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<h1>pcreapi man page</h1>

Return to the <a href="index.html">PCRE index page</a>.


This page is part of the PCRE HTML documentation. It was 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">PCRE NATIVE API</a>
<li><a name="TOC2" href="#SEC2">PCRE API OVERVIEW</a>
<li><a name="TOC3" href="#SEC3">NEWLINES</a>
<li><a name="TOC4" href="#SEC4">MULTITHREADING</a>
<li><a name="TOC5" href="#SEC5">SAVING PRECOMPILED PATTERNS FOR LATER USE</a>
<li><a name="TOC6" href="#SEC6">CHECKING BUILD-TIME OPTIONS</a>
<li><a name="TOC7" href="#SEC7">COMPILING A PATTERN</a>
<li><a name="TOC8" href="#SEC8">COMPILATION ERROR CODES</a>
<li><a name="TOC9" href="#SEC9">STUDYING A PATTERN</a>
<li><a name="TOC10" href="#SEC10">LOCALE SUPPORT</a>
<li><a name="TOC11" href="#SEC11">INFORMATION ABOUT A PATTERN</a>
<li><a name="TOC12" href="#SEC12">OBSOLETE INFO FUNCTION</a>
<li><a name="TOC13" href="#SEC13">REFERENCE COUNTS</a>
<li><a name="TOC14" href="#SEC14">MATCHING A PATTERN: THE TRADITIONAL FUNCTION</a>
<li><a name="TOC15" href="#SEC15">EXTRACTING CAPTURED SUBSTRINGS BY NUMBER</a>
<li><a name="TOC16" href="#SEC16">EXTRACTING CAPTURED SUBSTRINGS BY NAME</a>
<li><a name="TOC17" href="#SEC17">DUPLICATE SUBPATTERN NAMES</a>
<li><a name="TOC18" href="#SEC18">FINDING ALL POSSIBLE MATCHES</a>
<li><a name="TOC19" href="#SEC19">MATCHING A PATTERN: THE ALTERNATIVE FUNCTION</a>
<li><a name="TOC20" href="#SEC20">SEE ALSO</a>
<li><a name="TOC21" href="#SEC21">AUTHOR</a>
<li><a name="TOC22" href="#SEC22">REVISION</a>
</ul>
 <a name="SEC1" href="#TOC1">PCRE NATIVE API</a> 

#include &#60;pcre.h&#62;


pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);


pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
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_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);


int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);


int pcre_copy_substring(const char *subject, int *ovector,
int stringcount, int stringnumber, char *buffer,
int buffersize);


int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);


int pcre_get_stringnumber(const pcre *code,
const char *name);


int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);


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, int
*firstcharptr);


int pcre_refcount(pcre *code, int adjust);


int pcre_config(int what, void *where);


char *pcre_version(void);


void *(*pcre_malloc)(size_t);


void (*pcre_free)(void *);


void *(*pcre_stack_malloc)(size_t);


void (*pcre_stack_free)(void *);


int (*pcre_callout)(pcre_callout_block *);

 <a name="SEC2" href="#TOC1">PCRE API OVERVIEW</a> 

PCRE has its own native API, which is described in this document. There are
also some wrapper functions that correspond to the POSIX regular expression
API. These are described in the
<a href="pcreposix.html">pcreposix</a>
documentation. Both of these APIs define a set of C function calls. A C++
wrapper is distributed with PCRE. It is documented in the
<a href="pcrecpp.html">pcrecpp</a>
page.


The native API C function prototypes are defined in the header file
pcre.h, and on Unix systems the library itself is called libpcre.
It can normally be accessed by adding -lpcre to the command for linking
an application that uses PCRE. 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 of PCRE.


The functions pcre_compile(), pcre_compile2(), pcre_study(),
and pcre_exec() are used for compiling and matching regular expressions
in a Perl-compatible manner. A sample program that demonstrates the simplest
way of using them is provided in the file called pcredemo.c in the source
distribution. The
<a href="pcresample.html">pcresample</a>
documentation describes how to compile and run it.


A second matching function, pcre_dfa_exec(), which is not
Perl-compatible, is also provided. This uses a different algorithm for the
matching. The alternative algorithm finds all possible matches (at a given
point in the subject), and scans the subject just once. However, this algorithm
does not return captured substrings. A description of the two matching
algorithms and their advantages and disadvantages is given in the
<a href="pcrematching.html">pcrematching</a>
documentation.


In addition to the main compiling and matching functions, there are convenience
functions for extracting captured substrings from a subject string that is
matched by pcre_exec(). They are:
<pre>
 pcre_copy_substring()
 pcre_copy_named_substring()
 pcre_get_substring()
 pcre_get_named_substring()
 pcre_get_substring_list()
 pcre_get_stringnumber()
 pcre_get_stringtable_entries()
</pre>
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 to build a set of character tables
in the current locale for passing to pcre_compile(), pcre_exec(),
or pcre_dfa_exec(). This is an optional facility that is provided for
specialist use. Most commonly, no special tables are passed, in which case
internal tables that are generated when PCRE is built are used.


The function pcre_fullinfo() is used to find out information about a
compiled pattern; pcre_info() is an obsolete version that 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 function pcre_refcount() maintains a reference count in a data block
containing a compiled pattern. This is provided for the benefit of
object-oriented applications.


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.


The global variables pcre_stack_malloc and pcre_stack_free are also
indirections to memory management functions. These special functions are used
only when PCRE is compiled to use the heap for remembering data, instead of
recursive function calls, when running the pcre_exec() function. See the
<a href="pcrebuild.html">pcrebuild</a>
documentation for details of how to do this. It is a non-standard way of
building PCRE, for use in environments that have limited stacks. Because of the
greater use of memory management, it runs more slowly. Separate functions are
provided so that special-purpose external code can be used for this case. When
used, these functions are always called in a stack-like manner (last obtained,
first freed), and always for memory blocks of the same size. There is a
discussion about PCRE's stack usage in the
<a href="pcrestack.html">pcrestack</a>
documentation.


The global variable pcre_callout initially contains NULL. It can be set
by the caller to a "callout" function, which PCRE will then call at specified
points during a matching operation. Details are given in the
<a href="pcrecallout.html">pcrecallout</a>
documentation.
<a name="newlines"></a>
 <a name="SEC3" href="#TOC1">NEWLINES</a> 

PCRE supports five different conventions for indicating line breaks in
strings: a single CR (carriage return) character, a single LF (linefeed)
character, the two-character sequence CRLF, any of the three preceding, or any
Unicode newline sequence. The Unicode newline sequences are the three just
mentioned, plus the single characters VT (vertical tab, U+000B), FF (formfeed,
U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
(paragraph separator, U+2029).


Each of the first three conventions is used by at least one operating system as
its standard newline sequence. When PCRE is built, a default can be specified.
The default default is LF, which is the Unix standard. When PCRE is run, the
default can be overridden, either when a pattern is compiled, or when it is
matched.


At compile time, the newline convention can be specified by the options
argument of pcre_compile(), or it can be specified by special text at the
start of the pattern itself; this overrides any other settings. See the
<a href="pcrepattern.html">pcrepattern</a>
page for details of the special character sequences.


In the PCRE documentation the word "newline" is used to mean "the character or
pair of characters that indicate a line break". The choice of newline
convention affects the handling of the dot, circumflex, and dollar
metacharacters, the handling of #-comments in /x mode, and, when CRLF is a
recognized line ending sequence, the match position advancement for a
non-anchored pattern. There is more detail about this in the
<a href="#execoptions">section on pcre_exec() options</a>
below.


