+++ /dev/null
-NAME
- pcre - Perl-compatible regular expressions.
-
-
-
-SYNOPSIS
- #include <pcre.h>
-
- pcre *pcre_compile(const char *pattern, int options,
- const char **errptr, int *erroffset,
- const unsigned char *tableptr);
-
- pcre_extra *pcre_study(const pcre *code, int options,
- const char **errptr);
-
- int pcre_exec(const pcre *code, const pcre_extra *extra,
- const char *subject, int length, int startoffset,
- int options, int *ovector, int ovecsize);
-
- int pcre_copy_substring(const char *subject, int *ovector,
- int stringcount, int stringnumber, char *buffer,
- int buffersize);
-
- int pcre_get_substring(const char *subject, int *ovector,
- int stringcount, int stringnumber,
- const char **stringptr);
-
- int pcre_get_substring_list(const char *subject,
- int *ovector, int stringcount, const char ***listptr);
-
- void pcre_free_substring(const char *stringptr);
-
- void pcre_free_substring_list(const char **stringptr);
-
- const unsigned char *pcre_maketables(void);
-
- int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
- int what, void *where);
-
- int pcre_info(const pcre *code, int *optptr, *firstcharptr);
-
- char *pcre_version(void);
-
- void *(*pcre_malloc)(size_t);
-
- void (*pcre_free)(void *);
-
-
-
-
-DESCRIPTION
- The PCRE library is a set of functions that implement regu-
- lar 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, with some additional features from later versions.
- This includes some experimental, incomplete support for
- UTF-8 encoded strings. Details of exactly what is and what
- is not supported are given below.
-
- 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 regular expression API. These are
- described in the pcreposix documentation.
-
- The native API function prototypes are defined in the header
- file pcre.h, and on Unix systems the library itself is
- called libpcre.a, so can be accessed by adding -lpcre to the
- command for linking an application which 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 pcre_compile(), pcre_study(), and pcre_exec()
- are used for compiling and matching regular expressions.
-
- The functions pcre_copy_substring(), pcre_get_substring(),
- and pcre_get_substring_list() are convenience functions for
- extracting captured substrings from a matched subject
- string; pcre_free_substring() and pcre_free_substring_list()
- are also provided, to free the memory used for extracted
- strings.
-
- The function pcre_maketables() is used (optionally) to build
- a set of character tables in the current locale for passing
- to pcre_compile().
-
- The function pcre_fullinfo() is used to find out information
- about a compiled pattern; pcre_info() is an obsolete version
- which returns only some of the available information, but is
- retained for backwards compatibility. The function
- pcre_version() returns a pointer to a string containing the
- version of PCRE and its date of release.
-
- The global variables pcre_malloc and pcre_free initially
- contain the entry points of the standard malloc() and free()
- functions respectively. PCRE calls the memory management
- functions via these variables, so a calling program can
- replace them if it wishes to intercept the calls. This
- should be done before calling any PCRE functions.
-
-
-
-MULTI-THREADING
- The PCRE functions can be used in multi-threading
-
-
-
-
-
-SunOS 5.8 Last change: 2
-
-
-
- applications, with the proviso that the memory management
- functions pointed to by pcre_malloc and pcre_free 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.
-
-
-
-COMPILING A PATTERN
- The function pcre_compile() 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 pattern. A
- pointer to a single block of memory that is obtained via
- pcre_malloc is returned. This contains the compiled code and
- related data. The pcre type is defined for this for conveni-
- ence, but in fact pcre is just a typedef for void, 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.
-
- 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 just a single character,
- negated or not) requires 33 bytes, and repeat quantifiers
- with a minimum greater than one or a bounded maximum cause
- the relevant portions of the compiled pattern to be repli-
- cated.
-
- The options argument contains independent bits that affect
- the compilation. It should be zero if no options are
- required. Some of the options, in particular, those that are
- compatible with Perl, can also be set and unset from within
- the pattern (see the detailed description of regular expres-
- sions below). For these options, the contents of the options
- 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.
-
- If errptr is NULL, pcre_compile() returns NULL immediately.
- Otherwise, if compilation of a pattern fails, pcre_compile()
- returns NULL, and sets the variable pointed to by errptr to
- point to a textual 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
- erroffset, which must not be NULL. If it is, an immediate
- error is given.
-
- If the final argument, tableptr, is NULL, PCRE uses a
- default set of character tables which are built when it is
- compiled, using the default C locale. Otherwise, tableptr
- must be the result of a call to pcre_maketables(). See the
- section on locale support below.
-
- The following option bits are defined in the header file:
-
- PCRE_ANCHORED
-
- If this bit is set, the pattern is forced to be "anchored",
- that is, it is constrained to match only at the start of the
- string which is being searched (the "subject string"). This
- effect can also be achieved by appropriate constructs in the
- pattern itself, which is the only way to do it in Perl.
-
- PCRE_CASELESS
-
- If this bit is set, letters in the pattern match both upper
- and lower case 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. Without this
- option, a dollar also matches immediately before the final
- character if it is a newline (but not before any other new-
- lines). The PCRE_DOLLAR_ENDONLY option is ignored if
- PCRE_MULTILINE is 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. Without it, new-
- lines are excluded. This option is equivalent to Perl's /s
- option. A negative class such as [^a] always matches a new-
- line character, independent of the setting of this option.
-
- PCRE_EXTENDED
-
- If this bit is set, whitespace data characters in the pat-
- tern are totally ignored except when escaped or inside a
- character class, and characters between an unescaped # out-
- side 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. 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 sub-
- pattern.
-
- PCRE_EXTRA
-
- This option was invented in order to turn on additional
- functionality of PCRE 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. It can also be set by a (?X) option setting
- within a pattern.
-
- PCRE_MULTILINE
-
- By default, PCRE treats the subject string as consisting of
- a single "line" of 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 (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 immedi-
- ately before any newline in the subject string, respec-
- tively, as well as at the very start and end. This is
- equivalent to Perl's /m option. If there are no "\n" charac-
- ters in a subject string, or no occurrences of ^ or $ in a
- pattern, setting PCRE_MULTILINE has no effect.
-
- PCRE_UNGREEDY
-
- 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.
