trunk/doc/pcreapi.3

.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE NATIVE API"
.rs
.sp
.B #include <pcre.h>
.PP
.SM
.br
.B pcre *pcre_compile(const char *\fIpattern\fP, int \fIoptions\fP,
.ti +5n
.B const char **\fIerrptr\fP, int *\fIerroffset\fP,
.ti +5n
.B const unsigned char *\fItableptr\fP);
.PP
.br
.B pcre_extra *pcre_study(const pcre *\fIcode\fP, int \fIoptions\fP,
.ti +5n
.B const char **\fIerrptr\fP);
.PP
.br
.B int pcre_exec(const pcre *\fIcode\fP, "const pcre_extra *\fIextra\fP,"
.ti +5n
.B "const char *\fIsubject\fP," int \fIlength\fP, int \fIstartoffset\fP,
.ti +5n
.B int \fIoptions\fP, int *\fIovector\fP, int \fIovecsize\fP);
.PP
.br
.B int pcre_copy_named_substring(const pcre *\fIcode\fP,
.ti +5n
.B const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, const char *\fIstringname\fP,
.ti +5n
.B char *\fIbuffer\fP, int \fIbuffersize\fP);
.PP
.br
.B int pcre_copy_substring(const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, int \fIstringnumber\fP, char *\fIbuffer\fP,
.ti +5n
.B int \fIbuffersize\fP);
.PP
.br
.B int pcre_get_named_substring(const pcre *\fIcode\fP,
.ti +5n
.B const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, const char *\fIstringname\fP,
.ti +5n
.B const char **\fIstringptr\fP);
.PP
.br
.B int pcre_get_stringnumber(const pcre *\fIcode\fP,
.ti +5n
.B const char *\fIname\fP);
.PP
.br
.B int pcre_get_substring(const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, int \fIstringnumber\fP,
.ti +5n
.B const char **\fIstringptr\fP);
.PP
.br
.B int pcre_get_substring_list(const char *\fIsubject\fP,
.ti +5n
.B int *\fIovector\fP, int \fIstringcount\fP, "const char ***\fIlistptr\fP);"
.PP
.br
.B void pcre_free_substring(const char *\fIstringptr\fP);
.PP
.br
.B void pcre_free_substring_list(const char **\fIstringptr\fP);
.PP
.br
.B const unsigned char *pcre_maketables(void);
.PP
.br
.B int pcre_fullinfo(const pcre *\fIcode\fP, "const pcre_extra *\fIextra\fP,"
.ti +5n
.B int \fIwhat\fP, void *\fIwhere\fP);
.PP
.br
.B int pcre_info(const pcre *\fIcode\fP, int *\fIoptptr\fP, int
.B *\fIfirstcharptr\fP);
.PP
.br
.B int pcre_config(int \fIwhat\fP, void *\fIwhere\fP);
.PP
.br
.B char *pcre_version(void);
.PP
.br
.B void *(*pcre_malloc)(size_t);
.PP
.br
.B void (*pcre_free)(void *);
.PP
.br
.B void *(*pcre_stack_malloc)(size_t);
.PP
.br
.B void (*pcre_stack_free)(void *);
.PP
.br
.B int (*pcre_callout)(pcre_callout_block *);
.
.
.SH "PCRE API OVERVIEW"
.rs
.sp
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
.\" HREF
\fBpcreposix\fP
.\"
documentation.
.P
The native API function prototypes are defined in the header file \fBpcre.h\fP,
and on Unix systems the library itself is called \fBlibpcre\fP. It can
normally be accessed by adding \fB-lpcre\fP 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.
.P
The functions \fBpcre_compile()\fP, \fBpcre_study()\fP, and \fBpcre_exec()\fP
are used for compiling and matching regular expressions. A sample program that
demonstrates the simplest way of using them is provided in the file called
\fIpcredemo.c\fP in the source distribution. The
.\" HREF
\fBpcresample\fP
.\"
documentation describes how to run it.
.P
In addition to the main compiling and matching functions, there are convenience
functions for extracting captured substrings from a matched subject string.
They are:
.sp
  \fBpcre_copy_substring()\fP
  \fBpcre_copy_named_substring()\fP
  \fBpcre_get_substring()\fP
  \fBpcre_get_named_substring()\fP
  \fBpcre_get_substring_list()\fP
  \fBpcre_get_stringnumber()\fP
.sp
\fBpcre_free_substring()\fP and \fBpcre_free_substring_list()\fP are also
provided, to free the memory used for extracted strings.
.P
The function \fBpcre_maketables()\fP is used to build a set of character tables
in the current locale for passing to \fBpcre_compile()\fP or \fBpcre_exec()\fP.
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.
.P
The function \fBpcre_fullinfo()\fP is used to find out information about a
compiled pattern; \fBpcre_info()\fP is an obsolete version that returns only
some of the available information, but is retained for backwards compatibility.
