diff options
Diffstat (limited to 'rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.txt')
| -rw-r--r-- | rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.txt | 2315 |
1 files changed, 2315 insertions, 0 deletions
diff --git a/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.txt b/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.txt new file mode 100644 index 00000000..95f148f3 --- /dev/null +++ b/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.txt @@ -0,0 +1,2315 @@ +NAME + pcre - Perl-compatible regular expressions. + + + +SYNOPSIS + #include <pcre.h> + + pcre *pcre_compile(const char *pattern, int options, + const char **errptr, int *erroffset, + const unsigned char *tableptr); + + pcre_extra *pcre_study(const pcre *code, int options, + const char **errptr); + + int pcre_exec(const pcre *code, const pcre_extra *extra, + const char *subject, int length, int startoffset, + int options, int *ovector, int ovecsize); + + int pcre_copy_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, char *buffer, + int buffersize); + + int pcre_get_substring(const char *subject, int *ovector, + int stringcount, int stringnumber, + const char **stringptr); + + int pcre_get_substring_list(const char *subject, + int *ovector, int stringcount, const char ***listptr); + + void pcre_free_substring(const char *stringptr); + + void pcre_free_substring_list(const char **stringptr); + + const unsigned char *pcre_maketables(void); + + int pcre_fullinfo(const pcre *code, const pcre_extra *extra, + int what, void *where); + + int pcre_info(const pcre *code, int *optptr, *firstcharptr); + + char *pcre_version(void); + + void *(*pcre_malloc)(size_t); + + void (*pcre_free)(void *); + + + + +DESCRIPTION + The PCRE library is a set of functions that implement regu- + lar expression pattern matching using the same syntax and + semantics as Perl 5, with just a few differences (see + + below). The current implementation corresponds to Perl + 5.005, with some additional features from later versions. + This includes some experimental, incomplete support for + UTF-8 encoded strings. Details of exactly what is and what + is not supported are given below. + + PCRE has its own native API, which is described in this + document. There is also a set of wrapper functions that + correspond to the POSIX regular expression API. These are + described in the pcreposix documentation. + + The native API function prototypes are defined in the header + file pcre.h, and on Unix systems the library itself is + called libpcre.a, so can be accessed by adding -lpcre to the + command for linking an application which calls it. The + header file defines the macros PCRE_MAJOR and PCRE_MINOR to + contain the major and minor release numbers for the library. + Applications can use these to include support for different + releases. + + The functions pcre_compile(), pcre_study(), and pcre_exec() + are used for compiling and matching regular expressions. A + sample program that demonstrates the simplest way of using + them is given in the file pcredemo.c. The last section of + this man page describes how to run it. + + The functions pcre_copy_substring(), pcre_get_substring(), + and pcre_get_substring_list() are convenience functions for + extracting captured substrings from a matched subject + string; pcre_free_substring() and pcre_free_substring_list() + are also provided, to free the memory used for extracted + strings. + + The function pcre_maketables() is used (optionally) to build + a set of character tables in the current locale for passing + to pcre_compile(). + + The function pcre_fullinfo() is used to find out information + about a compiled pattern; pcre_info() is an obsolete version + which returns only some of the available information, but is + retained for backwards compatibility. The function + pcre_version() returns a pointer to a string containing the + version of PCRE and its date of release. + + The global variables pcre_malloc and pcre_free initially + contain the entry points of the standard malloc() and free() + functions respectively. PCRE calls the memory management + functions via these variables, so a calling program can + replace them if it wishes to intercept the calls. This + should be done before calling any PCRE functions. + + + +MULTI-THREADING + The PCRE functions can be used in multi-threading applica- + tions, with the proviso that the memory management functions + pointed to by pcre_malloc and pcre_free are shared by all + threads. + + The compiled form of a regular expression is not altered + during matching, so the same compiled pattern can safely be + used by several threads at once. + + + +COMPILING A PATTERN + The function pcre_compile() is called to compile a pattern + into an internal form. The pattern is a C string terminated + by a binary zero, and is passed in the argument pattern. A + pointer to a single block of memory that is obtained via + pcre_malloc is returned. This contains the compiled code and + related data. The pcre type is defined for 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. + + Although the compiled code of a PCRE regex is relocatable, + that is, it does not depend on memory location, the complete + pcre data block is not fully relocatable, because it con- + tains a copy of the tableptr argument, which is an address + (see below). + + The size of a compiled pattern is roughly proportional to + the length of the pattern string, except that each character + class (other than those containing just a single character, + negated or not) requires 33 bytes, and repeat quantifiers + with a minimum greater than one or a bounded maximum cause + the relevant portions of the compiled pattern to be repli- + cated. + + The options argument contains independent bits that affect + the compilation. It should be zero if no options are + required. Some of the options, in particular, those that are + compatible with Perl, can also be set and unset from within + the pattern (see the detailed description of regular expres- + sions below). For these options, the contents of the options + argument specifies their initial settings at the start of + compilation and execution. The PCRE_ANCHORED option can be + set at the time of matching as well as at compile time. + + If errptr is NULL, pcre_compile() returns NULL immediately. + Otherwise, if compilation of a pattern fails, pcre_compile() + returns NULL, and sets the variable pointed to by errptr to + point to a textual error message. The offset from the start + of the pattern to the character where the error was + discovered is placed in the variable pointed to by + erroffset, which must not be NULL. If it is, an immediate + error is given. + + If the final argument, tableptr, is NULL, PCRE uses a + default set of character tables which are built when it is + compiled, using the default C locale. Otherwise, tableptr + must be the result of a call to pcre_maketables(). See the + section on locale support below. + + This code fragment shows a typical straightforward call to + pcre_compile(): + + 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 */ + + The following option bits are defined in the header file: + + PCRE_ANCHORED + + If this bit is set, the pattern is forced to be "anchored", + that is, it is constrained to match only at the start of the + string which is being searched (the "subject string"). This + effect can also be achieved by appropriate constructs in the + pattern itself, which is the only way to do it in Perl. + + PCRE_CASELESS + + If this bit is set, letters in the pattern match both upper + and lower case letters. It is equivalent to Perl's /i + option. + + PCRE_DOLLAR_ENDONLY + + If this bit is set, a dollar metacharacter in the pattern + matches only at the end of the subject string. Without this + option, a dollar also matches immediately before the final + character if it is a newline (but not before any other new- + lines). The PCRE_DOLLAR_ENDONLY option is ignored if + PCRE_MULTILINE is set. There is no equivalent to this option + in Perl. + + PCRE_DOTALL + + If this bit is set, a dot metacharater in the pattern + matches all characters, including newlines. Without it, new- + lines are excluded. This option is equivalent to Perl's /s + option. A negative class such as [^a] always matches a new- + line character, independent of the setting of this option. + + PCRE_EXTENDED + + If this bit is set, whitespace data characters in the pat- + tern are totally ignored except when escaped or inside a + character class, and characters between an unescaped # out- + side a character class and the next newline character, + inclusive, are also ignored. This is equivalent to Perl's /x + option, and makes it possible to include comments inside + complicated patterns. Note, however, that this applies only + to data characters. Whitespace characters may never appear + within special character sequences in a pattern, for example + within the sequence (?( which introduces a conditional sub- + pattern. + + PCRE_EXTRA + + This option was invented in order to turn on additional + functionality of PCRE that is incompatible with Perl, but it + is currently of very little use. When set, any backslash in + a pattern that is followed by a letter that has no special + meaning causes an error, thus reserving these combinations + for future expansion. By default, as in Perl, a backslash + followed by a letter with no special meaning is treated as a + literal. There are at present no other features controlled + by this option. It can also be set by a (?X) option setting + within a pattern. + + PCRE_MULTILINE + + By default, PCRE treats the subject string as consisting of + a single "line" of characters (even if it actually contains + several newlines). The "start of line" metacharacter (^) + matches only at the start of the string, while the "end of + line" metacharacter ($) matches only at the end of the + string, or before a terminating newline (unless + PCRE_DOLLAR_ENDONLY is set). This is the same as Perl. + + When PCRE_MULTILINE it is set, the "start of line" and "end + of line" constructs match immediately following or immedi- + ately before any newline in the subject string, respec- + tively, as well as at the very start and end. This is + equivalent to Perl's /m option. If there are no "\n" charac- + ters in a subject string, or no occurrences of ^ or $ in a + pattern, setting PCRE_MULTILINE has no effect. + + PCRE_UNGREEDY + + This option inverts the "greediness" of the quantifiers so + that they are not greedy by default, but become greedy if + followed by "?". It is not compatible with Perl. It can also + be set by a (?U) option setting within the pattern. + + PCRE_UTF8 + + This option causes PCRE to regard both the pattern and the + subject as strings of UTF-8 characters instead of just byte + strings. However, it is available only if PCRE has been + built to include UTF-8 support. If not, the use of this + option provokes an error. Support for UTF-8 is new, experi- + mental, and incomplete. Details of exactly what it entails + are