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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. +</P> +<P> +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 <B>pcreposix</B> documentation. +</P> +<P> +The native API function prototypes are defined in the header file <B>pcre.h</B>, +and on Unix systems the library itself is called <B>libpcre.a</B>, so can be +accessed by adding <B>-lpcre</B> 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. +</P> +<P> +The functions <B>pcre_compile()</B>, <B>pcre_study()</B>, and <B>pcre_exec()</B> +are used for compiling and matching regular expressions. A sample program that +demonstrates the simplest way of using them is given in the file +<I>pcredemo.c</I>. The last section of this man page describes how to run it. +</P> +<P> +The functions <B>pcre_copy_substring()</B>, <B>pcre_get_substring()</B>, and +<B>pcre_get_substring_list()</B> are convenience functions for extracting +captured substrings from a matched subject string; <B>pcre_free_substring()</B> +and <B>pcre_free_substring_list()</B> are also provided, to free the memory used +for extracted strings. +</P> +<P> +The function <B>pcre_maketables()</B> is used (optionally) to build a set of +character tables in the current locale for passing to <B>pcre_compile()</B>. +</P> +<P> +The function <B>pcre_fullinfo()</B> is used to find out information about a +compiled pattern; <B>pcre_info()</B> is an obsolete version which returns only +some of the available information, but is retained for backwards compatibility. +The function <B>pcre_version()</B> returns a pointer to a string containing the +version of PCRE and its date of release. +</P> +<P> +The global variables <B>pcre_malloc</B> and <B>pcre_free</B> initially contain +the entry points of the standard <B>malloc()</B> and <B>free()</B> 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. +</P> +<LI><A NAME="SEC4" HREF="#TOC1">MULTI-THREADING</A> +<P> +The PCRE functions can be used in multi-threading applications, with the +proviso that the memory management functions pointed to by <B>pcre_malloc</B> +and <B>pcre_free</B> are shared by all threads. +</P> +<P> +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. +</P> +<LI><A NAME="SEC5" HREF="#TOC1">COMPILING A PATTERN</A> +<P> +The function <B>pcre_compile()</B> 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 <I>pattern</I>. A pointer to a single block of memory +that is obtained via <B>pcre_malloc</B> is returned. This contains the compiled +code and related data. The <B>pcre</B> type is defined for the returned block; +this is a typedef for a structure whose contents are not externally defined. It +is up to the caller to free the memory when it is no longer required. +</P> +<P> +Although the compiled code of a PCRE regex is relocatable, that is, it does not +depend on memory location, the complete <B>pcre</B> data block is not +fully relocatable, because it contains a copy of the <I>tableptr</I> argument, +which is an address (see below). +</P> +<P> +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 replicated. +</P> +<P> +The <I>options</I> 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 expressions +below). For these options, the contents of the <I>options</I> 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. +</P> +<P> +If <I>errptr</I> is NULL, <B>pcre_compile()</B> returns NULL immediately. +Otherwise, if compilation of a pattern fails, <B>pcre_compile()</B> returns +NULL, and sets the variable pointed to by <I>errptr</I> 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 +<I>erroffset</I>, which must not be NULL. If it is, an immediate error is given. +</P> +<P> +If the final argument, <I>tableptr</I>, is NULL, PCRE uses a default set of +character tables which are built when it is compiled, using the default C +locale. Otherwise, <I>tableptr</I> must be the result of a call to +<B>pcre_maketables()</B>. See the section on locale support below. +</P> +<P> +This code fragment shows a typical straightforward call to <B>pcre_compile()</B>: +</P> +<P> +<PRE> + pcre *re; + const char *error; + int erroffset; + re = pcre_compile( + "^A.*Z", /* the pattern */ + 0, /* default options */ + &error, /* for error message */ + &erroffset, /* for error offset */ + NULL); /* use default character tables */ +</PRE> +</P> +<P> +The following option bits are defined in the header file: +</P> +<P> +<PRE> + PCRE_ANCHORED +</PRE> +</P> +<P> +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. +</P> +<P> +<PRE> + PCRE_CASELESS +</PRE> +</P> +<P> +If this bit is set, letters in the pattern match both upper and lower case +letters. It is equivalent to Perl's /i option. +</P> +<P> +<PRE> + PCRE_DOLLAR_ENDONLY +</PRE> +</P> +<P> +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 newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is +set. There is no equivalent to this option in Perl. +</P> +<P> +<PRE> + PCRE_DOTALL +</PRE> +</P> +<P> +If this bit is set, a dot metacharater in the pattern matches all characters, +including newlines. Without it, newlines are excluded. This option is +equivalent to Perl's /s option. A negative class such as [^a] always matches a +newline character, independent of the setting of this option. +</P> +<P> +<PRE> + PCRE_EXTENDED +</PRE> +</P> +<P> +If this bit is set, whitespace data characters in the pattern are totally +ignored except when escaped or inside a character class, and characters between +an unescaped # outside a character class and the next newline character, +inclusive, are also ignored. This is equivalent to Perl's /x option, and makes +it possible to include comments inside complicated patterns. Note, however, +that this applies only to data characters. Whitespace characters may never +appear within special character sequences in a pattern, for example within the +sequence (?( which introduces a conditional subpattern. +</P> +<P> +<PRE> + PCRE_EXTRA +</PRE> +</P> +<P> +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. +</P> +<P> +<PRE> + PCRE_MULTILINE +</PRE> +</P> +<P> +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. +</P> +<P> +When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs +match immediately following or immediately before any newline in the subject +string, respectively, as well as at the very start and end. This is equivalent +to Perl's /m option. If there are no "\n" characters in a subject string, or +no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no +effect. +</P> +<P> +<PRE> + PCRE_UNGREEDY +</PRE> +</P> +<P> +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. +</P> +<P> +<PRE> + PCRE_UTF8 +</PRE> +</P> +<P> +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, experimental, and incomplete. +Details of exactly what it entails are given below. +</P> +<LI><A NAME="SEC6" HREF="#TOC1">STUDYING A PATTERN</A> +<P> +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 <B>pcre_study()</B> takes a pointer to a compiled pattern as its first +argument, and returns a pointer to a <B>pcre_extra</B> block (another typedef +for a structure with hidden contents) containing additional information about +the pattern; this can be passed to <B>pcre_exec()</B>. If no additional +information is available, NULL is returned. +</P> +<P> +The second argument contains option bits. At present, no options are defined +for <B>pcre_study()</B>, and