The function \fBpcre_version()\fP returns a pointer to a string containing the
version of PCRE and its date of release.
.P
The global variables \fBpcre_malloc\fP and \fBpcre_free\fP initially contain
the entry points of the standard \fBmalloc()\fP and \fBfree()\fP functions,
respectively. PCRE calls the memory management functions via these variables,
so a calling program can replace them if it wishes to intercept the calls. This
should be done before calling any PCRE functions.
.P
The global variables \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP 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. This 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.
.P
The global variable \fBpcre_callout\fP 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
.\" HREF
\fBpcrecallout\fP
.\"
documentation.
.
.
.SH MULTITHREADING
.rs
.sp
The PCRE functions can be used in multi-threading applications, with the
proviso that the memory management functions pointed to by \fBpcre_malloc\fP,
\fBpcre_free\fP, \fBpcre_stack_malloc\fP, and \fBpcre_stack_free\fP, and the
callout function pointed to by \fBpcre_callout\fP, are shared by all threads.
.P
The compiled form of a regular expression is not altered during matching, so
the same compiled pattern can safely be used by several threads at once.
.
.
.SH "SAVING PRECOMPILED PATTERNS FOR LATER USE"
.rs
.sp
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
.\" HREF
\fBpcreprecompile\fP
.\"
documentation.
.
.
.SH "CHECKING BUILD-TIME OPTIONS"
.rs
.sp
.B int pcre_config(int \fIwhat\fP, void *\fIwhere\fP);
.PP
The function \fBpcre_config()\fP makes it possible for a PCRE client to
discover which optional features have been compiled into the PCRE library. The
.\" HREF
\fBpcrebuild\fP
.\"
documentation has more details about these optional features.
.P
The first argument for \fBpcre_config()\fP 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:
.sp
  PCRE_CONFIG_UTF8
.sp
The output is an integer that is set to one if UTF-8 support is available;
otherwise it is set to zero.
.sp
  PCRE_CONFIG_UNICODE_PROPERTIES
.sp
The output is an integer that is set to one if support for Unicode character
properties is available; otherwise it is set to zero.
.sp
  PCRE_CONFIG_NEWLINE
.sp
The output is an integer that is set to the value of the code that is used for
the newline character. It is either linefeed (10) or carriage return (13), and
should normally be the standard character for your operating system.
.sp
  PCRE_CONFIG_LINK_SIZE
.sp
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.
.sp
  PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
.sp
The output is an integer that contains the threshold above which the POSIX
interface uses \fBmalloc()\fP for output vectors. Further details are given in
the
.\" HREF
\fBpcreposix\fP
.\"
documentation.
.sp
  PCRE_CONFIG_MATCH_LIMIT
.sp
The output is an integer that gives the default limit for the number of
internal matching function calls in a \fBpcre_exec()\fP execution. Further
details are given with \fBpcre_exec()\fP below.
.sp
  PCRE_CONFIG_STACKRECURSE
.sp
The output is an integer that is set to one if internal recursion 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, \fBpcre_stack_malloc\fP and \fBpcre_stack_free\fP are
called to manage memory blocks on the heap, thus avoiding the use of the stack.
.
.
.SH "COMPILING A PATTERN"
.rs
.sp
.B pcre *pcre_compile(const char *\fIpattern\fP, int \fIoptions\fP,
.ti +5n
.B const char **\fIerrptr\fP, int *\fIerroffset\fP,
.ti +5n
.B const unsigned char *\fItableptr\fP);
.P
The function \fBpcre_compile()\fP 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 \fIpattern\fP argument. A pointer to a single block of memory
that is obtained via \fBpcre_malloc\fP is returned. This contains the compiled
code and related data. The \fBpcre\fP type is defined for the returned block;
this is a typedef for a structure whose contents are not externally defined. It
is up to the caller to free the memory when it is no longer required.
.P
Although the compiled code of a PCRE regex is relocatable, that is, it does not
depend on memory location, the complete \fBpcre\fP data block is not
fully relocatable, because it may contain a copy of the \fItableptr\fP
argument, which is an address (see below).
.P
The \fIoptions\fP argument contains independent bits 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
.\" HREF
\fBpcrepattern\fP
.\"
documentation). For these options, the contents of the \fIoptions\fP argument
specifies their initial settings at the start of compilation and execution. The
PCRE_ANCHORED option can be set at the time of matching as well as at compile
time.
.P
If \fIerrptr\fP is NULL, \fBpcre_compile()\fP returns NULL immediately.
Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fP returns
NULL, and sets the variable pointed to by \fIerrptr\fP 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
\fIerroffset\fP, which must not be NULL. If it is, an immediate error is given.
