pcre.3

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the following negative numbers:

  PCRE_ERROR_NULL       the argument \fIcode\fR was NULL
                        the argument \fIwhere\fR was NULL
  PCRE_ERROR_BADMAGIC   the "magic number" was not found
  PCRE_ERROR_BADOPTION  the value of \fIwhat\fR was invalid

The possible values for the third argument are defined in \fBpcre.h\fR, and are
as follows:

  PCRE_INFO_OPTIONS

Return a copy of the options with which the pattern was compiled. The fourth
argument should point to au \fBunsigned long int\fR variable. These option bits
are those specified in the call to \fBpcre_compile()\fR, modified by any
top-level option settings within the pattern itself, and with the PCRE_ANCHORED
bit forcibly set if the form of the pattern implies that it can match only at
the start of a subject string.

  PCRE_INFO_SIZE

Return the size of the compiled pattern, that is, the value that was passed as
the argument to \fBpcre_malloc()\fR when PCRE was getting memory in which to
place the compiled data. The fourth argument should point to a \fBsize_t\fR
variable.

  PCRE_INFO_CAPTURECOUNT

Return the number of capturing subpatterns in the pattern. The fourth argument
should point to an \fbint\fR variable.

  PCRE_INFO_BACKREFMAX

Return the number of the highest back reference in the pattern. The fourth
argument should point to an \fBint\fR variable. Zero is returned if there are
no back references.

  PCRE_INFO_FIRSTCHAR

Return information about the first character of any matched string, for a
non-anchored pattern. If there is a fixed first character, e.g. from a pattern
such as (cat|cow|coyote), it is returned in the integer pointed to by
\fIwhere\fR. Otherwise, if either

(a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
starts with "^", or

(b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set
(if it were set, the pattern would be anchored),

-1 is returned, indicating that the pattern matches only at the start of a
subject string or after any "\\n" within the string. Otherwise -2 is returned.
For anchored patterns, -2 is returned.

  PCRE_INFO_FIRSTTABLE

If the pattern was studied, and this resulted in the construction of a 256-bit
table indicating a fixed set of characters for the first character in any
matching string, a pointer to the table is returned. Otherwise NULL is
returned. The fourth argument should point to an \fBunsigned char *\fR
variable.

  PCRE_INFO_LASTLITERAL

For a non-anchored pattern, return the value of the rightmost literal character
which must exist in any matched string, other than at its start. The fourth
argument should point to an \fBint\fR variable. If there is no such character,
or if the pattern is anchored, -1 is returned. For example, for the pattern
/a\\d+z\\d+/ the returned value is 'z'.

The \fBpcre_info()\fR 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()\fR instead. The yield of
\fBpcre_info()\fR is the number of capturing subpatterns, or one of the
following negative numbers:

  PCRE_ERROR_NULL       the argument \fIcode\fR was NULL
  PCRE_ERROR_BADMAGIC   the "magic number" was not found

If the \fIoptptr\fR 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 \fIfirstcharptr\fR argument is not NULL,
it is used to pass back information about the first character of any matched
string (see PCRE_INFO_FIRSTCHAR above).


.SH MATCHING A PATTERN
The function \fBpcre_exec()\fR is called to match a subject string against a
pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
pattern has been studied, the result of the study should be passed in the
\fIextra\fR argument. Otherwise this must be NULL.

The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
unused bits must be zero. However, if a pattern was compiled with
PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
cannot be made unachored at matching time.

There are also three further options that can be set only at matching time:

  PCRE_NOTBOL

The first character of the string is not the beginning of a line, so the
circumflex metacharacter should not match before it. Setting this without
PCRE_MULTILINE (at compile time) causes circumflex never to match.

  PCRE_NOTEOL

The end of the string is not the end of a line, so the 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.

  PCRE_NOTEMPTY

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

  a?b?

is applied to a string not beginning with "a" or "b", it matches the empty
string at the start of the subject. With PCRE_NOTEMPTY set, this match is not
valid, so PCRE searches further into the string for occurrences of "a" or "b".

Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a special case
of a pattern match of the empty string within its \fBsplit()\fR function, and
when using the /g modifier. It is possible to emulate Perl's behaviour after
matching a null string by first trying the match again at the same offset with
PCRE_NOTEMPTY set, and then if that fails by advancing the starting offset (see
below) and trying an ordinary match again.

The subject string is passed as a pointer in \fIsubject\fR, a length in
\fIlength\fR, and a starting offset in \fIstartoffset\fR. Unlike the pattern
string, it may contain binary zero characters. When the starting offset is
zero, the search for a match starts at the beginning of the subject, and this
is by far the most common case.

