/[pcre]/code/tags/pcre-3.2/doc/pcre.3

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-revision 41 by nigel,
Sat Feb 24 21:39:17 2007 UTC
+revision 43 by nigel,
Sat Feb 24 21:39:21 2007 UTC
 Line 47 
 pcre - Perl-compatible regular expressio
  .B const unsigned char *pcre_maketables(void);
  .PP
  .br
+ .B int pcre_fullinfo(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
+ .ti +5n
+ .B int \fIwhat\fR, void *\fIwhere\fR);
+ .PP
+ .br
  .B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
  .B *\fIfirstcharptr\fR);
  .PP
-Line 64 
 pcre - Perl-compatible regular expressio
+Line 69 
 pcre - Perl-compatible regular expressio
  .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 5.005.
+ differences (see below). The current implementation corresponds to Perl 5.005,
+ with some additional features from the Perl development release.
  PCRE has its own native API, which is described in this document. There is also
- a set of wrapper functions that correspond to the POSIX API. These are
+ a set of wrapper functions that correspond to the POSIX regular expression API.
- described in the \fBpcreposix\fR documentation.
+ These are described in the \fBpcreposix\fR documentation.
  The native API function prototypes are defined in the header file \fBpcre.h\fR,
  and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be
  accessed by adding \fB-lpcre\fR to the command for linking an application which
- calls it.
+ calls it. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to
+ contain the major and minor release numbers for the library. Applications can
+ use these to include support for different releases.
  The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
  are used for compiling and matching regular expressions, while
-Line 83 
 captured substrings from a matched subje
+Line 91 
 captured substrings from a matched subje
  \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
+ The function \fBpcre_fullinfo()\fR is used to find out information about a
- pattern, while the function \fBpcre_version()\fR returns a pointer to a string
+ compiled pattern; \fBpcre_info()\fR is an obsolete version which returns only
- containing the version of PCRE and its date of release.
+ some of the available information, but is retained for backwards compatibility.
+ The function \fBpcre_version()\fR returns a pointer to a string containing the
+ version of PCRE and its date of release.
  The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
  the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions
-Line 182 
 sequence (?( which introduces a conditio
+Line 192 
 sequence (?( which introduces a conditio
    PCRE_EXTRA
- This option turns on additional functionality of PCRE that is incompatible with
+ This option was invented in order to turn on additional functionality of PCRE
- Perl. Any backslash in a pattern that is followed by a letter that has no
+ that is incompatible with Perl, but it is currently of very little use. When
+ set, 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.
+ controlled by this option. It can also be set by a (?X) option setting within a
+ pattern.
    PCRE_MULTILINE
-Line 261 
 memory containing the tables remains ava
+Line 273 
 memory containing the tables remains ava
  .SH INFORMATION ABOUT A PATTERN
- The \fBpcre_info()\fR function returns information about a compiled pattern.
+ The \fBpcre_fullinfo()\fR function returns information about a compiled
- Its yield is the number of capturing subpatterns, or one of the following
+ pattern. It replaces the obsolete \fBpcre_info()\fR function, which is
- negative numbers:
+ nevertheless retained for backwards compability (and is documented below).
+ The first argument for \fBpcre_fullinfo()\fR is a pointer to the compiled
+ pattern. The second argument is the result of \fBpcre_study()\fR, or NULL if
+ the pattern was not studied. The third argument specifies which piece of
+ information is required, while the fourth argument is a pointer to a variable
+ to receive the data. The yield of the function is zero for success, or one of
+ the following negative numbers:
    PCRE_ERROR_NULL       the argument \fIcode\fR was NULL
+                         the argument \fIwhere\fR was NULL
    PCRE_ERROR_BADMAGIC   the "magic number" was not found
+   PCRE_ERROR_BADOPTION  the value of \fIwhat\fR was invalid
- If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
+ The possible values for the third argument are defined in \fBpcre.h\fR, and are
- pattern was compiled is placed in the integer it points to. These option bits
+ as follows:
+   PCRE_INFO_OPTIONS
+ Return a copy of the options with which the pattern was compiled. The fourth
+ argument should point to au \fBunsigned long int\fR variable. These option bits
  are those specified in the call to \fBpcre_compile()\fR, modified by any
  top-level option settings within the pattern itself, and with the PCRE_ANCHORED
- bit set if the form of the pattern implies that it can match only at the start
+ bit forcibly set if the form of the pattern implies that it can match only at
- of a subject string.
+ the start of a subject string.
- If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
+   PCRE_INFO_SIZE
- it is used to pass back information about the first character of any matched
- string. If there is a fixed first character, e.g. from a pattern such as
+ Return the size of the compiled pattern, that is, the value that was passed as
- (cat|cow|coyote), then it is returned in the integer pointed to by
+ the argument to \fBpcre_malloc()\fR when PCRE was getting memory in which to
- \fIfirstcharptr\fR. Otherwise, if either
+ place the compiled data. The fourth argument should point to a \fBsize_t\fR
+ variable.