.P
If the final argument, \fItableptr\fP, is NULL, PCRE uses a default set of
character tables that are built when PCRE is compiled, using the default C
locale. Otherwise, \fItableptr\fP must be an address that is the result of a
call to \fBpcre_maketables()\fP. This value is stored with the compiled
pattern, and used again by \fBpcre_exec()\fP, unless another table pointer is
passed to it. For more discussion, see the section on locale support below.
.P
This code fragment shows a typical straightforward call to \fBpcre_compile()\fP:
.sp
  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 */
.sp
The following names for option bits are defined in the \fBpcre.h\fP header
file:
.sp
  PCRE_ANCHORED
.sp
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.
.sp
  PCRE_AUTO_CALLOUT
.sp
If this bit is set, \fBpcre_compile()\fP automatically inserts callout items,
all with number 255, before each pattern item. For discussion of the callout
facility, see the
.\" HREF
\fBpcrecallout\fP
.\"
documentation.
.sp
  PCRE_CASELESS
.sp
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. When running in UTF-8 mode, case support for
high-valued characters is available only when PCRE is built with Unicode
character property support.
.sp
  PCRE_DOLLAR_ENDONLY
.sp
If this bit is set, a dollar metacharacter in the pattern matches only at the
end of the subject string. Without this option, a dollar also matches
immediately before the final character if it is a newline (but not before any
other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
set. There is no equivalent to this option in Perl, and no way to set it within
a pattern.
.sp
  PCRE_DOTALL
.sp
If this bit is set, a dot metacharater in the pattern matches all characters,
including newlines. Without it, newlines are excluded. This option is
equivalent to Perl's /s option, and it can be changed within a pattern by a
(?s) option setting. A negative class such as [^a] always matches a newline
character, independent of the setting of this option.
.sp
  PCRE_EXTENDED
.sp
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 character,
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.
.P
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.
.sp
  PCRE_EXTRA
.sp
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.
.sp
  PCRE_MULTILINE
.sp
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.
.P
When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
match immediately following or immediately before any newline in the subject
string, respectively, as well as at the very start and end. This is equivalent
to Perl's /m option, and it can be changed within a pattern by a (?m) option
setting. If there are no "\en" characters in a subject string, or no
occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no effect.
.sp
  PCRE_NO_AUTO_CAPTURE
.sp
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.
.sp
  PCRE_UNGREEDY
.sp
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.
.sp
  PCRE_UTF8
.sp
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
.\" HTML <a href="pcre.html#utf8support">
.\" </a>
section on UTF-8 support
.\"
in the main
.\" HREF
\fBpcre\fP
.\"
page.
.sp
  PCRE_NO_UTF8_CHECK
.sp
When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
automatically checked. If an invalid UTF-8 sequence of bytes is found,
\fBpcre_compile()\fP 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 \fBpcre_exec()\fP, to suppress the
UTF-8 validity checking of subject strings.
.
.
.SH "STUDYING A PATTERN"
.rs
.sp
.B pcre_extra *pcre_study(const pcre *\fIcode\fP, int \fIoptions\fP,
.ti +5n
.B const char **\fIerrptr\fP);
.PP
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 \fBpcre_study()\fP takes a pointer to a compiled pattern as its first
argument. If studying the pattern produces additional information that will
help speed up matching, \fBpcre_study()\fP returns a pointer to a
\fBpcre_extra\fP block, in which the \fIstudy_data\fP field points to the
results of the study.
.P
The returned value from \fBpcre_study()\fP can be passed directly to
\fBpcre_exec()\fP. However, a \fBpcre_extra\fP block also contains other
fields that can be set by the caller before the block is passed; these are
described
.\" HTML <a href="#extradata">
.\" </a>
below
.\"
in the section on matching a pattern.
.P
If studying the pattern does not produce any additional information,
\fBpcre_study()\fP returns NULL. In that circumstance, if the calling program
wants to pass any of the other fields to \fBpcre_exec()\fP, it must set up its
own \fBpcre_extra\fP block.
.P
The second argument of \fBpcre_study()\fP contains option bits. At present, no
options are defined, and this argument should always be zero.
.P
The third argument for \fBpcre_study()\fP 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 points to a textual error message. You should
therefore test the error pointer for NULL after calling \fBpcre_study()\fP, to
be sure that it has run successfully.
.P
This is a typical call to \fBpcre_study\fP():
.sp
  pcre_extra *pe;
  pe = pcre_study(
    re,             /* result of pcre_compile() */
    0,              /* no options exist */
    &error);        /* set to NULL or points to a message */
.sp
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.
.
.
.\" HTML <a name="localesupport"></a>
.SH "LOCALE SUPPORT"
.rs
.sp
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 \ew or \ed, but
can be tested with \ep if PCRE is built with Unicode character property
support.)
.P
An internal set of tables is created in the default C locale when PCRE is
built. This is used when the final argument of \fBpcre_compile()\fP is NULL,
and is sufficient for many applications. An alternative set of tables can,
however, be supplied. 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.