A non-zero starting offset is useful when searching for another match in the
same subject by calling \fBpcre_exec()\fR again after a previous success.
Setting \fIstartoffset\fR differs from just passing over a shortened string and
setting PCRE_NOTBOL in the case of a pattern that begins with any kind of
lookbehind. For example, consider the pattern

  \\Biss\\B

which finds occurrences of "iss" in the middle of words. (\\B matches only if
the current position in the subject is not a word boundary.) When applied to
the string "Mississipi" the first call to \fBpcre_exec()\fR finds the first
occurrence. If \fBpcre_exec()\fR 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
\fBpcre_exec()\fR is passed the entire string again, but with \fIstartoffset\fR
set to 4, it finds the second occurrence of "iss" because it is able to look
behind the starting point to discover that it is preceded by a letter.

If a non-zero starting offset is passed when the pattern is anchored, one
attempt to match at the given offset is tried. This can only succeed if the
pattern does not require the match to be at the start of the subject.

In general, a pattern matches a certain portion of the 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 integer offsets
whose address is passed in \fIovector\fR. The number of elements in the vector
is passed in \fIovecsize\fR. 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()\fR while
matching capturing subpatterns, and is not available for passing back
information. The length passed in \fIovecsize\fR should always be a multiple of
three. If it is not, it is rounded down.

When a match has been successful, information about captured substrings is
returned in pairs of integers, starting at the beginning of \fIovector\fR, 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]\fR and \fIovector[1]\fR, 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()\fR
is the number of pairs that have been set. If there are no capturing
subpatterns, the return value from a successful match is 1, indicating that
just the first pair of offsets has been set.

Some convenience functions are provided for extracting the captured substrings
as separate strings. These are described in the following section.

It is possible for an capturing subpattern number \fIn+1\fR to match some
part of the subject when subpattern \fIn\fR has not been used at all. For
example, if the string "abc" is matched against the pattern (a|(z))(bc)
subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
values corresponding to the unused subpattern are set to -1.

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

If the vector is too small to hold all the captured substrings, it is used as
far as possible (up to two-thirds of its length), and the function returns a
value of zero. In particular, if the substring offsets are not of interest,
\fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
\fIovecsize\fR as zero. However, if the pattern contains back references and
the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
to get additional memory for use during matching. Thus it is usually advisable
to supply an \fIovector\fR.

Note that \fBpcre_info()\fR can be used to find out how many capturing
subpatterns there are in a compiled pattern. The smallest size for
\fIovector\fR that will allow for \fIn\fR captured substrings in addition to
the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.

If \fBpcre_exec()\fR fails, it returns a negative number. The following are
defined in the header file:

  PCRE_ERROR_NOMATCH        (-1)

The subject string did not match the pattern.

  PCRE_ERROR_NULL           (-2)

Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
NULL and \fIovecsize\fR was not zero.

  PCRE_ERROR_BADOPTION      (-3)

An unrecognized bit was set in the \fIoptions\fR argument.

  PCRE_ERROR_BADMAGIC       (-4)

PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
the case when it is passed a junk pointer. This is the error it gives when the
magic number isn't present.

  PCRE_ERROR_UNKNOWN_NODE   (-5)

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.

  PCRE_ERROR_NOMEMORY       (-6)

If a pattern contains back references, but the \fIovector\fR that is passed to
\fBpcre_exec()\fR 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()\fR fails, this error is given. The memory is freed at
the end of matching.


.SH EXTRACTING CAPTURED SUBSTRINGS
Captured substrings can be accessed directly by using the offsets returned by
\fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
\fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
\fBpcre_get_substring_list()\fR are provided for extracting captured substrings
as new, separate, zero-terminated strings. A substring that contains a binary
zero is correctly extracted and has a further zero added on the end, but the
result does not, of course, function as a C string.

The first three arguments are the same for all three functions: \fIsubject\fR
is the subject string which has just been successfully matched, \fIovector\fR
is a pointer to the vector of integer offsets that was passed to
\fBpcre_exec()\fR, and \fIstringcount\fR 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\fR if it
is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
ran out of space in \fIovector\fR, the value passed as \fIstringcount\fR should
be the size of the vector divided by three.

The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
extract a single substring, whose number is given as \fIstringnumber\fR. A
value of zero extracts the substring that matched the entire pattern, while
higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
the string is placed in \fIbuffer\fR, whose length is given by
\fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is
obtained via \fBpcre_malloc\fR, and its address is returned via
\fIstringptr\fR. The yield of the function is the length of the string, not
including the terminating zero, or one of

  PCRE_ERROR_NOMEMORY       (-6)

The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get