this argument should always be zero. +</P> +<P> +The third argument for <B>pcre_study()</B> 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. +</P> +<P> +This is a typical call to <B>pcre_study</B>(): +</P> +<P> +<PRE> + pcre_extra *pe; + pe = pcre_study( + re, /* result of pcre_compile() */ + 0, /* no options exist */ + &error); /* set to NULL or points to a message */ +</PRE> +</P> +<P> +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. +</P> +<LI><A NAME="SEC7" HREF="#TOC1">LOCALE SUPPORT</A> +<P> +PCRE handles caseless matching, and determines whether characters are letters, +digits, or whatever, by reference to a set of tables. The library contains a +default set of tables which is created in the default C locale when PCRE is +compiled. This is used when the final argument of <B>pcre_compile()</B> is NULL, +and is sufficient for many applications. +</P> +<P> +An alternative set of tables can, however, be supplied. Such tables are built +by calling the <B>pcre_maketables()</B> function, which has no arguments, in the +relevant locale. The result can then be passed to <B>pcre_compile()</B> 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: +</P> +<P> +<PRE> + setlocale(LC_CTYPE, "fr"); + tables = pcre_maketables(); + re = pcre_compile(..., tables); +</PRE> +</P> +<P> +The tables are built in memory that is obtained via <B>pcre_malloc</B>. The +pointer that is passed to <B>pcre_compile</B> is saved with the compiled +pattern, and the same tables are used via this pointer by <B>pcre_study()</B> +and <B>pcre_exec()</B>. Thus for any single pattern, compilation, studying and +matching all happen in the same locale, but different patterns can be compiled +in different locales. It is the caller's responsibility to ensure that the +memory containing the tables remains available for as long as it is needed. +</P> +<LI><A NAME="SEC8" HREF="#TOC1">INFORMATION ABOUT A PATTERN</A> +<P> +The <B>pcre_fullinfo()</B> function returns information about a compiled +pattern. It replaces the obsolete <B>pcre_info()</B> function, which is +nevertheless retained for backwards compability (and is documented below). +</P> +<P> +The first argument for <B>pcre_fullinfo()</B> is a pointer to the compiled +pattern. The second argument is the result of <B>pcre_study()</B>, 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: +</P> +<P> +<PRE> + PCRE_ERROR_NULL the argument <I>code</I> was NULL + the argument <I>where</I> was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + PCRE_ERROR_BADOPTION the value of <I>what</I> was invalid +</PRE> +</P> +<P> +Here is a typical call of <B>pcre_fullinfo()</B>, to obtain the length of the +compiled pattern: +</P> +<P> +<PRE> + int rc; + unsigned long int length; + rc = pcre_fullinfo( + re, /* result of pcre_compile() */ + pe, /* result of pcre_study(), or NULL */ + PCRE_INFO_SIZE, /* what is required */ + &length); /* where to put the data */ +</PRE> +</P> +<P> +The possible values for the third argument are defined in <B>pcre.h</B>, and are +as follows: +</P> +<P> +<PRE> + PCRE_INFO_OPTIONS +</PRE> +</P> +<P> +Return a copy of the options with which the pattern was compiled. The fourth +argument should point to an <B>unsigned long int</B> variable. These option bits +are those specified in the call to <B>pcre_compile()</B>, 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. +</P> +<P> +<PRE> + PCRE_INFO_SIZE +</PRE> +</P> +<P> +Return the size of the compiled pattern, that is, the value that was passed as +the argument to <B>pcre_malloc()</B> when PCRE was getting memory in which to +place the compiled data. The fourth argument should point to a <B>size_t</B> +variable. +</P> +<P> +<PRE> + PCRE_INFO_CAPTURECOUNT +</PRE> +</P> +<P> +Return the number of capturing subpatterns in the pattern. The fourth argument +should point to an \fbint\fR variable. +</P> +<P> +<PRE> + PCRE_INFO_BACKREFMAX +</PRE> +</P> +<P> +Return the number of the highest back reference in the pattern. The fourth +argument should point to an <B>int</B> variable. Zero is returned if there are +no back references. +</P> +<P> +<PRE> + PCRE_INFO_FIRSTCHAR +</PRE> +</P> +<P> +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 +<I>where</I>. Otherwise, if either +</P> +<P> +(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch +starts with "^", or +</P> +<P> +(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set +(if it were set, the pattern would be anchored), +</P> +<P> +-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. +</P> +<P> +<PRE> + PCRE_INFO_FIRSTTABLE +</PRE> +</P> +<P> +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 <B>unsigned char *</B> +variable. +</P> +<P> +<PRE> + PCRE_INFO_LASTLITERAL +</PRE> +</P> +<P> +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 <B>int</B> 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'. +</P> +<P> +The <B>pcre_info()</B> function is now obsolete because its interface is too +restrictive to return all the available data about a compiled pattern. New +programs should use <B>pcre_fullinfo()</B> instead. The yield of +<B>pcre_info()</B> is the number of capturing subpatterns, or one of the +following negative numbers: +</P> +<P> +<PRE> + PCRE_ERROR_NULL the argument <I>code</I> was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found +</PRE> +</P> +<P> +If the <I>optptr</I> 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). +</P> +<P> +If the pattern is not anchored and the <I>firstcharptr</I> argument is not NULL, +it is used to pass back information about the first character of any matched +string (see PCRE_INFO_FIRSTCHAR above). +</P> +<LI><A NAME="SEC9" HREF="#TOC1">MATCHING A PATTERN</A> +<P> +The function <B>pcre_exec()</B> is called to match a subject string against a +pre-compiled pattern, which is passed in the <I>code</I> argument. If the +pattern has been studied, the result of the study should be passed in the +<I>extra</I> argument. Otherwise this must be NULL. +</P> +<P> +Here is an example of a simple call to <B>pcre_exec()</B>: +</P> +<P> +<PRE> + int rc; + int ovector[30]; + rc = pcre_exec( + re, /* result of pcre_compile() */ + NULL, /* we didn't study the pattern */ + "some string", /* the subject string */ + 11, /* the length of the subject string */ + 0, /* start at offset 0 in the subject */ + 0, /* default options */ + ovector, /* vector for substring information */ + 30); /* number of elements in the vector */ +</PRE> +</P> +<P> +The PCRE_ANCHORED option can be passed in the <I>options</I> argument, 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. +</P> +<P> +There are also three further options that can be set only at matching time: +</P> +<P> +<PRE> + PCRE_NOTBOL +</PRE> +</P> +<P> +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. +</P> +<P> +<PRE> + PCRE_NOTEOL +</PRE> +</P> +<P> +The end of the string is not the end of a line, so the dollar metacharacter +should not match it nor (except in multiline mode) a newline immediately before +it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never +to match. +</P> +<P> +<PRE> + PCRE_NOTEMPTY +</PRE> +</P> +<P> +An empty string is not considered to be a valid match if this option is set. If +there are alternatives in the pattern, they are tried. If all the alternatives +match the empty string, the entire match fails. For example, if the pattern +</P> +<P> +<PRE> + a?b? +</PRE> +</P> +<P> +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". +</P> +<P> +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 <B>split()</B> 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. +</P> +<P> +The subject string is passed as a pointer in <I>subject</I>, a length in +<I>length</I>, and a starting offset in <I>startoffset</I>. Unlike the pattern +string, the subject may contain binary zero characters. 