given below. + + + +STUDYING A PATTERN + When a pattern is going to be used several times, it is + worth spending more time analyzing it in order to speed up + the time taken for matching. The function pcre_study() takes + a pointer to a compiled pattern as its first argument, and + returns a pointer to a pcre_extra block (another typedef for + a structure with hidden contents) containing additional + information about the pattern; this can be passed to + pcre_exec(). If no additional information is available, NULL + is returned. + + The second argument contains option bits. At present, no + options are defined for pcre_study(), and this argument + should always be zero. + + The third argument for pcre_study() is a pointer to an error + message. If studying succeeds (even if no data is returned), + the variable it points to is set to NULL. Otherwise it + points to a textual error message. + + This is a typical call to pcre_study(): + + pcre_extra *pe; + pe = pcre_study( + re, /* result of pcre_compile() */ + 0, /* no options exist */ + &error); /* set to NULL or points to a message */ + + At present, studying a pattern is useful only for non- + anchored patterns that do not have a single fixed starting + character. A bitmap of possible starting characters is + created. + + + +LOCALE SUPPORT + PCRE handles caseless matching, and determines whether char- + acters are letters, digits, or whatever, by reference to a + set of tables. The library contains a default set of tables + which is created in the default C locale when PCRE is com- + piled. This is used when the final argument of + pcre_compile() is NULL, and is sufficient for many applica- + tions. + + An alternative set of tables can, however, be supplied. Such + tables are built by calling the pcre_maketables() function, + which has no arguments, in the relevant locale. The result + can then be passed to pcre_compile() as often as necessary. + For example, to build and use tables that are appropriate + for the French locale (where accented characters with codes + greater than 128 are treated as letters), the following code + could be used: + + setlocale(LC_CTYPE, "fr"); + tables = pcre_maketables(); + re = pcre_compile(..., tables); + + The tables are built in memory that is obtained via + pcre_malloc. The pointer that is passed to pcre_compile is + saved with the compiled pattern, and the same tables are + used via this pointer by pcre_study() and pcre_exec(). Thus + for any single pattern, compilation, studying and matching + all happen in the same locale, but different patterns can be + compiled in different locales. It is the caller's responsi- + bility to ensure that the memory containing the tables + remains available for as long as it is needed. + + + +INFORMATION ABOUT A PATTERN + The pcre_fullinfo() function returns information about a + compiled pattern. It replaces the obsolete pcre_info() func- + tion, which is nevertheless retained for backwards compabil- + ity (and is documented below). + + The first argument for pcre_fullinfo() is a pointer to the + compiled pattern. The second argument is the result of + pcre_study(), or NULL if the pattern was not studied. The + third argument specifies which piece of information is + required, while the fourth argument is a pointer to a vari- + able to receive the data. The yield of the function is zero + for success, or one of the following negative numbers: + + PCRE_ERROR_NULL the argument code was NULL + the argument where was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + PCRE_ERROR_BADOPTION the value of what was invalid + + Here is a typical call of pcre_fullinfo(), to obtain the + length of the compiled pattern: + + 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 */ + + The possible values for the third argument are defined in + pcre.h, and are as follows: + + PCRE_INFO_OPTIONS + + Return a copy of the options with which the pattern was com- + piled. The fourth argument should point to an unsigned long + int variable. These option bits are those specified in the + call to pcre_compile(), modified by any top-level option + settings within the pattern itself, and with the + PCRE_ANCHORED bit forcibly set if the form of the pattern + implies that it can match only at the start of a subject + string. + + PCRE_INFO_SIZE + + Return the size of the compiled pattern, that is, the value + that was passed as the argument to pcre_malloc() when PCRE + was getting memory in which to place the compiled data. The + fourth argument should point to a size_t variable. + + PCRE_INFO_CAPTURECOUNT + + Return the number of capturing subpatterns in the pattern. + The fourth argument should point to an int variable. + + PCRE_INFO_BACKREFMAX + + Return the number of the highest back reference in the pat- + tern. The fourth argument should point to an int variable. + Zero is returned if there are no back references. + + PCRE_INFO_FIRSTCHAR + + Return information about the first character of any matched + string, for a non-anchored pattern. If there is a fixed + first character, e.g. from a pattern such as + (cat|cow|coyote), it is returned in the integer pointed to + by where. Otherwise, if either + + (a) the pattern was compiled with the PCRE_MULTILINE option, + and every branch starts with "^", or + + (b) every branch of the pattern starts with ".*" and + PCRE_DOTALL is not set (if it were set, the pattern would be + anchored), + + -1 is returned, indicating that the pattern matches only at + the start of a subject string or after any "\n" within the + string. Otherwise -2 is returned. For anchored patterns, -2 + is returned. + + PCRE_INFO_FIRSTTABLE + + If the pattern was studied, and this resulted in the con- + struction of a 256-bit table indicating a fixed set of char- + acters for the first character in any matching string, a + pointer to the table is returned. Otherwise NULL is + returned. The fourth argument should point to an unsigned + char * variable. + + PCRE_INFO_LASTLITERAL + + For a non-anchored pattern, return the value of the right- + most literal character which must exist in any matched + string, other than at its start. The fourth argument should + point to an int variable. If there is no such character, or + if the pattern is anchored, -1 is returned. For example, for + the pattern /a\d+z\d+/ the returned value is 'z'. + + The pcre_info() function is now obsolete because its inter- + face is too restrictive to return all the available data + about a compiled pattern. New programs should use + pcre_fullinfo() instead. The yield of pcre_info() is the + number of capturing subpatterns, or one of the following + negative numbers: + + PCRE_ERROR_NULL the argument code was NULL + PCRE_ERROR_BADMAGIC the "magic number" was not found + + If the optptr argument is not NULL, a copy of the options + with which the pattern was compiled is placed in the integer + it points to (see PCRE_INFO_OPTIONS above). + + If the pattern is not anchored and the firstcharptr argument + is not NULL, it is used to pass back information about the + first character of any matched string (see + PCRE_INFO_FIRSTCHAR above). + + + +MATCHING A PATTERN + The function pcre_exec() is called to match a subject string + + + + + +SunOS 5.8 Last change: 9 + + + + against a pre-compiled pattern, which is passed in the code + argument. If the pattern has been studied, the result of the + study should be passed in the extra argument. Otherwise this + must be NULL. + + Here is an example of a simple call to pcre_exec(): + + 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 */ + + The PCRE_ANCHORED option can be passed in the options argu- + ment, whose unused bits must be zero. However, if a pattern + was compiled with PCRE_ANCHORED, or turned out to be + anchored by virtue of its contents, it cannot be made + unachored at matching time. + + There are also three further options that can be set only at + matching time: + + PCRE_NOTBOL + + The first character of the string is not the beginning of a + line, so the circumflex metacharacter should not match + before it. Setting this without PCRE_MULTILINE (at compile + time) causes circumflex never to match. + + PCRE_NOTEOL + + The end of the string is not the end of a line, so the dol- + lar metacharacter should not match it nor (except in multi- + line mode) a newline immediately before it. Setting this + without PCRE_MULTILINE (at compile time) causes dollar never + to match. + + PCRE_NOTEMPTY + + An empty string is not considered to be a valid match if + this option is set. If there are alternatives in the pat- + tern, they are tried. If all the alternatives match the + empty string, the entire match fails. For example, if the + pattern + + a?b? + + is applied to a string not beginning with "a" or "b", it + matches the empty string at the start of the subject. With + PCRE_NOTEMPTY set, this match is not valid, so PCRE searches + further into the string for occurrences of "a" or "b". + + Perl has no direct equivalent of PCRE_NOTEMPTY, but it does + make a special case of a pattern match of the empty string + within its split() function, and when using the /g modifier. + It is possible to emulate Perl's behaviour after matching a + null string by first trying the match again at the same + offset with PCRE_NOTEMPTY set, and then if that fails by + advancing the starting offset (see below) and trying an + ordinary match again. + + The subject string is passed as a pointer in subject, a + length in length, and a starting offset in startoffset. + Unlike the pattern string, 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. + + A non-zero starting offset is useful when searching for + another match in the same subject by calling pcre_exec() + again after a previous success. Setting startoffset differs + from just passing over a shortened string and setting + PCRE_NOTBOL in the case of a pattern that begins with any + kind of lookbehind. For example, consider the pattern + + \Biss\B + + which finds occurrences of "iss" in the middle of words. (\B + matches only if the current position in the subject is not a + word boundary.) When applied to the string "Mississipi" the + first call to pcre_exec() finds the first occurrence. If + pcre_exec() is called again with just the remainder of the + subject, namely "issipi", it does not match, because \B is + always false at the start of the subject, which is deemed to + be a word boundary. However, if pcre_exec() is passed the + entire string again, but with startoffset set to 4, it finds + the second occurrence of "iss" because it is able to look + behind the starting point to discover that it is preceded by + a letter. + + If a non-zero starting offset is passed when the pattern is + anchored, one attempt to match at the given offset is tried. + This can only succeed if the pattern does not require the + match to be at the start of the subject. + + In general, a pattern matches a certain portion of the sub- + ject, and in addition, further substrings from the subject + may be picked out by parts of the pattern. Following the + usage