .rs  .rs
# Line 307  Those that are not part of an identified Line 358  Those that are not part of an identified
358  .P  .P
359  Arabic,  Arabic,
360  Armenian,  Armenian,
361    Balinese,
362  Bengali,  Bengali,
363  Bopomofo,  Bopomofo,
364  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 368  Canadian_Aboriginal,
368  Cherokee,  Cherokee,
369  Common,  Common,
370  Coptic,  Coptic,
371    Cuneiform,
372  Cypriot,  Cypriot,
373  Cyrillic,  Cyrillic,
374  Deseret,  Deseret,
# Line 345  Malayalam, Line 398  Malayalam,
398  Mongolian,  Mongolian,
399  Myanmar,  Myanmar,
400  New_Tai_Lue,  New_Tai_Lue,
401    Nko,
402  Ogham,  Ogham,
403  Old_Italic,  Old_Italic,
404  Old_Persian,  Old_Persian,
405  Oriya,  Oriya,
406  Osmanya,  Osmanya,
407    Phags_Pa,
408    Phoenician,
409  Runic,  Runic,
410  Shavian,  Shavian,
411  Sinhala,  Sinhala,
# Line 466  properties in PCRE. Line 522  properties in PCRE.
522  .SS "Simple assertions"  .SS "Simple assertions"
523  .rs  .rs
524  .sp  .sp
525  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
526  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
527  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
528  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 534  The backslashed assertions are:
534  .sp  .sp
535    \eb     matches at a word boundary    \eb     matches at a word boundary
536    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
537    \eA     matches at start of subject    \eA     matches at the start of the subject
538    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
539    \ez     matches at end of subject            also matches before a newline at the end of the subject
540    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
541      \eG     matches at the first matching position in the subject
542  .sp  .sp
543  These assertions may not appear in character classes (but note that \eb has a  These assertions may not appear in character classes (but note that \eb has a
544  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 578  end of the subject in both modes, and if Line 635  end of the subject in both modes, and if
635  .sp  .sp
636  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
637  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
638  line. In UTF-8 mode, the matched character may be more than one byte long. When  line. In UTF-8 mode, the matched character may be more than one byte long.
639  a line ending is defined as a single character (CR or LF), dot never matches  .P
640  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
641  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
642  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
643    (including isolated CRs and LFs). When any Unicode line endings are being
644    recognized, dot does not match CR or LF or any of the other line ending
645    characters.
646  .P  .P
647  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
648  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
649  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
650    to match it.
651  .P  .P
652  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
653  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
# Line 597  special meaning in a character class. Line 658  special meaning in a character class.
658  .rs  .rs
659  .sp  .sp
660  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
661  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
662  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  characters. The feature is provided in Perl in order to match individual bytes
663  Because it breaks up UTF-8 characters into individual bytes, what remains in  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
664  the string may be a malformed UTF-8 string. For this reason, the \eC escape  what remains in the string may be a malformed UTF-8 string. For this reason,
665  sequence is best avoided.  the \eC escape sequence is best avoided.
666  .P  .P
667  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
668  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 652  If you want to use caseless matching for Line 713  If you want to use caseless matching for
713  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
714  UTF-8 support.  UTF-8 support.
715  .P  .P
716  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
717  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
718  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
719  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
720  .P  .P
721  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
722  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 790  If the change is placed right at the sta Line 851  If the change is placed right at the sta
851  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
852  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
853  .P  .P
854  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
855  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
856  .sp  .sp
857    (a(?i)b)c    (a(?i)b)c
858  .sp  .sp
# Line 824  Turning part of a pattern into a subpatt Line 885  Turning part of a pattern into a subpatt
885    cat(aract|erpillar|)    cat(aract|erpillar|)
886  .sp  .sp
887  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
888  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
889  .sp  .sp
890  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
891  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
# Line 849  the string "the white queen" is matched Line 910  the string "the white queen" is matched
910    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
911  .sp  .sp
912  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
913  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
914  .P  .P
915  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
916  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 871  the above patterns match "SUNDAY" as wel Line 931  the above patterns match "SUNDAY" as wel
931  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
932  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
933  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
934  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
935  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
936    introduced it at release 4.0, using the Python syntax. PCRE now supports both
937    the Perl and the Python syntax.
938    .P
939    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
940    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
941  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
942  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
943  .\" </a>  .\" </a>
# Line 890  conditions, Line 955  conditions,
955  can be made by name as well as by number.  can be made by name as well as by number.