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. +</P> +<P> +A non-zero starting offset is useful when searching for another match in the +same subject by calling <B>pcre_exec()</B> again after a previous success. +Setting <I>startoffset</I> 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 +</P> +<P> +<PRE> + \Biss\B +</PRE> +</P> +<P> +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 <B>pcre_exec()</B> finds the first +occurrence. If <B>pcre_exec()</B> 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 +<B>pcre_exec()</B> is passed the entire string again, but with <I>startoffset</I> +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. +</P> +<P> +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. +</P> +<P> +In general, a pattern matches a certain portion of the subject, and in +addition, further substrings from the subject may be picked out by parts of the +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 subpattern that do not cause substrings to be captured. +</P> +<P> +Captured substrings are returned to the caller via a vector of integer offsets +whose address is passed in <I>ovector</I>. The number of elements in the vector +is passed in <I>ovecsize</I>. 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 <B>pcre_exec()</B> while +matching capturing subpatterns, and is not available for passing back +information. The length passed in <I>ovecsize</I> should always be a multiple of +three. If it is not, it is rounded down. +</P> +<P> +When a match has been successful, information about captured substrings is +returned in pairs of integers, starting at the beginning of <I>ovector</I>, 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, <I>ovector[0]</I> and <I>ovector[1]</I>, 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 <B>pcre_exec()</B> +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. +</P> +<P> +Some convenience functions are provided for extracting the captured substrings +as separate strings. These are described in the following section. +</P> +<P> +It is possible for an capturing subpattern number <I>n+1</I> to match some +part of the subject when subpattern <I>n</I> 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. +</P> +<P> +If a capturing subpattern is matched repeatedly, it is the last portion of the +string that it matched that gets returned. +</P> +<P> +If the vector is too small to hold all the captured substrings, 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, +<B>pcre_exec()</B> may be called with <I>ovector</I> passed as NULL and +<I>ovecsize</I> as zero. However, if the pattern contains back references and +the <I>ovector</I> 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 <I>ovector</I>. +</P> +<P> +Note that <B>pcre_info()</B> can be used to find out how many capturing +subpatterns there are in a compiled pattern. The smallest size for +<I>ovector</I> that will allow for <I>n</I> captured substrings in addition to +the offsets of the substring matched by the whole pattern is (<I>n</I>+1)*3. +</P> +<P> +If <B>pcre_exec()</B> fails, it returns a negative number. The following are +defined in the header file: +</P> +<P> +<PRE> + PCRE_ERROR_NOMATCH (-1) +</PRE> +</P> +<P> +The subject string did not match the pattern. +</P> +<P> +<PRE> + PCRE_ERROR_NULL (-2) +</PRE> +</P> +<P> +Either <I>code</I> or <I>subject</I> was passed as NULL, or <I>ovector</I> was +NULL and <I>ovecsize</I> was not zero. +</P> +<P> +<PRE> + PCRE_ERROR_BADOPTION (-3) +</PRE> +</P> +<P> +An unrecognized bit was set in the <I>options</I> argument. +</P> +<P> +<PRE> + PCRE_ERROR_BADMAGIC (-4) +</PRE> +</P> +<P> +PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch +the case when it is passed a junk pointer. This is the error it gives when the +magic number isn't present. +</P> +<P> +<PRE> + PCRE_ERROR_UNKNOWN_NODE (-5) +</PRE> +</P> +<P> +While running the pattern match, an unknown item was encountered in the +compiled pattern. This error could be caused by a bug in PCRE or by overwriting +of the compiled pattern. +</P> +<P> +<PRE> + PCRE_ERROR_NOMEMORY (-6) +</PRE> +</P> +<P> +If a pattern contains back references, but the <I>ovector</I> that is passed to +<B>pcre_exec()</B> 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 <B>pcre_malloc()</B> fails, this error is given. The memory is freed at +the end of matching. +</P> +<LI><A NAME="SEC10" HREF="#TOC1">EXTRACTING CAPTURED SUBSTRINGS</A> +<P> +Captured substrings can be accessed directly by using the offsets returned by +<B>pcre_exec()</B> in <I>ovector</I>. For convenience, the functions +<B>pcre_copy_substring()</B>, <B>pcre_get_substring()</B>, and +<B>pcre_get_substring_list()</B> 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. +</P> +<P> +The first three arguments are the same for all three functions: <I>subject</I> +is the subject string which has just been successfully matched, <I>ovector</I> +is a pointer to the vector of integer offsets that was passed to +<B>pcre_exec()</B>, and <I>stringcount</I> 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 <B>pcre_exec</B> if it +is greater than zero. If <B>pcre_exec()</B> returned zero, indicating that it +ran out of space in <I>ovector</I>, the value passed as <I>stringcount</I> should +be the size of the vector divided by three. +</P> +<P> +The functions <B>pcre_copy_substring()</B> and <B>pcre_get_substring()</B> +extract a single substring, whose number is given as <I>stringnumber</I>. A +value of zero extracts the substring that matched the entire pattern, while +higher values extract the captured substrings. For <B>pcre_copy_substring()</B>, +the string is placed in <I>buffer</I>, whose length is given by +<I>buffersize</I>, while for <B>pcre_get_substring()</B> a new block of memory is +obtained via <B>pcre_malloc</B>, and its address is returned via +<I>stringptr</I>. The yield of the function is the length of the string, not +including the terminating zero, or one of +</P> +<P> +<PRE> + PCRE_ERROR_NOMEMORY (-6) +</PRE> +</P> +<P> +The buffer was too small for <B>pcre_copy_substring()</B>, or the attempt to get +memory failed for <B>pcre_get_substring()</B>. +</P> +<P> +<PRE> + PCRE_ERROR_NOSUBSTRING (-7) +</PRE> +</P> +<P> +There is no substring whose number is <I>stringnumber</I>. +</P> +<P> +The <B>pcre_get_substring_list()</B> function extracts all available substrings +and builds a list of pointers to them. All this is done in a single block of +memory which is obtained via <B>pcre_malloc</B>. The address of the memory block +is returned via <I>listptr</I>, 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 +</P> +<P> +<PRE> + PCRE_ERROR_NOMEMORY (-6) +</PRE> +</P> +<P> +if the attempt to get the memory block failed. +</P> +<P> +When any of these functions encounter a substring that is unset, which can +happen when capturing subpattern number <I>n+1</I> matches some part of the +subject, but subpattern <I>n</I> 