in Jeffrey Friedl's book, this is called "capturing" + in what follows, and the phrase "capturing subpattern" is + used for a fragment of a pattern that picks out a substring. + PCRE supports several other kinds of parenthesized subpat- + tern that do not cause substrings to be captured. + + Captured substrings are returned to the caller via a vector + of integer offsets whose address is passed in ovector. The + number of elements in the vector is passed in ovecsize. The + first two-thirds of the vector is used to pass back captured + substrings, each substring using a pair of integers. The + remaining third of the vector is used as workspace by + pcre_exec() while matching capturing subpatterns, and is not + available for passing back information. The length passed in + ovecsize should always be a multiple of three. If it is not, + it is rounded down. + + When a match has been successful, information about captured + substrings is returned in pairs of integers, starting at the + beginning of ovector, and continuing up to two-thirds of its + length at the most. The first element of a pair is set to + the offset of the first character in a substring, and the + second is set to the offset of the first character after the + end of a substring. The first pair, ovector[0] and ovec- + tor[1], identify the portion of the subject string matched + by the entire pattern. The next pair is used for the first + capturing subpattern, and so on. The value returned by + pcre_exec() is the number of pairs that have been set. If + there are no capturing subpatterns, the return value from a + successful match is 1, indicating that just the first pair + of offsets has been set. + + Some convenience functions are provided for extracting the + captured substrings as separate strings. These are described + in the following section. + + It is possible for an capturing subpattern number n+1 to + match some part of the subject when subpattern n has not + been used at all. For example, if the string "abc" is + matched against the pattern (a|(z))(bc) subpatterns 1 and 3 + are matched, but 2 is not. When this happens, both offset + values corresponding to the unused subpattern are set to -1. + + If a capturing subpattern is matched repeatedly, it is the + last portion of the string that it matched that gets + returned. + + If the vector is too small to hold all the captured sub- + strings, it is used as far as possible (up to two-thirds of + its length), and the function returns a value of zero. In + particular, if the substring offsets are not of interest, + pcre_exec() may be called with ovector passed as NULL and + ovecsize as zero. However, if the pattern contains back + references and the ovector isn't big enough to remember the + related substrings, PCRE has to get additional memory for + use during matching. Thus it is usually advisable to supply + an ovector. + + Note that pcre_info() can be used to find out how many cap- + turing subpatterns there are in a compiled pattern. The + smallest size for ovector that will allow for n captured + substrings in addition to the offsets of the substring + matched by the whole pattern is (n+1)*3. + + If pcre_exec() fails, it returns a negative number. The fol- + lowing are defined in the header file: + + PCRE_ERROR_NOMATCH (-1) + + The subject string did not match the pattern. + + PCRE_ERROR_NULL (-2) + + Either code or subject was passed as NULL, or ovector was + NULL and ovecsize was not zero. + + PCRE_ERROR_BADOPTION (-3) + + An unrecognized bit was set in the options argument. + + PCRE_ERROR_BADMAGIC (-4) + + PCRE stores a 4-byte "magic number" at the start of the com- + piled code, to catch the case when it is passed a junk + pointer. This is the error it gives when the magic number + isn't present. + + PCRE_ERROR_UNKNOWN_NODE (-5) + + While running the pattern match, an unknown item was encoun- + tered in the compiled pattern. This error could be caused by + a bug in PCRE or by overwriting of the compiled pattern. + + PCRE_ERROR_NOMEMORY (-6) + + If a pattern contains back references, but the ovector that + is passed to pcre_exec() is not big enough to remember the + referenced substrings, PCRE gets a block of memory at the + start of matching to use for this purpose. If the call via + pcre_malloc() fails, this error is given. The memory is + freed at the end of matching. + + + + +EXTRACTING CAPTURED SUBSTRINGS + Captured substrings can be accessed directly by using the + offsets returned by pcre_exec() in ovector. For convenience, + the functions pcre_copy_substring(), pcre_get_substring(), + and pcre_get_substring_list() are provided for extracting + captured substrings as new, separate, zero-terminated + strings. A substring that contains a binary zero is + correctly extracted and has a further zero added on the end, + but the result does not, of course, function as a C string. + + The first three arguments are the same for all three func- + tions: subject is the subject string which has just been + successfully matched, ovector is a pointer to the vector of + integer offsets that was passed to pcre_exec(), and + stringcount is the number of substrings that were captured + by the match, including the substring that matched the + entire regular expression. This is the value returned by + pcre_exec if it is greater than zero. If pcre_exec() + returned zero, indicating that it ran out of space in ovec- + tor, the value passed as stringcount should be the size of + the vector divided by three. + + The functions pcre_copy_substring() and pcre_get_substring() + extract a single substring, whose number is given as string- + number. A value of zero extracts the substring that matched + the entire pattern, while higher values extract the captured + substrings. For pcre_copy_substring(), the string is placed + in buffer, whose length is given by buffersize, while for + pcre_get_substring() a new block of memory is obtained via + pcre_malloc, and its address is returned via stringptr. The + yield of the function is the length of the string, not + including the terminating zero, or one of + + PCRE_ERROR_NOMEMORY (-6) + + The buffer was too small for pcre_copy_substring(), or the + attempt to get memory failed for pcre_get_substring(). + + PCRE_ERROR_NOSUBSTRING (-7) + + There is no substring whose number is stringnumber. + + The pcre_get_substring_list() function extracts all avail- + able substrings and builds a list of pointers to them. All + this is done in a single block of memory which is obtained + via pcre_malloc. The address of the memory block is returned + via listptr, which is also the start of the list of string + pointers. The end of the list is marked by a NULL pointer. + The yield of the function is zero if all went well, or + + PCRE_ERROR_NOMEMORY (-6) + + if the attempt to get the memory block failed. + + When any of these functions encounter a substring that is + unset, which can happen when capturing subpattern number n+1 + matches some part of the subject, but subpattern n has not + been used at all, they return an empty string. This can be + distinguished from a genuine zero-length substring by + inspecting the appropriate offset in ovector, which is nega- + tive for unset substrings. + + The two convenience functions pcre_free_substring() and + pcre_free_substring_list() can be used to free the memory + returned by a previous call of pcre_get_substring() or + pcre_get_substring_list(), respectively. They do nothing + more than call the function pointed to by pcre_free, which + of course could be called directly from a C program. How- + ever, PCRE is used in some situations where it is linked via + a special interface to another programming language which + cannot use pcre_free directly; it is for these cases that + the functions are provided. + + + +LIMITATIONS + There are some size limitations in PCRE but it is hoped that + they will never in practice be relevant. The maximum length + of a compiled pattern is 65539 (sic) bytes. All values in + repeating quantifiers must be less than 65536. There max- + imum 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 sub- + pattern, including capturing subpatterns, assertions, and + other types of subpattern, is 200. + + The maximum length of a subject string is the largest posi- + tive number that an integer variable can hold. However, PCRE + uses recursion to handle subpatterns and indefinite repeti- + tion. This means that the available stack space may limit + the size of a subject string that can be processed by cer- + tain patterns. + + + +DIFFERENCES FROM PERL + The differences described here are with respect to Perl + 5.005. + + 1. By default, a whitespace character is any character that + the C library function isspace() recognizes, though it is + possible to compile PCRE with alternative character type + tables. Normally isspace() matches space, formfeed, newline, + carriage return, horizontal tab, and vertical tab. Perl 5 no + longer includes vertical tab in its set of whitespace char- + acters. The \v escape that was in the Perl documentation for + a long time was never in fact recognized. However, the char- + acter itself was treated as whitespace at least up to 5.002. + In 5.004 and 5.005 it does not match \s. + + 2. PCRE does not allow repeat quantifiers on lookahead + assertions. Perl permits them, but they do not mean what you + might think. For example, (?!a){3} does not assert that the + next three characters are not "a". It just asserts that the + next character is not "a" three times. + + 3. Capturing subpatterns that occur inside negative looka- + head assertions are counted, but their entries in the + offsets vector are never set. Perl sets its numerical vari- + ables from any such patterns that are matched before the + assertion fails to match something (thereby succeeding), but + only if the negative lookahead assertion contains just one + branch. + + 4. Though binary zero characters are supported in the sub- + ject string, they are not allowed in a pattern string + because it is passed as a normal C string, terminated by + zero. The escape sequence "\0" can be used in the pattern to + represent a binary zero. + + 5. The following Perl escape sequences are not supported: + \l, \u, \L, \U, \E, \Q. In fact these are implemented by + Perl's general string-handling and are not part of its pat- + tern matching engine. + + 6. The Perl \G assertion is not supported as it is not + relevant to single pattern matches. + + 7. Fairly obviously, PCRE does not support the (?{code}) and + (?p{code}) constructions. However, there is some experimen- + tal support for recursive patterns using the non-Perl item + (?R). + + 8. There are at the time of writing some oddities in Perl + 5.005_02 concerned with the settings of captured strings + when part of a pattern is repeated. For example, matching + "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value + "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 + unset. However, if the pattern is changed to + /^(aa(b(b))?)