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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 167 by ph10, Wed May 9 15:53:54 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 268  in the Line 291  in the
291  .\" HREF  .\" HREF
292  \fBpcreapi\fP  \fBpcreapi\fP
293  .\"  .\"
294  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
295  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
296    accented letters, and these are matched by \ew.
297  .P  .P
298  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
299  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
# Line 277  character property support is available. Line 301  character property support is available.
301  discouraged.  discouraged.
302  .  .
303  .  .
304    .SS "Newline sequences"
305    .rs
306    .sp
307    Outside a character class, the escape sequence \eR matches any Unicode newline
308    sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
309    the following:
310    .sp
311      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
312    .sp
313    This is an example of an "atomic group", details of which are given
314    .\" HTML <a href="#atomicgroup">
315    .\" </a>
316    below.
317    .\"
318    This particular group matches either the two-character sequence CR followed by
319    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
320    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
321    line, U+0085). The two-character sequence is treated as a single unit that
322    cannot be split.
323    .P
324    In UTF-8 mode, two additional characters whose codepoints are greater than 255
325    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
326    Unicode character property support is not needed for these characters to be
327    recognized.
328    .P
329    Inside a character class, \eR matches the letter "R".
330    .
331    .
332  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
333  .SS Unicode character properties  .SS Unicode character properties
334  .rs  .rs
# Line 307  Those that are not part of an identified Line 359  Those that are not part of an identified
359  .P  .P
360  Arabic,  Arabic,
361  Armenian,  Armenian,
362    Balinese,
363  Bengali,  Bengali,
364  Bopomofo,  Bopomofo,
365  Braille,  Braille,
# Line 316  Canadian_Aboriginal, Line 369  Canadian_Aboriginal,
369  Cherokee,  Cherokee,
370  Common,  Common,
371  Coptic,  Coptic,
372    Cuneiform,
373  Cypriot,  Cypriot,
374  Cyrillic,  Cyrillic,
375  Deseret,  Deseret,
# Line 345  Malayalam, Line 399  Malayalam,
399  Mongolian,  Mongolian,
400  Myanmar,  Myanmar,
401  New_Tai_Lue,  New_Tai_Lue,
402    Nko,
403  Ogham,  Ogham,
404  Old_Italic,  Old_Italic,
405  Old_Persian,  Old_Persian,
406  Oriya,  Oriya,
407  Osmanya,  Osmanya,
408    Phags_Pa,
409    Phoenician,
410  Runic,  Runic,
411  Shavian,  Shavian,
412  Sinhala,  Sinhala,
# Line 466  properties in PCRE. Line 523  properties in PCRE.
523  .SS "Simple assertions"  .SS "Simple assertions"
524  .rs  .rs
525  .sp  .sp
526  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
527  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,
528  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
529  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 535  The backslashed assertions are:
535  .sp  .sp
536    \eb     matches at a word boundary    \eb     matches at a word boundary
537    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
538    \eA     matches at start of subject    \eA     matches at the start of the subject
539    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
540    \ez     matches at end of subject            also matches before a newline at the end of the subject
541    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
542      \eG     matches at the first matching position in the subject
543  .sp  .sp
544  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
545  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 636  end of the subject in both modes, and if
636  .sp  .sp
637  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
638  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
639  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.
640  a line ending is defined as a single character (CR or LF), dot never matches  .P
641  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
642  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
643  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
644    (including isolated CRs and LFs). When any Unicode line endings are being
645    recognized, dot does not match CR or LF or any of the other line ending
646    characters.
647  .P  .P
648  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
649  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
650  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
651    to match it.
652  .P  .P
653  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
654  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 659  special meaning in a character class.
659  .rs  .rs
660  .sp  .sp
661  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
662  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
663  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
664  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,
665  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,
666  sequence is best avoided.  the \eC escape sequence is best avoided.
667  .P  .P
668  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
669  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 652  If you want to use caseless matching for Line 714  If you want to use caseless matching for
714  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
715  UTF-8 support.  UTF-8 support.
716  .P  .P
717  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
718  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
719  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
720  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
721  .P  .P
722  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
723  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 679  example [\ex{100}-\ex{2ff}]. Line 741  example [\ex{100}-\ex{2ff}].
741  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
742  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
743  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
744  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
745  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
746  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
747  property support.  property support.
# Line 790  If the change is placed right at the sta Line 852  If the change is placed right at the sta
852  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
853  \fBpcre_fullinfo()\fP function).  \fBpcre_fullinfo()\fP function).
854  .P  .P
855  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
856  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
857  .sp  .sp
858    (a(?i)b)c    (a(?i)b)c
859  .sp  .sp
# Line 824  Turning part of a pattern into a subpatt Line 886  Turning part of a pattern into a subpatt
886    cat(aract|erpillar|)    cat(aract|erpillar|)
887  .sp  .sp
888  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
889  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
890  .sp  .sp
891  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
892  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 911  the string "the white queen" is matched
911    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
912  .sp  .sp
913  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
914  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.  
