/[pcre]/code/tags/pcre-2.06/pcre.3

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-revision 11 by nigel,
Sat Feb 24 21:38:17 2007 UTC
+revision 27 by nigel,
Sat Feb 24 21:38:49 2007 UTC
 Line 8 
 pcre - Perl-compatible regular expressio
  .br
  .B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
  .ti +5n
- .B const char **\fIerrptr\fR, int *\fIerroffset\fR);
+ .B const char **\fIerrptr\fR, int *\fIerroffset\fR,
+ .ti +5n
+ .B const unsigned char *\fItableptr\fR);
+ .PP
+ .br
+ .B const unsigned char *pcre_maketables(void);
  .PP
  .br
  .B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
-Line 34 
 pcre - Perl-compatible regular expressio
+Line 39 
 pcre - Perl-compatible regular expressio
  .PP
  .br
  .B void (*pcre_free)(void *);
- .PP
- .br
- .B unsigned char *pcre_cbits[128];
- .PP
- .br
- .B unsigned char *pcre_ctypes[256];
- .PP
- .br
- .B unsigned char *pcre_fcc[256];
- .PP
- .br
- .B unsigned char *pcre_lcc[256];
  .SH DESCRIPTION
  The PCRE library is a set of functions that implement regular expression
  pattern matching using the same syntax and semantics as Perl 5, with just a few
- differences (see below). The current implementation corresponds to Perl 5.004.
+ differences (see below). The current implementation corresponds to Perl 5.005.
  PCRE has its own native API, which is described in this man page. There is also
  a set of wrapper functions that correspond to the POSIX API. See
-Line 60 
 a set of wrapper functions that correspo
+Line 53 
 a set of wrapper functions that correspo
  The three functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and
  \fBpcre_exec()\fR are used for compiling and matching regular expressions. The
- function \fBpcre_info()\fR is used to find out information about a compiled
+ function \fBpcre_maketables()\fR is used (optionally) to build a set of
+ character tables in the current locale for passing to \fBpcre_compile()\fR.
+ The function \fBpcre_info()\fR is used to find out information about a compiled
  pattern, while the function \fBpcre_version()\fR returns a pointer to a string
  containing the version of PCRE and its date of release.
-Line 70 
 respectively. PCRE calls the memory mana
+Line 66 
 respectively. PCRE calls the memory mana
  so a calling program can replace them if it wishes to intercept the calls. This
  should be done before calling any PCRE functions.
- The other global variables are character tables. They are initialized when PCRE
- is compiled, from source that is generated by reference to the C character type
- functions, but which the maintainer of PCRE is free to modify. In principle
- they could also be modified at runtime. See PCRE's README file for more
- details.
  .SH MULTI-THREADING
  The PCRE functions can be used in multi-threading applications, with the
- proviso that the character tables and the memory management functions pointed
+ proviso that the memory management functions pointed to by \fBpcre_malloc\fR
- to by \fBpcre_malloc\fR and \fBpcre_free\fR will be shared by all threads.
+ and \fBpcre_free\fR are shared by all threads.
  The compiled form of a regular expression is not altered during matching, so
  the same compiled pattern can safely be used by several threads at once.
-Line 89 
 the same compiled pattern can safely be
+Line 79 
 the same compiled pattern can safely be
  .SH COMPILING A PATTERN
  The function \fBpcre_compile()\fR is called to compile a pattern into an
  internal form. The pattern is a C string terminated by a binary zero, and
- is passed in the argument \fIpattern\fR. A pointer to the compiled code block
+ is passed in the argument \fIpattern\fR. A pointer to a single block of memory
- is returned. The \fBpcre\fR type is defined for this for convenience, but in
+ that is obtained via \fBpcre_malloc\fR is returned. This contains the
- fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the contents of the
+ compiled code and related data. The \fBpcre\fR type is defined for this for
- block are not defined.
+ convenience, but in fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the
+ contents of the block are not externally defined. It is up to the caller to
+ free the memory when it is no longer required.
  .PP
  The size of a compiled pattern is roughly proportional to the length of the
  pattern string, except that each character class (other than those containing
-Line 101 
 quantifiers with a minimum greater than
+Line 93 
 quantifiers with a minimum greater than
  relevant portions of the compiled pattern to be replicated.
  .PP
  The \fIoptions\fR argument contains independent bits that affect the
- compilation. It should be zero if no options are required. Those options that
+ compilation. It should be zero if no options are required. Some of the options,
- are compabible with Perl can also be set at compile time from within the
+ in particular, those that are compatible with Perl, can also be set and unset
- pattern (see the detailed description of regular expressions below) and all
+ from within the pattern (see the detailed description of regular expressions
- options except PCRE_EXTENDED and PCRE_EXTRA can be set at the time of matching.
+ below). For these options, the contents of the \fIoptions\fR argument specifies
+ their initial settings at the start of compilation and execution. The
+ PCRE_ANCHORED option can be set at the time of matching as well as at compile
+ time.
  .PP
  If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
  Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
  NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
- error message.
+ error message. The offset from the start of the pattern to the character where
+ the error was discovered is placed in the variable pointed to by
- The offset from the start of the pattern to the character where the error was
+ \fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
- discovered is placed in the variable pointed to by \fIerroffset\fR, which must
+ .PP
- not be NULL. If it is, an immediate error is given.
+ If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
+ character tables which are built when it is compiled, using the default C
+ locale. Otherwise, \fItableptr\fR must be the result of a call to
+ \fBpcre_maketables()\fR. See the section on locale support below.
  .PP
  The following option bits are defined in the header file:
-Line 127 
 constructs in the pattern itself, which
+Line 125 
 constructs in the pattern itself, which
    PCRE_CASELESS
  If this bit is set, letters in the pattern match both upper and lower case
- letters in any subject string. It is equivalent to Perl's /i option.
+ letters. It is equivalent to Perl's /i option.
    PCRE_DOLLAR_ENDONLY
  If this bit is set, a dollar metacharacter in the pattern matches only at the
- end of the subject string. By default, it also matches immediately before the
+ end of the subject string. Without this option, a dollar also matches
- final character if it is a newline (but not before any other newlines). The
+ immediately before the final character if it is a newline (but not before any
- PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set. There is no
+ other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
- equivalent to this option in Perl.
+ set. There is no equivalent to this option in Perl.