956  .P  .P
957  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
958  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
959  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
960  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
961  captured substring by name.  is also a convenience function for extracting a captured substring by name.
962  .P  .P
963  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
964  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. This can
# Line 902  match. Suppose you want to match the nam Line 967  match. Suppose you want to match the nam
967  abbreviation or as the full name, and in both cases you want to extract the  abbreviation or as the full name, and in both cases you want to extract the
968  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
969  .sp  .sp
970    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
971    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
972    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
973    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
974    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
975  .sp  .sp
976  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
977  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
978  for the first, and in this example, the only, subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
979  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
980  make a reference to a non-unique named subpattern from elsewhere in the  make a reference to a non-unique named subpattern from elsewhere in the
981  pattern, the one that corresponds to the lowest number is used. For further  pattern, the one that corresponds to the lowest number is used. For further
# Line 928  Repetition is specified by quantifiers, Line 993  Repetition is specified by quantifiers,
993  items:  items:
994  .sp  .sp
995    a literal data character    a literal data character
996    the . metacharacter    the dot metacharacter
997    the \eC escape sequence    the \eC escape sequence
998    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
999      the \eR escape sequence
1000    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1001    a character class    a character class
1002    a back reference (see next section)    a back reference (see next section)
# Line 968  which may be several bytes long (and the Line 1034  which may be several bytes long (and the
1034  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1035  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1036  .P  .P
1037  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1038  quantifiers have single-character abbreviations:  abbreviations:
1039  .sp  .sp
1040    *    is equivalent to {0,}    *    is equivalent to {0,}
1041    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1083  own right. Because it has two uses, it c
1083  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1084  way the rest of the pattern matches.  way the rest of the pattern matches.
1085  .P  .P
1086  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1087  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1088  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1089  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1093  is greater than 1 or with a limited maxi
1093  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1094  .P  .P
1095  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1096  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1097  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1098  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1099  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 1039  alternatively using ^ to indicate anchor Line 1105  alternatively using ^ to indicate anchor
1105  .P  .P
1106  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1107  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1108  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1109  succeed. Consider, for example:  succeeds. Consider, for example:
1110  .sp  .sp
1111    (.*)abc\e1    (.*)abc\e1
1112  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1132  matches "aba" the value of the second ca
1132  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1133  .rs  .rs
1134  .sp  .sp
1135  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1136  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1137  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1138  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1139  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1140  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1141  .P  .P
1142  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1143  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1149  item, and then with 4, and so on, before
1149  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1150  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1151  .P  .P
1152  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1153  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1154  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1155  .sp  .sp
# Line 1115  previous example can be rewritten as Line 1181  previous example can be rewritten as
1181  .sp  .sp
1182  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1183  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
1184  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1185  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1186  .P  difference; possessive quantifiers should be slightly faster.
1187  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1188  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1189  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1190  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1191    package, and PCRE copied it from there. It ultimately found its way into Perl
1192    at release 5.10.
1193    .P
1194    PCRE has an optimization that automatically "possessifies" certain simple
1195    pattern constructs. For example, the sequence A+B is treated as A++B because
1196    there is no point in backtracking into a sequence of A's when B must follow.
1197  .P  .P
1198  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1199  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
# Line 1167  numbers less than 10. A "forward back re Line 1239  numbers less than 10. A "forward back re
1239  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1240  in an earlier iteration.  in an earlier iteration.
1241  .P  .P
1242  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1243  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1244  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1245  "Non-printing characters"  "Non-printing characters"
1246  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1247  .\" </a>  .\" </a>
1248  above  above
1249  .\"  .\"
1250  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1251    no such problem when named parentheses are used. A back reference to any
1252    subpattern is possible using named parentheses (see below).
1253    .P
1254    Another way of avoiding the ambiguity inherent in the use of digits following a
1255    backslash is to use the \eg escape sequence, which is a feature introduced in
1256    Perl 5.10. This escape must be followed by a positive or a negative number,
1257    optionally enclosed in braces. These examples are all identical:
1258    .sp
1259      (ring), \e1
1260      (ring), \eg1
1261      (ring), \eg{1}
1262    .sp
1263    A positive number specifies an absolute reference without the ambiguity that is
1264    present in the older syntax. It is also useful when literal digits follow the
1265    reference. A negative number is a relative reference. Consider this example:
1266    .sp
1267      (abc(def)ghi)\eg{-1}
1268    .sp
1269    The sequence \eg{-1} is a reference to the most recently started capturing
1270    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1271    would be equivalent to \e1. The use of relative references can be helpful in
1272    long patterns, and also in patterns that are created by joining together
1273    fragments that contain references within themselves.