+$/ then $2 (and $3) are set. + + In Perl 5.004 $2 is set in both cases, and that is also true + of PCRE. If in the future Perl changes to a consistent state + that is different, PCRE may change to follow. + + 9. Another as yet unresolved discrepancy is that in Perl + 5.005_02 the pattern /^(a)?(?(1)a|b)+$/ matches the string + "a", whereas in PCRE it does not. However, in both Perl and + PCRE /^(a)?a/ matched against "a" leaves $1 unset. + + 10. PCRE provides some extensions to the Perl regular + expression facilities: + + (a) Although lookbehind assertions must match fixed length + strings, each alternative branch of a lookbehind assertion + can match a different length of string. Perl 5.005 requires + them all to have the same length. + + (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not + set, the $ meta- character matches only at the very end of + the string. + + (c) If PCRE_EXTRA is set, a backslash followed by a letter + with no special meaning is faulted. + + (d) If PCRE_UNGREEDY is set, the greediness of the repeti- + tion quantifiers is inverted, that is, by default they are + not greedy, but if followed by a question mark they are. + + (e) PCRE_ANCHORED can be used to force a pattern to be tried + only at the start of the subject. + + (f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options + for pcre_exec() have no Perl equivalents. + + (g) The (?R) construct allows for recursive pattern matching + (Perl 5.6 can do this using the (?p{code}) construct, which + PCRE cannot of course support.) + + + +REGULAR EXPRESSION DETAILS + The syntax and semantics of the regular expressions sup- + ported by PCRE are described below. Regular expressions are + also described in the Perl documentation and in a number of + other books, some of which have copious examples. Jeffrey + Friedl's "Mastering Regular Expressions", published by + O'Reilly (ISBN 1-56592-257), covers them in great detail. + + The description here is intended as reference documentation. + The basic operation of PCRE is on strings of bytes. However, + there is the beginnings of some support for UTF-8 character + strings. To use this support you must configure PCRE to + include it, and then call pcre_compile() with the PCRE_UTF8 + option. How this affects the pattern matching is described + in the final section of this document. + + A regular expression is a pattern that is matched against a + subject string from left to right. Most characters stand for + themselves in a pattern, and match the corresponding charac- + ters in the subject. As a trivial example, the pattern + + The quick brown fox + + matches a portion of a subject string that is identical to + itself. The power of regular expressions comes from the + ability to include alternatives and repetitions in the pat- + tern. These are encoded in the pattern by the use of meta- + characters, which do not stand for themselves but instead + are interpreted in some special way. + + There are two different sets of meta-characters: those that + are recognized anywhere in the pattern except within square + brackets, and those that are recognized in square brackets. + Outside square brackets, the meta-characters are as follows: + + \ general escape character with several uses + ^ assert start of subject (or line, in multiline + mode) + $ assert end of subject (or line, in multiline mode) + . match any character except newline (by default) + [ start character class definition + | start of alternative branch + ( start subpattern + ) end subpattern + ? extends the meaning of ( + also 0 or 1 quantifier + also quantifier minimizer + * 0 or more quantifier + + 1 or more quantifier + { start min/max quantifier + + Part of a pattern that is in square brackets is called a + "character class". In a character class the only meta- + characters are: + + \ general escape character + ^ negate the class, but only if the first character + - indicates character range + ] terminates the character class + + The following sections describe the use of each of the + meta-characters. + + + +BACKSLASH + The backslash character has several uses. Firstly, if it is + followed by a non-alphameric character, it takes away any + special meaning that character may have. This use of + + backslash as an escape character applies both inside and + outside character classes. + + For example, if you want to match a "*" character, you write + "\*" in the pattern. This applies whether or not the follow- + ing character would otherwise be interpreted as a meta- + character, so it is always safe to precede a non-alphameric + with "\" to specify that it stands for itself. In particu- + lar, if you want to match a backslash, you write "\\". + + If a pattern is compiled with the PCRE_EXTENDED option, whi- + tespace in the pattern (other than in a character class) and + characters between a "#" outside a character class and the + next newline character are ignored. An escaping backslash + can be used to include a whitespace or "#" character as part + of the pattern. + + A second use of backslash provides a way of encoding non- + printing characters in patterns in a visible manner. There + is no restriction on the appearance of non-printing charac- + ters, apart from the binary zero that terminates a pattern, + but when a pattern is being prepared by text editing, it is + usually easier to use one of the following escape sequences + than the binary character it represents: + + \a alarm, that is, the BEL character (hex 07) + \cx "control-x", where x is any character + \e escape (hex 1B) + \f formfeed (hex 0C) + \n newline (hex 0A) + \r carriage return (hex 0D) + \t tab (hex 09) + \xhh character with hex code hh + \ddd character with octal code ddd, or backreference + + The precise effect of "\cx" is as follows: if "x" is a lower + case letter, it is converted to upper case. Then bit 6 of + the character (hex 40) is inverted. Thus "\cz" becomes hex + 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B. + + After "\x", up to two hexadecimal digits are read (letters + can be in upper or lower case). + + After "\0" up to two further octal digits are read. In both + cases, if there are fewer than two digits, just those that + are present are used. Thus the sequence "\0\x\07" specifies + two binary zeros followed by a BEL character. Make sure you + supply two digits after the initial zero if the character + that follows is itself an octal digit. + + The handling of a backslash followed by a digit other than 0 + is complicated. Outside a character class, PCRE reads it + and any following digits as a decimal number. If the number + is less than 10, or if there have been at least that many + previous capturing left parentheses in the expression, the + entire sequence is taken as a back reference. A description + of how this works is given later, following the discussion + of parenthesized subpatterns. + + Inside a character class, or if the decimal number is + greater than 9 and there have not been that many capturing + subpatterns, PCRE re-reads up to three octal digits follow- + ing the backslash, and generates a single byte from the + least significant 8 bits of the value. Any subsequent digits + stand for themselves. For example: + + \040 is another way of writing a space + \40 is the same, provided there are fewer than 40 + previous capturing subpatterns + \7 is always a back reference + \11 might be a back reference, or another way of + writing a tab + \011 is always a tab + \0113 is a tab followed by the character "3" + \113 is the character with octal code 113 (since there + can be no more than 99 back references) + \377 is a byte consisting entirely of 1 bits + \81 is either a back reference, or a binary zero + followed by the two characters "8" and "1" + + Note that octal values of 100 or greater must not be intro- + duced by a leading zero, because no more than three octal + digits are ever read. + + All the sequences that define a single byte value can be + used both inside and outside character classes. In addition, + inside a character class, the sequence "\b" is interpreted + as the backspace character (hex 08). Outside a character + class it has a different meaning (see below). + + The third use of backslash is for specifying generic charac- + ter types: + + \d any decimal digit + \D any character that is not a decimal digit + \s any whitespace character + \S any character that is not a whitespace character + \w any "word" character + \W any "non-word" character + + Each pair of escape sequences partitions the complete set of + characters into two disjoint sets. Any given character + matches one, and only one, of each pair. + + A "word" character is any letter or digit or the underscore + character, that is, any character which can be part of a + Perl "word". The definition of letters and digits is con- + trolled by PCRE's character tables, and may vary if locale- + specific matching is taking place (see "Locale support" + above). For example, in the "fr" (French) locale, some char- + acter codes greater than 128 are used for accented letters, + and these are matched by \w. + + These character type sequences can appear both inside and + outside character classes. They each match one character of + the appropriate type. If the current matching point is at + the end of the subject string, all of them fail, since there + is no character to match. + + The fourth use of backslash is for certain simple asser- + tions. An assertion specifies a condition that has to be met + at a particular point in a match, without consuming any + characters from the subject string. The use of subpatterns + for more complicated assertions is described below. The + backslashed assertions are + + \b word boundary + \B not a word boundary + \A start of subject (independent of multiline mode) + \Z end of subject or newline at end (independent of + multiline mode) + \z end of subject (independent of multiline mode) + + These assertions may not appear in character classes (but + note that "\b" has a different meaning, namely the backspace + character, inside a character class). + + A word boundary is a position in the subject string where + the current character and the previous character do not both + match \w or \W (i.e. one matches \w and the other matches + \W), or the start or end of the string if the first or last + character matches \w, respectively. + + The \A, \Z, and \z assertions differ from the traditional + circumflex and dollar (described below) in that they only + ever match at the very start and end of the subject string, + whatever options are set. They are not affected by the + PCRE_NOTBOL or PCRE_NOTEOL options. If the startoffset argu- + ment of pcre_exec() is non-zero, \A can never match. The + difference between \Z and \z is that \Z matches before a + newline that is the last character of the string as well as + at the end of the string, whereas \z matches only at the + end. + + + +CIRCUMFLEX AND DOLLAR + Outside a character class, in the default matching mode, the + circumflex character is an assertion which is true only if + the current matching point