The choice of newline convention does not affect the interpretation of
the \n or \r escape sequences, nor does it affect what \R matches, which is
controlled in a similar way, but by separate options.

 <a name="SEC4" href="#TOC1">MULTITHREADING</a> 

The PCRE functions can be used in multi-threading applications, with the
proviso that the memory management functions pointed to by pcre_malloc,
pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
callout function pointed to by pcre_callout, 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.

 <a name="SEC5" href="#TOC1">SAVING PRECOMPILED PATTERNS FOR LATER USE</a> 

The compiled form of a regular expression can be saved and re-used at a later
time, possibly by a different program, and even on a host other than the one on
which it was compiled. Details are given in the
<a href="pcreprecompile.html">pcreprecompile</a>
documentation. However, compiling a regular expression with one version of PCRE
for use with a different version is not guaranteed to work and may cause
crashes.

 <a name="SEC6" href="#TOC1">CHECKING BUILD-TIME OPTIONS</a> 

int pcre_config(int what, void *where);


The function pcre_config() makes it possible for a PCRE client to
discover which optional features have been compiled into the PCRE library. The
<a href="pcrebuild.html">pcrebuild</a>
documentation has more details about these optional features.


The first argument for pcre_config() is an integer, specifying which
information is required; the second argument is a pointer to a variable into
which the information is placed. The following information is available:
<pre>
 PCRE_CONFIG_UTF8
</pre>
The output is an integer that is set to one if UTF-8 support is available;
otherwise it is set to zero.
<pre>
 PCRE_CONFIG_UNICODE_PROPERTIES
</pre>
The output is an integer that is set to one if support for Unicode character
properties is available; otherwise it is set to zero.
<pre>
 PCRE_CONFIG_NEWLINE
</pre>
The output is an integer whose value specifies the default character sequence
that is recognized as meaning "newline". The four values that are supported
are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, and -1 for ANY. The
default should normally be the standard sequence for your operating system.
<pre>
 PCRE_CONFIG_BSR
</pre>
The output is an integer whose value indicates what character sequences the \R
escape sequence matches by default. A value of 0 means that \R matches any
Unicode line ending sequence; a value of 1 means that \R matches only CR, LF,
or CRLF. The default can be overridden when a pattern is compiled or matched.
<pre>
 PCRE_CONFIG_LINK_SIZE
</pre>
The output is an integer that contains the number of bytes used for internal
linkage in compiled regular expressions. The value is 2, 3, or 4. Larger values
allow larger regular expressions to be compiled, at the expense of slower
matching. The default value of 2 is sufficient for all but the most massive
patterns, since it allows the compiled pattern to be up to 64K in size.
<pre>
 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
</pre>
The output is an integer that contains the threshold above which the POSIX
interface uses malloc() for output vectors. Further details are given in
the
<a href="pcreposix.html">pcreposix</a>
documentation.
<pre>
 PCRE_CONFIG_MATCH_LIMIT
</pre>
The output is an integer that gives the default limit for the number of
internal matching function calls in a pcre_exec() execution. Further
details are given with pcre_exec() below.
<pre>
 PCRE_CONFIG_MATCH_LIMIT_RECURSION
</pre>
The output is an integer that gives the default limit for the depth of
recursion when calling the internal matching function in a pcre_exec()
execution. Further details are given with pcre_exec() below.
<pre>
 PCRE_CONFIG_STACKRECURSE
</pre>
The output is an integer that is set to one if internal recursion when running
pcre_exec() is implemented by recursive function calls that use the stack
to remember their state. This is the usual way that PCRE is compiled. The
output is zero if PCRE was compiled to use blocks of data on the heap instead
of recursive function calls. In this case, pcre_stack_malloc and
pcre_stack_free are called to manage memory blocks on the heap, thus
avoiding the use of the stack.

 <a name="SEC7" href="#TOC1">COMPILING A PATTERN</a> 

pcre *pcre_compile(const char *pattern, int options,
const char **errptr, int *erroffset,
const unsigned char *tableptr);
pcre *pcre_compile2(const char *pattern, int options,
int *errorcodeptr,
const char **errptr, int *erroffset,
const unsigned char *tableptr);


Either of the functions pcre_compile() or pcre_compile2() can be
called to compile a pattern into an internal form. The only difference between
the two interfaces is that pcre_compile2() has an additional argument,
errorcodeptr, via which a numerical error code can be returned.


The pattern is a C string terminated by a binary zero, and is passed in the
pattern argument. 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 (via pcre_free) 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 may contain a copy of the tableptr
argument, which is an address (see below).


The options argument contains various bit settings that affect the
compilation. It should be zero if no options are required. The available
options are described below. Some of them, in particular, those that are
compatible with Perl, can also be set and unset from within the pattern (see
the detailed description in the
<a href="pcrepattern.html">pcrepattern</a>
documentation). For these options, the contents of the options argument
specifies their initial settings at the start of compilation and execution. The
PCRE_ANCHORED and PCRE_NEWLINE_xxx options 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. This is a static string that is part of the library. You must
not try to free it. 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 pcre_compile2() is used instead of pcre_compile(), and the
errorcodeptr argument is not NULL, a non-zero error code number is
returned via this argument in the event of an error. This is in addition to the
textual error message. Error codes and messages are listed below.


If the final argument, tableptr, is NULL, PCRE uses a default set of
character tables that are built when PCRE is compiled, using the default C
locale. Otherwise, tableptr must be an address that is the result of a
call to pcre_maketables(). This value is stored with the compiled
pattern, and used again by pcre_exec(), unless another table pointer is
passed to it. For more discussion, see the section on locale support below.


This code fragment shows a typical straightforward call to pcre_compile():
<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>
The following names for option bits are defined in the pcre.h header
file:
<pre>
 PCRE_ANCHORED
</pre>
If this bit is set, the pattern is forced to be "anchored", that is, it is
constrained to match only at the first matching point in the string that 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.
<pre>
 PCRE_AUTO_CALLOUT
</pre>
If this bit is set, pcre_compile() automatically inserts callout items,
all with number 255, before each pattern item. For discussion of the callout
facility, see the
<a href="pcrecallout.html">pcrecallout</a>
documentation.
<pre>
 PCRE_BSR_ANYCRLF
 PCRE_BSR_UNICODE
</pre>
These options (which are mutually exclusive) control what the \R escape
sequence matches. The choice is either to match only CR, LF, or CRLF, or to
match any Unicode newline sequence. The default is specified when PCRE is
built. It can be overridden from within the pattern, or by setting an option
when a compiled pattern is matched.
<pre>
 PCRE_CASELESS
</pre>
If this bit is set, letters in the pattern match both upper and lower case
letters. It is equivalent to Perl's /i option, and it can be changed within a
pattern by a (?i) option setting. In UTF-8 mode, PCRE always understands the
concept of case for characters whose values are less than 128, so caseless
matching is always possible. For characters with higher values, the concept of
case is supported if PCRE is compiled with Unicode property support, but not
otherwise. If you want to use caseless matching for characters 128 and above,
you must ensure that PCRE is compiled with Unicode property support as well as
with UTF-8 support.
<pre>
 PCRE_DOLLAR_ENDONLY
</pre>
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 a newline at the end of the string (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, and no way to set it within a
pattern.
<pre>
 PCRE_DOTALL
</pre>
If this bit is set, a dot metacharater in the pattern matches all characters,
including those that indicate newline. Without it, a dot does not match when
the current position is at a newline. This option is equivalent to Perl's /s
option, and it can be changed within a pattern by a (?s) option setting. A
negative class such as [^a] always matches newline characters, independent of
the setting of this option.
<pre>
 PCRE_DUPNAMES
</pre>
If this bit is set, names used to identify capturing subpatterns need not be
unique. This can be helpful for certain types of pattern when it is known that
only one instance of the named subpattern can ever be matched. There are more
details of named subpatterns below; see also the
<a href="pcrepattern.html">pcrepattern</a>
documentation.
<pre>
 PCRE_EXTENDED
</pre>
If this bit is set, whitespace data characters in the pattern are totally
ignored except when escaped or inside a character class. Whitespace does not
include the VT character (code 11). In addition, characters between an
unescaped # outside a character class and the next newline, inclusive, are also
ignored. This is equivalent to Perl's /x option, and it can be changed within a
pattern by a (?x) option setting.