1274  .P  .P
1275  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1276  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1292  back reference, the case of letters is r
1292  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1293  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1294  .P  .P
1295  Back references to named subpatterns use the Python syntax (?P=name). We could  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'
1296  rewrite the above example as follows:  or the Python syntax (?P=name). We could rewrite the above example in either of
1297    the following ways:
1298  .sp  .sp
1299      (?<p1>(?i)rah)\es+\ek<p1>
1300    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1301  .sp  .sp
1302  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
# Line 1324  lengths, but it is acceptable if rewritt Line 1421  lengths, but it is acceptable if rewritt
1421    (?<=abc|abde)    (?<=abc|abde)
1422  .sp  .sp
1423  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1424  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1425  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1426  match is deemed to fail.  assertion fails.
1427  .P  .P
1428  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1429  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1430  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1431  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1432  .P  .P
1433  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1434  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching at the end of the subject string. Consider a simple
1435  such as  pattern such as
1436  .sp  .sp
1437    abcd$    abcd$
1438  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1448  then all but the last two characters, an
1448  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1449  if the pattern is written as  if the pattern is written as
1450  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1451    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1452  .sp  .sp
1453  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1454  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1455  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1456  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1413  If the condition is satisfied, the yes-p Line 1506  If the condition is satisfied, the yes-p
1506  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1507  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1508  .P  .P
1509  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1510  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1511  underscores, the condition is satisfied if the capturing subpattern of that  .
1512  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1513  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1514  named subpattern; if it cannot find one and the text consists entirely of  .sp
1515  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
1516  zero. Using subpattern names that consist entirely of digits is not  condition is true if the capturing subpattern of that number has previously
1517  recommended.  matched.
1518  .P  .P
1519  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1520  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1437  or not. If they did, that is, if subject Line 1530  or not. If they did, that is, if subject
1530  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
1531  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1532  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1533  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1534  named subpattern gives this:  .
1535    .SS "Checking for a used subpattern by name"
1536    .rs
1537    .sp
1538    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1539    subpattern by name. For compatibility with earlier versions of PCRE, which had
1540    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1541    there is a possible ambiguity with this syntax, because subpattern names may
1542    consist entirely of digits. PCRE looks first for a named subpattern; if it
1543    cannot find one and the name consists entirely of digits, PCRE looks for a
1544    subpattern of that number, which must be greater than zero. Using subpattern
1545    names that consist entirely of digits is not recommended.
1546    .P
1547    Rewriting the above example to use a named subpattern gives this:
1548  .sp  .sp
1549    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1550    .sp
1551    .
1552    .SS "Checking for pattern recursion"
1553    .rs
1554  .sp  .sp
1555  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
1556  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
1557  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
1558  Recursive patterns are described in the next section.  letter R, for example:
1559    .sp
1560      (?(R3)...) or (?(R&name)...)
1561    .sp
1562    the condition is true if the most recent recursion is into the subpattern whose
1563    number or name is given. This condition does not check the entire recursion
1564    stack.
1565    .P
1566    At "top level", all these recursion test conditions are false. Recursive
1567    patterns are described below.
1568    .
1569    .SS "Defining subpatterns for use by reference only"
1570    .rs
1571    .sp
1572    If the condition is the string (DEFINE), and there is no subpattern with the
1573    name DEFINE, the condition is always false. In this case, there may be only one
1574    alternative in the subpattern. It is always skipped if control reaches this
1575    point in the pattern; the idea of DEFINE is that it can be used to define
1576    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1577    is described below.) For example, a pattern to match an IPv4 address could be
1578    written like this (ignore whitespace and line breaks):
1579    .sp
1580      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1581      \eb (?&byte) (\e.(?&byte)){3} \eb
1582    .sp
1583    The first part of the pattern is a DEFINE group inside which a another group
1584    named "byte" is defined. This matches an individual component of an IPv4
1585    address (a number less than 256). When matching takes place, this part of the
1586    pattern is skipped because DEFINE acts like a false condition.