    PCRE_DOTALL
  If this bit is set, a dot metacharater in the pattern matches all characters,
- including newlines. By default, newlines are excluded. This option is
+ including newlines. Without it, newlines are excluded. This option is
  equivalent to Perl's /s option. A negative class such as [^a] always matches a
  newline character, independent of the setting of this option.
    PCRE_EXTENDED
- If this bit is set, whitespace characters in the pattern are totally ignored
+ If this bit is set, whitespace data characters in the pattern are totally
- except when escaped or inside a character class, and characters between an
+ ignored except when escaped or inside a character class, and characters between
- unescaped # outside a character class and the next newline character,
+ an unescaped # outside a character class and the next newline character,
  inclusive, are also ignored. This is equivalent to Perl's /x option, and makes
- it possible to include comments inside complicated patterns.
+ it possible to include comments inside complicated patterns. Note, however,
+ that this applies only to data characters. Whitespace characters may never
+ appear within special character sequences in a pattern, for example within the
+ sequence (?( which introduces a conditional subpattern.
+   PCRE_EXTRA
+ This option turns on additional functionality of PCRE that is incompatible with
+ Perl. Any backslash in a pattern that is followed by a letter that has no
+ special meaning causes an error, thus reserving these combinations for future
+ expansion. By default, as in Perl, a backslash followed by a letter with no
+ special meaning is treated as a literal. There are at present no other features
+ controlled by this option.
    PCRE_MULTILINE
-Line 158 
 By default, PCRE treats the subject stri
+Line 168 
 By default, PCRE treats the subject stri
  characters (even if it actually contains several newlines). The "start of line"
  metacharacter (^) matches only at the start of the string, while the "end of
  line" metacharacter ($) matches only at the end of the string, or before a
- terminating newline. This is the same as Perl.
+ terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as
+ Perl.
  When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
  match immediately following or immediately before any newline in the subject
-Line 167 
 to Perl's /m option. If there are no "\\
+Line 178 
 to Perl's /m option. If there are no "\\
  no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
  effect.
-   PCRE_EXTRA
+   PCRE_UNGREEDY
- This option turns on additional functionality of PCRE that is incompatible with
- Perl. Any backslash in a pattern that is followed by a letter that has no
- special meaning causes an error, thus reserving these combinations for future
- expansion. By default, as in Perl, a backslash followed by a letter with no
- special meaning is treated as a literal. There are two extra features currently
- provided, and both are in some sense experimental additions that are useful for
- influencing the progress of a match.
-   (1) The sequence \\X inserts a Prolog-like "cut" into the expression.
-   (2) Once a subpattern enclosed in (?>subpat) brackets has matched,
-       backtracking never goes back into the pattern.
- See below for further details of both of these.
+ This option inverts the "greediness" of the quantifiers so that they are not
+ greedy by default, but become greedy if followed by "?". It is not compatible
+ with Perl. It can also be set by a (?U) option setting within the pattern.
  .SH STUDYING A PATTERN
-Line 195 
 typedef) containing additional informati
+Line 194 
 typedef) containing additional informati
  passed to \fBpcre_exec()\fR. If no additional information is available, NULL
  is returned.
- The second argument contains option bits. The only one currently supported is
+ The second argument contains option bits. At present, no options are defined
- PCRE_CASELESS. It forces the studying to be done in a caseless manner, even if
+ for \fBpcre_study()\fR, and this argument should always be zero.
- the original pattern was compiled without PCRE_CASELESS. When the result of
- \fBpcre_study()\fR is passed to \fBpcre_exec()\fR, it is used only if its
- caseless state is the same as that of the matching process. A pattern that is
- compiled without PCRE_CASELESS can be studied with and without PCRE_CASELESS,
- and the appropriate data passed to \fBpcre_exec()\fR with and without the
- PCRE_CASELESS flag.
  The third argument for \fBpcre_study()\fR is a pointer to an error message. If
  studying succeeds (even if no data is returned), the variable it points to is
-Line 213 
 not have a single fixed starting charact
+Line 206 
 not have a single fixed starting charact
  characters is created.
+ .SH LOCALE SUPPORT
+ PCRE handles caseless matching, and determines whether characters are letters,
+ digits, or whatever, by reference to a set of tables. The library contains a
+ default set of tables which is created in the default C locale when PCRE is
+ compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
+ and is sufficient for many applications.
+ An alternative set of tables can, however, be supplied. Such tables are built
+ by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
+ relevant locale. The result can then be passed to \fBpcre_compile()\ as often
+ as necessary. For example, to build and use tables that are appropriate for the
+ French locale (where accented characters with codes greater than 128 are
+ treated as letters), the following code could be used:
+   setlocale(LC_CTYPE, "fr");
+   tables = pcre_maketables();
+   re = pcre_compile(..., tables);
+ The tables are built in memory that is obtained via \fBpcre_malloc\fR. The
+ pointer that is passed to \fBpcre_compile\fR is saved with the compiled
+ pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
+ and \fBpcre_match()\fR. Thus for any single pattern, compilation, studying and
+ matching all happen in the same locale, but different patterns can be compiled
+ in different locales. It is the caller's responsibility to ensure that the
+ memory containing the tables remains available for as long as it is needed.
  .SH MATCHING A PATTERN
  The function \fBpcre_exec()\fR is called to match a subject string against a
  pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
-Line 222 
 pattern has been studied, the result of
+Line 242 
 pattern has been studied, the result of
  The subject string is passed as a pointer in \fIsubject\fR and a length in
  \fIlength\fR. Unlike the pattern string, it may contain binary zero characters.
- The options PCRE_ANCHORED, PCRE_CASELESS, PCRE_DOLLAR_ENDONLY, PCRE_DOTALL, and
+ The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
- PCRE_MULTILINE can be passed in the \fIoptions\fR argument, whose unused bits
+ unused bits must be zero. However, if a pattern was compiled with
- must be zero. However, if a pattern is compiled with any of these options, they
+ PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
- cannot be unset when it is obeyed.
+ cannot be made unachored at matching time.
  There are also two further options that can be set only at matching time:
-Line 233 
 There are also two further options that
+Line 253 
 There are also two further options that
  The first character of the string is not the beginning of a line, so the
  circumflex metacharacter should not match before it. Setting this without
- PCRE_MULTILINE (at either compile or match time) causes circumflex never to
+ PCRE_MULTILINE (at compile time) causes circumflex never to match.
- match.
    PCRE_NOTEOL
  The end of the string is not the end of a line, so the dollar metacharacter
- should not match it. Setting this without PCRE_MULTILINE (at either compile or
+ should not match it nor (except in multiline mode) a newline immediately before
- match time) causes dollar never to match.