915  .P  .P
916  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
917  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 932  the above patterns match "SUNDAY" as wel
932  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
933  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
934  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
935  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
936  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
937    introduced it at release 4.0, using the Python syntax. PCRE now supports both
938    the Perl and the Python syntax.
939    .P
940    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
941    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
942  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
943  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
944  .\" </a>  .\" </a>
# Line 890  conditions, Line 956  conditions,
956  can be made by name as well as by number.  can be made by name as well as by number.
957  .P  .P
958  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
959  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
960  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
961  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
962  captured substring by name.  is also a convenience function for extracting a captured substring by name.
963  .P  .P
964  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
965  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 968  match. Suppose you want to match the nam
968  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
969  abbreviation. This pattern (ignoring the line breaks) does the job:  abbreviation. This pattern (ignoring the line breaks) does the job:
970  .sp  .sp
971    (?P<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
972    (?P<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
973    (?P<DN>Wed)(?:nesday)?|    (?<DN>Wed)(?:nesday)?|
974    (?P<DN>Thu)(?:rsday)?|    (?<DN>Thu)(?:rsday)?|
975    (?P<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
976  .sp  .sp
977  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.
978  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
979  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
980  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
981  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
982  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 994  Repetition is specified by quantifiers,
994  items:  items:
995  .sp  .sp
996    a literal data character    a literal data character
997    the . metacharacter    the dot metacharacter
998    the \eC escape sequence    the \eC escape sequence
999    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1000      the \eR escape sequence
1001    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1002    a character class    a character class
1003    a back reference (see next section)    a back reference (see next section)
# Line 968  which may be several bytes long (and the Line 1035  which may be several bytes long (and the
1035  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
1036  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1037  .P  .P
1038  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1039  quantifiers have single-character abbreviations:  abbreviations:
1040  .sp  .sp
1041    *    is equivalent to {0,}    *    is equivalent to {0,}
1042    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1084  own right. Because it has two uses, it c
1084  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
1085  way the rest of the pattern matches.  way the rest of the pattern matches.
1086  .P  .P
1087  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),
1088  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
1089  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
1090  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1094  is greater than 1 or with a limited maxi
1094  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1095  .P  .P
1096  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
1097  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
1098  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1099  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
1100  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 1106  alternatively using ^ to indicate anchor
1106  .P  .P
1107  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1108  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1109  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
1110  succeed. Consider, for example:  succeeds. Consider, for example:
1111  .sp  .sp
1112    (.*)abc\e1    (.*)abc\e1
1113  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1133  matches "aba" the value of the second ca
1133  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1134  .rs  .rs
1135  .sp  .sp
1136  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1137  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
1138  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
1139  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
1140  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
1141  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1142  .P  .P
1143  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
1144  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1150  item, and then with 4, and so on, before
1150  (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
1151  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.
1152  .P  .P
1153  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
1154  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
1155  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1156  .sp  .sp
# Line 1115  previous example can be rewritten as Line 1182  previous example can be rewritten as
1182  .sp  .sp
1183  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1184  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
1185  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
1186  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1187  .P  difference; possessive quantifiers should be slightly faster.
1188  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1189  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.
1190  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
1191  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1192    package, and PCRE copied it from there. It ultimately found its way into Perl
1193    at release 5.10.
1194    .P
1195    PCRE has an optimization that automatically "possessifies" certain simple
1196    pattern constructs. For example, the sequence A+B is treated as A++B because
1197    there is no point in backtracking into a sequence of A's when B must follow.
1198  .P  .P
1199  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
1200  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 1240  numbers less than 10. A "forward back re
1240  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
1241  in an earlier iteration.  in an earlier iteration.
1242  .P  .P
1243  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
1244  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
1245  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1246  "Non-printing characters"  "Non-printing characters"
1247  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1248  .\" </a>  .\" </a>
1249  above  above
1250  .\"  .\"
1251  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1252    no such problem when named parentheses are used. A back reference to any
1253    subpattern is possible using named parentheses (see below).
1254    .P
1255    Another way of avoiding the ambiguity inherent in the use of digits following a
1256    backslash is to use the \eg escape sequence, which is a feature introduced in
1257    Perl 5.10. This escape must be followed by a positive or a negative number,
1258    optionally enclosed in braces. These examples are all identical:
1259    .sp
1260      (ring), \e1
1261      (ring), \eg1
1262      (ring), \eg{1}
1263    .sp
1264    A positive number specifies an absolute reference without the ambiguity that is
1265    present in the older syntax. It is also useful when literal digits follow the
1266    reference. A negative number is a relative reference. Consider this example:
1267    .sp
1268      (abc(def)ghi)\eg{-1}
1269    .sp
1270    The sequence \eg{-1} is a reference to the most recently started capturing
1271    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1272    would be equivalent to \e1. The use of relative references can be helpful in
1273    long patterns, and also in patterns that are created by joining together
1274    fragments that contain references within themselves.