This option 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.
<pre>
 PCRE_EXTRA
</pre>
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. (Perl can, however, be persuaded to
give a warning for this.) There are at present no other features controlled by
this option. It can also be set by a (?X) option setting within a pattern.
<pre>
 PCRE_FIRSTLINE
</pre>
If this option is set, an unanchored pattern is required to match before or at
the first newline in the subject string, though the matched text may continue
over the newline.
<pre>
 PCRE_JAVASCRIPT_COMPAT
</pre>
If this option is set, PCRE's behaviour is changed in some ways so that it is
compatible with JavaScript rather than Perl. The changes are as follows:


(1) A lone closing square bracket in a pattern causes a compile-time error,
because this is illegal in JavaScript (by default it is treated as a data
character). Thus, the pattern AB]CD becomes illegal when this option is set.


(2) At run time, a back reference to an unset subpattern group matches an empty
string (by default this causes the current matching alternative to fail). A
pattern such as (\1)(a) succeeds when this option is set (assuming it can find
an "a" in the subject), whereas it fails by default, for Perl compatibility.
<pre>
 PCRE_MULTILINE
</pre>
By default, PCRE treats the subject string as consisting of a single line of
characters (even if it actually contains 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 immediately before internal newlines in the
subject string, respectively, as well as at the very start and end. This is
equivalent to Perl's /m option, and it can be changed within a pattern by a
(?m) option setting. If there are no newlines in a subject string, or no
occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no effect.
<pre>
 PCRE_NEWLINE_CR
 PCRE_NEWLINE_LF
 PCRE_NEWLINE_CRLF
 PCRE_NEWLINE_ANYCRLF
 PCRE_NEWLINE_ANY
</pre>
These options override the default newline definition that was chosen when PCRE
was built. Setting the first or the second specifies that a newline is
indicated by a single character (CR or LF, respectively). Setting
PCRE_NEWLINE_CRLF specifies that a newline is indicated by the two-character
CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies that any of the three
preceding sequences should be recognized. Setting PCRE_NEWLINE_ANY specifies
that any Unicode newline sequence should be recognized. The Unicode newline
sequences are the three just mentioned, plus the single characters VT (vertical
tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
separator, U+2028), and PS (paragraph separator, U+2029). The last two are
recognized only in UTF-8 mode.


The newline setting in the options word uses three bits that are treated
as a number, giving eight possibilities. Currently only six are used (default
plus the five values above). This means that if you set more than one newline
option, the combination may or may not be sensible. For example,
PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to PCRE_NEWLINE_CRLF, but
other combinations may yield unused numbers and cause an error.


The only time that a line break is specially recognized when compiling a
pattern is if PCRE_EXTENDED is set, and an unescaped # outside a character
class is encountered. This indicates a comment that lasts until after the next
line break sequence. In other circumstances, line break sequences are treated
as literal data, except that in PCRE_EXTENDED mode, both CR and LF are treated
as whitespace characters and are therefore ignored.


The newline option that is set at compile time becomes the default that is used
for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
<pre>
 PCRE_NO_AUTO_CAPTURE
</pre>
If this option is set, it disables the use of numbered capturing parentheses in
the pattern. Any opening parenthesis that is not followed by ? behaves as if it
were followed by ?: but named parentheses can still be used for capturing (and
they acquire numbers in the usual way). There is no equivalent of this option
in Perl.
<pre>
 PCRE_UNGREEDY
</pre>
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.
<pre>
 PCRE_UTF8
</pre>
This option causes PCRE to regard both the pattern and the subject as strings
of UTF-8 characters instead of single-byte character strings. However, it is
available only when PCRE is built to include UTF-8 support. If not, the use
of this option provokes an error. Details of how this option changes the
behaviour of PCRE are given in the
<a href="pcre.html#utf8support">section on UTF-8 support</a>
in the main
<a href="pcre.html">pcre</a>
page.
<pre>
 PCRE_NO_UTF8_CHECK
</pre>
When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
automatically checked. There is a discussion about the
<a href="pcre.html#utf8strings">validity of UTF-8 strings</a>
in the main
<a href="pcre.html">pcre</a>
page. If an invalid UTF-8 sequence of bytes is found, pcre_compile()
returns an error. If you already know that your pattern is valid, and you want
to skip this check for performance reasons, you can set the PCRE_NO_UTF8_CHECK
option. When it is set, the effect of passing an invalid UTF-8 string as a
pattern is undefined. It may cause your program to crash. Note that this option
can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress
the UTF-8 validity checking of subject strings.

 <a name="SEC8" href="#TOC1">COMPILATION ERROR CODES</a> 

The following table lists the error codes than may be returned by
pcre_compile2(), along with the error messages that may be returned by
both compiling functions. As PCRE has developed, some error codes have fallen
out of use. To avoid confusion, they have not been re-used.
<pre>
 0 no error
 1 \ at end of pattern
 2 \c at end of pattern
 3 unrecognized character follows \
 4 numbers out of order in {} quantifier
 5 number too big in {} quantifier
 6 missing terminating ] for character class
 7 invalid escape sequence in character class
 8 range out of order in character class
 9 nothing to repeat
 10 [this code is not in use]
 11 internal error: unexpected repeat
 12 unrecognized character after (? or (?-
 13 POSIX named classes are supported only within a class
 14 missing )
 15 reference to non-existent subpattern
 16 erroffset passed as NULL
 17 unknown option bit(s) set
 18 missing ) after comment
 19 [this code is not in use]
 20 regular expression is too large
 21 failed to get memory
 22 unmatched parentheses
 23 internal error: code overflow
 24 unrecognized character after (?&#60;
 25 lookbehind assertion is not fixed length
 26 malformed number or name after (?(
 27 conditional group contains more than two branches
 28 assertion expected after (?(
 29 (?R or (?[+-]digits must be followed by )
 30 unknown POSIX class name
 31 POSIX collating elements are not supported
 32 this version of PCRE is not compiled with PCRE_UTF8 support
 33 [this code is not in use]
 34 character value in \x{...} sequence is too large
 35 invalid condition (?(0)
 36 \C not allowed in lookbehind assertion
 37 PCRE does not support \L, \l, \N, \U, or \u
 38 number after (?C is &#62; 255
 39 closing ) for (?C expected
 40 recursive call could loop indefinitely
 41 unrecognized character after (?P
 42 syntax error in subpattern name (missing terminator)
 43 two named subpatterns have the same name
 44 invalid UTF-8 string
 45 support for \P, \p, and \X has not been compiled
 46 malformed \P or \p sequence
 47 unknown property name after \P or \p
 48 subpattern name is too long (maximum 32 characters)
 49 too many named subpatterns (maximum 10000)
 50 [this code is not in use]
 51 octal value is greater than \377 (not in UTF-8 mode)
 52 internal error: overran compiling workspace
 53 internal error: previously-checked referenced subpattern not found
 54 DEFINE group contains more than one branch
 55 repeating a DEFINE group is not allowed
 56 inconsistent NEWLINE options
 57 \g is not followed by a braced, angle-bracketed, or quoted
 name/number or by a plain number
 58 a numbered reference must not be zero
 59 (*VERB) with an argument is not supported
 60 (*VERB) not recognized
 61 number is too big
 62 subpattern name expected
 63 digit expected after (?+
 64 ] is an invalid data character in JavaScript compatibility mode
</pre>
The numbers 32 and 10000 in errors 48 and 49 are defaults; different values may
be used if the limits were changed when PCRE was built.