+ it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never
+ to match.
  In general, a pattern matches a certain portion of the subject, and in
  addition, further substrings from the subject may be picked out by parts of the
-Line 251 
 kinds of parenthesized subpattern that d
+Line 271 
 kinds of parenthesized subpattern that d
  Captured substrings are returned to the caller via a vector of integer offsets
  whose address is passed in \fIovector\fR. The number of elements in the vector
- is passed in \fIovecsize\fR. This should always be an even number, because the
+ is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass
- elements are used in pairs. If an odd number is passed, it is rounded down.
+ back captured substrings, each substring using a pair of integers. The
+ remaining third of the vector is used as workspace by \fBpcre_exec()\fR while
- The first element of a pair is set to the offset of the first character in a
+ matching capturing subpatterns, and is not available for passing back
- substring, and the second is set to the offset of the first character after the
+ information. The length passed in \fIovecsize\fR should always be a multiple of
- end of a substring. The first pair, \fIovector[0]\fR and \fIovector[1]\fR,
+ three. If it is not, it is rounded down.
- identify the portion of the subject string matched by the entire pattern. The
- next pair is used for the first capturing subpattern, and so on. The value
+ When a match has been successful, information about captured substrings is
- returned by \fBpcre_exec()\fR is the number of pairs that have been set. If
+ returned in pairs of integers, starting at the beginning of \fIovector\fR, and
- there are no capturing subpatterns, the return value from a successful match
+ continuing up to two-thirds of its length at the most. The first element of a
- is 1, indicating that just the first pair of offsets has been set.
+ pair is set to the offset of the first character in a substring, and the second
+ is set to the offset of the first character after the end of a substring. The
+ first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
+ subject string matched by the entire pattern. The next pair is used for the
+ first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
+ is the number of pairs that have been set. If there are no capturing
+ subpatterns, the return value from a successful match is 1, indicating that
+ just the first pair of offsets has been set.
  It is possible for an capturing subpattern number \fIn+1\fR to match some
  part of the subject when subpattern \fIn\fR has not been used at all. For
- example, if the string "abc" is matched against the pattern "(a|(z))(bc)",
+ example, if the string "abc" is matched against the pattern (a|(z))(bc)
  subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
  values corresponding to the unused subpattern are set to -1.
-Line 273 
 If a capturing subpattern is matched rep
+Line 300 
 If a capturing subpattern is matched rep
  string that it matched that gets returned.
  If the vector is too small to hold all the captured substrings, it is used as
- far as possible, and the function returns a value of zero. In particular, if
+ far as possible (up to two-thirds of its length), and the function returns a
- the substring offsets are not of interest, \fBpcre_exec()\fR may be called with
+ value of zero. In particular, if the substring offsets are not of interest,
- \fIovector\fR passed as NULL and \fIovecsize\fR as zero. However, if the
+ \fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
- pattern contains back references and the \fIovector\fR isn't big enough to
+ \fIovecsize\fR as zero. However, if the pattern contains back references and
- remember the related substrings, PCRE has to get additional memory for use
+ the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
- during matching. Thus it is usually advisable to supply an \fIovector\fR.
+ to get additional memory for use during matching. Thus it is usually advisable
+ to supply an \fIovector\fR.
  Note that \fBpcre_info()\fR can be used to find out how many capturing
- subpatterns there are in a compiled pattern.
+ subpatterns there are in a compiled pattern. The smallest size for
+ \fIovector\fR that will allow for \fIn\fR captured substrings in addition to
+ the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
  If \fBpcre_exec()\fR fails, it returns a negative number. The following are
  defined in the header file:
-Line 290 
 defined in the header file:
+Line 320 
 defined in the header file:
  The subject string did not match the pattern.
-   PCRE_ERROR_BADREF         (-2)
+   PCRE_ERROR_NULL           (-2)
- There was a back-reference in the pattern to a capturing subpattern that had
- not previously been set.
-   PCRE_ERROR_NULL           (-3)
  Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
  NULL and \fIovecsize\fR was not zero.
-   PCRE_ERROR_BADOPTION      (-4)
+   PCRE_ERROR_BADOPTION      (-3)
  An unrecognized bit was set in the \fIoptions\fR argument.
-   PCRE_ERROR_BADMAGIC       (-5)
+   PCRE_ERROR_BADMAGIC       (-4)
  PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
  the case when it is passed a junk pointer. This is the error it gives when the
  magic number isn't present.
-   PCRE_ERROR_UNKNOWN_NODE   (-6)
+   PCRE_ERROR_UNKNOWN_NODE   (-5)
  While running the pattern match, an unknown item was encountered in the
  compiled pattern. This error could be caused by a bug in PCRE or by overwriting
  of the compiled pattern.
-   PCRE_ERROR_NOMEMORY       (-7)
+   PCRE_ERROR_NOMEMORY       (-6)
  If a pattern contains back references, but the \fIovector\fR that is passed to
  \fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE
-Line 353 
 The maximum length of a compiled pattern
+Line 378 
 The maximum length of a compiled pattern
  All values in repeating quantifiers must be less than 65536.
  The maximum number of capturing subpatterns is 99.
  The maximum number of all parenthesized subpatterns, including capturing
- subpatterns and assertions, is 200.
+ subpatterns, assertions, and other types of subpattern, is 200.
  The maximum length of a subject string is the largest positive number that an
  integer variable can hold. However, PCRE uses recursion to handle subpatterns
-Line 362 
 the size of a subject string that can be
+Line 387 
 the size of a subject string that can be
  .SH DIFFERENCES FROM PERL
- The differences described here are with respect to Perl 5.004.
+ The differences described here are with respect to Perl 5.005.
 . By default, a whitespace character is any character that the C library
  function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
-Line 371 
 formfeed, newline, carriage return, hori
+Line 396 
 formfeed, newline, carriage return, hori
  no longer includes vertical tab in its set of whitespace characters. The \\v
  escape that was in the Perl documentation for a long time was never in fact
  recognized. However, the character itself was treated as whitespace at least
- up to 5.002. In 5.004 it does not match \\s.
+ up to 5.002. In 5.004 and 5.005 it does not match \\s.
 . PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
- them, but they do not mean what you might think. For example, "(?!a){3}" does
+ them, but they do not mean what you might think. For example, (?!a){3} does
  not assert that the next three characters are not "a". It just asserts that the
  next character is not "a" three times.