-
- PCRE_UTF8
-
- This option causes PCRE to regard both the pattern and the
- subject as strings of UTF-8 characters instead of just byte
- strings. However, it is available only if PCRE has been
- built to include UTF-8 support. If not, the use of this
- option provokes an error. Support for UTF-8 is new, experi-
- mental, and incomplete. Details of exactly what it entails
- are given below.
-
-
-
-STUDYING A PATTERN
- When a pattern is going to be used several times, it is
- worth spending more time analyzing it in order to speed up
- the time taken for matching. The function pcre_study() takes
-
- a pointer to a compiled pattern as its first argument, and
- returns a pointer to a pcre_extra block (another void
- typedef) containing additional information about the pat-
- tern; this can be passed to pcre_exec(). If no additional
- information is available, NULL is returned.
-
- The second argument contains option bits. At present, no
- options are defined for pcre_study(), and this argument
- should always be zero.
-
- The third argument for pcre_study() is a pointer to an error
- message. If studying succeeds (even if no data is returned),
- the variable it points to is set to NULL. Otherwise it
- points to a textual error message.
-
- At present, studying a pattern is useful only for non-
- anchored patterns that do not have a single fixed starting
- character. A bitmap of possible starting characters is
- created.
-
-
-
-LOCALE SUPPORT
- PCRE handles caseless matching, and determines whether char-
- acters 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 com-
- piled. This is used when the final argument of
- pcre_compile() is NULL, and is sufficient for many applica-
- tions.
-
- An alternative set of tables can, however, be supplied. Such
- tables are built by calling the pcre_maketables() function,
- which has no arguments, in the relevant locale. The result
- can then be passed to pcre_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
- pcre_malloc. The pointer that is passed to pcre_compile is
- saved with the compiled pattern, and the same tables are
- used via this pointer by pcre_study() and pcre_exec(). 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 responsi-
- bility to ensure that the memory containing the tables
- remains available for as long as it is needed.
-
-
-
-INFORMATION ABOUT A PATTERN
- The pcre_fullinfo() function returns information about a
- compiled pattern. It replaces the obsolete pcre_info() func-
- tion, which is nevertheless retained for backwards compabil-
- ity (and is documented below).
-
- The first argument for pcre_fullinfo() is a pointer to the
- compiled pattern. The second argument is the result of
- pcre_study(), 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 vari-
- able 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 code was NULL
- the argument where was NULL
- PCRE_ERROR_BADMAGIC the "magic number" was not found
- PCRE_ERROR_BADOPTION the value of what was invalid
-
- The possible values for the third argument are defined in
- pcre.h, and are as follows:
-
- PCRE_INFO_OPTIONS
-
- Return a copy of the options with which the pattern was com-
- piled. The fourth argument should point to au unsigned long
- int variable. These option bits are those specified in the
- call to pcre_compile(), modified by any top-level option
- settings within the pattern itself, and with the
- PCRE_ANCHORED bit forcibly set if the form of the pattern
- implies that it can match only at the start of a subject
- string.
-
- PCRE_INFO_SIZE
-
- Return the size of the compiled pattern, that is, the value
- that was passed as the argument to pcre_malloc() when PCRE
- was getting memory in which to place the compiled data. The
- fourth argument should point to a size_t variable.
-
- PCRE_INFO_CAPTURECOUNT
-
- Return the number of capturing subpatterns in the pattern.
- The fourth argument should point to an int variable.
-
- PCRE_INFO_BACKREFMAX
-
- Return the number of the highest back reference in the
- pattern. The fourth argument should point to an int vari-
- able. 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), it is returned in the integer pointed to
- by where. Otherwise, if either
-
- (a) the pattern was compiled with the PCRE_MULTILINE option,
- and every branch starts with "^", or
-
- (b) every branch of the pattern starts with ".*" and
- PCRE_DOTALL is not set (if it were set, the pattern would be
- anchored),
-
- -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. For anchored patterns, -2
- is returned.
-
- PCRE_INFO_FIRSTTABLE
-
- If the pattern was studied, and this resulted in the con-
- struction of a 256-bit table indicating a fixed set of char-
- acters 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 unsigned
- char * variable.
-
- PCRE_INFO_LASTLITERAL
-
- For a non-anchored pattern, return the value of the right-
- most literal character which must exist in any matched
- string, other than at its start. The fourth argument should
- point to an int 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 pcre_info() function is now obsolete because its inter-
- face is too restrictive to return all the available data
- about a compiled pattern. New programs should use
- pcre_fullinfo() instead. The yield of pcre_info() is the
- number of capturing subpatterns, or one of the following
- negative numbers:
-
- PCRE_ERROR_NULL the argument code was NULL
- PCRE_ERROR_BADMAGIC the "magic number" was not found
-
- If the optptr 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 firstcharptr argument
- is not NULL, it is used to pass back information about the
- first character of any matched string (see
- PCRE_INFO_FIRSTCHAR above).
-
-
-
-MATCHING A PATTERN
- The function pcre_exec() is called to match a subject string
- against a pre-compiled pattern, which is passed in the code
- argument. If the pattern has been studied, the result of the
- study should be passed in the extra argument. Otherwise this
- must be NULL.
-
- The PCRE_ANCHORED option can be passed in the options argu-
- ment, whose unused bits must be zero. However, if a pattern
- was compiled with PCRE_ANCHORED, or turned out to be
- anchored by virtue of its contents, it cannot be made
- unachored at matching time.
-
- There are also three further options that can be set only at
- matching time:
-
- PCRE_NOTBOL
-
- 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 compile
- time) causes circumflex never to match.
-
- PCRE_NOTEOL
-
- The end of the string is not the end of a line, so the dol-
- lar metacharacter should not match it nor (except in multi-
- line mode) a newline immediately before it. Setting this
- without PCRE_MULTILINE (at compile time) causes dollar never
- to match.
-
- PCRE_NOTEMPTY
-
- An empty string is not considered to be a valid match if
- this option is set. If there are alternatives in the pat-
- tern, they are tried. If all the alternatives match the
- empty string, the entire match fails. For example, if the
- pattern
-
- a?b?