 <a name="SEC9" href="#TOC1">STUDYING A PATTERN</a> 

pcre_extra *pcre_study(const pcre *code, int options
const char **errptr);


If a compiled 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. If studying the pattern produces additional information that will
help speed up matching, pcre_study() returns a pointer to a
pcre_extra block, in which the study_data field points to the
results of the study.


The returned value from pcre_study() can be passed directly to
pcre_exec(). However, a pcre_extra block also contains other
fields that can be set by the caller before the block is passed; these are
described
<a href="#extradata">below</a>
in the section on matching a pattern.


If studying the pattern does not produce any additional information
pcre_study() returns NULL. In that circumstance, if the calling program
wants to pass any of the other fields to pcre_exec(), it must set up its
own pcre_extra block.


The second argument of pcre_study() contains option bits. At present, no
options are defined, and this argument should always be zero.


The third argument for pcre_study() is a pointer for an error message. If
studying succeeds (even if no data is returned), the variable it points to is
set to NULL. Otherwise it is set to point to a textual error message. This is a
static string that is part of the library. You must not try to free it. You
should test the error pointer for NULL after calling pcre_study(), to be
sure that it has run successfully.


This is a typical call to pcre_study():
<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>
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
bytes is created.
<a name="localesupport"></a>
 <a name="SEC10" href="#TOC1">LOCALE SUPPORT</a> 

PCRE handles caseless matching, and determines whether characters are letters,
digits, or whatever, by reference to a set of tables, indexed by character
value. When running in UTF-8 mode, this applies only to characters with codes
less than 128. Higher-valued codes never match escapes such as \w or \d, but
can be tested with \p if PCRE is built with Unicode character property
support. The use of locales with Unicode is discouraged. If you are handling
characters with codes greater than 128, you should either use UTF-8 and
Unicode, or use locales, but not try to mix the two.


PCRE contains an internal set of tables that are used when the final argument
of pcre_compile() is NULL. These are sufficient for many applications.
Normally, the internal tables recognize only ASCII characters. However, when
PCRE is built, it is possible to cause the internal tables to be rebuilt in the
default "C" locale of the local system, which may cause them to be different.


The internal tables can always be overridden by tables supplied by the
application that calls PCRE. These may be created in a different locale from
the default. As more and more applications change to using Unicode, the need
for this locale support is expected to die away.


External 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() or pcre_exec() as often as necessary. For
example, to build and use tables that are appropriate for the French locale
(where accented characters with values greater than 128 are treated as letters),
the following code could be used:
<pre>
 setlocale(LC_CTYPE, "fr_FR");
 tables = pcre_maketables();
 re = pcre_compile(..., tables);
</pre>
The locale name "fr_FR" is used on Linux and other Unix-like systems; if you
are using Windows, the name for the French locale is "french".


When pcre_maketables() runs, the tables are built in memory that is
obtained via pcre_malloc. It is the caller's responsibility to ensure
that the memory containing the tables remains available for as long as it is
needed.


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 normally also by pcre_exec(). Thus, by default, 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 possible to pass a table pointer or NULL (indicating the use of the
internal tables) to pcre_exec(). Although not intended for this purpose,
this facility could be used to match a pattern in a different locale from the
one in which it was compiled. Passing table pointers at run time is discussed
below in the section on matching a pattern.

 <a name="SEC11" href="#TOC1">INFORMATION ABOUT A PATTERN</a> 

int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
int what, void *where);


The pcre_fullinfo() function returns information about a compiled
pattern. It replaces the obsolete pcre_info() function, which is
nevertheless retained for backwards compability (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, and 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:
<pre>
 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
</pre>
The "magic number" is placed at the start of each compiled pattern as an simple
check against passing an arbitrary memory pointer. Here is a typical call of
pcre_fullinfo(), to obtain the length of the compiled pattern:
<pre>
 int rc;
 size_t 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>
The possible values for the third argument are defined in pcre.h, and are
as follows:
<pre>
 PCRE_INFO_BACKREFMAX
</pre>
Return the number of the highest back reference in the pattern. The fourth
argument should point to an int variable. Zero is returned if there are
no back references.
<pre>
 PCRE_INFO_CAPTURECOUNT
</pre>
Return the number of capturing subpatterns in the pattern. The fourth argument
should point to an int variable.
<pre>
 PCRE_INFO_DEFAULT_TABLES
</pre>
Return a pointer to the internal default character tables within PCRE. The
fourth argument should point to an unsigned char * variable. This
information call is provided for internal use by the pcre_study()
function. External callers can cause PCRE to use its internal tables by passing
a NULL table pointer.
<pre>
 PCRE_INFO_FIRSTBYTE
</pre>
Return information about the first byte of any matched string, for a
non-anchored pattern. The fourth argument should point to an int
variable. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name is
still recognized for backwards compatibility.)


If there is a fixed first byte, for example, from a pattern such as
(cat|cow|coyote), its value is returned. 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 newline within the string. Otherwise -2 is
returned. For anchored patterns, -2 is returned.
<pre>
 PCRE_INFO_FIRSTTABLE
</pre>
If the pattern was studied, and this resulted in the construction of a 256-bit
table indicating a fixed set of bytes for the first byte 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.
<pre>
 PCRE_INFO_HASCRORLF
</pre>
Return 1 if the pattern contains any explicit matches for CR or LF characters,
otherwise 0. The fourth argument should point to an int variable. An
explicit match is either a literal CR or LF character, or \r or \n.
<pre>
 PCRE_INFO_JCHANGED
</pre>
Return 1 if the (?J) or (?-J) option setting is used in the pattern, otherwise
0. The fourth argument should point to an int variable. (?J) and
(?-J) set and unset the local PCRE_DUPNAMES option, respectively.
<pre>
 PCRE_INFO_LASTLITERAL
</pre>
Return the value of the rightmost literal byte that must exist in any matched
string, other than at its start, if such a byte has been recorded. The fourth
argument should point to an int variable. If there is no such byte, -1 is
returned. For anchored patterns, a last literal byte is recorded only if it
follows something of variable length. For example, for the pattern
/^a\d+z\d+/ the returned value is "z", but for /^a\dz\d/ the returned value
is -1.
<pre>
 PCRE_INFO_NAMECOUNT
 PCRE_INFO_NAMEENTRYSIZE
 PCRE_INFO_NAMETABLE
</pre>
PCRE supports the use of named as well as numbered capturing parentheses. The
names are just an additional way of identifying the parentheses, which still
acquire numbers. Several convenience functions such as
pcre_get_named_substring() are provided for extracting captured
substrings by name. It is also possible to extract the data directly, by first
converting the name to a number in order to access the correct pointers in the
output vector (described with pcre_exec() below). To do the conversion,
you need to use the name-to-number map, which is described by these three
values.