-Line 396 
 are not part of its pattern matching eng
+Line 421 
 are not part of its pattern matching eng
 . The Perl \\G assertion is not supported as it is not relevant to single
  pattern matches.
-. If a backreference can never be matched, PCRE diagnoses an error. In a case
+. Fairly obviously, PCRE does not support the (?{code}) construction.
- like
-   /(123)\\2/
+. There are at the time of writing some oddities in Perl 5.005_02 concerned
+ with the settings of captured strings when part of a pattern is repeated. For
+ example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
+ "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
+ the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set.
- the error occurs at compile time. Perl gives no compile time error; version
+ In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
-.004 either always fails to match, or gives a segmentation fault at runtime.
+ future Perl changes to a consistent state that is different, PCRE may change to
- In more complicated cases such as
+ follow.
-   /(1)(2)(3)(4)(5)(6)(7)(8)(9)(10\\10)/
+. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
+ /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
+ However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
- PCRE returns PCRE_ERROR_BADREF at run time. Perl always fails to match.
+. PCRE provides some extensions to the Perl regular expression facilities:
-. PCRE provides some extensions to the Perl regular expression facilities:
+ (a) Although lookbehind assertions must match fixed length strings, each
+ alternative branch of a lookbehind assertion can match a different length of
+ string. Perl 5.005 requires them all to have the same length.
- (a) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
+ (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
  character matches only at the very end of the string.
- (b) If PCRE_EXTRA is set, the \\X assertion (a Prolog-like "cut") is
+ (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
- recognized, and a backslash followed by a letter with no special meaning is
+ meaning is faulted.
- faulted. There is also a new kind of parenthesized subpattern starting with (?>
- which has a block on backtracking into it once it has matched.
+ (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
+ inverted, that is, by default they are not greedy, but if followed by a
+ question mark they are.
  .SH REGULAR EXPRESSION DETAILS
-Line 484 
 non-alphameric with "\\" to specify that
+Line 518 
 non-alphameric with "\\" to specify that
  if you want to match a backslash, you write "\\\\".
  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
- pattern and characters between a "#" outside a character class and the next
+ pattern (other than in a character class) and characters between a "#" outside
- newline character are ignored. An escaping backslash can be used to include a
+ a character class and the next newline character are ignored. An escaping
- whitespace or "#" character as part of the pattern.
+ backslash can be used to include a whitespace or "#" character as part of the
+ pattern.
  A second use of backslash provides a way of encoding non-printing characters
  in patterns in a visible manner. There is no restriction on the appearance of
-Line 503 
 represents:
+Line 538 
 represents:
    \\r     carriage return (hex 0D)
    \\t     tab (hex 09)
    \\xhh   character with hex code hh
-   \\ddd   character with octal code ddd or backreference
+   \\ddd   character with octal code ddd, or backreference
  The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
-Line 516 
 lower case).
+Line 551 
 lower case).
  After "\\0" up to two further octal digits are read. In both cases, if there
  are fewer than two digits, just those that are present are used. Thus the
  sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
- Make sure you supply two digits if the character that follows could otherwise
+ Make sure you supply two digits after the initial zero if the character that
- be taken as another digit.
+ follows is itself an octal digit.
  The handling of a backslash followed by a digit other than 0 is complicated.
  Outside a character class, PCRE reads it and any following digits as a decimal
-Line 567 
 Each pair of escape sequences partitions
+Line 602 
 Each pair of escape sequences partitions
  two disjoint sets. Any given character matches one, and only one, of each pair.
  A "word" character is any letter or digit or the underscore character, that is,
- any character which can be part of a Perl "word". These character type
+ any character which can be part of a Perl "word". The definition of letters and
- sequences can appear both inside and outside character classes. They each match
+ digits is controlled by PCRE's character tables, and may vary if locale-
- one character of the appropriate type. If the current matching point is at the
+ specific matching is taking place (see "Locale support" above). For example, in
- end of the subject string, all of them fail, since there is no character to
+ the "fr" (French) locale, some character codes greater than 128 are used for
- match.
+ accented letters, and these are matched by \\w.
- The fourth use of backslash is for certain assertions. An assertion specifies a
+ These character type sequences can appear both inside and outside character
- condition that has to be met at a particular point in a match, without
+ classes. They each match one character of the appropriate type. If the current
- consuming any characters from the subject string. The backslashed assertions
+ matching point is at the end of the subject string, all of them fail, since
- are
+ there is no character to match.
+ The fourth use of backslash is for certain simple assertions. An assertion
+ specifies a condition that has to be met at a particular point in a match,
+ without consuming any characters from the subject string. The use of
+ subpatterns for more complicated assertions is described below. The backslashed
+ assertions are
    \\b     word boundary
    \\B     not a word boundary
    \\A     start of subject (independent of multiline mode)
-   \\Z     end of subject (independent of multiline mode)
+   \\Z     end of subject or newline at end (independent of multiline mode)
+   \\z     end of subject (independent of multiline mode)
- Assertions may not appear in character classes (but note that "\\b" has a
+ These assertions may not appear in character classes (but note that "\\b" has a
  different meaning, namely the backspace character, inside a character class).
  A word boundary is a position in the subject string where the current character
- and the previous character do not both match "\\w" or "\\W" (i.e. one matches
+ and the previous character do not both match \\w or \\W (i.e. one matches
- "\\w" and the other matches "\\W"), or the start or end of the string if the
+ \\w and the other matches \\W), or the start or end of the string if the
- first or last character matches "\\w", respectively. More complicated
+ first or last character matches \\w, respectively.
- assertions are also supported (see below).
+ The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
- The "\\A" and "\\Z" assertions differ from the traditional "^" and "$"
+ dollar (described below) in that they only ever match at the very start and end
- (described below) in that they only ever match at the very start and end of the
+ of the subject string, whatever options are set. They are not affected by the
- subject string, respectively, whatever options are set.
+ PCRE_NOTBOL or PCRE_NOTEOL options. The difference between \\Z and \\z is that
+ \\Z matches before a newline that is the last character of the string as well
- When the PCRE_EXTRA flag is set on a call to \fBpcre_compile()\fR, the
+ as at the end of the string, whereas \\z matches only at the end.
- additional assertion \\X, which has no equivalent in Perl, is recognized.