-
- is applied to a string not beginning with "a" or "b", it
- matches the empty string at the start of the subject. With
- PCRE_NOTEMPTY set, this match is not valid, so PCRE searches
- further into the string for occurrences of "a" or "b".
-
- Perl has no direct equivalent of PCRE_NOTEMPTY, but it does
- make a special case of a pattern match of the empty string
- within its split() function, and when using the /g modifier.
- It is possible to emulate Perl's behaviour after matching a
- null string by first trying the match again at the same
- offset with PCRE_NOTEMPTY set, and then if that fails by
- advancing the starting offset (see below) and trying an
- ordinary match again.
-
- The subject string is passed as a pointer in subject, a
- length in length, and a starting offset in startoffset.
- Unlike the pattern string, it may contain binary zero char-
- acters. When the starting offset is zero, the search for a
- match starts at the beginning of the subject, and this is by
- far the most common case.
-
- A non-zero starting offset is useful when searching for
- another match in the same subject by calling pcre_exec()
- again after a previous success. Setting startoffset differs
- from just passing over a shortened string and setting
- PCRE_NOTBOL in the case of a pattern that begins with any
- kind of lookbehind. For example, consider the pattern
-
- \Biss\B
-
- which finds occurrences of "iss" in the middle of words. (\B
- matches only if the current position in the subject is not a
- word boundary.) When applied to the string "Mississipi" the
- first call to pcre_exec() finds the first occurrence. If
- pcre_exec() is called again with just the remainder of the
- subject, namely "issipi", it does not match, because \B is
- always false at the start of the subject, which is deemed to
- be a word boundary. However, if pcre_exec() is passed the
- entire string again, but with startoffset set to 4, it finds
- the second occurrence of "iss" because it is able to look
- behind the starting point to discover that it is preceded by
- a letter.
-
- If a non-zero starting offset is passed when the pattern is
- anchored, one attempt to match at the given offset is tried.
- This can only succeed if the pattern does not require the
- match to be at the start of the subject.
-
- In general, a pattern matches a certain portion of the sub-
- ject, and in addition, further substrings from the subject
- may be picked out by parts of the pattern. Following the
- usage in Jeffrey Friedl's book, this is called "capturing"
- in what follows, and the phrase "capturing subpattern" is
- used for a fragment of a pattern that picks out a substring.
- PCRE supports several other kinds of parenthesized subpat-
- tern that do not cause substrings to be captured.
-
- Captured substrings are returned to the caller via a vector
- of integer offsets whose address is passed in ovector. The
- number of elements in the vector is passed in ovecsize. The
- first two-thirds of the vector is used to pass back captured
- substrings, each substring using a pair of integers. The
- remaining third of the vector is used as workspace by
- pcre_exec() while matching capturing subpatterns, and is not
- available for passing back information. The length passed in
- ovecsize should always be a multiple of three. If it is not,
- it is rounded down.
-
- When a match has been successful, information about captured
- substrings is returned in pairs of integers, starting at the
- beginning of ovector, and continuing up to two-thirds of its
- length at the most. The first element of a 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, ovector[0] and ovec-
- tor[1], 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
- pcre_exec() 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.
-
- Some convenience functions are provided for extracting the
- captured substrings as separate strings. These are described
- in the following section.
-
- It is possible for an capturing subpattern number n+1 to
- match some part of the subject when subpattern n has not
- been used at all. For 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.
-
- If a capturing subpattern is matched repeatedly, it is the
- last portion of the string that it matched that gets
- returned.
-
- If the vector is too small to hold all the captured sub-
- strings, it is used as far as possible (up to two-thirds of
- its length), and the function returns a value of zero. In
- particular, if the substring offsets are not of interest,
- pcre_exec() may be called with ovector passed as NULL and
- ovecsize as zero. However, if the pattern contains back
- references and the ovector isn't big enough to remember the
- related substrings, PCRE has to get additional memory for
- use during matching. Thus it is usually advisable to supply
- an ovector.
-
- Note that pcre_info() can be used to find out how many cap-
- turing subpatterns there are in a compiled pattern. The
- smallest size for ovector that will allow for n captured
- substrings in addition to the offsets of the substring
- matched by the whole pattern is (n+1)*3.
-
- If pcre_exec() fails, it returns a negative number. The fol-
- lowing are defined in the header file:
-
- PCRE_ERROR_NOMATCH (-1)
-
- The subject string did not match the pattern.
-
- PCRE_ERROR_NULL (-2)
-
- Either code or subject was passed as NULL, or ovector was
- NULL and ovecsize was not zero.
-
- PCRE_ERROR_BADOPTION (-3)
-
- An unrecognized bit was set in the options argument.
-
- PCRE_ERROR_BADMAGIC (-4)
-
- PCRE stores a 4-byte "magic number" at the start of the com-
- piled 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 (-5)
-
- While running the pattern match, an unknown item was encoun-
- tered in the compiled pattern. This error could be caused by
- a bug in PCRE or by overwriting of the compiled pattern.
-
- PCRE_ERROR_NOMEMORY (-6)
-
- If a pattern contains back references, but the ovector that
- is passed to pcre_exec() is not big enough to remember the
- referenced substrings, PCRE gets a block of memory at the
- start of matching to use for this purpose. If the call via
- pcre_malloc() fails, this error is given. The memory is
- freed at the end of matching.
-
-
-
-EXTRACTING CAPTURED SUBSTRINGS
- Captured substrings can be accessed directly by using the
-
-
-
-
-
-SunOS 5.8 Last change: 12
-
-
-
- offsets returned by pcre_exec() in ovector. For convenience,
- the functions pcre_copy_substring(), pcre_get_substring(),
- and pcre_get_substring_list() are provided for extracting
- captured substrings as new, separate, zero-terminated
- strings. A substring that contains a binary zero is
- correctly extracted and has a further zero added on the end,
- but the result does not, of course, function as a C string.
-
- The first three arguments are the same for all three func-
- tions: subject is the subject string which has just been
- successfully matched, ovector is a pointer to the vector of
- integer offsets that was passed to pcre_exec(), and
- stringcount is the number of substrings that were captured
- by the match, including the substring that matched the
- entire regular expression. This is the value returned by
- pcre_exec if it is greater than zero. If pcre_exec()
- returned zero, indicating that it ran out of space in ovec-
- tor, the value passed as stringcount should be the size of
- the vector divided by three.