is at the start of the subject + string. If the startoffset argument of pcre_exec() is non- + zero, circumflex can never match. Inside a character class, + circumflex has an entirely different meaning (see below). + + Circumflex need not be the first character of the pattern if + a number of alternatives are involved, but it should be the + first thing in each alternative in which it appears if the + pattern is ever to match that branch. If all possible alter- + natives start with a circumflex, that is, if the pattern is + constrained to match only at the start of the subject, it is + said to be an "anchored" pattern. (There are also other con- + structs that can cause a pattern to be anchored.) + + A dollar character is an assertion which is true only if the + current matching point is at the end of the subject string, + or immediately before a newline character that is the last + character in the string (by default). Dollar need not be the + last character of the pattern if a number of alternatives + are involved, but it should be the last item in any branch + in which it appears. Dollar has no special meaning in a + character class. + + The meaning of dollar can be changed so that it matches only + at the very end of the string, by setting the + PCRE_DOLLAR_ENDONLY option at compile or matching time. This + does not affect the \Z assertion. + + The meanings of the circumflex and dollar characters are + changed if the PCRE_MULTILINE option is set. When this is + the case, they match immediately after and immediately + before an internal "\n" character, respectively, in addition + to matching at the start and end of the subject string. For + example, the pattern /^abc$/ matches the subject string + "def\nabc" in multiline mode, but not otherwise. Conse- + quently, patterns that are anchored in single line mode + because all branches start with "^" are not anchored in mul- + tiline mode, and a match for circumflex is possible when the + startoffset argument of pcre_exec() is non-zero. The + PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is + set. + + Note that the sequences \A, \Z, and \z can be used to match + the start and end of the subject in both modes, and if all + branches of a pattern start with \A it is always anchored, + whether PCRE_MULTILINE is set or not. + + + +FULL STOP (PERIOD, DOT) + Outside a character class, a dot in the pattern matches any + one character in the subject, including a non-printing char- + acter, but not (by default) newline. If the PCRE_DOTALL + option is set, dots match newlines as well. The handling of + dot is entirely independent of the handling of circumflex + and dollar, the only relationship being that they both + involve newline characters. Dot has no special meaning in a + character class. + + + +SQUARE BRACKETS + An opening square bracket introduces a character class, ter- + minated by a closing square bracket. A closing square + bracket on its own is not special. If a closing square + bracket is required as a member of the class, it should be + the first data character in the class (after an initial cir- + cumflex, if present) or escaped with a backslash. + + A character class matches a single character in the subject; + the character must be in the set of characters defined by + the class, unless the first character in the class is a cir- + cumflex, in which case the subject character must not be in + the set defined by the class. If a circumflex is actually + required as a member of the class, ensure it is not the + first character, or escape it with a backslash. + + For example, the character class [aeiou] matches any lower + case vowel, while [^aeiou] matches any character that is not + a lower case vowel. Note that a circumflex is just a con- + venient notation for specifying the characters which are in + the class by enumerating those that are not. It is not an + assertion: it still consumes a character from the subject + string, and fails if the current pointer is at the end of + the string. + + When caseless matching is set, any letters in a class + represent both their upper case and lower case versions, so + for example, a caseless [aeiou] matches "A" as well as "a", + and a caseless [^aeiou] does not match "A", whereas a case- + ful version would. + + The newline character is never treated in any special way in + character classes, whatever the setting of the PCRE_DOTALL + or PCRE_MULTILINE options is. A class such as [^a] will + always match a newline. + + The minus (hyphen) character can be used to specify a range + of characters in a character class. For example, [d-m] + matches any letter between d and m, inclusive. If a minus + character is required in a class, it must be escaped with a + backslash or appear in a position where it cannot be inter- + preted as indicating a range, typically as the first or last + character in the class. + + It is not possible to have the literal character "]" as the + end character of a range. A pattern such as [W-]46] is + interpreted as a class of two characters ("W" and "-") fol- + lowed by a literal string "46]", so it would match "W46]" or + "-46]". However, if the "]" is escaped with a backslash it + is interpreted as the end of range, so [W-\]46] is inter- + preted as a single class containing a range followed by two + separate characters. The octal or hexadecimal representation + of "]" can also be used to end a range. + + Ranges operate in ASCII collating sequence. They can also be + used for characters specified numerically, for example + [\000-\037]. If a range that includes letters is used when + caseless matching is set, it matches the letters in either + case. For example, [W-c] is equivalent to [][\^_`wxyzabc], + matched caselessly, and if character tables for the "fr" + locale are in use, [\xc8-\xcb] matches accented E characters + in both cases. + + The character types \d, \D, \s, \S, \w, and \W may also + appear in a character class, and add the characters that + they match to the class. For example, [\dABCDEF] matches any + hexadecimal digit. A circumflex can conveniently be used + with the upper case character types to specify a more res- + tricted set of characters than the matching lower case type. + For example, the class [^\W_] matches any letter or digit, + but not underscore. + + All non-alphameric characters other than \, -, ^ (at the + start) and the terminating ] are non-special in character + classes, but it does no harm if they are escaped. + + + +POSIX CHARACTER CLASSES + Perl 5.6 (not yet released at the time of writing) is going + to support the POSIX notation for character classes, which + uses names enclosed by [: and :] within the enclosing + square brackets. PCRE supports this notation. For example, + + [01[:alpha:]%] + + matches "0", "1", any alphabetic character, or "%". The sup- + ported class names are + + alnum letters and digits + alpha letters + ascii character codes 0 - 127 + cntrl control characters + digit decimal digits (same as \d) + graph printing characters, excluding space + lower lower case letters + print printing characters, including space + punct printing characters, excluding letters and digits + space white space (same as \s) + upper upper case letters + word "word" characters (same as \w) + xdigit hexadecimal digits + + The names "ascii" and "word" are Perl extensions. Another + Perl extension is negation, which is indicated by a ^ char- + acter after the colon. For example, + + [12[:^digit:]] + + matches "1", "2", or any non-digit. PCRE (and Perl) also + 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. + + + +VERTICAL BAR + Vertical bar characters are used to separate alternative + patterns. For example, the pattern + + gilbert|sullivan + + matches either "gilbert" or "sullivan". Any number of alter- + natives may appear, and an empty alternative is permitted + (matching the empty string). The matching process tries + each alternative in turn, from left to right, and the first + one that succeeds is used. If the alternatives are within a + subpattern (defined below), "succeeds" means matching the + rest of the main pattern as well as the alternative in the + subpattern. + + + +INTERNAL OPTION SETTING + The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, + and PCRE_EXTENDED can be changed from within the pattern by + a sequence of Perl option letters enclosed between "(?" and + ")". The option letters are + + i for PCRE_CASELESS + m for PCRE_MULTILINE + s for PCRE_DOTALL + x for PCRE_EXTENDED + + For example, (?im) sets caseless, multiline matching. It is + also possible to unset these options by preceding the letter + with a hyphen, and a combined setting and unsetting such as + (?im-sx), which sets PCRE_CASELESS and PCRE_MULTILINE while + unsetting PCRE_DOTALL and PCRE_EXTENDED, is also permitted. + If a letter appears both before and after the hyphen, the + option is unset. + + The scope of these option changes depends on where in the + pattern the setting occurs. For settings that are outside + any subpattern (defined below), the effect is the same as if + the options were set or unset at the start of matching. The + following patterns all behave in exactly the same way: + + (?i)abc + a(?i)bc + ab(?i)c + abc(?i) + + which in turn is the same as compiling the pattern abc with + PCRE_CASELESS set. In other words, such "top level" set- + tings apply to the whole pattern (unless there are other + changes inside subpatterns). If there is more than one set- + ting of the same option at top level, the rightmost setting + is used. + + If an option change occurs inside a subpattern, the effect + is different. This is a change of behaviour in Perl 5.005. + An option change inside a subpattern affects only that part + of the subpattern that follows it, so + + (a(?i)b)c + + matches abc and aBc and no other strings (assuming + PCRE_CASELESS is not used). By this means, options can be + made to have different settings in different parts of the + pattern. Any changes made in one alternative do carry on + into subsequent branches within the same subpattern. For + example, + + (a(?i)b|c) + + matches "ab", "aB", "c", and "C", even though when matching + "C" the first branch is abandoned before the option setting. + This is because the effects of option settings happen at + compile time. There would be some very weird behaviour oth- + erwise. + + The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can + be changed in the same way as the Perl-compatible options by + using the characters U and X respectively. The (?X) flag + setting is special in that it must always occur earlier in + the pattern than any of the additional features it turns on, + even when it is at top level. It is best put at the start. + + + +SUBPATTERNS + Subpatterns are delimited by parentheses (round brackets), + which can be nested. Marking part of a pattern as a subpat- + tern does two things: + + 1. It localizes a set of alternatives. For example, the pat- + tern + + cat(aract|erpillar|) + + matches one of the words "cat", "cataract", or "caterpil- + lar". Without