1587  .P  .P
1588  If the condition is not a sequence of digits or (R), it must be an assertion.  The rest of the pattern uses references to the named group to match the four
1589    dot-separated components of an IPv4 address, insisting on a word boundary at
1590    each end.
1591    .
1592    .SS "Assertion conditions"
1593    .rs
1594    .sp
1595    If the condition is not in any of the above formats, it must be an assertion.
1596  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1597  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1598  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1628  next newline in the pattern.
1628  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1629  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1630  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1631  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1632  that allows regular expressions to recurse (amongst other things). It does this  .P
1633  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
1634  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
1635  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1636    pattern using code interpolation to solve the parentheses problem can be
1637    created like this:
1638  .sp  .sp
1639    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1640  .sp  .sp
1641  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
1642  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1643  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1644  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1645  .P  supports special syntax for recursion of the entire pattern, and also for
1646  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1647  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1648  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1649  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
1650  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1651  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1652  A recursive subpattern call is always treated as an atomic group. That is, once  call, which is described in the next section.) The special item (?R) or (?0) is
1653  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1654  it contains untried alternatives and there is a subsequent matching failure.  .P
1655    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1656    treated as an atomic group. That is, once it has matched some of the subject
1657    string, it is never re-entered, even if it contains untried alternatives and
1658    there is a subsequent matching failure.
1659  .P  .P
1660  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1661  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
# Line 1524  pattern, so instead you could use this: Line 1675  pattern, so instead you could use this:
1675  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
1676  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern. In a larger pattern, keeping track of
1677  parenthesis numbers can be tricky. It may be more convenient to use named  parenthesis numbers can be tricky. It may be more convenient to use named
1678  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier
1679  the Python syntax that PCRE uses for named parentheses (Perl does not provide  syntax (?P>name) is also supported. We could rewrite the above example as
1680  named parentheses). We could rewrite the above example as follows:  follows:
1681  .sp  .sp
1682    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1683  .sp  .sp
1684  This particular example pattern contains nested unlimited repeats, and so the  If there is more than one subpattern with the same name, the earliest one is
1685  use of atomic grouping for matching strings of non-parentheses is important  used. This particular example pattern contains nested unlimited repeats, and so
1686    the use of atomic grouping for matching strings of non-parentheses is important
1687  when applying the pattern to strings that do not match. For example, when this  when applying the pattern to strings that do not match. For example, when this
1688  pattern is applied to  pattern is applied to
1689  .sp  .sp
# Line 1545  before failure can be reported. Line 1697  before failure can be reported.
1697  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1698  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1699  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1700  the next section and the  below and the
1701  .\" HREF  .\" HREF
1702  \fBpcrecallout\fP  \fBpcrecallout\fP
1703  .\"  .\"
# Line 1584  is the actual recursive call. Line 1736  is the actual recursive call.
1736  .sp  .sp
1737  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
1738  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
1739  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1740  pattern  before or after the reference. An earlier example pointed out that the pattern
1741  .sp  .sp
1742    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1743  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 1747  matches "sense and sensibility" and "res
1747    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1748  .sp  .sp
1749  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
1750  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
 which they refer. However, named references can refer to later subpatterns.  
1751  .P  .P
1752  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1753  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
1754  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
1755  matching failure.  matching failure.
1756    .P
1757    When a subpattern is used as a subroutine, processing options such as
1758    case-independence are fixed when the subpattern is defined. They cannot be
1759    changed for different calls. For example, consider this pattern:
1760    .sp
1761      (abc)(?i:(?1))
1762    .sp
1763    It matches "abcabc". It does not match "abcABC" because the change of
1764    processing option does not affect the called subpattern.
1765  .  .
1766  .  .
1767  .SH CALLOUTS  .SH CALLOUTS
# Line 1638  description of the interface to the call Line 1798  description of the interface to the call
1798  \fBpcrecallout\fP  \fBpcrecallout\fP
1799  .\"  .\"
1800  documentation.  documentation.
1801    .
1802    .
1803    .SH "SEE ALSO"
1804    .rs
1805    .sp
1806    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1807  .P  .P
1808  .in 0  .in 0
1809  Last updated: 06 June 2006  Last updated: 06 December 2006
1810  .br  .br
1811  Copyright (c) 1997-2006 University of Cambridge.  Copyright (c) 1997-2006 University of Cambridge.
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