.P
External tables are built by calling the \fBpcre_maketables()\fP function,
which has no arguments, in the relevant locale. The result can then be passed
to \fBpcre_compile()\fP or \fBpcre_exec()\fP 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:
.sp
  setlocale(LC_CTYPE, "fr_FR");
  tables = pcre_maketables();
  re = pcre_compile(..., tables);
.sp
When \fBpcre_maketables()\fP runs, the tables are built in memory that is
obtained via \fBpcre_malloc\fP. It is the caller's responsibility to ensure
that the memory containing the tables remains available for as long as it is
needed.
.P
The pointer that is passed to \fBpcre_compile()\fP is saved with the compiled
pattern, and the same tables are used via this pointer by \fBpcre_study()\fP
and normally also by \fBpcre_exec()\fP. 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.
.P
It is possible to pass a table pointer or NULL (indicating the use of the
internal tables) to \fBpcre_exec()\fP. 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.
.
.
.SH "INFORMATION ABOUT A PATTERN"
.rs
.sp
.B int pcre_fullinfo(const pcre *\fIcode\fP, "const pcre_extra *\fIextra\fP,"
.ti +5n
.B int \fIwhat\fP, void *\fIwhere\fP);
.PP
The \fBpcre_fullinfo()\fP function returns information about a compiled
pattern. It replaces the obsolete \fBpcre_info()\fP function, which is
nevertheless retained for backwards compability (and is documented below).
.P
The first argument for \fBpcre_fullinfo()\fP is a pointer to the compiled
pattern. The second argument is the result of \fBpcre_study()\fP, 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:
.sp
  PCRE_ERROR_NULL       the argument \fIcode\fP was NULL
                        the argument \fIwhere\fP was NULL
  PCRE_ERROR_BADMAGIC   the "magic number" was not found
  PCRE_ERROR_BADOPTION  the value of \fIwhat\fP was invalid
.sp
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
\fBpcre_fullinfo()\fP, to obtain the length of the compiled pattern:
.sp
  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 */
.sp
The possible values for the third argument are defined in \fBpcre.h\fP, and are
as follows:
.sp
  PCRE_INFO_BACKREFMAX
.sp
Return the number of the highest back reference in the pattern. The fourth
argument should point to an \fBint\fP variable. Zero is returned if there are
no back references.
.sp
  PCRE_INFO_CAPTURECOUNT
.sp
Return the number of capturing subpatterns in the pattern. The fourth argument
should point to an \fBint\fP variable.
.sp
  PCRE_INFO_DEFAULTTABLES
.sp
Return a pointer to the internal default character tables within PCRE. The
fourth argument should point to an \fBunsigned char *\fP variable. This
information call is provided for internal use by the \fBpcre_study()\fP
function. External callers can cause PCRE to use its internal tables by passing
a NULL table pointer.
.sp
  PCRE_INFO_FIRSTBYTE
.sp
Return information about the first byte of any matched string, for a
non-anchored pattern. (This option used to be called PCRE_INFO_FIRSTCHAR; the
old name is still recognized for backwards compatibility.)
.P
If there is a fixed first byte, for example, from a pattern such as
(cat|cow|coyote), it is returned in the integer pointed to by \fIwhere\fP.
Otherwise, if either
.sp
(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
starts with "^", or
.sp
(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set
(if it were set, the pattern would be anchored),
.sp
-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.
.sp
  PCRE_INFO_FIRSTTABLE
.sp
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 \fBunsigned char *\fP variable.
.sp
  PCRE_INFO_LASTLITERAL
.sp
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 \fBint\fP 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\ed+z\ed+/ the returned value is "z", but for /^a\edz\ed/ the returned value
is -1.
.sp
  PCRE_INFO_NAMECOUNT
  PCRE_INFO_NAMEENTRYSIZE
  PCRE_INFO_NAMETABLE
.sp
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. A convenience function called \fBpcre_get_named_substring()\fP
is provided for extracting an individual captured substring 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
\fBpcre_exec()\fP below). To do the conversion, you need to use the
name-to-number map, which is described by these three values.
.P
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 \fBint\fP 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 \fBchar\fP). 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. For example, consider the following pattern (assume
PCRE_EXTENDED is set, so white space - including newlines - is ignored):
.sp
.\" JOIN
  (?P<date> (?P<year>(\ed\ed)?\ed\ed) -
  (?P<month>\ed\ed) - (?P<day>\ed\ed) )
.sp
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 ??:
.sp
  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 ??
.sp
When writing code to extract data from named subpatterns using the
name-to-number map, remember that the length of each entry is likely to be
different for each compiled pattern.