+   PCRE_INFO_CAPTURECOUNT
+ Return the number of capturing subpatterns in the pattern. The fourth argument
+ should point to an \fbint\fR variable.
+   PCRE_INFO_BACKREFMAX
+ Return the number of the highest back reference in the pattern. The fourth
+ argument should point to an \fBint\fR variable. Zero is returned if there are
+ no back references.
+   PCRE_INFO_FIRSTCHAR
+ Return information about the first character of any matched string, for a
+ non-anchored pattern. If there is a fixed first character, e.g. from a pattern
+ such as (cat|cow|coyote), then it is returned in the integer pointed to by
+ \fIwhere\fR. Otherwise, if either
  (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
  starts with "^", or
-Line 289 
 starts with "^", or
+Line 334 
 starts with "^", or
  then -1 is returned, indicating that the pattern matches only at the
  start of a subject string or after any "\\n" within the string. Otherwise -2 is
- returned.
+ returned. For anchored patterns, -2 is returned.
+   PCRE_INFO_FIRSTTABLE
+ If the pattern was studied, and this resulted in the construction of a 256-bit
+ table indicating a fixed set of characters for the first character in any
+ matching string, a pointer to the table is returned. Otherwise NULL is
+ returned. The fourth argument should point to an \fBunsigned char *\fR
+ variable.
+   PCRE_INFO_LASTLITERAL
+ For a non-anchored pattern, return the value of the rightmost literal character
+ which must exist in any matched string, other than at its start. The fourth
+ argument should point to an \fBint\fR variable. If there is no such character,
+ or if the pattern is anchored, -1 is returned. For example, for the pattern
+ /a\\d+z\\d+/ the returned value is 'z'.
+ The \fBpcre_info()\fR function is now obsolete because its interface is too
+ restrictive to return all the available data about a compiled pattern. New
+ programs should use \fBpcre_fullinfo()\fR instead. The yield of
+ \fBpcre_info()\fR is the number of capturing subpatterns, or one of the
+ following negative numbers:
+   PCRE_ERROR_NULL       the argument \fIcode\fR was NULL
+   PCRE_ERROR_BADMAGIC   the "magic number" was not found
+ If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
+ pattern was compiled is placed in the integer it points to (see
+ PCRE_INFO_OPTIONS above).
+ If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
+ it is used to pass back information about the first character of any matched
+ string (see PCRE_INFO_FIRSTCHAR above).
  .SH MATCHING A PATTERN
-Line 564 
 are not part of its pattern matching eng
+Line 642 
 are not part of its pattern matching eng
 . The Perl \\G assertion is not supported as it is not relevant to single
  pattern matches.
-. Fairly obviously, PCRE does not support the (?{code}) construction.
+. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
+ constructions. However, there is some experimental support for recursive
+ patterns using the non-Perl item (?R).
 . 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
-Line 602 
 of the subject.
+Line 682 
 of the subject.
  (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
  \fBpcre_exec()\fR have no Perl equivalents.
+ (g) The (?R) construct allows for recursive pattern matching (Perl 5.6 can do
+ this using the (?p{code}) construct, which PCRE cannot of course support.)
  .SH REGULAR EXPRESSION DETAILS
  The syntax and semantics of the regular expressions supported by PCRE are
  described below. Regular expressions are also described in the Perl
  documentation and in a number of other books, some of which have copious
  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
- O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description
+ O'Reilly (ISBN 1-56592-257), covers them in great detail. The description
  here is intended as reference documentation.
  A regular expression is a pattern that is matched against a subject string from
-Line 906 
 terminating ] are non-special in charact
+Line 989 
 terminating ] are non-special in charact
  are escaped.
+ .SH POSIX CHARACTER CLASSES
+ Perl 5.6 (not yet released at the time of writing) is going to support the
+ POSIX notation for character classes, which uses names enclosed by [: and :]
+ within the enclosing square brackets. PCRE supports this notation. For example,
+   [01[:alpha:]%]
+ matches "0", "1", any alphabetic character, or "%". The supported class names
+ are
+   alnum    letters and digits
+   alpha    letters
+   ascii    character codes 0 - 127
+   cntrl    control characters
+   digit    decimal digits (same as \\d)
+   graph    printing characters, excluding space
+   lower    lower case letters
+   print    printing characters, including space
+   punct    printing characters, excluding letters and digits
+   space    white space (same as \\s)
+   upper    upper case letters
+   word     "word" characters (same as \\w)
+   xdigit   hexadecimal digits
+ The names "ascii" and "word" are Perl extensions. Another Perl extension is
+ negation, which is indicated by a ^ character after the colon. For example,
+   [12[:^digit:]]
+ matches "1", "2", or any non-digit. PCRE (and Perl) also recogize the POSIX
+ syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
+ supported, and an error is given if they are encountered.