-
- The functions pcre_copy_substring() and pcre_get_substring()
- extract a single substring, whose number is given as string-
- number. A value of zero extracts the substring that matched
- the entire pattern, while higher values extract the captured
- substrings. For pcre_copy_substring(), the string is placed
- in buffer, whose length is given by buffersize, while for
- pcre_get_substring() a new block of memory is obtained via
- pcre_malloc, and its address is returned via stringptr. The
- yield of the function is the length of the string, not
- including the terminating zero, or one of
-
- PCRE_ERROR_NOMEMORY (-6)
-
- The buffer was too small for pcre_copy_substring(), or the
- attempt to get memory failed for pcre_get_substring().
-
- PCRE_ERROR_NOSUBSTRING (-7)
-
- There is no substring whose number is stringnumber.
-
- The pcre_get_substring_list() function extracts all avail-
- able substrings and builds a list of pointers to them. All
- this is done in a single block of memory which is obtained
- via pcre_malloc. The address of the memory block is returned
- via listptr, which is also the start of the list of string
- pointers. The end of the list is marked by a NULL pointer.
- The yield of the function is zero if all went well, or
-
- PCRE_ERROR_NOMEMORY (-6)
-
- if the attempt to get the memory block failed.
-
- When any of these functions encounter a substring that is
- unset, which can happen when capturing subpattern number n+1
- matches some part of the subject, but subpattern n has not
- been used at all, they return an empty string. This can be
- distinguished from a genuine zero-length substring by
- inspecting the appropriate offset in ovector, which is nega-
- tive for unset substrings.
-
- The two convenience functions pcre_free_substring() and
- pcre_free_substring_list() can be used to free the memory
- returned by a previous call of pcre_get_substring() or
- pcre_get_substring_list(), respectively. They do nothing
- more than call the function pointed to by pcre_free, which
- of course could be called directly from a C program. How-
- ever, PCRE is used in some situations where it is linked via
- a special interface to another programming language which
- cannot use pcre_free directly; it is for these cases that
- the functions are provided.
-
-
-
-LIMITATIONS
- There are some size limitations in PCRE but it is hoped that
- they will never in practice be relevant. The maximum length
- of a compiled pattern is 65539 (sic) bytes. 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 sub-
- patterns, assertions, and other types of subpattern, is 200.
-
- The maximum length of a subject string is the largest posi-
- tive number that an integer variable can hold. However, PCRE
- uses recursion to handle subpatterns and indefinite repeti-
- tion. This means that the available stack space may limit
- the size of a subject string that can be processed by cer-
- tain patterns.
-
-
-
-DIFFERENCES FROM PERL
- The differences described here are with respect to Perl
- 5.005.
-
- 1. By default, a whitespace character is any character that
- the C library function isspace() recognizes, though it is
- possible to compile PCRE with alternative character type
- tables. Normally isspace() matches space, formfeed, newline,
- carriage return, horizontal tab, and vertical tab. Perl 5 no
- longer includes vertical tab in its set of whitespace char-
- acters. The \v escape that was in the Perl documentation for
- a long time was never in fact recognized. However, the char-
- acter itself was treated as whitespace at least up to 5.002.
- In 5.004 and 5.005 it does not match \s.
-
- 2. 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 not assert that the
- next three characters are not "a". It just asserts that the
- next character is not "a" three times.
-
- 3. Capturing subpatterns that occur inside negative looka-
- head assertions are counted, but their entries in the
- offsets vector are never set. Perl sets its numerical vari-
- ables from any such patterns that are matched before the
- assertion fails to match something (thereby succeeding), but
- only if the negative lookahead assertion contains just one
- branch.
-
- 4. Though binary zero characters are supported in the sub-
- ject string, they are not allowed in a pattern string
- because it is passed as a normal C string, terminated by
- zero. The escape sequence "\0" can be used in the pattern to
- represent a binary zero.
-
- 5. The following Perl escape sequences are not supported:
- \l, \u, \L, \U, \E, \Q. In fact these are implemented by
- Perl's general string-handling and are not part of its pat-
- tern matching engine.
-
- 6. The Perl \G assertion is not supported as it is not
- relevant to single pattern matches.
-
- 7. Fairly obviously, PCRE does not support the (?{code}) and
- (?p{code}) constructions. However, there is some experimen-
- tal support for recursive patterns using the non-Perl item
- (?R).
-
- 8. 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) are set.
-
- In Perl 5.004 $2 is set in both cases, and that is also true
- of PCRE. If in the future Perl changes to a consistent state
- that is different, PCRE may change to follow.
-
- 9. Another as yet unresolved discrepancy is that in Perl
- 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string
- "a", whereas in PCRE it does not. However, in both Perl and
- PCRE /^(a)?a/ matched against "a" leaves $1 unset.
-
- 10. 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.
-
- (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.
-
- (c) If PCRE_EXTRA is set, a backslash followed by a letter
- with no special meaning is faulted.
-
- (d) If PCRE_UNGREEDY is set, the greediness of the repeti-
- tion quantifiers is inverted, that is, by default they are
- not greedy, but if followed by a question mark they are.
-
- (e) PCRE_ANCHORED can be used to force a pattern to be tried
- only at the start of the subject.
-
- (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options
- for pcre_exec() 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.)
-
-
-
-REGULAR EXPRESSION DETAILS
- The syntax and semantics of the regular expressions sup-
- ported 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), covers them in great detail.
-
- The description here is intended as reference documentation.
- The basic operation of PCRE is on strings of bytes. However,
- there is the beginnings of some support for UTF-8 character
- strings. To use this support you must configure PCRE to
- include it, and then call pcre_compile() with the PCRE_UTF8
- option. How this affects the pattern matching is described
- in the final section of this document.
-
- A regular expression is a pattern that is matched against a
- subject string from left to right. Most characters stand for
- themselves in a pattern, and match the corresponding charac-
- ters in the subject. As a trivial example, the pattern
-
- The quick brown fox
-
- matches a portion of a subject string that is identical to
- itself. The power of regular expressions comes from the
- ability to include alternatives and repetitions in the pat-
- tern. These are encoded in the pattern by the use of meta-
- characters, which do not stand for themselves but instead
- are interpreted in some special way.