1275  .P  .P
1276  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1277  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 1293  back reference, the case of letters is r
1293  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
1294  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1295  .P  .P
1296  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'
1297  rewrite the above example as follows:  or the Python syntax (?P=name). We could rewrite the above example in either of
1298    the following ways:
1299  .sp  .sp
1300      (?<p1>(?i)rah)\es+\ek<p1>
1301    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1302  .sp  .sp
1303  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 1422  lengths, but it is acceptable if rewritt
1422    (?<=abc|abde)    (?<=abc|abde)
1423  .sp  .sp
1424  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1425  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
1426  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1427  match is deemed to fail.  assertion fails.
1428  .P  .P
1429  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)
1430  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1431  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
1432  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1433  .P  .P
1434  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1435  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
1436  such as  pattern such as
1437  .sp  .sp
1438    abcd$    abcd$
1439  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1449  then all but the last two characters, an
1449  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,
1450  if the pattern is written as  if the pattern is written as
1451  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1452    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1453  .sp  .sp
1454  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
1455  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
1456  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1457  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 1507  If the condition is satisfied, the yes-p
1507  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
1508  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1509  .P  .P
1510  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1511  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1512  underscores, the condition is satisfied if the capturing subpattern of that  .
1513  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1514  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1515  named subpattern; if it cannot find one and the text consists entirely of  .sp
1516  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
1517  zero. Using subpattern names that consist entirely of digits is not  condition is true if the capturing subpattern of that number has previously
1518  recommended.  matched. An alternative notation is to precede the digits with a plus or minus
1519    sign. In this case, the subpattern number is relative rather than absolute.
1520    The most recently opened parentheses can be referenced by (?(-1), the next most
1521    recent by (?(-2), and so on. In looping constructs it can also make sense to
1522    refer to subsequent groups with constructs such as (?(+2).
1523  .P  .P
1524  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1525  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 1535  or not. If they did, that is, if subject
1535  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
1536  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1537  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1538  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
1539  named subpattern gives this:  .P
1540    If you were embedding this pattern in a larger one, you could use a relative
1541    reference:
1542    .sp
1543      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1544  .sp  .sp
1545    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )  This makes the fragment independent of the parentheses in the larger pattern.
1546    .
1547    .SS "Checking for a used subpattern by name"
1548    .rs
1549    .sp
1550    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1551    subpattern by name. For compatibility with earlier versions of PCRE, which had
1552    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1553    there is a possible ambiguity with this syntax, because subpattern names may
1554    consist entirely of digits. PCRE looks first for a named subpattern; if it
1555    cannot find one and the name consists entirely of digits, PCRE looks for a
1556    subpattern of that number, which must be greater than zero. Using subpattern
1557    names that consist entirely of digits is not recommended.
1558    .P
1559    Rewriting the above example to use a named subpattern gives this:
1560    .sp
1561      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1562    .sp
1563    .
1564    .SS "Checking for pattern recursion"
1565    .rs
1566  .sp  .sp
1567  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,
1568  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
1569  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
1570  Recursive patterns are described in the next section.  letter R, for example:
1571    .sp
1572      (?(R3)...) or (?(R&name)...)
1573    .sp
1574    the condition is true if the most recent recursion is into the subpattern whose
1575    number or name is given. This condition does not check the entire recursion
1576    stack.
1577  .P  .P
1578  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false. Recursive
1579    patterns are described below.
1580    .
1581    .SS "Defining subpatterns for use by reference only"
1582    .rs
1583    .sp
1584    If the condition is the string (DEFINE), and there is no subpattern with the
1585    name DEFINE, the condition is always false. In this case, there may be only one
1586    alternative in the subpattern. It is always skipped if control reaches this
1587    point in the pattern; the idea of DEFINE is that it can be used to define
1588    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1589    is described below.) For example, a pattern to match an IPv4 address could be
1590    written like this (ignore whitespace and line breaks):
1591    .sp
1592      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1593      \eb (?&byte) (\e.(?&byte)){3} \eb
1594    .sp
1595    The first part of the pattern is a DEFINE group inside which a another group
1596    named "byte" is defined. This matches an individual component of an IPv4
1597    address (a number less than 256). When matching takes place, this part of the
1598    pattern is skipped because DEFINE acts like a false condition.
1599    .P
1600    The rest of the pattern uses references to the named group to match the four
1601    dot-separated components of an IPv4 address, insisting on a word boundary at
1602    each end.