- This operates like the "cut" operation in Prolog: it prevents the matching
- operation from backtracking past it. For example, if the expression
-   .*/foo
- is matched against the string "/this/string/is/not" then after the greedy .*
- has swallowed the whole string, PCRE keeps backtracking all the way to the
- beginning before failing. If, on the other hand, the expression is
-   .*/\\Xfoo
- then once it has discovered that "/not" is not "/foo", backtracking ceases, and
- the match fails. See also the section on "once-only" subpatterns below.
  .SH CIRCUMFLEX AND DOLLAR
- Outside a character class, the circumflex character is an assertion which is
+ Outside a character class, in the default matching mode, the circumflex
- true only if the current matching point is at the start of the subject string,
+ character is an assertion which is true only if the current matching point is
- in the default matching mode. Inside a character class, circumflex has an
+ at the start of the subject string. Inside a character class, circumflex has an
  entirely different meaning (see below).
  Circumflex need not be the first character of the pattern if a number of
-Line 637 
 Dollar has no special meaning in a chara
+Line 664 
 Dollar has no special meaning in a chara
  The meaning of dollar can be changed so that it matches only at the very end of
  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
- time.
+ time. This does not affect the \\Z assertion.
  The meanings of the circumflex and dollar characters are changed if the
- PCRE_MULTILINE option is set at compile or matching time. When this is the
+ PCRE_MULTILINE option is set. When this is the case, they match immediately
- case, they match immediately after and immediately before an internal "\\n"
+ after and immediately before an internal "\\n" character, respectively, in
- character, respectively, in addition to matching at the start and end of the
+ addition to matching at the start and end of the subject string. For example,
- subject string. For example, the pattern /^abc$/ matches the subject string
+ the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
- "def\\nabc" in multiline mode, but not otherwise. Consequently, patterns that
+ but not otherwise. Consequently, patterns that are anchored in single line mode
- are anchored in single line mode because all branches start with "^" are not
+ because all branches start with "^" are not anchored in multiline mode. The
- anchored in multiline mode. The PCRE_DOLLAR_ENDONLY option is ignored if
+ PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
- PCRE_MULTILINE is set.
+ Note that the sequences \\A, \\Z, and \\z can be used to match the start and
- Note that the sequences "\\A" and "\\Z" can be used to match the start and end
+ end of the subject in both modes, and if all branches of a pattern start with
- of the subject in both modes, and if all branches of a pattern start with "\\A"
+ \\A is it always anchored, whether PCRE_MULTILINE is set or not.
- is it always anchored.
  .SH FULL STOP (PERIOD, DOT)
-Line 668 
 An opening square bracket introduces a c
+Line 694 
 An opening square bracket introduces a c
  square bracket. A closing square bracket on its own is not special. If a
  closing square bracket is required as a member of the class, it should be the
  first data character in the class (after an initial circumflex, if present) or
- escaped with \\.
+ escaped with a backslash.
  A character class matches a single character in the subject; the character must
  be in the set of characters defined by the class, unless the first character in
  the class is a circumflex, in which case the subject character must not be in
  the set defined by the class. If a circumflex is actually required as a member
- of the class, ensure it is not the first character, or escape it with \\.
+ of the class, ensure it is not the first character, or escape it with a
+ backslash.
  For example, the character class [aeiou] matches any lower case vowel, while
  [^aeiou] matches any character that is not a lower case vowel. Note that a
-Line 683 
 are in the class by enumerating those th
+Line 710 
 are in the class by enumerating those th
  still consumes a character from the subject string, and fails if the current
  pointer is at the end of the string.
+ When caseless matching is set, any letters in a class represent both their
+ upper case and lower case versions, so for example, a caseless [aeiou] matches
+ "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
+ caseful version would.
  The newline character is never treated in any special way in character classes,
  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
  such as [^a] will always match a newline.
-Line 690 
 such as [^a] will always match a newline
+Line 722 
 such as [^a] will always match a newline
  The minus (hyphen) character can be used to specify a range of characters in a
  character class. For example, [d-m] matches any letter between d and m,
  inclusive. If a minus character is required in a class, it must be escaped with
- \\ or appear in a position where it cannot be interpreted as indicating a
+ a backslash or appear in a position where it cannot be interpreted as
- range, typically as the first or last character in the class. It is not
+ indicating a range, typically as the first or last character in the class. It
- possible to have the character "]" as the end character of a range, since a
+ is not possible to have the character "]" as the end character of a range,
- sequence such as [w-] is interpreted as a class of two characters. The octal or
+ since a sequence such as [w-] is interpreted as a class of two characters. The
- hexadecimal representation of "]" can, however, be used to end a range.
+ octal or hexadecimal representation of "]" can, however, be used to end a
+ range.
  Ranges operate in ASCII collating sequence. They can also be used for
- characters specified numerically, for example [\\000-\\037]. If a range such as
+ characters specified numerically, for example [\\000-\\037]. If a range that
- [W-c] is used when PCRE_CASELESS is set, it matches the letters involved in
+ includes letters is used when caseless matching is set, it matches the letters
- either case.
+ in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
+ caselessly, and if character tables for the "fr" locale are in use,
+ [\\xc8-\\xcb] matches accented E characters in both cases.
  The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
  character class, and add the characters that they match to the class. For
- example, the class [^\\W_] matches any letter or digit.
+ example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
+ conveniently be used with the upper case character types to specify a more
+ restricted set of characters than the matching lower case type. For example,
+ the class [^\\W_] matches any letter or digit, but not underscore.
  All non-alphameric characters other than \\, -, ^ (at the start) and the
  terminating ] are non-special in character classes, but it does no harm if they
-Line 711 
 are escaped.
+Line 749 
 are escaped.
  .SH VERTICAL BAR
- Vertical bar characters are used to separate alternative patterns. The matching
+ Vertical bar characters are used to separate alternative patterns. For example,
- process tries all the alternatives in turn. For example, the pattern
+ the pattern
    gilbert|sullivan
- matches either "gilbert" or "sullivan". Any number of alternatives can be used,
+ matches either "gilbert" or "sullivan". Any number of alternatives may appear,
  and an empty alternative is permitted (matching the empty string).
+ The matching process tries each alternative in turn, from left to right,
+ and the first one that succeeds is used. If the alternatives are within a
+ subpattern (defined below), "succeeds" means matching the rest of the main
+ pattern as well as the alternative in the subpattern.