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 <I>ovector</I>, which is negative for unset +substrings. +</P> +<P> +The two convenience functions <B>pcre_free_substring()</B> and +<B>pcre_free_substring_list()</B> can be used to free the memory returned by +a previous call of <B>pcre_get_substring()</B> or +<B>pcre_get_substring_list()</B>, respectively. They do nothing more than call +the function pointed to by <B>pcre_free</B>, which of course could be called +directly from a C program. However, PCRE is used in some situations where it is +linked via a special interface to another programming language which cannot use +<B>pcre_free</B> directly; it is for these cases that the functions are +provided. +</P> +<LI><A NAME="SEC11" HREF="#TOC1">LIMITATIONS</A> +<P> +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. +There maximum number of capturing subpatterns is 65535. +There is no limit to the number of non-capturing subpatterns, but the maximum +depth of nesting of all kinds of parenthesized subpattern, including capturing +subpatterns, assertions, and other types of subpattern, is 200. +</P> +<P> +The maximum length of a subject string is the largest positive number that an +integer variable can hold. However, PCRE uses recursion to handle subpatterns +and indefinite repetition. This means that the available stack space may limit +the size of a subject string that can be processed by certain patterns. +</P> +<LI><A NAME="SEC12" HREF="#TOC1">DIFFERENCES FROM PERL</A> +<P> +The differences described here are with respect to Perl 5.005. +</P> +<P> +1. By default, a whitespace character is any character that the C library +function <B>isspace()</B> recognizes, though it is possible to compile PCRE with +alternative character type tables. Normally <B>isspace()</B> matches space, +formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5 +no longer includes vertical tab in its set of whitespace characters. The \v +escape that was in the Perl documentation for a long time was never in fact +recognized. However, the character itself was treated as whitespace at least +up to 5.002. In 5.004 and 5.005 it does not match \s. +</P> +<P> +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. +</P> +<P> +3. Capturing subpatterns that occur inside negative lookahead assertions are +counted, but their entries in the offsets vector are never set. Perl sets its +numerical variables 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. +</P> +<P> +4. Though binary zero characters are supported in the subject 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. +</P> +<P> +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 pattern matching engine. +</P> +<P> +6. The Perl \G assertion is not supported as it is not relevant to single +pattern matches. +</P> +<P> +7. 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). +</P> +<P> +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. +</P> +<P> +In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the +future Perl changes to a consistent state that is different, PCRE may change to +follow. +</P> +<P> +9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern +/^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not. +However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset. +</P> +<P> +10. PCRE provides some extensions to the Perl regular expression facilities: +</P> +<P> +(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. +</P> +<P> +(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. +</P> +<P> +(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special +meaning is faulted. +</P> +<P> +(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is +inverted, that is, by default they are not greedy, but if followed by a +question mark they are. +</P> +<P> +(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start +of the subject. +</P> +<P> +(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for +<B>pcre_exec()</B> have no Perl equivalents. +</P> +<P> +(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.) +</P> +<LI><A NAME="SEC13" HREF="#TOC1">REGULAR EXPRESSION DETAILS</A> +<P> +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), covers them in great detail. +</P> +<P> +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 <B>pcre_compile()</B> with the +PCRE_UTF8 option. How this affects the pattern matching is described in the +final section of this document. +</P> +<P> +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 characters in the subject. As a trivial example, the pattern +</P> +<P> +<PRE> + The quick brown fox +</PRE> +</P> +<P> +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 pattern. These are encoded in the pattern by the use of +<I>meta-characters</I>, which do not stand for themselves but instead are +interpreted in some special way. +</P> +<P> +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: +</P> +<P> +<PRE> + \ 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 +</PRE> +</P> +<P> +Part of a pattern that is in square brackets is called a "character class". In +a character class the only meta-characters are: +</P> +<P> +<PRE> + \ general escape character + ^ negate the class, but only if the first character + - indicates character range + ] terminates the character class +</PRE> +</P> +<P> +The following sections describe the use of each of the meta-characters. +</P> +<LI><A NAME="SEC14" HREF="#TOC1">BACKSLASH</A> +<P> +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. +</P> +<P> +For example, if you want to match a "*" character, you write "\*" in the +pattern. This applies whether or not the following 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 particular, +if you want to match a backslash, you write "\\". +</P> +<P> +If a pattern is compiled with the PCRE_EXTENDED option, whitespace 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. +</P> +<P> +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 characters, 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: +</P> +<P> +<PRE> + \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 +</PRE> +</P> +<P> +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. +</P> +<P> +After "\x", up to two hexadecimal digits are read (letters can be in upper or +lower case). +</P> +<P> +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. +</P> +<P> +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 <I>back reference</I>. A description of how this works is given +later, following the discussion of parenthesized subpatterns. +</P> +<P> +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 following the backslash, and generates a single byte from the least +significant 8 bits of the value. Any subsequent digits stand for themselves. +For example: +</P> +<P> +<PRE> + \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" +</PRE> +</P> +<P> +Note that octal values of 100 or greater must not be introduced by a leading +zero, because no more than three octal digits are ever read. +</P> +<P> +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). +</P> +<P> +The third use of backslash is for specifying generic character types: +</P> +<P> +<PRE> + \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 +</PRE> +</P> +<P> +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. +</P> +<P> +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 controlled 