1603    .
1604    .SS "Assertion conditions"
1605    .rs
1606    .sp
1607    If the condition is not in any of the above formats, it must be an assertion.
1608  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1609  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1610  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 1640  next newline in the pattern.
1640  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1641  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1642  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
1643  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
1644  that allows regular expressions to recurse (amongst other things). It does this  .P
1645  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
1646  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
1647  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1648    pattern using code interpolation to solve the parentheses problem can be
1649    created like this:
1650  .sp  .sp
1651    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1652  .sp  .sp
1653  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
1654  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1655  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1656  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1657  .P  supports special syntax for recursion of the entire pattern, and also for
1658  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1659  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.
1660  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1661  "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
1662  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1663  .P  provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1664  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
1665  it has matched some of the subject string, it is never re-entered, even if  a recursive call of the entire regular expression.
1666  it contains untried alternatives and there is a subsequent matching failure.  .P
1667    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1668    treated as an atomic group. That is, once it has matched some of the subject
1669    string, it is never re-entered, even if it contains untried alternatives and
1670    there is a subsequent matching failure.
1671  .P  .P
1672  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
1673  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
# Line 1522  pattern, so instead you could use this: Line 1685  pattern, so instead you could use this:
1685    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1686  .sp  .sp
1687  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
1688  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1689  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1690  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1691  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. (A Perl 5.10 feature.)
1692  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1693  .sp  most recently opened parentheses preceding the recursion. In other words, a
1694    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
1695  .sp  it is encountered.
1696  This particular example pattern contains nested unlimited repeats, and so the  .P
1697  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1698  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
1699  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1700    "subroutine" calls, as described in the next section.
1701    .P
1702    An alternative approach is to use named parentheses instead. The Perl syntax
1703    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1704    could rewrite the above example as follows:
1705    .sp
1706      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1707    .sp
1708    If there is more than one subpattern with the same name, the earliest one is
1709    used.
1710    .P
1711    This particular example pattern that we have been looking at contains nested
1712    unlimited repeats, and so the use of atomic grouping for matching strings of
1713    non-parentheses is important when applying the pattern to strings that do not
1714    match. For example, when this pattern is applied to
1715  .sp  .sp
1716    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1717  .sp  .sp
# Line 1545  before failure can be reported. Line 1723  before failure can be reported.
1723  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
1724  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.
1725  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
1726  the next section and the  below and the
1727  .\" HREF  .\" HREF
1728  \fBpcrecallout\fP  \fBpcrecallout\fP
1729  .\"  .\"
# Line 1584  is the actual recursive call. Line 1762  is the actual recursive call.
1762  .sp  .sp
1763  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
1764  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
1765  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1766  pattern  before or after the reference. A numbered reference can be absolute or
1767    relative, as in these examples:
1768    .sp
1769      (...(absolute)...)...(?2)...
1770      (...(relative)...)...(?-1)...
1771      (...(?+1)...(relative)...
1772    .sp
1773    An earlier example pointed out that the pattern
1774  .sp  .sp
1775    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1776  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 1780  matches "sense and sensibility" and "res
1780    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1781  .sp  .sp
1782  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
1783  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.  
1784  .P  .P
1785  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1786  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
1787  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
1788  matching failure.  matching failure.
1789    .P
1790    When a subpattern is used as a subroutine, processing options such as
1791    case-independence are fixed when the subpattern is defined. They cannot be
1792    changed for different calls. For example, consider this pattern:
1793    .sp
1794      (abc)(?i:(?-1))
1795    .sp
1796    It matches "abcabc". It does not match "abcABC" because the change of
1797    processing option does not affect the called subpattern.
1798  .  .
1799  .  .
1800  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 1815  function is to be called. If you want to
1815  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1816  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1817  .sp  .sp
1818    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1819  .sp  .sp
1820  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1821  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1638  description of the interface to the call Line 1831  description of the interface to the call
1831  \fBpcrecallout\fP  \fBpcrecallout\fP
1832  .\"  .\"
1833  documentation.  documentation.
1834  .P  .
1835  .in 0  .
1836  Last updated: 06 June 2006  .SH "SEE ALSO"
1837  .br  .rs
1838  Copyright (c) 1997-2006 University of Cambridge.  .sp
1839    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1840    .
1841    .
1842    .SH AUTHOR
1843    .rs
1844    .sp
1845    .nf
1846    Philip Hazel
1847    University Computing Service
1848    Cambridge CB2 3QH, England.
1849    .fi
1850    .
1851    .
1852    .SH REVISION
1853    .rs
1854    .sp
1855    .nf
1856    Last updated: 09 May 2007
1857    Copyright (c) 1997-2007 University of Cambridge.
1858    .fi

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