+ .SH INTERNAL OPTION SETTING
+ The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
+ can be changed from within the pattern by a sequence of Perl option letters
+ enclosed between "(?" and ")". The option letters are
+   i  for PCRE_CASELESS
+   m  for PCRE_MULTILINE
+   s  for PCRE_DOTALL
+   x  for PCRE_EXTENDED
+ For example, (?im) sets caseless, multiline matching. It is also possible to
+ unset these options by preceding the letter with a hyphen, and a combined
+ setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
+ PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
+ permitted. If a letter appears both before and after the hyphen, the option is
+ unset.
+ The scope of these option changes depends on where in the pattern the setting
+ occurs. For settings that are outside any subpattern (defined below), the
+ effect is the same as if the options were set or unset at the start of
+ matching. The following patterns all behave in exactly the same way:
+   (?i)abc
+   a(?i)bc
+   ab(?i)c
+   abc(?i)
+ which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
+ In other words, such "top level" settings apply to the whole pattern (unless
+ there are other changes inside subpatterns). If there is more than one setting
+ of the same option at top level, the rightmost setting is used.
+ If an option change occurs inside a subpattern, the effect is different. This
+ is a change of behaviour in Perl 5.005. An option change inside a subpattern
+ affects only that part of the subpattern that follows it, so
+   (a(?i)b)c
+ matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
+ By this means, options can be made to have different settings in different
+ parts of the pattern. Any changes made in one alternative do carry on
+ into subsequent branches within the same subpattern. For example,
+   (a(?i)b|c)
+ matches "ab", "aB", "c", and "C", even though when matching "C" the first
+ branch is abandoned before the option setting. This is because the effects of
+ option settings happen at compile time. There would be some very weird
+ behaviour otherwise.
+ The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
+ same way as the Perl-compatible options by using the characters U and X
+ respectively. The (?X) flag setting is special in that it must always occur
+ earlier in the pattern than any of the additional features it turns on, even
+ when it is at top level. It is best put at the start.
  .SH SUBPATTERNS
-Line 757 
 the captured substrings are "white queen
+Line 856 
 the captured substrings are "white queen
 . The maximum number of captured substrings is 99, and the maximum number of
  all subpatterns, both capturing and non-capturing, is 200.
+ As a convenient shorthand, if any option settings are required at the start of
- .SH BACK REFERENCES
+ a non-capturing subpattern, the option letters may appear between the "?" and
- Outside a character class, a backslash followed by a digit greater than 0 (and
+ the ":". Thus the two patterns
- possibly further digits) is a back reference to a capturing subpattern earlier
- (i.e. to its left) in the pattern, provided there have been that many previous
+   (?i:saturday|sunday)
- capturing left parentheses. However, if the decimal number following the
+   (?:(?i)saturday|sunday)
- backslash is less than 10, it is always taken as a back reference, and causes
- an error if there have not been that many previous capturing left parentheses.
+ match exactly the same set of strings. Because alternative branches are tried
- See the section entitled "Backslash" above for further details of the handling
+ from left to right, and options are not reset until the end of the subpattern
- of digits following a backslash.
+ is reached, an option setting in one branch does affect subsequent branches, so
+ the above patterns match "SUNDAY" as well as "Saturday".
- A back reference matches whatever actually matched the capturing subpattern in
- the current subject string, rather than anything matching the subpattern
- itself. So the pattern
-    (sens|respons)e and \\1ibility
- matches "sense and sensibility" and "response and responsibility", but not
- "sense and responsibility".
- There may be more than one back reference to the same subpattern. If a
- subpattern has not actually been used in a particular match, then any back
- references to it always fail. For example, the pattern
-   (a|(bc))\\2
- always fails if it starts to match "a" rather than "bc". Because there may be
- up to 99 back references, all digits following the backslash are taken
- as part of a potential back reference number. If the pattern continues with a
- digit character, then some delimiter must be used to terminate the back
- reference. If the PCRE_EXTENDED option is set, this can be whitespace.
- Otherwise an empty comment can be used.
  .SH REPETITION
-Line 798 
 items:
+Line 876 
 items:
    a single character, possibly escaped
    the . metacharacter
    a character class
-   a back reference
+   a back reference (see next section)
-   a parenthesized subpattern
+   a parenthesized subpattern (unless it is an assertion - see below)
  The general repetition quantifier specifies a minimum and maximum number of
  permitted matches, by giving the two numbers in curly brackets (braces),
-Line 821 
 matches at least 3 successive vowels, bu
+Line 899 
 matches at least 3 successive vowels, bu
  matches exactly 8 digits. An opening curly bracket that appears in a position
  where a quantifier is not allowed, or one that does not match the syntax of a
- quantifier, is taken as a literal character. For example, "{,6}" is not a
+ quantifier, is taken as a literal character. For example, {,6} is not a
  quantifier, but a literal string of four characters.
  The quantifier {0} is permitted, causing the expression to behave as if the
-Line 834 
 quantifiers have single-character abbrev
+Line 912 
 quantifiers have single-character abbrev
    +    is equivalent to {1,}
    ?    is equivalent to {0,1}
+ It is possible to construct infinite loops by following a subpattern that can
+ match no characters with a quantifier that has no upper limit, for example:
+   (a?)*
+ Earlier versions of Perl and PCRE used to give an error at compile time for
+ such patterns. However, because there are cases where this can be useful, such
+ patterns are now accepted, but if any repetition of the subpattern does in fact
+ match no characters, the loop is forcibly broken.
  By default, the quantifiers are "greedy", that is, they match as much as
  possible (up to the maximum number of permitted times), without causing the
  rest of the pattern to fail. The classic example of where this gives problems
-Line 861 
 quantifiers is not otherwise changed, ju
+Line 949 
 quantifiers is not otherwise changed, ju
  Do not confuse this use of question mark with its use as a quantifier in its
  own right. Because it has two uses, it can sometimes appear doubled, as in
-    \\d??\\d
+   \\d??\\d
  which matches one digit by preference, but can match two if that is the only
  way the rest of the pattern matches.
+ If the PCRE_UNGREEDY option is set (an option which is not available in Perl)
+ then the quantifiers are not greedy by default, but individual ones can be made
+ greedy by following them with a question mark. In other words, it inverts the
+ default behaviour.
  When a parenthesized subpattern is quantified with a minimum repeat count that
  is greater than 1 or with a limited maximum, more store is required for the
  compiled pattern, in proportion to the size of the minimum or maximum.
-Line 875 
 follows will be tried against every char
+Line 968 
 follows will be tried against every char
  PCRE treats this as though it were preceded by \\A.