the parentheses, it would match "cataract", + "erpillar" or the empty string. + + 2. It sets up the subpattern as a capturing subpattern (as + defined above). When the whole pattern matches, that por- + tion of the subject string that matched the subpattern is + passed back to the caller via the ovector argument of + pcre_exec(). Opening parentheses are counted from left to + right (starting from 1) to obtain the numbers of the captur- + ing subpatterns. + + For example, if the string "the red king" is matched against + the pattern + + the ((red|white) (king|queen)) + + the captured substrings are "red king", "red", and "king", + and are numbered 1, 2, and 3, respectively. + + The fact that plain parentheses fulfil two functions is not + always helpful. There are often times when a grouping sub- + pattern is required without a capturing requirement. If an + opening parenthesis is followed by "?:", the subpattern does + not do any capturing, and is not counted when computing the + number of any subsequent capturing subpatterns. For example, + if the string "the white queen" is matched against the pat- + tern + + the ((?:red|white) (king|queen)) + + the captured substrings are "white queen" and "queen", and + are numbered 1 and 2. The maximum number of captured sub- + strings is 99, and the maximum number of all subpatterns, + both capturing and non-capturing, is 200. + + As a convenient shorthand, if any option settings are + required at the start of a non-capturing subpattern, the + option letters may appear between the "?" and the ":". Thus + the two patterns + + (?i:saturday|sunday) + (?:(?i)saturday|sunday) + + match exactly the same set of strings. Because alternative + branches are tried from left to right, and options are not + reset until the end of the subpattern is reached, an option + setting in one branch does affect subsequent branches, so + the above patterns match "SUNDAY" as well as "Saturday". + + + +REPETITION + Repetition is specified by quantifiers, which can follow any + of the following items: + + a single character, possibly escaped + the . metacharacter + a character class + a back reference (see next section) + a parenthesized subpattern (unless it is an assertion - + see below) + + The general repetition quantifier specifies a minimum and + maximum number of permitted matches, by giving the two + numbers in curly brackets (braces), separated by a comma. + The numbers must be less than 65536, and the first must be + less than or equal to the second. For example: + + z{2,4} + + matches "zz", "zzz", or "zzzz". A closing brace on its own + is not a special character. If the second number is omitted, + but the comma is present, there is no upper limit; if the + second number and the comma are both omitted, the quantifier + specifies an exact number of required matches. Thus + + [aeiou]{3,} + + matches at least 3 successive vowels, but may match many + more, while + + \d{8} + + matches exactly 8 digits. An opening curly bracket that + appears in a position where a quantifier is not allowed, or + one that does not match the syntax of a quantifier, is taken + as a literal character. For example, {,6} is not a quantif- + ier, but a literal string of four characters. + The quantifier {0} is permitted, causing the expression to + behave as if the previous item and the quantifier were not + present. + + For convenience (and historical compatibility) the three + most common quantifiers have single-character abbreviations: + + * is equivalent to {0,} + + is equivalent to {1,} + ? is equivalent to {0,1} + + It is possible to construct infinite loops by following a + subpattern that can match no characters with a quantifier + that has no upper limit, for example: + + (a?)* + + Earlier versions of Perl and PCRE used to give an error at + compile time for such patterns. However, because there are + cases where this can be useful, such patterns are now + accepted, but if any repetition of the subpattern does in + fact match no characters, the loop is forcibly broken. + + By default, the quantifiers are "greedy", that is, they + match as much as possible (up to the maximum number of per- + mitted times), without causing the rest of the pattern to + fail. The classic example of where this gives problems is in + trying to match comments in C programs. These appear between + the sequences /* and */ and within the sequence, individual + * and / characters may appear. An attempt to match C com- + ments by applying the pattern + + /\*.*\*/ + + to the string + + /* first command */ not comment /* second comment */ + + fails, because it matches the entire string owing to the + greediness of the .* item. + + However, if a quantifier is followed by a question mark, it + ceases to be greedy, and instead matches the minimum number + of times possible, so the pattern + + /\*.*?\*/ + + does the right thing with the C comments. The meaning of the + various quantifiers is not otherwise changed, just the pre- + ferred number of matches. Do not confuse this use of ques- + tion mark with its use as a quantifier in its own right. + Because it has two uses, it can sometimes appear doubled, as + in + + \d??\d + + which matches one digit by preference, but can match two if + that is the only way the rest of the pattern matches. + + If the PCRE_UNGREEDY option is set (an option which is not + available in Perl), the quantifiers are not greedy by + default, but individual ones can be made greedy by following + them with a question mark. In other words, it inverts the + default behaviour. + + When a parenthesized subpattern is quantified with a minimum + repeat count that is greater than 1 or with a limited max- + imum, more store is required for the compiled pattern, in + proportion to the size of the minimum or maximum. + + If a pattern starts with .* or .{0,} and the PCRE_DOTALL + option (equivalent to Perl's /s) is set, thus allowing the . + to match newlines, the pattern is implicitly anchored, + because whatever follows will be tried against every charac- + ter position in the subject string, so there is no point in + retrying the overall match at any position after the first. + PCRE treats such a pattern as though it were preceded by \A. + In cases where it is known that the subject string contains + no newlines, it is worth setting PCRE_DOTALL when the pat- + tern begins with .* in order to obtain this optimization, or + alternatively using ^ to indicate anchoring explicitly. + + When a capturing subpattern is repeated, the value captured + is the substring that matched the final iteration. For exam- + ple, after + + (tweedle[dume]{3}\s*)+ + + has matched "tweedledum tweedledee" the value of the cap- + tured substring is "tweedledee". However, if there are + nested capturing subpatterns, the corresponding captured + values may have been set in previous iterations. For exam- + ple, after + + /(a|(b))+/ + + matches "aba" the value of the second captured substring is + "b". + + + +BACK REFERENCES + Outside a character class, a backslash followed by a digit + greater than 0 (and possibly further digits) is a back + + + + +SunOS 5.8 Last change: 30 + + + + reference to a capturing subpattern earlier (i.e. to its + left) in the pattern, provided there have been that many + previous capturing left parentheses. + + However, if the decimal number following the backslash is + less than 10, it is always taken as a back reference, and + causes an error only if there are not that many capturing + left parentheses in the entire pattern. In other words, the + parentheses that are referenced need not be to the left of + the reference for numbers less than 10. See the section + entitled "Backslash" above for further details of the han- + dling of digits following a backslash. + + A back reference matches whatever actually matched the cap- + turing subpattern in the current subject string, rather than + anything matching the subpattern itself. So the pattern + + (sens|respons)e and \1ibility + + matches "sense and sensibility" and "response and responsi- + bility", but not "sense and responsibility". If caseful + matching is in force at the time of the back reference, the + case of letters is relevant. For example, + + ((?i)rah)\s+\1 + + matches "rah rah" and "RAH RAH", but not "RAH rah", even + though the original capturing subpattern is matched case- + lessly. + + There may be more than one back reference to the same sub- + pattern. If a subpattern has not actually been used in a + particular match, any back references to it always fail. For + example, the pattern + + (a|(bc))\2 + + always fails if it starts to match "a" rather than "bc". + Because there may be up to 99 back references, all digits + following the backslash are taken as part of a potential + back reference number. If the pattern continues with a digit + character, some delimiter must be used to terminate the back + reference. If the PCRE_EXTENDED option is set, this can be + whitespace. Otherwise an empty comment can be used. + + A back reference that occurs inside the parentheses to which + it refers fails when the subpattern is first used, so, for + example, (a\1) never matches. However, such references can + be useful inside repeated subpatterns. For example, the pat- + tern + + (a|b\1)+ + + matches any number of "a"s and also "aba", "ababbaa" etc. At + each iteration of the subpattern, the back reference matches + the character string corresponding to the previous itera- + tion. In order for this to work, the pattern must be such + that the first iteration does not need to match the back + reference. This can be done using alternation, as in the + example above, or by a quantifier with a minimum of zero. + + + +ASSERTIONS + An assertion is a test on the characters following or + preceding the current matching point that does not actually + consume any characters. The simple assertions coded as \b, + \B, \A, \Z, \z, ^ and $ are described above. More compli- + cated assertions are coded as subpatterns. There are two + kinds: those that look ahead of the current position in the + subject string, and those that look behind it. + + An assertion subpattern is matched in the normal way, except + that it does not cause the current matching position to be + changed. Lookahead assertions start with (?= for positive + assertions and (?! for negative assertions. For example, + + \w+(?=;) + + matches a word followed by a semicolon, but does not include + the semicolon in the match, and + + foo(?!bar) + + matches any occurrence of "foo" that is not followed by + "bar". Note that the apparently similar pattern + + (?!foo)bar + + does not find an occurrence of "bar" that is preceded by + something other than "foo"; it finds any occurrence of "bar" + whatsoever, because the assertion (?!foo) is always true + when the next three characters are "bar". A lookbehind + assertion is needed to achieve this effect. + + Lookbehind assertions start with (?<= for positive asser- + tions and (?<! for negative assertions. For example, + + (?