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 character codes greater than 128 are used for +accented letters, and these are matched by \w. +</P> +<P> +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. +</P> +<P> +The fourth use of backslash is for certain simple assertions. An assertion +specifies a condition that has to be met at a particular point in a match, +without consuming any characters from the subject string. The use of +subpatterns for more complicated assertions is described below. The backslashed +assertions are +</P> +<P> +<PRE> + \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) +</PRE> +</P> +<P> +These assertions may not appear in character classes (but note that "\b" has a +different meaning, namely the backspace character, inside a character class). +</P> +<P> +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. +</P> +<P> +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 <I>startoffset</I> argument of +<B>pcre_exec()</B> 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. +</P> +<LI><A NAME="SEC15" HREF="#TOC1">CIRCUMFLEX AND DOLLAR</A> +<P> +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 <I>startoffset</I> argument of +<B>pcre_exec()</B> is non-zero, circumflex can never match. Inside a character +class, circumflex has an entirely different meaning (see below). +</P> +<P> +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 alternatives 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 constructs that can cause a pattern +to be anchored.) +</P> +<P> +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. +</P> +<P> +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. +</P> +<P> +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. Consequently, patterns that are anchored in single line mode +because all branches start with "^" are not anchored in multiline mode, and a +match for circumflex is possible when the <I>startoffset</I> argument of +<B>pcre_exec()</B> is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if +PCRE_MULTILINE is set. +</P> +<P> +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 it is always anchored, whether PCRE_MULTILINE is set or not. +</P> +<LI><A NAME="SEC16" HREF="#TOC1">FULL STOP (PERIOD, DOT)</A> +<P> +Outside a character class, a dot in the pattern matches any one character in +the subject, including a non-printing character, 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. +</P> +<LI><A NAME="SEC17" HREF="#TOC1">SQUARE BRACKETS</A> +<P> +An opening square bracket introduces a character class, terminated 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 circumflex, if present) or +escaped with a backslash. +</P> +<P> +A character class matches a single character in the subject; the character must +be in the set of characters defined by the class, unless the first character in +the class is a circumflex, in which case the subject character must not be in +the set defined by the class. If a circumflex is actually required as a member +of the class, ensure it is not the first character, or escape it with a +backslash. +</P> +<P> +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 convenient 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. +</P> +<P> +When caseless matching is set, any letters in a class represent both their +upper case and lower case versions, so for example, a caseless [aeiou] matches +"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a +caseful version would. +</P> +<P> +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. +</P> +<P> +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 interpreted as +indicating a range, typically as the first or last character in the class. +</P> +<P> +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 "-") followed 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 interpreted 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. +</P> +<P> +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. +</P> +<P> +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 +restricted set of characters than the matching lower case type. For example, +the class [^\W_] matches any letter or digit, but not underscore. +</P> +<P> +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. +</P> +<LI><A NAME="SEC18" HREF="#TOC1">POSIX CHARACTER CLASSES</A> +<P> +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, +</P> +<P> +<PRE> + [01[:alpha:]%] +</PRE> +</P> +<P> +matches "0", "1", any alphabetic character, or "%". The supported class names +are +</P> +<P> +<PRE> + 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 +</PRE> +</P> +<P> +The names "ascii" and "word" are Perl extensions. Another Perl extension is +negation, which is indicated by a ^ character after the colon. For example, +</P> +<P> +<PRE> + [12[:^digit:]] +</PRE> +</P> +<P> +matches "1", "2", or any non-digit. PCRE (and Perl) also recognize 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. +</P> +<LI><A NAME="SEC19" HREF="#TOC1">VERTICAL BAR</A> +<P> +Vertical bar characters are used to separate alternative patterns. For example, +the pattern +</P> +<P> +<PRE> + gilbert|sullivan +</PRE> +</P> +<P> +matches either "gilbert" or "sullivan". Any number of alternatives may appear, +and an empty alternative is permitted (matching the empty string). +The matching process tries each alternative in turn, from left to right, +and the first one that succeeds is used. If the alternatives are within a +subpattern (defined below), "succeeds" means matching the rest of the main +pattern as well as the alternative in the subpattern. +</P> +<LI><A NAME="SEC20" HREF="#TOC1">INTERNAL OPTION SETTING</A> +<P> +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 +</P> +<P> +<PRE> + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED +</PRE> +</P> +<P> +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. +</P> +<P> +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: +</P> +<P> +<PRE> + (?i)abc + a(?i)bc + ab(?i)c + abc(?i) +</PRE> +</P> +<P> +which in turn is the same as compiling the pattern abc with PCRE_CASELESS set. +In other words, such "top level" settings apply to the whole pattern (unless +there are other changes inside subpatterns). If there is more than one setting +of the same option at top level, the rightmost setting is used. +</P> +<P> +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 +</P> +<P> +<PRE> + (a(?i)b)c +</PRE> +</P> +<P> +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, +</P> +<P> +<PRE> + (a(?i)b|c) +</PRE> +</P> +<P> +matches "ab", "aB", "c", and "C", even though when matching "C" the first +branch is abandoned before the option setting. This is because the effects of +option settings happen at compile time. There would be some very weird +behaviour otherwise. +</P> +<P> +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. +</P> +<LI><A NAME="SEC21" HREF="#TOC1">SUBPATTERNS</A> +<P> +Subpatterns are delimited by parentheses (round brackets), which can be nested. +Marking part of a pattern as a subpattern does two things: +</P> +<P> +1. It localizes a set of alternatives. For example, the pattern +</P> +<P> +<PRE> + cat(aract|erpillar|) +</PRE> +</P> +<P> +matches one of the words "cat", "cataract", or "caterpillar". Without the +parentheses, it would match "cataract", "erpillar" or the empty string. +</P> +<P> +2. It sets up the subpattern as a capturing subpattern (as defined