  When a capturing subpattern is repeated, the value captured is the substring
- that matched the final iteration. For example,
+ that matched the final iteration. For example, after
+   (tweedle[dume]{3}\\s*)+
+ has matched "tweedledum tweedledee" the value of the captured substring is
+ "tweedledee". However, if there are nested capturing subpatterns, the
+ corresponding captured values may have been set in previous iterations. For
+ example, after
+   /(a|(b))+/
+ matches "aba" the value of the second captured substring is "b".
+ .SH BACK REFERENCES
+ Outside a character class, a backslash followed by a digit greater than 0 (and
+ possibly further digits) is a back reference to a capturing subpattern earlier
+ (i.e. to its left) in the pattern, provided there have been that many previous
+ capturing left parentheses.
+ However, if the decimal number following the backslash is less than 10, it is
+ always taken as a back reference, and causes an error only if there are not
+ that many capturing left parentheses in the entire pattern. In other words, the
+ parentheses that are referenced need not be to the left of the reference for
+ numbers less than 10. See the section entitled "Backslash" above for further
+ details of the handling of digits following a backslash.
+ A back reference matches whatever actually matched the capturing subpattern in
+ the current subject string, rather than anything matching the subpattern
+ itself. So the pattern
-    (\s*tweedle[dume]{3})+\\1
+   (sens|respons)e and \\1ibility
- matches "tweedledum tweedledee tweedledee" but not "tweedledum tweedledee
+ matches "sense and sensibility" and "response and responsibility", but not
- tweedledum".
+ "sense and responsibility". If caseful matching is in force at the time of the
+ back reference, then the case of letters is relevant. For example,
+   ((?i)rah)\\s+\\1
+ matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
+ capturing subpattern is matched caselessly.
+ There may be more than one back reference to the same subpattern. If a
+ subpattern has not actually been used in a particular match, then any back
+ references to it always fail. For example, the pattern
+   (a|(bc))\\2
+ always fails if it starts to match "a" rather than "bc". Because there may be
+ up to 99 back references, all digits following the backslash are taken
+ as part of a potential back reference number. If the pattern continues with a
+ digit character, then some delimiter must be used to terminate the back
+ reference. If the PCRE_EXTENDED option is set, this can be whitespace.
+ Otherwise an empty comment can be used.
+ A back reference that occurs inside the parentheses to which it refers fails
+ when the subpattern is first used, so, for example, (a\\1) never matches.
+ However, such references can be useful inside repeated subpatterns. For
+ example, the pattern
+   (a|b\\1)+
+ matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
+ the subpattern, the back reference matches the character string corresponding
+ to the previous iteration. In order for this to work, the pattern must be such
+ that the first iteration does not need to match the back reference. This can be
+ done using alternation, as in the example above, or by a quantifier with a
+ minimum of zero.
  .SH ASSERTIONS
- An assertion is a test on the characters following the current matching point
+ An assertion is a test on the characters following or preceding the current
- that does not actually consume any of those characters. The simple assertions
+ matching point that does not actually consume any characters. The simple
- coded as \\b, \\B, \\A, \\Z, ^ and $ are described above. More complicated
+ assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
- assertions are coded as subpatterns starting with (?= for positive assertions,
+ complicated assertions are coded as subpatterns. There are two kinds: those
- and (?! for negative assertions. For example,
+ that look ahead of the current position in the subject string, and those that
+ look behind it.
+ An assertion subpattern is matched in the normal way, except that it does not
+ cause the current matching position to be changed. Lookahead assertions start
+ with (?= for positive assertions and (?! for negative assertions. For example,
    \\w+(?=;)
-Line 904 
 apparently similar pattern
+Line 1064 
 apparently similar pattern
  does not find an occurrence of "bar" that is preceded by something other than
  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
- (?!foo) is always true when the next three characters are "bar".
+ (?!foo) is always true when the next three characters are "bar". A
+ lookbehind assertion is needed to achieve this effect.
+ Lookbehind assertions start with (?<= for positive assertions and (?<! for
+ negative assertions. For example,
+   (?<!foo)bar
+ does find an occurrence of "bar" that is not preceded by "foo". The contents of
+ a lookbehind assertion are restricted such that all the strings it matches must
+ have a fixed length. However, if there are several alternatives, they do not
+ all have to have the same fixed length. Thus
+   (?<=bullock|donkey)
+ is permitted, but
+   (?<!dogs?|cats?)
+ causes an error at compile time. Branches that match different length strings
+ are permitted only at the top level of a lookbehind assertion. This is an
+ extension compared with Perl 5.005, which requires all branches to match the
+ same length of string. An assertion such as
+   (?<=ab(c|de))
+ is not permitted, because its single top-level branch can match two different
+ lengths, but it is acceptable if rewritten to use two top-level branches:
+   (?<=abc|abde)
+ The implementation of lookbehind assertions is, for each alternative, to
+ temporarily move the current position back by the fixed width and then try to
+ match. If there are insufficient characters before the current position, the
+ match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
+ can be particularly useful for matching at the ends of strings; an example is
+ given at the end of the section on once-only subpatterns.
+ Several assertions (of any sort) may occur in succession. For example,
+   (?<=\\d{3})(?<!999)foo
+ matches "foo" preceded by three digits that are not "999". Furthermore,
+ assertions can be nested in any combination. For example,
+   (?<=(?<!foo)bar)baz
+ matches an occurrence of "baz" that is preceded by "bar" which in turn is not
+ preceded by "foo".
  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
  because it makes no sense to assert the same thing several times. If an
-Line 917 
 Assertions count towards the maximum of
+Line 1125 
 Assertions count towards the maximum of
  .SH ONCE-ONLY SUBPATTERNS
- The facility described in this section is available only when the PCRE_EXTRA
- option is set at compile time. It is an extension to Perl regular expressions.
  With both maximizing and minimizing repetition, failure of what follows
  normally causes the repeated item to be re-evaluated to see if a different
  number of repeats allows the rest of the pattern to match. Sometimes it is
  useful to prevent this, either to change the nature of the match, or to cause
- it fail earlier than it otherwise might when the author of the pattern knows
+ it fail earlier than it otherwise might, when the author of the pattern knows
  there is no point in carrying on.