  .SH VERTICAL BAR
  Vertical bar characters are used to separate alternative patterns. For example,
  the pattern
-Line 1352 
 pattern such as
+Line 1469 
 pattern such as
    abcd$
- when applied to a long string which does not match it. Because matching
+ when applied to a long string which does not match. Because matching proceeds
- proceeds from left to right, PCRE will look for each "a" in the subject and
+ from left to right, PCRE will look for each "a" in the subject and then see if
- then see if what follows matches the rest of the pattern. If the pattern is
+ what follows matches the rest of the pattern. If the pattern is specified as
- specified as
    ^.*abcd$
- then the initial .* matches the entire string at first, but when this fails, it
+ then the initial .* matches the entire string at first, but when this fails
- backtracks to match all but the last character, then all but the last two
+ (because there is no following "a"), it backtracks to match all but the last
- characters, and so on. Once again the search for "a" covers the entire string,
+ character, then all but the last two characters, and so on. Once again the
- from right to left, so we are no better off. However, if the pattern is written
+ search for "a" covers the entire string, from right to left, so we are no
- as
+ better off. However, if the pattern is written as
    ^(?>.*)(?<=abcd)
-Line 1372 
 string. The subsequent lookbehind assert
+Line 1488 
 string. The subsequent lookbehind assert
  characters. If it fails, the match fails immediately. For long strings, this
  approach makes a significant difference to the processing time.
+ When a pattern contains an unlimited repeat inside a subpattern that can itself
+ be repeated an unlimited number of times, the use of a once-only subpattern is
+ the only way to avoid some failing matches taking a very long time indeed.
+ The pattern
+   (\\D+|<\\d+>)*[!?]
+ matches an unlimited number of substrings that either consist of non-digits, or
+ digits enclosed in <>, followed by either ! or ?. When it matches, it runs
+ quickly. However, if it is applied to
+   aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+ it takes a long time before reporting failure. This is because the string can
+ be divided between the two repeats in a large number of ways, and all have to
+ be tried. (The example used [!?] rather than a single character at the end,
+ because both PCRE and Perl have an optimization that allows for fast failure
+ when a single character is used. They remember the last single character that
+ is required for a match, and fail early if it is not present in the string.)
+ If the pattern is changed to
+   ((?>\\D+)|<\\d+>)*[!?]
+ sequences of non-digits cannot be broken, and failure happens quickly.
  .SH CONDITIONAL SUBPATTERNS
  It is possible to cause the matching process to obey a subpattern
-Line 1431 
 character class introduces a comment tha
+Line 1572 
 character class introduces a comment tha
  character in the pattern.
+ .SH RECURSIVE PATTERNS
+ Consider the problem of matching a string in parentheses, allowing for
+ unlimited nested parentheses. Without the use of recursion, the best that can
+ be done is to use a pattern that matches up to some fixed depth of nesting. It
+ is not possible to handle an arbitrary nesting depth. Perl 5.6 has provided an
+ experimental facility that allows regular expressions to recurse (amongst other
+ things). It does this by interpolating Perl code in the expression at run time,
+ and the code can refer to the expression itself. A Perl pattern to solve the
+ parentheses problem can be created like this:
+   $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;
+ The (?p{...}) item interpolates Perl code at run time, and in this case refers
+ recursively to the pattern in which it appears. Obviously, PCRE cannot support
+ the interpolation of Perl code. Instead, the special item (?R) is provided for
+ the specific case of recursion. This PCRE pattern solves the parentheses
+ problem (assume the PCRE_EXTENDED option is set so that white space is
+ ignored):
+   \\( ( (?>[^()]+) | (?R) )* \\)
+ First it matches an opening parenthesis. Then it matches any number of
+ substrings which can either be a sequence of non-parentheses, or a recursive
+ match of the pattern itself (i.e. a correctly parenthesized substring). Finally
+ there is a closing parenthesis.
+ This particular example pattern contains nested unlimited repeats, and so the
+ use of a once-only subpattern for matching strings of non-parentheses is
+ important when applying the pattern to strings that do not match. For example,
+ when it is applied to
+   (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
+ it yields "no match" quickly. However, if a once-only subpattern is not used,
+ the match runs for a very long time indeed because there are so many different
+ ways the + and * repeats can carve up the subject, and all have to be tested
+ before failure can be reported.
+ The values set for any capturing subpatterns are those from the outermost level
+ of the recursion at which the subpattern value is set. If the pattern above is
+ matched against
+   (ab(cd)ef)
+ the value for the capturing parentheses is "ef", which is the last value taken
+ on at the top level. If additional parentheses are added, giving
+   \\( ( ( (?>[^()]+) | (?R) )* ) \\)
+      ^                        ^
+      ^                        ^
+ then the string they capture is "ab(cd)ef", the contents of the top level
+ parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
+ has to obtain extra memory to store data during a recursion, which it does by
+ using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no
+ memory can be obtained, it saves data for the first 15 capturing parentheses
+ only, as there is no way to give an out-of-memory error from within a
+ recursion.
  .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 1497 
 Cambridge CB2 3QG, England.
+Line 1697 
 Cambridge CB2 3QG, England.
  .br
  Phone: +44 1223 334714
- Last updated: 29 July 1999
+ Last updated: 27 January 2000
  .br
- Copyright (c) 1997-1999 University of Cambridge.
+ Copyright (c) 1997-2000 University of Cambridge.

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