.sp
  PCRE_INFO_OPTIONS
.sp
Return a copy of the options with which the pattern was compiled. The fourth
argument should point to an \fBunsigned long int\fP variable. These option bits
are those specified in the call to \fBpcre_compile()\fP, modified by any
top-level option settings within the pattern itself.
.P
A pattern is automatically anchored by PCRE if all of its top-level
alternatives begin with one of the following:
.sp
  ^     unless PCRE_MULTILINE is set
  \eA    always
  \eG    always
.\" JOIN
  .*    if PCRE_DOTALL is set and there are no back
          references to the subpattern in which .* appears
.sp
For such patterns, the PCRE_ANCHORED bit is set in the options returned by
\fBpcre_fullinfo()\fP.
.sp
  PCRE_INFO_SIZE
.sp
Return the size of the compiled pattern, that is, the value that was passed as
the argument to \fBpcre_malloc()\fP when PCRE was getting memory in which to
place the compiled data. The fourth argument should point to a \fBsize_t\fP
variable.
.sp
  PCRE_INFO_STUDYSIZE
.sp
Return the size of the data block pointed to by the \fIstudy_data\fP field in
a \fBpcre_extra\fP block. That is, it is the value that was passed to
\fBpcre_malloc()\fP when PCRE was getting memory into which to place the data
created by \fBpcre_study()\fP. The fourth argument should point to a
\fBsize_t\fP variable.
.
.
.SH "OBSOLETE INFO FUNCTION"
.rs
.sp
.B int pcre_info(const pcre *\fIcode\fP, int *\fIoptptr\fP, int
.B *\fIfirstcharptr\fP);
.PP
The \fBpcre_info()\fP function is now obsolete because its interface is too
restrictive to return all the available data about a compiled pattern. New
programs should use \fBpcre_fullinfo()\fP instead. The yield of
\fBpcre_info()\fP is the number of capturing subpatterns, or one of the
following negative numbers:
.sp
  PCRE_ERROR_NULL       the argument \fIcode\fP was NULL
  PCRE_ERROR_BADMAGIC   the "magic number" was not found
.sp
If the \fIoptptr\fP argument is not NULL, a copy of the options with which the
pattern was compiled is placed in the integer it points to (see
PCRE_INFO_OPTIONS above).
.P
If the pattern is not anchored and the \fIfirstcharptr\fP argument is not NULL,
it is used to pass back information about the first character of any matched
string (see PCRE_INFO_FIRSTBYTE above).
.
.
.SH "MATCHING A PATTERN"
.rs
.sp
.B int pcre_exec(const pcre *\fIcode\fP, "const pcre_extra *\fIextra\fP,"
.ti +5n
.B "const char *\fIsubject\fP," int \fIlength\fP, int \fIstartoffset\fP,
.ti +5n
.B int \fIoptions\fP, int *\fIovector\fP, int \fIovecsize\fP);
.P
The function \fBpcre_exec()\fP is called to match a subject string against a
compiled pattern, which is passed in the \fIcode\fP argument. If the
pattern has been studied, the result of the study should be passed in the
\fIextra\fP argument.
.P
In most applications, the pattern will have been compiled (and optionally
studied) in the same process that calls \fBpcre_exec()\fP. 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
.\" HREF
\fBpcreprecompile\fP
.\"
documentation.
.P
Here is an example of a simple call to \fBpcre_exec()\fP:
.sp
  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 in the vector (NOT size in bytes) */
.
.\" HTML <a name="extradata"></a>
.SS "Extra data for \fBpcre_exec()\fR"
.rs
.sp
If the \fIextra\fP argument is not NULL, it must point to a \fBpcre_extra\fP
data block. The \fBpcre_study()\fP 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 fields in a \fBpcre_extra\fP block are as
follows:
.sp
  unsigned long int \fIflags\fP;
  void *\fIstudy_data\fP;
  unsigned long int \fImatch_limit\fP;
  void *\fIcallout_data\fP;
  const unsigned char *\fItables\fP;
.sp
The \fIflags\fP field is a bitmap that specifies which of the other fields
are set. The flag bits are:
.sp
  PCRE_EXTRA_STUDY_DATA
  PCRE_EXTRA_MATCH_LIMIT
  PCRE_EXTRA_CALLOUT_DATA
  PCRE_EXTRA_TABLES
.sp
Other flag bits should be set to zero. The \fIstudy_data\fP field is set in the
\fBpcre_extra\fP block that is returned by \fBpcre_study()\fP, 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.
.P
The \fImatch_limit\fP 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.
.P
Internally, PCRE uses a function called \fBmatch()\fP which it calls repeatedly
(sometimes recursively). The limit is imposed on the number of times this
function is called during a match, which has the effect of limiting the amount
of recursion and backtracking that can take place. For patterns that are not
anchored, the count starts from zero for each position in the subject string.