-
- There are two different sets of meta-characters: those that
- are recognized anywhere in the pattern except within square
- brackets, and those that are recognized in square brackets.
- Outside square brackets, the meta-characters are as follows:
-
- \ general escape character with several uses
- ^ assert start of subject (or line, in multiline
- mode)
- $ assert end of subject (or line, in multiline mode)
- . match any character except newline (by default)
- [ start character class definition
- | start of alternative branch
- ( start subpattern
- ) end subpattern
- ? extends the meaning of (
- also 0 or 1 quantifier
- also quantifier minimizer
- * 0 or more quantifier
- + 1 or more quantifier
- { start min/max quantifier
-
- Part of a pattern that is in square brackets is called a
- "character class". In a character class the only meta-
- characters are:
-
- \ general escape character
- ^ negate the class, but only if the first character
- - indicates character range
- ] terminates the character class
-
- The following sections describe the use of each of the
- meta-characters.
-
-
-
-BACKSLASH
- The backslash character has several uses. Firstly, if it is
- followed by a non-alphameric character, it takes away any
- special meaning that character may have. This use of
- backslash as an escape character applies both inside and
- outside character classes.
-
- For example, if you want to match a "*" character, you write
- "\*" in the pattern. This applies whether or not the follow-
- ing character would otherwise be interpreted as a meta-
- character, so it is always safe to precede a non-alphameric
- with "\" to specify that it stands for itself. In particu-
- lar, if you want to match a backslash, you write "\\".
-
- If a pattern is compiled with the PCRE_EXTENDED option, whi-
- tespace in the pattern (other than in a character class) and
- characters between a "#" outside a character class and the
- next newline character are ignored. An escaping 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 non-printing charac-
- ters, apart from the binary zero that terminates a pattern,
- but when a pattern is being prepared by text editing, it is
- usually easier to use one of the following escape sequences
- than the binary character it represents:
-
- \a alarm, that is, the BEL character (hex 07)
- \cx "control-x", where x is any character
- \e escape (hex 1B)
- \f formfeed (hex 0C)
- \n newline (hex 0A)
- \r carriage return (hex 0D)
- \t tab (hex 09)
- \xhh character with hex code hh
- \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. Thus "\cz" becomes hex
- 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.
-
- After "\x", up to two hexadecimal digits are read (letters
- can be in upper or 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 after the initial zero if the character
- that 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 number. If the number
- is less than 10, or if there have been at least that many
- previous capturing left parentheses in the expression, the
- entire sequence is taken as a back reference. A description
- of how this works is given later, following the discussion
- of parenthesized subpatterns.
-
- Inside a character class, or if the decimal number is
- greater than 9 and there have not been that many capturing
- subpatterns, PCRE re-reads up to three octal digits follow-
- ing the backslash, and generates a single byte from the
- least significant 8 bits of the value. Any subsequent digits
- stand for themselves. For example:
-
- \040 is another way of writing a space
- \40 is the same, provided there are fewer than 40
- previous capturing subpatterns
- \7 is always a back reference
- \11 might be a back reference, or another way of
- writing a tab
- \011 is always a tab
- \0113 is a tab followed by the character "3"
- \113 is the character with octal code 113 (since there
- can be no more than 99 back references)
- \377 is a byte consisting entirely of 1 bits
- \81 is either a back reference, or a binary zero
- followed by the two characters "8" and "1"
-
- Note that octal values of 100 or greater must not be intro-
- duced by a leading zero, because no more than three octal
- digits are ever read.
-
- All the sequences that define a single byte value can be
- used both inside and outside character classes. In addition,
- inside a character class, the sequence "\b" is interpreted
- as the backspace character (hex 08). Outside a character
- class it has a different meaning (see below).
-
- The third use of backslash is for specifying generic charac-
- ter types:
-
- \d any decimal digit
- \D any character that is not a decimal digit
- \s any whitespace character
- \S any character that is not a whitespace character
- \w any "word" character
- \W any "non-word" character
-
- Each pair of escape sequences partitions the complete set of
- characters into 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". The definition of letters and digits is con-
- trolled by PCRE's character tables, and may vary if locale-
- specific matching is taking place (see "Locale support"
- above). For example, in the "fr" (French) locale, some char-
- acter codes greater than 128 are used for accented letters,
- and these are matched by \w.
-
- These character type sequences can appear both inside and
- outside character classes. They each match one character of
- the appropriate type. If the current matching point is at
- the end of the subject string, all of them fail, since there
- is no character to match.
-
- The fourth use of backslash is for certain simple asser-
- tions. 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 or newline at end (independent of
- multiline mode)
- \z end of subject (independent of multiline mode)
-
- 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 \w and the other matches
- \W), or the start or end of the string if the first or last
- character matches \w, respectively.
-
- The \A, \Z, and \z assertions differ from the traditional
- circumflex and dollar (described below) in that they only
- ever match at the very start and end of the subject string,
- whatever options are set. They are not affected by the
- PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu-
- ment of pcre_exec() is non-zero, \A can never match. The
- difference between \Z and \z is that \Z matches before a
- newline that is the last character of the string as well as
- at the end of the string, whereas \z matches only at the
- end.
-
-
-
-CIRCUMFLEX AND DOLLAR
- Outside a character class, in the default matching mode, the
- circumflex character is an assertion which is true only if
- the current matching point is at the start of the subject
-
- string. If the startoffset argument of pcre_exec() is non-
- zero, circumflex can never match. 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 alternatives are involved, but it should be the
- first thing in each alternative in which it appears if the
- pattern is ever to match that branch. If all possible alter-
- natives start with a circumflex, that is, if the pattern is
- constrained to match only at the start of the subject, it is
- said to be an "anchored" pattern. (There are also other con-
- structs that can cause a pattern to be anchored.)
-
- A dollar character is an assertion which is true only if the
- current matching point is at the end of the subject string,
- or immediately before a newline character that is the last
- character in the string (by default). Dollar need not be the
- last character of the pattern if a number of alternatives
- are involved, but it should be the last item in any branch
- in which it appears. Dollar has no special meaning in a
- character class.
