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diff --git a/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.html b/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.html deleted file mode 100644 index 3e9eb36b..00000000 --- a/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.html +++ /dev/null @@ -1,2669 +0,0 @@ -<HTML> -<HEAD> -<TITLE>pcre specification</TITLE> -</HEAD> -<body bgcolor="#FFFFFF" text="#00005A"> -<H1>pcre specification</H1> -This HTML document has been generated automatically from the original man page. -If there is any nonsense in it, please consult the man page in case the -conversion went wrong. -<UL> -<LI><A NAME="TOC1" HREF="#SEC1">NAME</A> -<LI><A NAME="TOC2" HREF="#SEC2">SYNOPSIS</A> -<LI><A NAME="TOC3" HREF="#SEC3">DESCRIPTION</A> -<LI><A NAME="TOC4" HREF="#SEC4">MULTI-THREADING</A> -<LI><A NAME="TOC5" HREF="#SEC5">COMPILING A PATTERN</A> -<LI><A NAME="TOC6" HREF="#SEC6">STUDYING A PATTERN</A> -<LI><A NAME="TOC7" HREF="#SEC7">LOCALE SUPPORT</A> -<LI><A NAME="TOC8" HREF="#SEC8">INFORMATION ABOUT A PATTERN</A> -<LI><A NAME="TOC9" HREF="#SEC9">MATCHING A PATTERN</A> -<LI><A NAME="TOC10" HREF="#SEC10">EXTRACTING CAPTURED SUBSTRINGS</A> -<LI><A NAME="TOC11" HREF="#SEC11">LIMITATIONS</A> -<LI><A NAME="TOC12" HREF="#SEC12">DIFFERENCES FROM PERL</A> -<LI><A NAME="TOC13" HREF="#SEC13">REGULAR EXPRESSION DETAILS</A> -<LI><A NAME="TOC14" HREF="#SEC14">BACKSLASH</A> -<LI><A NAME="TOC15" HREF="#SEC15">CIRCUMFLEX AND DOLLAR</A> -<LI><A NAME="TOC16" HREF="#SEC16">FULL STOP (PERIOD, DOT)</A> -<LI><A NAME="TOC17" HREF="#SEC17">SQUARE BRACKETS</A> -<LI><A NAME="TOC18" HREF="#SEC18">POSIX CHARACTER CLASSES</A> -<LI><A NAME="TOC19" HREF="#SEC19">VERTICAL BAR</A> -<LI><A NAME="TOC20" HREF="#SEC20">INTERNAL OPTION SETTING</A> -<LI><A NAME="TOC21" HREF="#SEC21">SUBPATTERNS</A> -<LI><A NAME="TOC22" HREF="#SEC22">REPETITION</A> -<LI><A NAME="TOC23" HREF="#SEC23">BACK REFERENCES</A> -<LI><A NAME="TOC24" HREF="#SEC24">ASSERTIONS</A> -<LI><A NAME="TOC25" HREF="#SEC25">ONCE-ONLY SUBPATTERNS</A> -<LI><A NAME="TOC26" HREF="#SEC26">CONDITIONAL SUBPATTERNS</A> -<LI><A NAME="TOC27" HREF="#SEC27">COMMENTS</A> -<LI><A NAME="TOC28" HREF="#SEC28">RECURSIVE PATTERNS</A> -<LI><A NAME="TOC29" HREF="#SEC29">PERFORMANCE</A> -<LI><A NAME="TOC30" HREF="#SEC30">UTF-8 SUPPORT</A> -<LI><A NAME="TOC31" HREF="#SEC31">SAMPLE PROGRAM</A> -<LI><A NAME="TOC32" HREF="#SEC32">AUTHOR</A> -</UL> -<LI><A NAME="SEC1" HREF="#TOC1">NAME</A> -<P> -pcre - Perl-compatible regular expressions. -</P> -<LI><A NAME="SEC2" HREF="#TOC1">SYNOPSIS</A> -<P> -<B>#include <pcre.h></B> -</P> -<P> -<B>pcre *pcre_compile(const char *<I>pattern</I>, int <I>options</I>,</B> -<B>const char **<I>errptr</I>, int *<I>erroffset</I>,</B> -<B>const unsigned char *<I>tableptr</I>);</B> -</P> -<P> -<B>pcre_extra *pcre_study(const pcre *<I>code</I>, int <I>options</I>,</B> -<B>const char **<I>errptr</I>);</B> -</P> -<P> -<B>int pcre_exec(const pcre *<I>code</I>, const pcre_extra *<I>extra</I>,</B> -<B>const char *<I>subject</I>, int <I>length</I>, int <I>startoffset</I>,</B> -<B>int <I>options</I>, int *<I>ovector</I>, int <I>ovecsize</I>);</B> -</P> -<P> -<B>int pcre_copy_substring(const char *<I>subject</I>, int *<I>ovector</I>,</B> -<B>int <I>stringcount</I>, int <I>stringnumber</I>, char *<I>buffer</I>,</B> -<B>int <I>buffersize</I>);</B> -</P> -<P> -<B>int pcre_get_substring(const char *<I>subject</I>, int *<I>ovector</I>,</B> -<B>int <I>stringcount</I>, int <I>stringnumber</I>,</B> -<B>const char **<I>stringptr</I>);</B> -</P> -<P> -<B>int pcre_get_substring_list(const char *<I>subject</I>,</B> -<B>int *<I>ovector</I>, int <I>stringcount</I>, const char ***<I>listptr</I>);</B> -</P> -<P> -<B>void pcre_free_substring(const char *<I>stringptr</I>);</B> -</P> -<P> -<B>void pcre_free_substring_list(const char **<I>stringptr</I>);</B> -</P> -<P> -<B>const unsigned char *pcre_maketables(void);</B> -</P> -<P> -<B>int pcre_fullinfo(const pcre *<I>code</I>, const pcre_extra *<I>extra</I>,</B> -<B>int <I>what</I>, void *<I>where</I>);</B> -</P> -<P> -<B>int pcre_info(const pcre *<I>code</I>, int *<I>optptr</I>, int</B> -<B>*<I>firstcharptr</I>);</B> -</P> -<P> -<B>char *pcre_version(void);</B> -</P> -<P> -<B>void *(*pcre_malloc)(size_t);</B> -</P> -<P> -<B>void (*pcre_free)(void *);</B> -</P> -<LI><A NAME="SEC3" HREF="#TOC1">DESCRIPTION</A> -<P> -The PCRE library is a set of functions that implement regular expression -pattern matching using the same syntax and semantics as Perl 5, with just a few -differences (see below). The current implementation corresponds to Perl 5.005, -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. |