The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT gives
the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size of each
entry; both of these return an int value. The entry size depends on the
length of the longest name. PCRE_INFO_NAMETABLE returns a pointer to the first
entry of the table (a pointer to char). The first two bytes of each entry
are the number of the capturing parenthesis, most significant byte first. The
rest of the entry is the corresponding name, zero terminated. The names are in
alphabetical order. When PCRE_DUPNAMES is set, duplicate names are in order of
their parentheses numbers. For example, consider the following pattern (assume
PCRE_EXTENDED is set, so white space - including newlines - is ignored):
<pre>
 (?&#60;date&#62; (?&#60;year&#62;(\d\d)?\d\d) - (?&#60;month&#62;\d\d) - (?&#60;day&#62;\d\d) )
</pre>
There are four named subpatterns, so the table has four entries, and each entry
in the table is eight bytes long. The table is as follows, with non-printing
bytes shows in hexadecimal, and undefined bytes shown as ??:
<pre>
 00 01 d a t e 00 ??
 00 05 d a y 00 ?? ??
 00 04 m o n t h 00
 00 02 y e a r 00 ??
</pre>
When writing code to extract data from named subpatterns using the
name-to-number map, remember that the length of the entries is likely to be
different for each compiled pattern.
<pre>
 PCRE_INFO_OKPARTIAL
</pre>
Return 1 if the pattern can be used for partial matching, otherwise 0. The
fourth argument should point to an int variable. The
<a href="pcrepartial.html">pcrepartial</a>
documentation lists the restrictions that apply to patterns when partial
matching is used.
<pre>
 PCRE_INFO_OPTIONS
</pre>
Return a copy of the options with which the pattern was compiled. 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 at the start of the pattern itself. In other words,
they are the options that will be in force when matching starts. For example,
if the pattern /(?im)abc(?-i)d/ is compiled with the PCRE_EXTENDED option, the
result is PCRE_CASELESS, PCRE_MULTILINE, and PCRE_EXTENDED.


A pattern is automatically anchored by PCRE if all of its top-level
alternatives begin with one of the following:
<pre>
 ^ unless PCRE_MULTILINE is set
 \A always
 \G always
 .* if PCRE_DOTALL is set and there are no back references to the subpattern in which .* appears
</pre>
For such patterns, the PCRE_ANCHORED bit is set in the options returned by
pcre_fullinfo().
<pre>
 PCRE_INFO_SIZE
</pre>
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.
<pre>
 PCRE_INFO_STUDYSIZE
</pre>
Return the size of the data block pointed to by the study_data field in
a pcre_extra block. That is, it is the value that was passed to
pcre_malloc() when PCRE was getting memory into which to place the data
created by pcre_study(). The fourth argument should point to a
size_t variable.

 <a name="SEC12" href="#TOC1">OBSOLETE INFO FUNCTION</a> 

int pcre_info(const pcre *code, int *optptr, int
*firstcharptr);


The pcre_info() function is now obsolete because its interface 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:
<pre>
 PCRE_ERROR_NULL the argument code was NULL
 PCRE_ERROR_BADMAGIC the "magic number" was not found
</pre>
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_FIRSTBYTE above).

 <a name="SEC13" href="#TOC1">REFERENCE COUNTS</a> 

int pcre_refcount(pcre *code, int adjust);


The pcre_refcount() function is used to maintain a reference count in the
data block that contains a compiled pattern. It is provided for the benefit of
applications that operate in an object-oriented manner, where different parts
of the application may be using the same compiled pattern, but you want to free
the block when they are all done.


When a pattern is compiled, the reference count field is initialized to zero.
It is changed only by calling this function, whose action is to add the
adjust value (which may be positive or negative) to it. The yield of the
function is the new value. However, the value of the count is constrained to
lie between 0 and 65535, inclusive. If the new value is outside these limits,
it is forced to the appropriate limit value.


Except when it is zero, the reference count is not correctly preserved if a
pattern is compiled on one host and then transferred to a host whose byte-order
is different. (This seems a highly unlikely scenario.)

 <a name="SEC14" href="#TOC1">MATCHING A PATTERN: THE TRADITIONAL FUNCTION</a> 

int pcre_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize);


The function pcre_exec() is called to match a subject string against a
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. This function is the main matching facility of the
library, and it operates in a Perl-like manner. For specialist use there is
also an alternative matching function, which is described
<a href="#dfamatch">below</a>
in the section about the pcre_dfa_exec() function.


In most applications, the pattern will have been compiled (and optionally
studied) in the same process that calls pcre_exec(). However, it is
possible to save compiled patterns and study data, and then use them later
in different processes, possibly even on different hosts. For a discussion
about this, see the
<a href="pcreprecompile.html">pcreprecompile</a>
documentation.


Here is an example of a simple call to pcre_exec():
<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 of integers for substring information */
 30); /* number of elements (NOT size in bytes) */
<a name="extradata"></a></PRE>

 
Extra data for pcre_exec()
 

If the extra argument is not NULL, it must point to a pcre_extra
data block. The pcre_study() function returns such a block (when it
doesn't return NULL), but you can also create one for yourself, and pass
additional information in it. The pcre_extra block contains the following
fields (not necessarily in this order):
<pre>
 unsigned long int flags;
 void *study_data;
 unsigned long int match_limit;
 unsigned long int match_limit_recursion;
 void *callout_data;
 const unsigned char *tables;
</pre>
The flags field is a bitmap that specifies which of the other fields
are set. The flag bits are:
<pre>
 PCRE_EXTRA_STUDY_DATA
 PCRE_EXTRA_MATCH_LIMIT
 PCRE_EXTRA_MATCH_LIMIT_RECURSION
 PCRE_EXTRA_CALLOUT_DATA
 PCRE_EXTRA_TABLES
</pre>
Other flag bits should be set to zero. The study_data field is set in the
pcre_extra block that is returned by pcre_study(), together with
the appropriate flag bit. You should not set this yourself, but you may add to
the block by setting the other fields and their corresponding flag bits.


The match_limit field provides a means of preventing PCRE from using up a
vast amount of resources when running patterns that are not going to match,
but which have a very large number of possibilities in their search trees. The
classic example is the use of nested unlimited repeats.


Internally, PCRE uses a function called match() which it calls repeatedly
(sometimes recursively). The limit set by match_limit is imposed on the
number of times this function is called during a match, which has the effect of
limiting the amount of backtracking that can take place. For patterns that are
not anchored, the count restarts from zero for each position in the subject
string.


The default value for the limit can be set when PCRE is built; the default
default is 10 million, which handles all but the most extreme cases. You can
override the default by suppling pcre_exec() with a pcre_extra
block in which match_limit is set, and PCRE_EXTRA_MATCH_LIMIT is set in
the flags field. If the limit is exceeded, pcre_exec() returns
PCRE_ERROR_MATCHLIMIT.