above). +When the whole pattern matches, that portion of the subject string that matched +the subpattern is passed back to the caller via the <I>ovector</I> argument of +<B>pcre_exec()</B>. Opening parentheses are counted from left to right (starting +from 1) to obtain the numbers of the capturing subpatterns. +</P> +<P> +For example, if the string "the red king" is matched against the pattern +</P> +<P> +<PRE> + the ((red|white) (king|queen)) +</PRE> +</P> +<P> +the captured substrings are "red king", "red", and "king", and are numbered 1, +2, and 3, respectively. +</P> +<P> +The fact that plain parentheses fulfil two functions is not always helpful. +There are often times when a grouping subpattern 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 pattern +</P> +<P> +<PRE> + the ((?:red|white) (king|queen)) +</PRE> +</P> +<P> +the captured substrings are "white queen" and "queen", and are numbered 1 and +2. The maximum number of captured substrings is 99, and the maximum number of +all subpatterns, both capturing and non-capturing, is 200. +</P> +<P> +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 +</P> +<P> +<PRE> + (?i:saturday|sunday) + (?:(?i)saturday|sunday) +</PRE> +</P> +<P> +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". +</P> +<LI><A NAME="SEC22" HREF="#TOC1">REPETITION</A> +<P> +Repetition is specified by quantifiers, which can follow any of the following +items: +</P> +<P> +<PRE> + 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) +</PRE> +</P> +<P> +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: +</P> +<P> +<PRE> + z{2,4} +</PRE> +</P> +<P> +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 +</P> +<P> +<PRE> + [aeiou]{3,} +</PRE> +</P> +<P> +matches at least 3 successive vowels, but may match many more, while +</P> +<P> +<PRE> + \d{8} +</PRE> +</P> +<P> +matches exactly 8 digits. An opening curly bracket that appears in a position +where a quantifier is not allowed, or one that does not match the syntax of a +quantifier, is taken as a literal character. For example, {,6} is not a +quantifier, but a literal string of four characters. +</P> +<P> +The quantifier {0} is permitted, causing the expression to behave as if the +previous item and the quantifier were not present. +</P> +<P> +For convenience (and historical compatibility) the three most common +quantifiers have single-character abbreviations: +</P> +<P> +<PRE> + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} +</PRE> +</P> +<P> +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: +</P> +<P> +<PRE> + (a?)* +</PRE> +</P> +<P> +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. +</P> +<P> +By default, the quantifiers are "greedy", that is, they match as much as +possible (up to the maximum number of permitted times), without causing the +rest of the pattern to fail. The classic example of where this gives problems +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 comments by applying the pattern +</P> +<P> +<PRE> + /\*.*\*/ +</PRE> +</P> +<P> +to the string +</P> +<P> +<PRE> + /* first command */ not comment /* second comment */ +</PRE> +</P> +<P> +fails, because it matches the entire string owing to the greediness of the .* +item. +</P> +<P> +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 +</P> +<P> +<PRE> + /\*.*?\*/ +</PRE> +</P> +<P> +does the right thing with the C comments. The meaning of the various +quantifiers is not otherwise changed, just the preferred number of matches. +Do not confuse this use of question mark with its use as a quantifier in its +own right. Because it has two uses, it can sometimes appear doubled, as in +</P> +<P> +<PRE> + \d??\d +</PRE> +</P> +<P> +which matches one digit by preference, but can match two if that is the only +way the rest of the pattern matches. +</P> +<P> +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. +</P> +<P> +When a parenthesized subpattern is quantified with a minimum repeat count that +is greater than 1 or with a limited maximum, more store is required for the +compiled pattern, in proportion to the size of the minimum or maximum. +</P> +<P> +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 +character 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 pattern +begins with .* in order to obtain this optimization, or alternatively using ^ +to indicate anchoring explicitly. +</P> +<P> +When a capturing subpattern is repeated, the value captured is the substring +that matched the final iteration. For example, after +</P> +<P> +<PRE> + (tweedle[dume]{3}\s*)+ +</PRE> +</P> +<P> +has matched "tweedledum tweedledee" the value of the captured substring is +"tweedledee". However, if there are nested capturing subpatterns, the +corresponding captured values may have been set in previous iterations. For +example, after +</P> +<P> +<PRE> + /(a|(b))+/ +</PRE> +</P> +<P> +matches "aba" the value of the second captured substring is "b". +</P> +<LI><A NAME="SEC23" HREF="#TOC1">BACK REFERENCES</A> +<P> +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. +</P> +<P> +However, if the decimal number following the backslash is less than 10, it is +always taken as a back reference, and causes an error only if there are not +that many capturing left parentheses in the entire pattern. In other words, the +parentheses that are referenced need not be to the left of the reference for +numbers less than 10. See the section entitled "Backslash" above for further +details of the handling of digits following a backslash. +</P> +<P> +A back reference matches whatever actually matched the capturing subpattern in +the current subject string, rather than anything matching the subpattern +itself. So the pattern +</P> +<P> +<PRE> + (sens|respons)e and \1ibility +</PRE> +</P> +<P> +matches "sense and sensibility" and "response and responsibility", 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, +</P> +<P> +<PRE> + ((?i)rah)\s+\1 +</PRE> +</P> +<P> +matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original +capturing subpattern is matched caselessly. +</P> +<P> +There may be more than one back reference to the same subpattern. If a +subpattern has not actually been used in a particular match, any back +references to it always fail. For example, the pattern +</P> +<P> +<PRE> + (a|(bc))\2 +</PRE> +</P> +<P> +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. +</P> +<P> +A back reference that occurs inside the parentheses to which it refers fails +when the subpattern is first used, so, for example, (a\1) never matches. +However, such references can be useful inside repeated subpatterns. For +example, the pattern +</P> +<P> +<PRE> + (a|b\1)+ +</PRE> +</P> +<P> +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. +</P> +<LI><A NAME="SEC24" HREF="#TOC1">ASSERTIONS</A> +<P> +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 +complicated 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. +</P> +<P> +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, +</P> +<P> +<PRE> + \w+(?=;) +</PRE> +</P> +<P> +matches a word followed by a semicolon, but does not include the semicolon in +the match, and +</P> +<P> +<PRE> + foo(?!bar) +</PRE> +</P> +<P> +matches any occurrence of "foo" that is not followed by "bar". Note that the +apparently similar pattern +</P> +<P> +<PRE> + (?!foo)bar +</PRE> +</P> +<P> +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. +</P> +<P> +Lookbehind assertions start with (?