.P
The default limit for the library can be set when PCRE is built; the default
default is 10 million, which handles all but the most extreme cases. You can
reduce the default by suppling \fBpcre_exec()\fP with a \fBpcre_extra\fP block
in which \fImatch_limit\fP is set to a smaller value, and
PCRE_EXTRA_MATCH_LIMIT is set in the \fIflags\fP field. If the limit is
exceeded, \fBpcre_exec()\fP returns PCRE_ERROR_MATCHLIMIT.
.P
The \fIpcre_callout\fP field is used in conjunction with the "callout" feature,
which is described in the
.\" HREF
\fBpcrecallout\fP
.\"
documentation.
.P
The \fItables\fP field is used to pass a character tables pointer to
\fBpcre_exec()\fP; 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 \fBpcre_compile()\fP via its \fItableptr\fP argument.
If NULL is passed to \fBpcre_exec()\fP 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 \fBpcre_exec()\fP is
called. See the
.\" HREF
\fBpcreprecompile\fP
.\"
documentation for a discussion of saving compiled patterns for later use.
.
.SS "Option bits for \fBpcre_exec()\fP"
.rs
.sp
The unused bits of the \fIoptions\fP argument for \fBpcre_exec()\fP must be
zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NOTBOL,
PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and PCRE_PARTIAL.
.sp
  PCRE_ANCHORED
.sp
The PCRE_ANCHORED option limits \fBpcre_exec()\fP 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.
.sp
  PCRE_NOTBOL
.sp
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 \eA.
.sp
  PCRE_NOTEOL
.sp
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 \eZ or \ez.
.sp
  PCRE_NOTEMPTY
.sp
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
.sp
  a?b?
.sp
is applied to a string not beginning with "a" or "b", it matches the empty
string at the start of the subject. With PCRE_NOTEMPTY set, this match is not
valid, so PCRE searches further into the string for occurrences of "a" or "b".
.P
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 \fBsplit()\fP 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 \fIpcredemo.c\fP sample program.
.sp
  PCRE_NO_UTF8_CHECK
.sp
When PCRE_UTF8 is set at compile time, the validity of the subject as a UTF-8
string is automatically checked when \fBpcre_exec()\fP is subsequently called.
The value of \fIstartoffset\fP is also checked to ensure that it points to the
start of a UTF-8 character. If an invalid UTF-8 sequence of bytes is found,
\fBpcre_exec()\fP returns the error PCRE_ERROR_BADUTF8. If \fIstartoffset\fP
contains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
.P
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 \fBpcre_exec()\fP. You might want to do this for the second and
subsequent calls to \fBpcre_exec()\fP 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 \fIstartoffset\fP 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 \fIstartoffset\fP that does not point to the start of a
UTF-8 character, is undefined. Your program may crash.
.sp
  PCRE_PARTIAL
.sp
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), \fBpcre_exec()\fP 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
.\" HREF
\fBpcrepartial\fP
.\"
documentation.
.
.SS "The string to be matched by \fBpcre_exec()\fP"
.rs
.sp
The subject string is passed to \fBpcre_exec()\fP as a pointer in
\fIsubject\fP, a length in \fIlength\fP, and a starting byte offset in
\fIstartoffset\fP. 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.
.P
A non-zero starting offset is useful when searching for another match in the
same subject by calling \fBpcre_exec()\fP again after a previous success.
Setting \fIstartoffset\fP 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
.sp
  \eBiss\eB
.sp
which finds occurrences of "iss" in the middle of words. (\eB matches only if
the current position in the subject is not a word boundary.) When applied to
the string "Mississipi" the first call to \fBpcre_exec()\fP finds the first
occurrence. If \fBpcre_exec()\fP is called again with just the remainder of the
subject, namely "issipi", it does not match, because \eB is always false at the
start of the subject, which is deemed to be a word boundary. However, if
\fBpcre_exec()\fP is passed the entire string again, but with \fIstartoffset\fP
set to 4, it finds the second occurrence of "iss" because it is able to look
behind the starting point to discover that it is preceded by a letter.
.P
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.
.
.SS "How \fBpcre_exec()\fP returns captured substrings"
.rs
.sp
In general, a pattern matches a certain portion of the subject, and in
addition, further substrings from the subject may be picked out by parts of the
pattern. Following the usage in Jeffrey Friedl's book, this is called
"capturing" in what follows, and the phrase "capturing subpattern" is used for
a fragment of a pattern that picks out a substring. PCRE supports several other
kinds of parenthesized subpattern that do not cause substrings to be captured.
.P
Captured substrings are returned to the caller via a vector of integer offsets
whose address is passed in \fIovector\fP. The number of elements in the vector
is passed in \fIovecsize\fP, which must be a non-negative number. \fBNote\fP:
this argument is NOT the size of \fIovector\fP in bytes.
.P
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 \fBpcre_exec()\fP while matching capturing subpatterns,
and is not available for passing back information. The length passed in
\fIovecsize\fP should always be a multiple of three. If it is not, it is
rounded down.