-
- 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. This
- does not affect the \Z assertion.
-
- The meanings of the circumflex and dollar characters are
- changed if the PCRE_MULTILINE option is set. When this is
- the case, they match immediately after and immediately
- before an internal "\n" character, respectively, in addition
- to matching at the start and end of the subject string. For
- example, the pattern /^abc$/ matches the subject string
- "def\nabc" in multiline mode, but not otherwise. Conse-
- quently, patterns that are anchored in single line mode
- because all branches start with "^" are not anchored in mul-
- tiline mode, and a match for circumflex is possible when the
- startoffset argument of pcre_exec() is non-zero. The
- PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
- set.
-
- Note that the sequences \A, \Z, and \z can be used to match
- the start and end of the subject in both modes, and if all
- branches of a pattern start with \A is it always anchored,
- whether PCRE_MULTILINE is set or not.
-
-
-
-FULL STOP (PERIOD, DOT)
- Outside a character class, a dot in the pattern matches any
- one character in the subject, including a non-printing char-
- acter, but not (by default) newline. If the PCRE_DOTALL
-
- option is set, dots match newlines as well. The handling of
- dot is entirely independent of the handling of circumflex
- and dollar, the only relationship being that they both
- involve newline characters. Dot has no special meaning in a
- character class.
-
-
-
-SQUARE BRACKETS
- An opening square bracket introduces a character class, ter-
- minated by a closing 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 cir-
- cumflex, if present) or 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 cir-
- cumflex, 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 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 circumflex is just a con-
- venient notation for specifying the characters which are in
- the class by enumerating those that are not. It is not an
- assertion: it 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 case-
- ful 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.
-
- 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 a
- backslash or appear in a position where it cannot be inter-
- preted as indicating a range, typically as the first or last
- character in the class.
-
- It is not possible to have the literal character "]" as the
- end character of a range. A pattern such as [W-]46] is
- interpreted as a class of two characters ("W" and "-") fol-
- lowed by a literal string "46]", so it would match "W46]" or
- "-46]". However, if the "]" is escaped with a backslash it
- is interpreted as the end of range, so [W-\]46] is inter-
- preted as a single class containing a range followed by two
- separate characters. The octal or hexadecimal representation
- of "]" can also 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 that includes letters is used when
- caseless matching is set, it matches the letters 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, [\dABCDEF] matches any
- hexadecimal digit. A circumflex can conveniently be used
- with the upper case character types to specify a more res-
- tricted 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 are escaped.
-
-
-
-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 sup-
- ported 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 ^ char-
- acter 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.
-
-
-
-VERTICAL BAR
- Vertical bar characters are used to separate alternative
- patterns. For example, the pattern
-
- gilbert|sullivan
-
- matches either "gilbert" or "sullivan". Any number of alter-
- natives 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.
-
-
-
-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" set-
- tings apply to the whole pattern (unless there are other
- changes inside subpatterns). If there is more than one set-
- ting 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 oth-
- erwise.
-
- 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.
-
-
-
-SUBPATTERNS
- Subpatterns are delimited by parentheses (round brackets),
- which can be nested. Marking part of a pattern as a subpat-
- tern does two things:
-
- 1. It localizes a set of alternatives. For example, the pat-
- tern
-
- cat(aract|erpillar|)
-
- matches one of the words "cat", "cataract", or "caterpil-
- lar". Without the parentheses, it would match "cataract",
- "erpillar" or the empty string.
-
- 2. It sets up the subpattern as a capturing subpattern (as
- defined above). When the whole pattern matches, that por-
- tion of the subject string that matched the subpattern is
- passed back to the caller via the ovector argument of
- pcre_exec(). Opening parentheses are counted from left to
- right (starting from 1) to obtain the numbers of the captur-
- ing subpatterns.
-
- For example, if the string "the red king" is matched against
- the pattern
-
- the ((red|white) (king|queen))
-
- the captured substrings are "red king", "red", and "king",
- and are numbered 1, 2, and 3.
-
- The fact that plain parentheses fulfil two functions is not
- always helpful. There are often times when a grouping sub-
- pattern is required without a capturing requirement. If an
- opening parenthesis is followed by "?:", the subpattern does
- not do any capturing, and is not counted when computing the
- number of any subsequent capturing subpatterns. For example,
- if the string "the white queen" is matched against the pat-
- tern
-
- the ((?:red|white) (king|queen))
-
- the captured substrings are "white queen" and "queen", and
- are numbered 1 and 2. The maximum number of captured sub-
- strings 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 a non-capturing subpattern, the
- option letters may appear between the "?" and the ":". Thus
- the two patterns
-
- (?i:saturday|sunday)
- (?:(?i)saturday|sunday)
-
- match exactly the same set of strings. Because alternative
- branches are tried from left to right, and options are not
- reset until the end of the subpattern is reached, an option
- setting in one branch does affect subsequent branches, so
- the above patterns match "SUNDAY" as well as "Saturday".
-
-
-
-REPETITION
- Repetition is specified by quantifiers, which can follow any
- of the following items:
-
- a single character, possibly escaped
- the . metacharacter
- a character class
- a back reference (see next section)
- 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), separated by a comma.
- The numbers must be less than 65536, and the first must be
- less than or equal to the second. For example:
-
- z{2,4}
-
- matches "zz", "zzz", or "zzzz". A closing brace on its own
- is not a special character. If the second number is omitted,
- but the comma is present, there is no upper limit; if the
- second number and the comma are both omitted, the quantifier
- specifies an exact number of required matches. Thus
-
- [aeiou]{3,}
-
- matches at least 3 successive vowels, but may match many
- more, while
-
- \d{8}
-
- 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 quantif-
- ier, but a literal string of four characters.
-
- The quantifier {0} is permitted, causing the expression to
- behave as if the previous item and the quantifier were not
- present.
-
- For convenience (and historical compatibility) the three
- most common quantifiers have single-character abbreviations:
-
- * is equivalent to {0,}
- + 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 per-
- mitted times), without causing the rest of the pattern to
- fail. The classic example of where this gives problems is in
- trying to match comments in C programs. These appear between
- the sequences /* and */ and within the sequence, individual
- * and / characters may appear. An attempt to match C com-
- ments by applying the pattern
-
- /\*.*\*/
-
- to the string
-
- /* first command */ not comment /* second comment */
-
- fails, because it matches the entire string owing to the
- greediness of the .* item.