  Consider, for example, the pattern \\d+foo when applied to the subject line
 bar
  After matching all 6 digits and then failing to match "foo", the normal
  action of the matcher is to try again with only 5 digits matching the \\d+
-Line 939 
 has matched, it is not to be re-evaluate
+Line 1144 
 has matched, it is not to be re-evaluate
  give up immediately on failing to match "foo" the first time. The notation is
  another kind of special parenthesis, starting with (?> as in this example:
-   (?>\d+)bar
+   (?>\\d+)bar
  This kind of parenthesis "locks up" the  part of the pattern it contains once
  it has matched, and a failure further into the pattern is prevented from
  backtracking into it. Backtracking past it to previous items, however, works as
  normal.
- For simple cases such as the above example, this feature can be though of as a
+ An alternative description is that a subpattern of this type matches the string
- maximizing repeat that must swallow everything it can. So, while both \\d+ and
+ of characters that an identical standalone pattern would match, if anchored at
- \\d+? are prepared to adjust the number of digits they match in order to make
+ the current point in the subject string.
- the rest of the pattern match, (?>\\d+) can only match an entire sequence of
- digits.
+ Once-only subpatterns are not capturing subpatterns. Simple cases such as the
+ above example can be though of as a maximizing repeat that must swallow
+ everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
+ number of digits they match in order to make the rest of the pattern match,
+ (?>\\d+) can only match an entire sequence of digits.
  This construction can of course contain arbitrarily complicated subpatterns,
- and it can be nested. Contrast with the \\X assertion, which is a Prolog-like
+ and it can be nested.
- "cut".
+ Once-only subpatterns can be used in conjunction with lookbehind assertions to
+ specify efficient matching at the end of the subject string. Consider a simple
+ pattern such as
+   abcd$
+ when applied to a long string which does not match it. Because matching
+ proceeds from left to right, PCRE will look for each "a" in the subject and
+ then see if what follows matches the rest of the pattern. If the pattern is
+ specified as
+   .*abcd$
+ then the initial .* matches the entire string at first, but when this fails, it
+ backtracks to match all but the last character, then all but the last two
+ characters, and so on. Once again the search for "a" covers the entire string,
+ from right to left, so we are no better off. However, if the pattern is written
+ as
+   (?>.*)(?<=abcd)
+ then there can be no backtracking for the .* item; it can match only the entire
+ string. The subsequent lookbehind assertion does a single test on the last four
+ characters. If it fails, the match fails immediately. For long strings, this
+ approach makes a significant difference to the processing time.
+ .SH CONDITIONAL SUBPATTERNS
+ It is possible to cause the matching process to obey a subpattern
+ conditionally or to choose between two alternative subpatterns, depending on
+ the result of an assertion, or whether a previous capturing subpattern matched
+ or not. The two possible forms of conditional subpattern are
+   (?(condition)yes-pattern)
+   (?(condition)yes-pattern|no-pattern)
+ If the condition is satisfied, the yes-pattern is used; otherwise the
+ no-pattern (if present) is used. If there are more than two alternatives in the
+ subpattern, a compile-time error occurs.
+ There are two kinds of condition. If the text between the parentheses consists
+ of a sequence of digits, then the condition is satisfied if the capturing
+ subpattern of that number has previously matched. Consider the following
+ pattern, which contains non-significant white space to make it more readable
+ (assume the PCRE_EXTENDED option) and to divide it into three parts for ease
+ of discussion:
+   ( \\( )?    [^()]+    (?(1) \\) )
+ The first part matches an optional opening parenthesis, and if that
+ character is present, sets it as the first captured substring. The second part
+ matches one or more characters that are not parentheses. The third part is a
+ conditional subpattern that tests whether the first set of parentheses matched
+ or not. If they did, that is, if subject started with an opening parenthesis,
+ the condition is true, and so the yes-pattern is executed and a closing
+ parenthesis is required. Otherwise, since no-pattern is not present, the
+ subpattern matches nothing. In other words, this pattern matches a sequence of
+ non-parentheses, optionally enclosed in parentheses.
+ If the condition is not a sequence of digits, it must be an assertion. This may
+ be a positive or negative lookahead or lookbehind assertion. Consider this
+ pattern, again containing non-significant white space, and with the two
+ alternatives on the second line:
+   (?(?=[^a-z]*[a-z])
+   \\d{2}[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )
+ The condition is a positive lookahead assertion that matches an optional
+ sequence of non-letters followed by a letter. In other words, it tests for the
+ presence of at least one letter in the subject. If a letter is found, the
+ subject is matched against the first alternative; otherwise it is matched
+ against the second. This pattern matches strings in one of the two forms
+ dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
  .SH COMMENTS
-Line 967 
 character class introduces a comment tha
+Line 1249 
 character class introduces a comment tha
  character in the pattern.
- .SH INTERNAL FLAG SETTING
- If the sequence (?i) occurs anywhere in a pattern, it has the effect of setting
- the PCRE_CASELESS option, that is, all letters are matched in a
- case-independent manner. The option applies to the whole pattern, not just to
- the portion that follows it.
- If the sequence (?m) occurs anywhere in a pattern, it has the effect of setting
- the PCRE_MULTILINE option, that is, subject strings matched by this pattern are
- treated as consisting of multiple lines.
- If the sequence (?s) occurs anywhere in a pattern, it has the effect of setting
- the PCRE_DOTALL option, so that dot metacharacters match newlines as well as
- all other characters.
- If the sequence (?x) occurs anywhere in a pattern, it has the effect of setting
- the PCRE_EXTENDED option, that is, whitespace is ignored and # introduces a
- comment that lasts till the next newline. The option applies to the whole
- pattern, not just to the portion that follows it.
- If more than one option is required, they can be specified jointly, for example
- as (?ix) or (?mi).
  .SH PERFORMANCE
  Certain items that may appear in patterns are more efficient than others. It is
  more efficient to use a character class like [aeiou] than a set of alternatives
-Line 998 
 required behaviour is usually the most e
+Line 1257 
 required behaviour is usually the most e
  contains a lot of discussion about optimizing regular expressions for efficient
  performance.
- The use of PCRE_MULTILINE causes additional processing and should be avoided
- when it is not necessary. Caseless matching of character classes is more
- efficient if PCRE_CASELESS is set when the pattern is compiled.
  .SH AUTHOR
  Philip Hazel <ph10@cam.ac.uk>
-Line 1014 
 Cambridge CB2 3QG, England.
+Line 1269 
 Cambridge CB2 3QG, England.
  .br
  Phone: +44 1223 334714
- Copyright (c) 1997 University of Cambridge.
+ Copyright (c) 1997-1999 University of Cambridge.

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