<= for positive assertions and (?<! for +negative assertions. For example, +</P> +<P> +<PRE> + (?<!foo)bar +</PRE> +</P> +<P> +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 +</P> +<P> +<PRE> + (?<=bullock|donkey) +</PRE> +</P> +<P> +is permitted, but +</P> +<P> +<PRE> + (?<!dogs?|cats?) +</PRE> +</P> +<P> +causes an error at compile time. Branches that match different length strings +are permitted only at the top level of a lookbehind assertion. This is an +extension compared with Perl 5.005, which requires all branches to match the +same length of string. An assertion such as +</P> +<P> +<PRE> + (?<=ab(c|de)) +</PRE> +</P> +<P> +is not permitted, because its single top-level branch can match two different +lengths, but it is acceptable if rewritten to use two top-level branches: +</P> +<P> +<PRE> + (?<=abc|abde) +</PRE> +</P> +<P> +The implementation of lookbehind assertions is, for each alternative, to +temporarily move the current position back by the fixed width and then try to +match. If there are insufficient characters before the current position, the +match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns +can be particularly useful for matching at the ends of strings; an example is +given at the end of the section on once-only subpatterns. +</P> +<P> +Several assertions (of any sort) may occur in succession. For example, +</P> +<P> +<PRE> + (?<=\d{3})(?<!999)foo +</PRE> +</P> +<P> +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 <I>not</I> 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 +</P> +<P> +<PRE> + (?<=\d{3}...)(?<!999)foo +</PRE> +</P> +<P> +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". +</P> +<P> +Assertions can be nested in any combination. For example, +</P> +<P> +<PRE> + (?<=(?<!foo)bar)baz +</PRE> +</P> +<P> +matches an occurrence of "baz" that is preceded by "bar" which in turn is not +preceded by "foo", while +</P> +<P> +<PRE> + (?<=\d{3}(?!999)...)foo +</PRE> +</P> +<P> +is another pattern which matches "foo" preceded by three digits and any three +characters that are not "999". +</P> +<P> +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. +</P> +<P> +Assertions count towards the maximum of 200 parenthesized subpatterns. +</P> +<LI><A NAME="SEC25" HREF="#TOC1">ONCE-ONLY SUBPATTERNS</A> +<P> +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. +</P> +<P> +Consider, for example, the pattern \d+foo when applied to the subject line +</P> +<P> +<PRE> + 123456bar +</PRE> +</P> +<P> +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: +</P> +<P> +<PRE> + (?>\d+)bar +</PRE> +</P> +<P> +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. +</P> +<P> +An alternative description is that a subpattern of this type matches the string +of characters that an identical standalone pattern would match, if anchored at +the current point in the subject string. +</P> +<P> +Once-only subpatterns are not capturing subpatterns. Simple cases such as the +above example can be thought of as a maximizing repeat that must swallow +everything it can. So, while both \d+ and \d+? are prepared to adjust the +number of digits they match in order to make the rest of the pattern match, +(?>\d+) can only match an entire sequence of digits. +</P> +<P> +This construction can of course contain arbitrarily complicated subpatterns, +and it can be nested. +</P> +<P> +Once-only subpatterns can be used in conjunction with lookbehind assertions to +specify efficient matching at the end of the subject string. Consider a simple +pattern such as +</P> +<P> +<PRE> + abcd$ +</PRE> +</P> +<P> +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 +</P> +<P> +<PRE> + ^.*abcd$ +</PRE> +</P> +<P> +the initial .* matches the entire string at first, but when this fails (because +there is no following "a"), it backtracks to match all but the last character, +then all but the last two characters, and so on. Once again the search for "a" +covers the entire string, from right to left, so we are no better off. However, +if the pattern is written as +</P> +<P> +<PRE> + ^(?>.*)(?<=abcd) +</PRE> +</P> +<P> +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. +</P> +<P> +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 +</P> +<P> +<PRE> + (\D+|<\d+>)*[!?] +</PRE> +</P> +<P> +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 +</P> +<P> +<PRE> + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +</PRE> +</P> +<P> +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 +</P> +<P> +<PRE> + ((?>\D+)|<\d+>)*[!?] +</PRE> +</P> +<P> +sequences of non-digits cannot be broken, and failure happens quickly. +</P> +<LI><A NAME="SEC26" HREF="#TOC1">CONDITIONAL SUBPATTERNS</A> +<P> +It is possible to cause the matching process to obey a subpattern +conditionally or to choose between two alternative subpatterns, depending on +the result of an assertion, or whether a previous capturing subpattern matched +or not. The two possible forms of conditional subpattern are +</P> +<P> +<PRE> + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) +</PRE> +</P> +<P> +If the condition is satisfied, the yes-pattern is used; otherwise the +no-pattern (if present) is used. If there are more than two alternatives in the +subpattern, a compile-time error occurs. +</P> +<P> +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: +</P> +<P> +<PRE> + ( \( )? [^()]+ (?(1) \) ) +</PRE> +</P> +<P> +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. +</P> +<P> +If the condition is not a sequence of digits, it must be an assertion. This may +be a positive or negative lookahead or lookbehind assertion. Consider this +pattern, again containing non-significant white space, and with the two +alternatives on the second line: +</P> +<P> +<PRE> + (?(?=[^a-z]*[a-z]) + \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) +</PRE> +</P> +<P> +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. +</P> +<LI><A NAME="SEC27" HREF="#TOC1">COMMENTS</A> +<P> +The sequence (?# marks the start of a comment which continues 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. +</P> +<P> +If the PCRE_EXTENDED option is set, an unescaped # character outside a +character class introduces a comment that continues up to the next newline +character in the pattern. +</P> +<LI><A NAME="SEC28" HREF="#TOC1">RECURSIVE PATTERNS</A> +<P> +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: +</P> +<P> +<PRE> + $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; +</PRE> +</P> +<P> +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): +</P> +<P> +<PRE> + \( ( (?>[^()]+) | (?R) )* \) +</PRE> +</P> +<P> +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. +</P> +<P> +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 +</P> +<P> +<PRE> + (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() +</PRE> +</P> +<P> +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. +</P> +<P> +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 +</P> +<P> +<PRE> + (ab(cd)ef) +</PRE> +</P> +<P> +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 +</P> +<P> +<PRE> + \( ( ( (?>[^()]+) | (?R) )* ) \) + ^ ^ + ^ ^ +</PRE> +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 <B>pcre_malloc</B>, freeing it via <B>pcre_free</B> 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. +</P> +<LI><A NAME="SEC29" HREF="#TOC1">PERFORMANCE</A> +<P> +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. +</P> +<P> +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 +</P> +<P> +<PRE> + (.