<!foo)bar + + does find an occurrence of "bar" that is not preceded by + "foo". The contents of a lookbehind assertion are restricted + such that all the strings it matches must have a fixed + length. However, if there are several alternatives, they do + not all have to have the same fixed length. Thus + + (?<=bullock|donkey) + + is permitted, but + + (?<!dogs?|cats?) + + causes an error at compile time. Branches that match dif- + ferent length strings are permitted only at the top level of + a lookbehind assertion. This is an extension compared with + Perl 5.005, which requires all branches to match the same + length of string. An assertion such as + + (?<=ab(c|de)) + + is not permitted, because its single top-level branch can + match two different lengths, but it is acceptable if rewrit- + ten to use two top-level branches: + + (?<=abc|abde) + + The implementation of lookbehind assertions is, for each + alternative, to temporarily move the current position back + by the fixed width and then try to match. If there are + insufficient characters before the current position, the + match is deemed to fail. Lookbehinds in conjunction with + once-only subpatterns can be particularly useful for match- + ing at the ends of strings; an example is given at the end + of the section on once-only subpatterns. + + Several assertions (of any sort) may occur in succession. + For example, + + (?<=\d{3})(?<!999)foo + + matches "foo" preceded by three digits that are not "999". + Notice that each of the assertions is applied independently + at the same point in the subject string. First there is a + check that the previous three characters are all digits, and + then there is a check that the same three characters are not + "999". This pattern does not match "foo" preceded by six + characters, the first of which are digits and the last three + of which are not "999". For example, it doesn't match + "123abcfoo". A pattern to do that is + + (?<=\d{3}...)(?<!999)foo + + This time the first assertion looks at the preceding six + characters, checking that the first three are digits, and + then the second assertion checks that the preceding three + characters are not "999". + + Assertions can be nested in any combination. For example, + + (?<=(?<!foo)bar)baz + + matches an occurrence of "baz" that is preceded by "bar" + which in turn is not preceded by "foo", while + + (?<=\d{3}(?!999)...)foo + + is another pattern which matches "foo" preceded by three + digits and any three characters that are not "999". + + Assertion subpatterns are not capturing subpatterns, and may + not be repeated, because it makes no sense to assert the + same thing several times. If any kind of assertion contains + capturing subpatterns within it, these are counted for the + purposes of numbering the capturing subpatterns in the whole + pattern. However, substring capturing is carried out only + for positive assertions, because it does not make sense for + negative assertions. + + Assertions count towards the maximum of 200 parenthesized + subpatterns. + + + +ONCE-ONLY SUBPATTERNS + With both maximizing and minimizing repetition, failure of + what follows normally causes the repeated item to be re- + evaluated to see if a different number of repeats allows the + rest of the pattern to match. Sometimes it is useful to + prevent this, either to change the nature of the match, or + to cause it fail earlier than it otherwise might, when the + author of the pattern knows there is no point in carrying + on. + + Consider, for example, the pattern \d+foo when applied to + the subject line + + 123456bar + + After matching all 6 digits and then failing to match "foo", + the normal action of the matcher is to try again with only 5 + digits matching the \d+ item, and then with 4, and so on, + before ultimately failing. Once-only subpatterns provide the + means for specifying that once a portion of the pattern has + matched, it is not to be re-evaluated in this way, so the + matcher would give up immediately on failing to match "foo" + the first time. The notation is another kind of special + parenthesis, starting with (?> as in this example: + + (?>\d+)bar + + This kind of parenthesis "locks up" the part of the pattern + it contains once it has matched, and a failure further into + the pattern is prevented from backtracking into it. Back- + tracking past it to previous items, however, works as nor- + mal. + + An alternative description is that a subpattern of this type + matches the string of characters that an identical stan- + dalone pattern would match, if anchored at the current point + in the subject string. + + Once-only subpatterns are not capturing subpatterns. Simple + cases such as the above example can be thought of as a max- + imizing repeat that must swallow everything it can. So, + while both \d+ and \d+? are prepared to adjust the number of + digits they match in order to make the rest of the pattern + match, (?>\d+) can only match an entire sequence of digits. + + This construction can of course contain arbitrarily compli- + cated subpatterns, and it can be nested. + + Once-only subpatterns can be used in conjunction with look- + behind assertions to specify efficient matching at the end + of the subject string. Consider a simple pattern such as + + abcd$ + + when applied to a long string which does not match. Because + matching proceeds from left to right, PCRE will look for + each "a" in the subject and then see if what follows matches + the rest of the pattern. If the pattern is specified as + + ^.*abcd$ + + the initial .* matches the entire string at first, but when + this fails (because there is no following "a"), it back- + tracks to match all but the last character, then all but the + last two characters, and so on. Once again the search for + "a" covers the entire string, from right to left, so we are + no better off. However, if the pattern is written as + + ^(?>.*)(?<=abcd) + + there can be no backtracking for the .* item; it can match + only the entire string. The subsequent lookbehind assertion + does a single test on the last four characters. If it fails, + the match fails immediately. For long strings, this approach + makes a significant difference to the processing time. + + When a pattern contains an unlimited repeat inside a subpat- + tern that can itself be repeated an unlimited number of + times, the use of a once-only subpattern is the only way to + avoid some failing matches taking a very long time indeed. + The pattern + + (\D+|<\d+>)*[!?] + + matches an unlimited number of substrings that either con- + sist of non-digits, or digits enclosed in <>, followed by + either ! or ?. When it matches, it runs quickly. However, if + it is applied to + + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + + it takes a long time before reporting failure. This is + because the string can be divided between the two repeats in + a large number of ways, and all have to be tried. (The exam- + ple used [!?] rather than a single character at the end, + because both PCRE and Perl have an optimization that allows + for fast failure when a single character is used. They + remember the last single character that is required for a + match, and fail early if it is not present in the string.) + If the pattern is changed to + + ((?>\D+)|<\d+>)*[!?] + + sequences of non-digits cannot be broken, and failure hap- + pens quickly. + + + +CONDITIONAL SUBPATTERNS + It is possible to cause the matching process to obey a sub- + pattern conditionally or to choose between two alternative + subpatterns, depending on the result of an assertion, or + whether a previous capturing subpattern matched or not. The + two possible forms of conditional subpattern are + + (?(condition)yes-pattern) + (?(condition)yes-pattern|no-pattern) + + If the condition is satisfied, the yes-pattern is used; oth- + erwise the no-pattern (if present) is used. If there are + more than two alternatives in the subpattern, a compile-time + error occurs. + + There are two kinds of condition. If the text between the + parentheses consists of a sequence of digits, the condition + is satisfied if the capturing subpattern of that number has + previously matched. The number must be greater than zero. + Consider the following pattern, which contains non- + significant white space to make it more readable (assume the + PCRE_EXTENDED option) and to divide it into three parts for + ease of discussion: + + ( \( )? [^()]+ (?(1) \) ) + + The first part matches an optional opening parenthesis, and + if that character is present, sets it as the first captured + substring. The second part matches one or more characters + that are not parentheses. The third part is a conditional + subpattern that tests whether the first set of parentheses + matched or not. If they did, that is, if subject started + with an opening parenthesis, the condition is true, and so + the yes-pattern is executed and a closing parenthesis is + required. Otherwise, since no-pattern is not present, the + subpattern matches nothing. In other words, this pattern + matches a sequence of non-parentheses, optionally enclosed + in parentheses. + + If the condition is not a sequence of digits, it must be an + assertion. This may be a positive or negative lookahead or + lookbehind assertion. Consider this pattern, again contain- + ing non-significant white space, and with the two alterna- + tives on the second line: + + (?(?=[^a-z]*[a-z]) + \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) + + The condition is a positive lookahead assertion that matches + an optional sequence of non-letters followed by a letter. In + other words, it tests for the presence of at least one + letter in the subject. If a letter is found, the subject is + matched against the first alternative; otherwise it is + matched against the second. This pattern matches strings in + one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are + letters and dd are digits. + + + +COMMENTS + The sequence (?# marks the start of a comment which contin- + ues up to the next closing parenthesis. Nested parentheses + are not permitted. The characters that make up a comment + play no part in the pattern matching at all. + + If the PCRE_EXTENDED option is set, an unescaped # character + outside a character class introduces a comment that contin- + ues up to the next newline character in the pattern. + + + +RECURSIVE PATTERNS + Consider the problem of matching a string in parentheses, + allowing for unlimited nested parentheses. Without the use + of recursion, the best that can be done is to use a pattern + that matches up to some fixed depth of nesting. It is not + possible to handle an arbitrary nesting depth. Perl 5.6 has + provided an experimental facility that allows regular + expressions to recurse (amongst other things). It does this + by interpolating Perl code in the expression at run time, + and the code can refer to the expression itself. A Perl pat- + tern to solve the parentheses problem can be created like + this: + + $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; + + The (?p{...