-
- However, if a quantifier is followed by a question mark, it
- ceases to be greedy, and instead matches the minimum number
- of times possible, so the pattern
-
- /\*.*?\*/
-
- does the right thing with the C comments. The meaning of the
- various quantifiers is not otherwise changed, just the pre-
- ferred number of matches. Do not confuse this use of ques-
- tion 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
-
- 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), 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 max-
- imum, more store is required for the compiled pattern, in
- proportion to the size of the minimum or maximum.
-
- If a pattern starts with .* or .{0,} and the PCRE_DOTALL
- option (equivalent to Perl's /s) is set, thus allowing the .
- to match newlines, the pattern is implicitly anchored,
- because whatever follows will be tried against every charac-
- ter position in the subject string, so there is no point in
- retrying the overall match at any position after the first.
- PCRE treats such a pattern as though it were preceded by \A.
- In cases where it is known that the subject string contains
- no newlines, it is worth setting PCRE_DOTALL when the pat-
- tern begins with .* in order to obtain this optimization, or
- alternatively using ^ to indicate anchoring explicitly.
-
- When a capturing subpattern is repeated, the value captured
- is the substring that matched the final iteration. For exam-
- ple, after
-
- (tweedle[dume]{3}\s*)+
-
- has matched "tweedledum tweedledee" the value of the cap-
- tured substring is "tweedledee". However, if there are
- nested capturing subpatterns, the corresponding captured
- values may have been set in previous iterations. For exam-
- ple, after
-
- /(a|(b))+/
-
- matches "aba" the value of the second captured substring is
- "b".
-
-
-
-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 han-
- dling of digits following a backslash.
-
- A back reference matches whatever actually matched the cap-
- turing 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 responsi-
- bility", but not "sense and responsibility". If caseful
- matching is in force at the time of the back reference, 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 case-
- lessly.
-
- There may be more than one back reference to the same sub-
- pattern. If a subpattern has not actually been used in a
- particular match, 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, 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 pat-
- tern
-
- (a|b\1)+
-
- matches any number of "a"s and also "aba", "ababbaa" 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.
-
-
-
-ASSERTIONS
- An assertion is a test on the characters following or
- preceding the current matching point that does not actually
- consume any characters. The simple assertions coded as \b,
- \B, \A, \Z, \z, ^ and $ are described above. More compli-
- cated assertions are coded as subpatterns. There are two
- kinds: those 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+(?=;)
-
- matches a word followed by a semicolon, but does not include
- the semicolon in the match, and
-
- foo(?!bar)
-
- matches any occurrence of "foo" that is not followed by
- "bar". Note that the apparently similar pattern
-
- (?!foo)bar
-
- 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". A lookbehind
- assertion is needed to achieve this effect.
-
- Lookbehind assertions start with (?<= for positive asser-
- tions 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 dif-
- ferent 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 rewrit-
- ten 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 match-
- ing 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".
- Notice that each of the assertions is applied independently
- at the same point in the subject string. First there is a
- check that the previous three characters are all digits, and
- then there is a check that the same three characters are not
- "999". This pattern does not match "foo" preceded by six
- characters, the first of which are digits and the last three
- of which are not "999". For example, it doesn't match
- "123abcfoo". A pattern to do that is
-
- (?<=\d{3}...)(?<!999)foo
-
- This time the first assertion looks at the preceding six
- characters, checking that the first three are digits, and
- then the second assertion checks that the preceding three
- characters are not "999".
-
- 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", while
-
- (?<=\d{3}(?!999)...)foo
-
- is another pattern which matches "foo" preceded by three
- digits and any three characters that are not "999".
-
- Assertion subpatterns are not capturing subpatterns, and may
- not be repeated, because it makes no sense to assert the
- same thing several times. If any kind of assertion contains
- capturing subpatterns within it, these are counted for the
- purposes of numbering the capturing subpatterns in the whole
- pattern. However, substring capturing is carried out only
- for positive assertions, because it does not make sense for
- negative assertions.
-
- Assertions count towards the maximum of 200 parenthesized
- subpatterns.
-
-
-
-ONCE-ONLY SUBPATTERNS
- 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 there is no point in carrying
- on.
-
- Consider, for example, the pattern \d+foo when applied to
- the subject line
-
- 123456bar
-
- 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+ item, and then with 4, and so on,
- before ultimately failing. Once-only subpatterns provide the
- means for specifying that once a portion of the pattern has
- matched, it is not to be re-evaluated in this way, so the
- matcher would 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
-
- 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.
-
- An alternative description is that a subpattern of this type
- matches the string of characters that an identical stan-
- dalone pattern would match, if anchored at the current point
- in the subject string.
-
- Once-only subpatterns are not capturing subpatterns. Simple
- cases such as the above example can be thought of as a max-
- imizing 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 compli-
- cated subpatterns, and it can be nested.
-
- Once-only subpatterns can be used in conjunction with look-
- behind 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. 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$
-
- the initial .* matches the entire string at first, but when
- this fails (because there is no following "a"), it back-
- tracks 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)
-
- 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.
-
- When a pattern contains an unlimited repeat inside a subpat-
- tern 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 con-
- sist 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 exam-
- ple 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 hap-
- pens quickly.
-
-
-
-CONDITIONAL SUBPATTERNS
- It is possible to cause the matching process to obey a sub-
- pattern 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; oth-
- erwise 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, the condition
- is satisfied if the capturing subpattern of that number has
- previously matched. The number must be greater than zero.
- 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 contain-
- ing non-significant white space, and with the two alterna-
- tives 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.
-
-
-
-COMMENTS
- The sequence (?# marks the start of a comment which contin-
- ues up to the next closing parenthesis. Nested parentheses
- are not permitted. The characters that make up a comment
- play no part in the pattern matching at all.
-
- If the PCRE_EXTENDED option is set, an unescaped # character
- outside a character class introduces a comment that contin-
- ues up to the next newline character in the pattern.