The match_limit_recursion field is similar to match_limit, but
instead of limiting the total number of times that match() is called, it
limits the depth of recursion. The recursion depth is a smaller number than the
total number of calls, because not all calls to match() are recursive.
This limit is of use only if it is set smaller than match_limit.


Limiting the recursion depth limits the amount of stack that can be used, or,
when PCRE has been compiled to use memory on the heap instead of the stack, the
amount of heap memory that can be used.


The default value for match_limit_recursion can be set when PCRE is
built; the default default is the same value as the default for
match_limit. You can override the default by suppling pcre_exec()
with a pcre_extra block in which match_limit_recursion is set, and
PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the limit
is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.


The pcre_callout field is used in conjunction with the "callout" feature,
which is described in the
<a href="pcrecallout.html">pcrecallout</a>
documentation.


The tables field is used to pass a character tables pointer to
pcre_exec(); this overrides the value that is stored with the compiled
pattern. A non-NULL value is stored with the compiled pattern only if custom
tables were supplied to pcre_compile() via its tableptr argument.
If NULL is passed to pcre_exec() using this mechanism, it forces PCRE's
internal tables to be used. This facility is helpful when re-using patterns
that have been saved after compiling with an external set of tables, because
the external tables might be at a different address when pcre_exec() is
called. See the
<a href="pcreprecompile.html">pcreprecompile</a>
documentation for a discussion of saving compiled patterns for later use.
<a name="execoptions"></a>
 
Option bits for pcre_exec()
 

The unused bits of the options argument for pcre_exec() must be
zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and PCRE_PARTIAL.
<pre>
 PCRE_ANCHORED
</pre>
The PCRE_ANCHORED option limits pcre_exec() to matching at the first
matching position. 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.
<pre>
 PCRE_BSR_ANYCRLF
 PCRE_BSR_UNICODE
</pre>
These options (which are mutually exclusive) control what the \R escape
sequence matches. The choice is either to match only CR, LF, or CRLF, or to
match any Unicode newline sequence. These options override the choice that was
made or defaulted when the pattern was compiled.
<pre>
 PCRE_NEWLINE_CR
 PCRE_NEWLINE_LF
 PCRE_NEWLINE_CRLF
 PCRE_NEWLINE_ANYCRLF
 PCRE_NEWLINE_ANY
</pre>
These options override the newline definition that was chosen or defaulted when
the pattern was compiled. For details, see the description of
pcre_compile() above. During matching, the newline choice affects the
behaviour of the dot, circumflex, and dollar metacharacters. It may also alter
the way the match position is advanced after a match failure for an unanchored
pattern.


When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is set, and a
match attempt for an unanchored pattern fails when the current position is at a
CRLF sequence, and the pattern contains no explicit matches for CR or LF
characters, the match position is advanced by two characters instead of one, in
other words, to after the CRLF.


The above rule is a compromise that makes the most common cases work as
expected. For example, if the pattern is .+A (and the PCRE_DOTALL option is not
set), it does not match the string "\r\nA" because, after failing at the
start, it skips both the CR and the LF before retrying. However, the pattern
[\r\n]A does match that string, because it contains an explicit CR or LF
reference, and so advances only by one character after the first failure.


An explicit match for CR of LF is either a literal appearance of one of those
characters, or one of the \r or \n escape sequences. Implicit matches such as
[^X] do not count, nor does \s (which includes CR and LF in the characters
that it matches).


Notwithstanding the above, anomalous effects may still occur when CRLF is a
valid newline sequence and explicit \r or \n escapes appear in the pattern.
<pre>
 PCRE_NOTBOL
</pre>
This option specifies that first character of the subject 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. This option affects only the behaviour of the circumflex
metacharacter. It does not affect \A.
<pre>
 PCRE_NOTEOL
</pre>
This option specifies that the end of the subject 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. This option affects only the
behaviour of the dollar metacharacter. It does not affect \Z or \z.
<pre>
 PCRE_NOTEMPTY
</pre>
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
<pre>
 a?b?
</pre>
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 and PCRE_ANCHORED, and then if that fails by advancing the
starting offset (see below) and trying an ordinary match again. There is some
code that demonstrates how to do this in the pcredemo.c sample program.
<pre>
 PCRE_NO_UTF8_CHECK
</pre>
When PCRE_UTF8 is set at compile time, the validity of the subject as a UTF-8
string is automatically checked when pcre_exec() is subsequently called.
The value of startoffset is also checked to ensure that it points to the
start of a UTF-8 character. There is a discussion about the validity of UTF-8
strings in the
<a href="pcre.html#utf8strings">section on UTF-8 support</a>
in the main
<a href="pcre.html">pcre</a>
page. If an invalid UTF-8 sequence of bytes is found, pcre_exec() returns
the error PCRE_ERROR_BADUTF8. If startoffset contains an invalid value,
PCRE_ERROR_BADUTF8_OFFSET is returned.


If you already know that your subject is valid, and you want to skip these
checks for performance reasons, you can set the PCRE_NO_UTF8_CHECK option when
calling pcre_exec(). You might want to do this for the second and
subsequent calls to pcre_exec() if you are making repeated calls to find
all the matches in a single subject string. However, you should be sure that
the value of startoffset points to the start of a UTF-8 character. When
PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8 string as a
subject, or a value of startoffset that does not point to the start of a
UTF-8 character, is undefined. Your program may crash.
<pre>
 PCRE_PARTIAL
</pre>
This option turns on the partial matching feature. If the subject string fails
to match the pattern, but at some point during the matching process the end of
the subject was reached (that is, the subject partially matches the pattern and
the failure to match occurred only because there were not enough subject
characters), pcre_exec() returns PCRE_ERROR_PARTIAL instead of
PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is used, there are restrictions on what
may appear in the pattern. These are discussed in the
<a href="pcrepartial.html">pcrepartial</a>
documentation.

 
The string to be matched by pcre_exec()
 

The subject string is passed to pcre_exec() as a pointer in
subject, a length (in bytes) in length, and a starting byte offset
in startoffset. In UTF-8 mode, the byte offset must point to the start of
a UTF-8 character. Unlike the pattern string, the subject may contain binary
zero bytes. 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
<pre>
 \Biss\B
</pre>
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 made. This can only succeed if the
pattern does not require the match to be at the start of the subject.

 
How pcre_exec() returns captured substrings
 

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.


Captured substrings are returned to the caller via a vector of integers whose
address is passed in ovector. The number of elements in the vector is
passed in ovecsize, which must be a non-negative number. Note: this
argument is NOT the size of ovector in bytes.


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 number passed in
ovecsize should always be a multiple of three. If it is not, it is
rounded down.


When a match is 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
each pair is set to the byte offset of the first character in a substring, and
the second is set to the byte offset of the first character after the end of a
substring. Note: these values are always byte offsets, even in UTF-8
mode. They are not character counts.


The first pair of integers, ovector[0] and ovector[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 one more than the highest numbered pair that has been set.
For example, if two substrings have been captured, the returned value is 3. 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.


If a capturing subpattern is matched repeatedly, it is the last portion of the
string that it matched that is returned.


If the vector is too small to hold all the captured substring offsets, it is
used as far as possible (up to two-thirds of its length), and the function
returns a value of zero. 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 is not 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.


The pcre_info() function can be used to find out how many capturing
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.


It is possible for 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) the return from the
function is 4, and subpatterns 1 and 3 are matched, but 2 is not. When this
happens, both values in the offset pairs corresponding to unused subpatterns
are set to -1.