}) item interpolates Perl code at run time, and + in this case refers recursively to the pattern in which it + appears. Obviously, PCRE cannot support the interpolation of + Perl code. Instead, the special item (?R) is provided for + the specific case of recursion. This PCRE pattern solves the + parentheses problem (assume the PCRE_EXTENDED option is set + so that white space is ignored): + + \( ( (?>[^()]+) | (?R) )* \) + + First it matches an opening parenthesis. Then it matches any + number of substrings which can either be a sequence of non- + parentheses, or a recursive match of the pattern itself + (i.e. a correctly parenthesized substring). Finally there is + a closing parenthesis. + + This particular example pattern contains nested unlimited + repeats, and so the use of a once-only subpattern for match- + ing strings of non-parentheses is important when applying + the pattern to strings that do not match. For example, when + it is applied to + + (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() + + it yields "no match" quickly. However, if a once-only sub- + pattern is not used, the match runs for a very long time + indeed because there are so many different ways the + and * + repeats can carve up the subject, and all have to be tested + before failure can be reported. + + The values set for any capturing subpatterns are those from + the outermost level of the recursion at which the subpattern + value is set. If the pattern above is matched against + + (ab(cd)ef) + + the value for the capturing parentheses is "ef", which is + the last value taken on at the top level. If additional + parentheses are added, giving + + \( ( ( (?>[^()]+) | (?R) )* ) \) + ^ ^ + ^ ^ the string they capture is + "ab(cd)ef", the contents of the top level parentheses. If + there are more than 15 capturing parentheses in a pattern, + PCRE has to obtain extra memory to store data during a + recursion, which it does by using pcre_malloc, freeing it + via pcre_free afterwards. If no memory can be obtained, it + saves data for the first 15 capturing parentheses only, as + there is no way to give an out-of-memory error from within a + recursion. + + + +PERFORMANCE + Certain items that may appear in patterns are more efficient + than others. It is more efficient to use a character class + like [aeiou] than a set of alternatives such as (a|e|i|o|u). + In general, the simplest construction that provides the + required behaviour is usually the most efficient. Jeffrey + Friedl's book contains a lot of discussion about optimizing + regular expressions for efficient performance. + + When a pattern begins with .* and the PCRE_DOTALL option is + set, the pattern is implicitly anchored by PCRE, since it + can match only at the start of a subject string. However, if + PCRE_DOTALL is not set, PCRE cannot make this optimization, + because the . metacharacter does not then match a newline, + and if the subject string contains newlines, the pattern may + match from the character immediately following one of them + instead of from the very start. For example, the pattern + + (.*) second + + matches the subject "first\nand second" (where \n stands for + a newline character) with the first captured substring being + "and". In order to do this, PCRE has to retry the match + starting after every newline in the subject. + + If you are using such a pattern with subject strings that do + not contain newlines, the best performance is obtained by + setting PCRE_DOTALL, or starting the pattern with ^.* to + indicate explicit anchoring. That saves PCRE from having to + scan along the subject looking for a newline to restart at. + + Beware of patterns that contain nested indefinite repeats. + These can take a long time to run when applied to a string + that does not match. Consider the pattern fragment + + (a+)* + + This can match "aaaa" in 33 different ways, and this number + increases very rapidly as the string gets longer. (The * + repeat can match 0, 1, 2, 3, or 4 times, and for each of + those cases other than 0, the + repeats can match different + numbers of times.) When the remainder of the pattern is such + that the entire match is going to fail, PCRE has in princi- + ple to try every possible variation, and this can take an + extremely long time. + + An optimization catches some of the more simple cases such + as + + (a+)*b + + where a literal character follows. Before embarking on the + standard matching procedure, PCRE checks that there is a "b" + later in the subject string, and if there is not, it fails + the match immediately. However, when there is no following + literal this optimization cannot be used. You can see the + difference by comparing the behaviour of + + (a+)*\d + + with the pattern above. The former gives a failure almost + instantly when applied to a whole line of "a" characters, + whereas the latter takes an appreciable time with strings + longer than about 20 characters. + + + +UTF-8 SUPPORT + Starting at release 3.3, PCRE has some support for character + strings encoded in the UTF-8 format. This is incomplete, and + is regarded as experimental. In order to use it, you must + configure PCRE to include UTF-8 support in the code, and, in + addition, you must call pcre_compile() with the PCRE_UTF8 + option flag. When you do this, both the pattern and any sub- + ject strings that are matched against it are treated as + UTF-8 strings instead of just strings of bytes, but only in + the cases that are mentioned below. + + If you compile PCRE with UTF-8 support, but do not use it at + run time, the library will be a bit bigger, but the addi- + tional run time overhead is limited to testing the PCRE_UTF8 + flag in several places, so should not be very large. + + PCRE assumes that the strings it is given contain valid + UTF-8 codes. It does not diagnose invalid UTF-8 strings. If + you pass invalid UTF-8 strings to PCRE, the results are + undefined. + + Running with PCRE_UTF8 set causes these changes in the way + PCRE works: + + 1. In a pattern, the escape sequence \x{...}, where the + 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. + + 2. The original hexadecimal escape sequence, \xhh, generates + a two-byte UTF-8 character if its value is greater than 127. + + 3. Repeat quantifiers are NOT correctly handled if they fol- + low a multibyte character. For example, \x{100}* and \xc3+ + do not work. If you want to repeat such characters, you must + enclose them in non-capturing parentheses, for example + (?:\x{100}), at present. + + 4. The dot metacharacter matches one UTF-8 character instead + of a single byte. + + 5. Unlike literal UTF-8 characters, the dot metacharacter + followed by a repeat quantifier does operate correctly on + UTF-8 characters instead of single bytes. + + 4. Although the \x{...} escape is permitted in a character + class, characters whose values are greater than 255 cannot + be included in a class. + + 5. A class is matched against a UTF-8 character instead of + just a single byte, but it can match only characters whose + values are less than 256. Characters with greater values + always fail to match a class. + + 6. Repeated classes work correctly on multiple characters. + + 7. Classes containing just a single character whose value is + greater than 127 (but less than 256), for example, [\x80] or + [^\x{93}], do not work because these are optimized into sin- + gle byte matches. In the first case, of course, the class + brackets are just redundant. + + 8. Lookbehind assertions move backwards in the subject by a + fixed number of characters instead of a fixed number of + bytes. Simple cases have been tested to work correctly, but + there may be hidden gotchas herein. + + 9. The character types such as \d and \w do not work + correctly with UTF-8 characters. They continue to test a + single byte. + + 10. Anything not explicitly mentioned here continues to work + in bytes rather than in characters. + + The following UTF-8 features of Perl 5.6 are not imple- + mented: + + 1. The escape sequence \C to match a single byte. + + 2. The use of Unicode tables and properties and escapes \p, + \P, and \X. + + + +SAMPLE PROGRAM + The code below is a simple, complete demonstration program, + to get you started with using PCRE. This code is also sup- + plied in the file pcredemo.c in the PCRE distribution. + + 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 charac- + ter tables are used. If matching succeeds, the program out- + puts the portion of the subject that matched, together with + the contents of any captured substrings. + + On a Unix system that has PCRE installed in /usr/local, you + can compile the demonstration program using a command like + this: + + gcc -o pcredemo pcredemo.c -I/usr/local/include + -L/usr/local/lib -lpcre + + Then you can run simple tests like this: + + ./pcredemo 'cat|dog' 'the cat sat on the mat' + + Note that there is a much more comprehensive test program, + called pcretest, which supports many more facilities for + testing regular expressions. The pcredemo program is pro- + vided as a simple coding example. + + On some operating systems (e.g. Solaris) you may get an + error like this when you try to run pcredemo: + + ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such + file or directory + + This is caused by the way shared library support works on + those systems. You need to add + + -R/usr/local/lib + + to the compile command to get round this problem. Here's the + code: + + #include <stdio.h> + #include <string.h> + #include <pcre.h> + + #define OVECCOUNT 30 /* should be a multiple of 3 */ + + int main(int argc, char **argv) + { + pcre *re; + const char *error; + int erroffset; + int ovector[OVECCOUNT]; + int rc, i; + + if (argc != 3) + { + printf("Two arguments required: a regex and a " + "subject string\n"); + return 1; + } + + /* Compile the regular expression in the first argument */ + + re = pcre_compile( + argv[1], /* the pattern */ + 0, /* default options */ + &error, /* for error message */ + &erroffset, /* for error offset */ + NULL); /* use default character tables */ + + /* Compilation failed: print the error message and exit */ + + if (re == NULL) + { + printf("PCRE compilation failed at offset %d: %s\n", + erroffset, error); + return 1; + } + + /* Compilation succeeded: match the subject in the second + argument */ + + 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 */ + + /* Matching failed: handle error cases */ + + 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; + } + + /* Match succeded */ + + printf("Match succeeded\n"); + + /* The output vector wasn't big enough */ + + if (rc == 0) + { + rc = OVECCOUNT/3; + printf("ovector only has room for %d captured " + substrings\n", rc - 1); + } + + /* Show substrings stored in the output vector */ + + 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); + } + + return 0; + } + + + +AUTHOR + Philip Hazel <ph10@cam.ac.uk> + University Computing Service, + New Museums Site, + Cambridge CB2 3QG, England. + Phone: +44 1223 334714 + + Last updated: 15 August 2001 + Copyright (c) 1997-2001 University of Cambridge. |