.P
When a match is successful, information about captured substrings is returned
in pairs of integers, starting at the beginning of \fIovector\fP, 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, \fIovector[0]\fP and \fIovector[1]\fP, 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 \fBpcre_exec()\fP
is the number of pairs that have been set. If there are no capturing
subpatterns, the return value from a successful match is 1, indicating that
just the first pair of offsets has been set.
.P
Some convenience functions are provided for extracting the captured substrings
as separate strings. These are described in the following section.
.P
It is possible for an capturing subpattern number \fIn+1\fP to match some
part of the subject when subpattern \fIn\fP has not been used at all. For
example, if the string "abc" is matched against the pattern (a|(z))(bc)
subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
values corresponding to the unused subpattern are set to -1.
.P
If a capturing subpattern is matched repeatedly, it is the last portion of the
string that it matched that is returned.
.P
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. In particular, if the substring offsets are not of
interest, \fBpcre_exec()\fP may be called with \fIovector\fP passed as NULL and
\fIovecsize\fP as zero. However, if the pattern contains back references and
the \fIovector\fP 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 \fIovector\fP.
.P
Note that \fBpcre_info()\fP can be used to find out how many capturing
subpatterns there are in a compiled pattern. The smallest size for
\fIovector\fP that will allow for \fIn\fP captured substrings, in addition to
the offsets of the substring matched by the whole pattern, is (\fIn\fP+1)*3.
.
.SS "Return values from \fBpcre_exec()\fP"
.rs
.sp
If \fBpcre_exec()\fP fails, it returns a negative number. The following are
defined in the header file:
.sp
  PCRE_ERROR_NOMATCH        (-1)
.sp
The subject string did not match the pattern.
.sp
  PCRE_ERROR_NULL           (-2)
.sp
Either \fIcode\fP or \fIsubject\fP was passed as NULL, or \fIovector\fP was
NULL and \fIovecsize\fP was not zero.
.sp
  PCRE_ERROR_BADOPTION      (-3)
.sp
An unrecognized bit was set in the \fIoptions\fP argument.
.sp
  PCRE_ERROR_BADMAGIC       (-4)
.sp
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.
.sp
  PCRE_ERROR_UNKNOWN_NODE   (-5)
.sp
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.
.sp
  PCRE_ERROR_NOMEMORY       (-6)
.sp
If a pattern contains back references, but the \fIovector\fP that is passed to
\fBpcre_exec()\fP 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 \fBpcre_malloc()\fP fails, this error is given. The memory is
automatically freed at the end of matching.
.sp
  PCRE_ERROR_NOSUBSTRING    (-7)
.sp
This error is used by the \fBpcre_copy_substring()\fP,
\fBpcre_get_substring()\fP, and \fBpcre_get_substring_list()\fP functions (see
below). It is never returned by \fBpcre_exec()\fP.
.sp
  PCRE_ERROR_MATCHLIMIT     (-8)
.sp
The recursion and backtracking limit, as specified by the \fImatch_limit\fP
field in a \fBpcre_extra\fP structure (or defaulted) was reached. See the
description above.
.sp
  PCRE_ERROR_CALLOUT        (-9)
.sp
This error is never generated by \fBpcre_exec()\fP itself. It is provided for
use by callout functions that want to yield a distinctive error code. See the
.\" HREF
\fBpcrecallout\fP
.\"
documentation for details.
.sp
  PCRE_ERROR_BADUTF8        (-10)
.sp
A string that contains an invalid UTF-8 byte sequence was passed as a subject.
.sp
  PCRE_ERROR_BADUTF8_OFFSET (-11)
.sp
The UTF-8 byte sequence that was passed as a subject was valid, but the value
of \fIstartoffset\fP did not point to the beginning of a UTF-8 character.
.sp
  PCRE_ERROR_PARTIAL (-12)
.sp
The subject string did not match, but it did match partially. See the
.\" HREF
\fBpcrepartial\fP
.\"
documentation for details of partial matching.
.sp
  PCRE_ERROR_BAD_PARTIAL (-13)
.sp
The PCRE_PARTIAL option was used with a compiled pattern containing items that
are not supported for partial matching. See the
.\" HREF
\fBpcrepartial\fP
.\"
documentation for details of partial matching.
.sp
  PCRE_ERROR_INTERNAL (-14)
.sp
An unexpected internal error has occurred. This error could be caused by a bug
in PCRE or by overwriting of the compiled pattern.
.sp
  PCRE_ERROR_BADCOUNT (-15)
.sp
This error is given if the value of the \fIovecsize\fP argument is negative.
.
.