*) second +</PRE> +</P> +<P> +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. +</P> +<P> +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. +</P> +<P> +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 +</P> +<P> +<PRE> + (a+)* +</PRE> +</P> +<P> +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 principle to try every possible +variation, and this can take an extremely long time. +</P> +<P> +An optimization catches some of the more simple cases such as +</P> +<P> +<PRE> + (a+)*b +</PRE> +</P> +<P> +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 +</P> +<P> +<PRE> + (a+)*\d +</PRE> +</P> +<P> +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. +</P> +<LI><A NAME="SEC30" HREF="#TOC1">UTF-8 SUPPORT</A> +<P> +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 <B>pcre_compile()</B> with the PCRE_UTF8 option +flag. When you do this, both the pattern and any subject 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. +</P> +<P> +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 additional run time overhead is limited +to testing the PCRE_UTF8 flag in several places, so should not be very large. +</P> +<P> +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. +</P> +<P> +Running with PCRE_UTF8 set causes these changes in the way PCRE works: +</P> +<P> +1. In a pattern, the escape sequence \x{...}, where the contents 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. +</P> +<P> +2. The original hexadecimal escape sequence, \xhh, generates a two-byte UTF-8 +character if its value is greater than 127. +</P> +<P> +3. Repeat quantifiers are NOT correctly handled if they follow 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. +</P> +<P> +4. The dot metacharacter matches one UTF-8 character instead of a single byte. +</P> +<P> +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. +</P> +<P> +4. Although the \x{...} escape is permitted in a character class, characters +whose values are greater than 255 cannot be included in a class. +</P> +<P> +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. +</P> +<P> +6. Repeated classes work correctly on multiple characters. +</P> +<P> +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 single byte matches. In the first case, of course, +the class brackets are just redundant. +</P> +<P> +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. +</P> +<P> +9. The character types such as \d and \w do not work correctly with UTF-8 +characters. They continue to test a single byte. +</P> +<P> +10. Anything not explicitly mentioned here continues to work in bytes rather +than in characters. +</P> +<P> +The following UTF-8 features of Perl 5.6 are not implemented: +</P> +<P> +1. The escape sequence \C to match a single byte. +</P> +<P> +2. The use of Unicode tables and properties and escapes \p, \P, and \X. +</P> +<LI><A NAME="SEC31" HREF="#TOC1">SAMPLE PROGRAM</A> +<P> +The code below is a simple, complete demonstration program, to get you started +with using PCRE. This code is also supplied in the file <I>pcredemo.c</I> in the +PCRE distribution. +</P> +<P> +The program compiles the regular expression that is its first argument, and +matches it against the subject string in its second argument. No options are +set, and default character tables are used. If matching succeeds, the program +outputs the portion of the subject that matched, together with the contents of +any captured substrings. +</P> +<P> +On a Unix system that has PCRE installed in <I>/usr/local</I>, you can compile +the demonstration program using a command like this: +</P> +<P> +<PRE> + gcc -o pcredemo pcredemo.c -I/usr/local/include -L/usr/local/lib -lpcre +</PRE> +</P> +<P> +Then you can run simple tests like this: +</P> +<P> +<PRE> + ./pcredemo 'cat|dog' 'the cat sat on the mat' +</PRE> +</P> +<P> +Note that there is a much more comprehensive test program, called +<B>pcretest</B>, which supports many more facilities for testing regular +expressions. The <B>pcredemo</B> program is provided as a simple coding example. +</P> +<P> +On some operating systems (e.g. Solaris) you may get an error like this when +you try to run <B>pcredemo</B>: +</P> +<P> +<PRE> + ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or directory +</PRE> +</P> +<P> +This is caused by the way shared library support works on those systems. You +need to add +</P> +<P> +<PRE> + -R/usr/local/lib +</PRE> +</P> +<P> +to the compile command to get round this problem. Here's the code: +</P> +<P> +<PRE> + #include <stdio.h> + #include <string.h> + #include <pcre.h> +</PRE> +</P> +<P> +<PRE> + #define OVECCOUNT 30 /* should be a multiple of 3 */ +</PRE> +</P> +<P> +<PRE> + int main(int argc, char **argv) + { + pcre *re; + const char *error; + int erroffset; + int ovector[OVECCOUNT]; + int rc, i; +</PRE> +</P> +<P> +<PRE> + if (argc != 3) + { + printf("Two arguments required: a regex and a " + "subject string\n"); + return 1; + } +</PRE> +</P> +<P> +<PRE> + /* Compile the regular expression in the first argument */ +</PRE> +</P> +<P> +<PRE> + re = pcre_compile( + argv[1], /* the pattern */ + 0, /* default options */ + &error, /* for error message */ + &erroffset, /* for error offset */ + NULL); /* use default character tables */ +</PRE> +</P> +<P> +<PRE> + /* Compilation failed: print the error message and exit */ +</PRE> +</P> +<P> +<PRE> + if (re == NULL) + { + printf("PCRE compilation failed at offset %d: %s\n", + erroffset, error); + return 1; + } +</PRE> +</P> +<P> +<PRE> + /* Compilation succeeded: match the subject in the second + argument */ +</PRE> +</P> +<P> +<PRE> + rc = pcre_exec( + re, /* the compiled pattern */ + NULL, /* we didn't study the pattern */ + argv[2], /* the subject string */ + (int)strlen(argv[2]), /* the length of the subject */ + 0, /* start at offset 0 in the subject */ + 0, /* default options */ + ovector, /* vector for substring information */ + OVECCOUNT); /* number of elements in the vector */ +</PRE> +</P> +<P> +<PRE> + /* Matching failed: handle error cases */ +</PRE> +</P> +<P> +<PRE> + if (rc < 0) + { + switch(rc) + { + case PCRE_ERROR_NOMATCH: printf("No match\n"); break; + /* + Handle other special cases if you like + */ + default: printf("Matching error %d\n", rc); break; + } + return 1; + } +</PRE> +</P> +<P> +<PRE> + /* Match succeded */ +</PRE> +</P> +<P> +<PRE> + printf("Match succeeded\n"); +</PRE> +</P> +<P> +<PRE> + /* The output vector wasn't big enough */ +</PRE> +</P> +<P> +<PRE> + if (rc == 0) + { + rc = OVECCOUNT/3; + printf("ovector only has room for %d captured " + substrings\n", rc - 1); + } +</PRE> +</P> +<P> +<PRE> + /* Show substrings stored in the output vector */ +</PRE> +</P> +<P> +<PRE> + for (i = 0; i < rc; i++) + { + char *substring_start = argv[2] + ovector[2*i]; + int substring_length = ovector[2*i+1] - ovector[2*i]; + printf("%2d: %.*s\n", i, substring_length, + substring_start); + } +</PRE> +</P> +<P> +<PRE> + return 0; + } +</PRE> +</P> +<LI><A NAME="SEC32" HREF="#TOC1">AUTHOR</A> +<P> +Philip Hazel <ph10@cam.ac.uk> +<BR> +University Computing Service, +<BR> +New Museums Site, +<BR> +Cambridge CB2 3QG, England. +<BR> +Phone: +44 1223 334714 +</P> +<P> +Last updated: 15 August 2001 +<BR> +Copyright (c) 1997-2001 University of Cambridge. |