Offset values that correspond to unused subpatterns at the end of the
expression are also set to -1. For example, if the string "abc" is matched
against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not matched. The
return from the function is 2, because the highest used capturing subpattern
number is 1. However, you can refer to the offsets for the second and third
capturing subpatterns if you wish (assuming the vector is large enough, of
course).


Some convenience functions are provided for extracting the captured substrings
as separate strings. These are described below.
<a name="errorlist"></a>
 
Error return values from pcre_exec()
 

If pcre_exec() fails, it returns a negative number. The following are
defined in the header file:
<pre>
 PCRE_ERROR_NOMATCH (-1)
</pre>
The subject string did not match the pattern.
<pre>
 PCRE_ERROR_NULL (-2)
</pre>
Either code or subject was passed as NULL, or ovector was
NULL and ovecsize was not zero.
<pre>
 PCRE_ERROR_BADOPTION (-3)
</pre>
An unrecognized bit was set in the options argument.
<pre>
 PCRE_ERROR_BADMAGIC (-4)
</pre>
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 and to detect when a pattern that was
compiled in an environment of one endianness is run in an environment with the
other endianness. This is the error that PCRE gives when the magic number is
not present.
<pre>
 PCRE_ERROR_UNKNOWN_OPCODE (-5)
</pre>
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.
<pre>
 PCRE_ERROR_NOMEMORY (-6)
</pre>
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
automatically freed at the end of matching.
<pre>
 PCRE_ERROR_NOSUBSTRING (-7)
</pre>
This error is used by the pcre_copy_substring(),
pcre_get_substring(), and pcre_get_substring_list() functions (see
below). It is never returned by pcre_exec().
<pre>
 PCRE_ERROR_MATCHLIMIT (-8)
</pre>
The backtracking limit, as specified by the match_limit field in a
pcre_extra structure (or defaulted) was reached. See the description
above.
<pre>
 PCRE_ERROR_CALLOUT (-9)
</pre>
This error is never generated by pcre_exec() itself. It is provided for
use by callout functions that want to yield a distinctive error code. See the
<a href="pcrecallout.html">pcrecallout</a>
documentation for details.
<pre>
 PCRE_ERROR_BADUTF8 (-10)
</pre>
A string that contains an invalid UTF-8 byte sequence was passed as a subject.
<pre>
 PCRE_ERROR_BADUTF8_OFFSET (-11)
</pre>
The UTF-8 byte sequence that was passed as a subject was valid, but the value
of startoffset did not point to the beginning of a UTF-8 character.
<pre>
 PCRE_ERROR_PARTIAL (-12)
</pre>
The subject string did not match, but it did match partially. See the
<a href="pcrepartial.html">pcrepartial</a>
documentation for details of partial matching.
<pre>
 PCRE_ERROR_BADPARTIAL (-13)
</pre>
The PCRE_PARTIAL option was used with a compiled pattern containing items that
are not supported for partial matching. See the
<a href="pcrepartial.html">pcrepartial</a>
documentation for details of partial matching.
<pre>
 PCRE_ERROR_INTERNAL (-14)
</pre>
An unexpected internal error has occurred. This error could be caused by a bug
in PCRE or by overwriting of the compiled pattern.
<pre>
 PCRE_ERROR_BADCOUNT (-15)
</pre>
This error is given if the value of the ovecsize argument is negative.
<pre>
 PCRE_ERROR_RECURSIONLIMIT (-21)
</pre>
The internal recursion limit, as specified by the match_limit_recursion
field in a pcre_extra structure (or defaulted) was reached. See the
description above.
<pre>
 PCRE_ERROR_BADNEWLINE (-23)
</pre>
An invalid combination of PCRE_NEWLINE_xxx options was given.


Error numbers -16 to -20 and -22 are not used by pcre_exec().

 <a name="SEC15" href="#TOC1">EXTRACTING CAPTURED SUBSTRINGS BY NUMBER</a> 

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);


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. These functions identify substrings
by number. The next section describes functions for extracting named
substrings.


A substring that contains a binary zero is correctly extracted and has a
further zero added on the end, but the result is not, of course, a C string.
However, you can process such a string by referring to the length that is
returned by pcre_copy_substring() and pcre_get_substring().
Unfortunately, the interface to pcre_get_substring_list() is not adequate
for handling strings containing binary zeros, because the end of the final
string is not independently indicated.


The first three arguments are the same for all three of these functions:
subject is the subject string that 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 ovector, the value passed as stringcount should be the
number of elements in the vector divided by three.


The functions pcre_copy_substring() and pcre_get_substring()
extract a single substring, whose number is given as stringnumber. A
value of zero extracts the substring that matched the entire pattern, whereas
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 these error codes:
<pre>
 PCRE_ERROR_NOMEMORY (-6)
</pre>
The buffer was too small for pcre_copy_substring(), or the attempt to get
memory failed for pcre_get_substring().
<pre>
 PCRE_ERROR_NOSUBSTRING (-7)
</pre>
There is no substring whose number is stringnumber.


The pcre_get_substring_list() function extracts all available substrings
and builds a list of pointers to them. All this is done in a single block of
memory that 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 the error code
<pre>
 PCRE_ERROR_NOMEMORY (-6)
</pre>
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 negative 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. However, PCRE is used in some situations where it is
linked via a special interface to another programming language that cannot use
pcre_free directly; it is for these cases that the functions are
provided.

 <a name="SEC16" href="#TOC1">EXTRACTING CAPTURED SUBSTRINGS BY NAME</a> 

int pcre_get_stringnumber(const pcre *code,
const char *name);


int pcre_copy_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
char *buffer, int buffersize);


int pcre_get_named_substring(const pcre *code,
const char *subject, int *ovector,
int stringcount, const char *stringname,
const char **stringptr);


To extract a substring by name, you first have to find associated number.
For example, for this pattern
<pre>
 (a+)b(?&#60;xxx&#62;\d+)...
</pre>
the number of the subpattern called "xxx" is 2. If the name is known to be
unique (PCRE_DUPNAMES was not set), you can find the number from the name by
calling pcre_get_stringnumber(). The first argument is the compiled
pattern, and the second is the name. The yield of the function is the
subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no subpattern of
that name.


Given the number, you can extract the substring directly, or use one of the
functions described in the previous section. For convenience, there are also
two functions that do the whole job.


Most of the arguments of pcre_copy_named_substring() and
pcre_get_named_substring() are the same as those for the similarly named
functions that extract by number. As these are described in the previous
section, they are not re-described here. There are just two differences:


First, instead of a substring number, a substring name is given. Second, there
is an extra argument, given at the start, which is a pointer to the compiled
pattern. This is needed in order to gain access to the name-to-number
translation table.


These functions call pcre_get_stringnumber(), and if it succeeds, they
then call pcre_copy_substring() or pcre_get_substring(), as
appropriate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names,
the behaviour may not be what you want (see the next section).

 <a name="SEC17" href="#TOC1">DUPLICATE SUBPATTERN NAMES</a> 

int pcre_get_stringtable_entries(const pcre *code,
const char *name, char **first, char **last);


When a pattern is compiled with the PCRE_DUPNAMES option, names for subpatterns
are not required to be unique. Normally, patterns with duplicate names are such
that in any one match, only one of the named subpatterns participates. An
example is shown in the
<a href="pcrepattern.html">pcrepattern</a>
documentation.