-
-
-
-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 pat-
- tern 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 match-
- ing 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 sub-
- pattern 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
-
- \( ( ( (?>[^()]+) | (?R) )* ) \)
- ^ ^
- ^ ^ 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 pcre_malloc, freeing it
- via pcre_free 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.
-
-
-
-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 such as (a|e|i|o|u).
- In general, the simplest construction that provides the
- required behaviour is usually the most efficient. Jeffrey
- Friedl's book contains a lot of discussion about optimizing
- regular expressions for efficient performance.
-
- When a pattern begins with .* and the PCRE_DOTALL option is
- set, the pattern is implicitly anchored by PCRE, since it
- can match only at the start of a subject string. However, if
- PCRE_DOTALL is not set, PCRE cannot make this optimization,
- because the . metacharacter does not then match a newline,
- and if the subject string contains newlines, the pattern may
- match from the character immediately following one of them
- instead of from the very start. For example, the pattern
-
- (.*) second
-
- matches the subject "first\nand second" (where \n stands for
- a newline character) with the first captured substring being
- "and". In order to do this, PCRE has to retry the match
- starting after every newline in the subject.
-
- If you are using such a pattern with subject strings that do
- not contain newlines, the best performance is obtained by
- setting PCRE_DOTALL, or starting the pattern with ^.* to
- indicate explicit anchoring. That saves PCRE from having to
- scan along the subject looking for a newline to restart at.
-
- Beware of patterns that contain nested indefinite repeats.
- These can take a long time to run when applied to a string
- that does not match. Consider the pattern fragment
-
- (a+)*
-
- This can match "aaaa" in 33 different ways, and this number
- increases very rapidly as the string gets longer. (The *
- repeat can match 0, 1, 2, 3, or 4 times, and for each of
- those cases other than 0, the + repeats can match different
- numbers of times.) When the remainder of the pattern is such
- that the entire match is going to fail, PCRE has in princi-
- ple to try every possible variation, and this can take an
- extremely long time.
-
- An optimization catches some of the more simple cases such
- as
-
- (a+)*b
-
- where a literal character follows. Before embarking on the
- standard matching procedure, PCRE checks that there is a "b"
- later in the subject string, and if there is not, it fails
- the match immediately. However, when there is no following
- literal this optimization cannot be used. You can see the
- difference by comparing the behaviour of
-
- (a+)*\d
-
- with the pattern above. The former gives a failure almost
- instantly when applied to a whole line of "a" characters,
- whereas the latter takes an appreciable time with strings
- longer than about 20 characters.
-
-
-
-UTF-8 SUPPORT
- Starting at release 3.3, PCRE has some support for character
- strings encoded in the UTF-8 format. This is incomplete, and
- is regarded as experimental. In order to use it, you must
- configure PCRE to include UTF-8 support in the code, and, in
- addition, you must call pcre_compile() with the PCRE_UTF8
- option flag. When you do this, both the pattern and any sub-
- ject strings that are matched against it are treated as
- UTF-8 strings instead of just strings of bytes, but only in
- the cases that are mentioned below.
-
- If you compile PCRE with UTF-8 support, but do not use it at
- run time, the library will be a bit bigger, but the addi-
- tional run time overhead is limited to testing the PCRE_UTF8
- flag in several places, so should not be very large.
-
- PCRE assumes that the strings it is given contain valid
- UTF-8 codes. It does not diagnose invalid UTF-8 strings. If
- you pass invalid UTF-8 strings to PCRE, the results are
- undefined.
-
- Running with PCRE_UTF8 set causes these changes in the way
- PCRE works:
-
- 1. In a pattern, the escape sequence \x{...}, where the con-
- tents of the braces is a string of hexadecimal digits, is
- interpreted as a UTF-8 character whose code number is the
- given hexadecimal number, for example: \x{1234}. This
- inserts from one to six literal bytes into the pattern,
- using the UTF-8 encoding. If a non-hexadecimal digit appears
- between the braces, the item is not recognized.
-
- 2. The original hexadecimal escape sequence, \xhh, generates
- a two-byte UTF-8 character if its value is greater than 127.
-
- 3. Repeat quantifiers are NOT correctly handled if they fol-
- low a multibyte character. For example, \x{100}* and \xc3+
- do not work. If you want to repeat such characters, you must
- enclose them in non-capturing parentheses, for example
- (?:\x{100}), at present.
-
- 4. The dot metacharacter matches one UTF-8 character instead
- of a single byte.
-
- 5. Unlike literal UTF-8 characters, the dot metacharacter
- followed by a repeat quantifier does operate correctly on
- UTF-8 characters instead of single bytes.
-
- 4. Although the \x{...} escape is permitted in a character
- class, characters whose values are greater than 255 cannot
- be included in a class.
-
- 5. A class is matched against a UTF-8 character instead of
- just a single byte, but it can match only characters whose
- values are less than 256. Characters with greater values
- always fail to match a class.
-
- 6. Repeated classes work correctly on multiple characters.
-
- 7. Classes containing just a single character whose value is
- greater than 127 (but less than 256), for example, [\x80] or
- [^\x{93}], do not work because these are optimized into sin-
- gle byte matches. In the first case, of course, the class
- brackets are just redundant.
-
- 8. Lookbehind assertions move backwards in the subject by a
- fixed number of characters instead of a fixed number of
- bytes. Simple cases have been tested to work correctly, but
- there may be hidden gotchas herein.
-
- 9. The character types such as \d and \w do not work
- correctly with UTF-8 characters. They continue to test a
- single byte.
-
- 10. Anything not explicitly mentioned here continues to work
- in bytes rather than in characters.
-
- The following UTF-8 features of Perl 5.6 are not imple-
- mented:
- 1. The escape sequence \C to match a single byte.
-
- 2. The use of Unicode tables and properties and escapes \p,
- \P, and \X.
-
-
-
-AUTHOR
- Philip Hazel <ph10@cam.ac.uk>
- University Computing Service,
- New Museums Site,
- Cambridge CB2 3QG, England.
- Phone: +44 1223 334714
-
- Last updated: 28 August 2000,
- the 250th anniversary of the death of J.S. Bach.
- Copyright (c) 1997-2000 University of Cambridge.
projects / privoxy.git / blobdiff