.SH "EXTRACTING CAPTURED SUBSTRINGS BY NUMBER"
.rs
.sp
.B int pcre_copy_substring(const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, int \fIstringnumber\fP, char *\fIbuffer\fP,
.ti +5n
.B int \fIbuffersize\fP);
.PP
.br
.B int pcre_get_substring(const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, int \fIstringnumber\fP,
.ti +5n
.B const char **\fIstringptr\fP);
.PP
.br
.B int pcre_get_substring_list(const char *\fIsubject\fP,
.ti +5n
.B int *\fIovector\fP, int \fIstringcount\fP, "const char ***\fIlistptr\fP);"
.PP
Captured substrings can be accessed directly by using the offsets returned by
\fBpcre_exec()\fP in \fIovector\fP. For convenience, the functions
\fBpcre_copy_substring()\fP, \fBpcre_get_substring()\fP, and
\fBpcre_get_substring_list()\fP 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.
.P
The first three arguments are the same for all three of these functions:
\fIsubject\fP is the subject string that has just been successfully matched,
\fIovector\fP is a pointer to the vector of integer offsets that was passed to
\fBpcre_exec()\fP, and \fIstringcount\fP 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 \fBpcre_exec()\fP if it is greater
than zero. If \fBpcre_exec()\fP returned zero, indicating that it ran out of
space in \fIovector\fP, the value passed as \fIstringcount\fP should be the
number of elements in the vector divided by three.
.P
The functions \fBpcre_copy_substring()\fP and \fBpcre_get_substring()\fP
extract a single substring, whose number is given as \fIstringnumber\fP. A
value of zero extracts the substring that matched the entire pattern, whereas
higher values extract the captured substrings. For \fBpcre_copy_substring()\fP,
the string is placed in \fIbuffer\fP, whose length is given by
\fIbuffersize\fP, while for \fBpcre_get_substring()\fP a new block of memory is
obtained via \fBpcre_malloc\fP, and its address is returned via
\fIstringptr\fP. The yield of the function is the length of the string, not
including the terminating zero, or one of
.sp
  PCRE_ERROR_NOMEMORY       (-6)
.sp
The buffer was too small for \fBpcre_copy_substring()\fP, or the attempt to get
memory failed for \fBpcre_get_substring()\fP.
.sp
  PCRE_ERROR_NOSUBSTRING    (-7)
.sp
There is no substring whose number is \fIstringnumber\fP.
.P
The \fBpcre_get_substring_list()\fP 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 \fBpcre_malloc\fP. The address of the memory block
is returned via \fIlistptr\fP, 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
.sp
  PCRE_ERROR_NOMEMORY       (-6)
.sp
if the attempt to get the memory block failed.
.P
When any of these functions encounter a substring that is unset, which can
happen when capturing subpattern number \fIn+1\fP matches some part of the
subject, but subpattern \fIn\fP 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 \fIovector\fP, which is negative for unset
substrings.
.P
The two convenience functions \fBpcre_free_substring()\fP and
\fBpcre_free_substring_list()\fP can be used to free the memory returned by
a previous call of \fBpcre_get_substring()\fP or
\fBpcre_get_substring_list()\fP, respectively. They do nothing more than call
the function pointed to by \fBpcre_free\fP, which of course could be called
directly from a C program. However, PCRE is used in some situations where it is
linked via a special interface to another programming language which cannot use
\fBpcre_free\fP directly; it is for these cases that the functions are
provided.
.
.
.SH "EXTRACTING CAPTURED SUBSTRINGS BY NAME"
.rs
.sp
.B int pcre_get_stringnumber(const pcre *\fIcode\fP,
.ti +5n
.B const char *\fIname\fP);
.PP
.br
.B int pcre_copy_named_substring(const pcre *\fIcode\fP,
.ti +5n
.B const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, const char *\fIstringname\fP,
.ti +5n
.B char *\fIbuffer\fP, int \fIbuffersize\fP);
.PP
.br
.B int pcre_get_named_substring(const pcre *\fIcode\fP,
.ti +5n
.B const char *\fIsubject\fP, int *\fIovector\fP,
.ti +5n
.B int \fIstringcount\fP, const char *\fIstringname\fP,
.ti +5n
.B const char **\fIstringptr\fP);
.PP
To extract a substring by name, you first have to find associated number.
For example, for this pattern
.sp
  (a+)b(?<xxx>\ed+)...
.sp
the number of the subpattern called "xxx" is 2. You can find the number from
the name by calling \fBpcre_get_stringnumber()\fP. 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.
.P
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.
.P
Most of the arguments of \fIpcre_copy_named_substring()\fP and
\fIpcre_get_named_substring()\fP 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:
.P
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.
.P
These functions call \fBpcre_get_stringnumber()\fP, and if it succeeds, they
then call \fIpcre_copy_substring()\fP or \fIpcre_get_substring()\fP, as
appropriate.
.P
.in 0
Last updated: 09 September 2004
.br
Copyright (c) 1997-2004 University of Cambridge.