When duplicates are present, pcre_copy_named_substring() and
pcre_get_named_substring() return the first substring corresponding to
the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING (-7) is
returned; no data is returned. The pcre_get_stringnumber() function
returns one of the numbers that are associated with the name, but it is not
defined which it is.


If you want to get full details of all captured substrings for a given name,
you must use the pcre_get_stringtable_entries() function. The first
argument is the compiled pattern, and the second is the name. The third and
fourth are pointers to variables which are updated by the function. After it
has run, they point to the first and last entries in the name-to-number table
for the given name. The function itself returns the length of each entry, or
PCRE_ERROR_NOSUBSTRING (-7) if there are none. The format of the table is
described above in the section entitled Information about a pattern.
Given all the relevant entries for the name, you can extract each of their
numbers, and hence the captured data, if any.

 <a name="SEC18" href="#TOC1">FINDING ALL POSSIBLE MATCHES</a> 

The traditional matching function uses a similar algorithm to Perl, which stops
when it finds the first match, starting at a given point in the subject. If you
want to find all possible matches, or the longest possible match, consider
using the alternative matching function (see below) instead. If you cannot use
the alternative function, but still need to find all possible matches, you
can kludge it up by making use of the callout facility, which is described in
the
<a href="pcrecallout.html">pcrecallout</a>
documentation.


What you have to do is to insert a callout right at the end of the pattern.
When your callout function is called, extract and save the current matched
substring. Then return 1, which forces pcre_exec() to backtrack and try
other alternatives. Ultimately, when it runs out of matches, pcre_exec()
will yield PCRE_ERROR_NOMATCH.
<a name="dfamatch"></a>
 <a name="SEC19" href="#TOC1">MATCHING A PATTERN: THE ALTERNATIVE FUNCTION</a> 

int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
const char *subject, int length, int startoffset,
int options, int *ovector, int ovecsize,
int *workspace, int wscount);


The function pcre_dfa_exec() is called to match a subject string against
a compiled pattern, using a matching algorithm that scans the subject string
just once, and does not backtrack. This has different characteristics to the
normal algorithm, and is not compatible with Perl. Some of the features of PCRE
patterns are not supported. Nevertheless, there are times when this kind of
matching can be useful. For a discussion of the two matching algorithms, see
the
<a href="pcrematching.html">pcrematching</a>
documentation.


The arguments for the pcre_dfa_exec() function are the same as for
pcre_exec(), plus two extras. The ovector argument is used in a
different way, and this is described below. The other common arguments are used
in the same way as for pcre_exec(), so their description is not repeated
here.


The two additional arguments provide workspace for the function. The workspace
vector should contain at least 20 elements. It is used for keeping track of
multiple paths through the pattern tree. More workspace will be needed for
patterns and subjects where there are a lot of potential matches.


Here is an example of a simple call to pcre_dfa_exec():
<pre>
 int rc;
 int ovector[10];
 int wspace[20];
 rc = pcre_dfa_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 of integers for substring information */
 10, /* number of elements (NOT size in bytes) */
 wspace, /* working space vector */
 20); /* number of elements (NOT size in bytes) */
</PRE>

 
Option bits for pcre_dfa_exec()
 

The unused bits of the options argument for pcre_dfa_exec() must be
zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL,
PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last three of these are
the same as for pcre_exec(), so their description is not repeated here.
<pre>
 PCRE_PARTIAL
</pre>
This has the same general effect as it does for pcre_exec(), but the
details are slightly different. When PCRE_PARTIAL is set for
pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
PCRE_ERROR_PARTIAL if the end of the subject is reached, there have been no
complete matches, but there is still at least one matching possibility. The
portion of the string that provided the partial match is set as the first
matching string.
<pre>
 PCRE_DFA_SHORTEST
</pre>
Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to stop as
soon as it has found one match. Because of the way the alternative algorithm
works, this is necessarily the shortest possible match at the first possible
matching point in the subject string.
<pre>
 PCRE_DFA_RESTART
</pre>
When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and returns
a partial match, it is possible to call it again, with additional subject
characters, and have it continue with the same match. The PCRE_DFA_RESTART
option requests this action; when it is set, the workspace and
wscount options must reference the same vector as before because data
about the match so far is left in them after a partial match. There is more
discussion of this facility in the
<a href="pcrepartial.html">pcrepartial</a>
documentation.

 
Successful returns from pcre_dfa_exec()
 

When pcre_dfa_exec() succeeds, it may have matched more than one
substring in the subject. Note, however, that all the matches from one run of
the function start at the same point in the subject. The shorter matches are
all initial substrings of the longer matches. For example, if the pattern
<pre>
 &#60;.*&#62;
</pre>
is matched against the string
<pre>
 This is &#60;something&#62; &#60;something else&#62; &#60;something further&#62; no more
</pre>
the three matched strings are
<pre>
 &#60;something&#62;
 &#60;something&#62; &#60;something else&#62;
 &#60;something&#62; &#60;something else&#62; &#60;something further&#62;
</pre>
On success, the yield of the function is a number greater than zero, which is
the number of matched substrings. The substrings themselves are returned in
ovector. Each string uses two elements; the first is the offset to the
start, and the second is the offset to the end. In fact, all the strings have
the same start offset. (Space could have been saved by giving this only once,
but it was decided to retain some compatibility with the way pcre_exec()
returns data, even though the meaning of the strings is different.)


The strings are returned in reverse order of length; that is, the longest
matching string is given first. If there were too many matches to fit into
ovector, the yield of the function is zero, and the vector is filled with
the longest matches.

 
Error returns from pcre_dfa_exec()
 

The pcre_dfa_exec() function returns a negative number when it fails.
Many of the errors are the same as for pcre_exec(), and these are
described
<a href="#errorlist">above.</a>
There are in addition the following errors that are specific to
pcre_dfa_exec():
<pre>
 PCRE_ERROR_DFA_UITEM (-16)
</pre>
This return is given if pcre_dfa_exec() encounters an item in the pattern
that it does not support, for instance, the use of \C or a back reference.
<pre>
 PCRE_ERROR_DFA_UCOND (-17)
</pre>
This return is given if pcre_dfa_exec() encounters a condition item that
uses a back reference for the condition, or a test for recursion in a specific
group. These are not supported.
<pre>
 PCRE_ERROR_DFA_UMLIMIT (-18)
</pre>
This return is given if pcre_dfa_exec() is called with an extra
block that contains a setting of the match_limit field. This is not
supported (it is meaningless).
<pre>
 PCRE_ERROR_DFA_WSSIZE (-19)
</pre>
This return is given if pcre_dfa_exec() runs out of space in the
workspace vector.
<pre>
 PCRE_ERROR_DFA_RECURSE (-20)
</pre>
When a recursive subpattern is processed, the matching function calls itself
recursively, using private vectors for ovector and workspace. This
error is given if the output vector is not large enough. This should be
extremely rare, as a vector of size 1000 is used.

 <a name="SEC20" href="#TOC1">SEE ALSO</a> 

pcrebuild(3), pcrecallout(3), pcrecpp(3)(3),
pcrematching(3), pcrepartial(3), pcreposix(3),
pcreprecompile(3), pcresample(3), pcrestack(3).

 <a name="SEC21" href="#TOC1">AUTHOR</a> 

Philip Hazel
 
University Computing Service
 
Cambridge CB2 3QH, England.
 

 <a name="SEC22" href="#TOC1">REVISION</a> 

Last updated: 24 August 2008
 
Copyright &copy; 1997-2008 University of Cambridge.
 

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