upload http

JIRA: BOTTLENECK-10

Change-Id: I7598427ff904df438ce77c2819ee48ac75ffa8da
Signed-off-by: hongbotian <hongbo.tianhongbo@huawei.com>

1 files changed, 1991 insertions, 0 deletions

diff --git a/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.3 b/rubbos/app/httpd-2.0.64/srclib/pcre/doc/pcre.3
new file mode 100644
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+.TH PCRE 3
+.SH NAME
+pcre - Perl-compatible regular expressions.
+.SH SYNOPSIS
+.B #include <pcre.h>
+.PP
+.SM
+.br
+.B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
+.ti +5n
+.B const char **\fIerrptr\fR, int *\fIerroffset\fR,
+.ti +5n
+.B const unsigned char *\fItableptr\fR);
+.PP
+.br
+.B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
+.ti +5n
+.B const char **\fIerrptr\fR);
+.PP
+.br
+.B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
+.ti +5n
+.B "const char *\fIsubject\fR," int \fIlength\fR, int \fIstartoffset\fR,
+.ti +5n
+.B int \fIoptions\fR, int *\fIovector\fR, int \fIovecsize\fR);
+.PP
+.br
+.B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR,
+.ti +5n
+.B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR,
+.ti +5n
+.B int \fIbuffersize\fR);
+.PP
+.br
+.B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR,
+.ti +5n
+.B int \fIstringcount\fR, int \fIstringnumber\fR,
+.ti +5n
+.B const char **\fIstringptr\fR);
+.PP
+.br
+.B int pcre_get_substring_list(const char *\fIsubject\fR,
+.ti +5n
+.B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);"
+.PP
+.br
+.B void pcre_free_substring(const char *\fIstringptr\fR);
+.PP
+.br
+.B void pcre_free_substring_list(const char **\fIstringptr\fR);
+.PP
+.br
+.B const unsigned char *pcre_maketables(void);
+.PP
+.br
+.B int pcre_fullinfo(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
+.ti +5n
+.B int \fIwhat\fR, void *\fIwhere\fR);
+.PP
+.br
+.B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
+.B *\fIfirstcharptr\fR);
+.PP
+.br
+.B char *pcre_version(void);
+.PP
+.br
+.B void *(*pcre_malloc)(size_t);
+.PP
+.br
+.B void (*pcre_free)(void *);
+
+
+
+.SH DESCRIPTION
+The PCRE library is a set of functions that implement regular expression
+pattern matching using the same syntax and semantics as Perl 5, with just a few
+differences (see below). The current implementation corresponds to Perl 5.005,
+with some additional features from later versions. This includes some
+experimental, incomplete support for UTF-8 encoded strings. Details of exactly
+what is and what is not supported are given below.
+
+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 \fBpcreposix\fR documentation.
+
+The native API function prototypes are defined in the header file \fBpcre.h\fR,
+and on Unix systems the library itself is called \fBlibpcre.a\fR, so can be
+accessed by adding \fB-lpcre\fR to the command for linking an application which
+calls it. The header file defines the macros PCRE_MAJOR and PCRE_MINOR to
+contain the major and minor release numbers for the library. Applications can
+use these to include support for different releases.
+
+The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
+are used for compiling and matching regular expressions. A sample program that
+demonstrates the simplest way of using them is given in the file
+\fIpcredemo.c\fR. The last section of this man page describes how to run it.
+
+The functions \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
+\fBpcre_get_substring_list()\fR are convenience functions for extracting
+captured substrings from a matched subject string; \fBpcre_free_substring()\fR
+and \fBpcre_free_substring_list()\fR are also provided, to free the memory used
+for extracted strings.
+
+The function \fBpcre_maketables()\fR is used (optionally) to build a set of
+character tables in the current locale for passing to \fBpcre_compile()\fR.
+
+The function \fBpcre_fullinfo()\fR is used to find out information about a
+compiled pattern; \fBpcre_info()\fR is an obsolete version which returns only
+some of the available information, but is retained for backwards compatibility.
+The function \fBpcre_version()\fR returns a pointer to a string containing the
+version of PCRE and its date of release.
+
+The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
+the entry points of the standard \fBmalloc()\fR and \fBfree()\fR 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.
+
+
+.SH MULTI-THREADING
+The PCRE functions can be used in multi-threading applications, with the
+proviso that the memory management functions pointed to by \fBpcre_malloc\fR
+and \fBpcre_free\fR 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.
+
+
+.SH COMPILING A PATTERN
+The function \fBpcre_compile()\fR is called to compile a pattern into an
+internal form. The pattern is a C string terminated by a binary zero, and
+is passed in the argument \fIpattern\fR. A pointer to a single block of memory
+that is obtained via \fBpcre_malloc\fR is returned. This contains the compiled
+code and related data. The \fBpcre\fR type is defined for the returned block;
+this is a typedef for a structure whose contents are not externally defined. It
+is up to the caller to free the memory when it is no longer required.
+
+Although the compiled code of a PCRE regex is relocatable, that is, it does not
+depend on memory location, the complete \fBpcre\fR data block is not
+fully relocatable, because it contains a copy of the \fItableptr\fR argument,
+which is an address (see below).
+
+The size of a compiled pattern is roughly proportional to the length of the
+pattern string, except that each character class (other than those containing
+just a single character, negated or not) requires 33 bytes, and repeat
+quantifiers with a minimum greater than one or a bounded maximum cause the
+relevant portions of the compiled pattern to be replicated.
+
+The \fIoptions\fR argument contains independent bits that affect the
+compilation. It should be zero if no options are required. Some of the options,
+in particular, those that are compatible with Perl, can also be set and unset
+from within the pattern (see the detailed description of regular expressions
+below). For these options, the contents of the \fIoptions\fR argument specifies
+their initial settings at the start of compilation and execution. The
+PCRE_ANCHORED option can be set at the time of matching as well as at compile
+time.
+
+If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
+Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
+NULL, and sets the variable pointed to by \fIerrptr\fR 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\fR, which must not be NULL. If it is, an immediate error is given.
+
+If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
+character tables which are built when it is compiled, using the default C
+locale. Otherwise, \fItableptr\fR must be the result of a call to
+\fBpcre_maketables()\fR. See the section on locale support below.
+
+This code fragment shows a typical straightforward call to \fBpcre_compile()\fR:
+
+  pcre *re;
+  const char *error;
+  int erroffset;
+  re = pcre_compile(
+    "^A.*Z",          /* the pattern */
+    0,                /* default options */
+    &error,           /* for error message */
+    &erroffset,       /* for error offset */
+    NULL);            /* use default character tables */
+
+The following option bits are defined in the header file:
+
+  PCRE_ANCHORED
+
+If this bit is set, the pattern is forced to be "anchored", that is, it is
+constrained to match only at the start of the string which is being searched
+(the "subject string"). This effect can also be achieved by appropriate
+constructs in the pattern itself, which is the only way to do it in Perl.
+
+  PCRE_CASELESS
+
+If this bit is set, letters in the pattern match both upper and lower case
+letters. It is equivalent to Perl's /i option.
+
+  PCRE_DOLLAR_ENDONLY
+
+If this bit is set, a dollar metacharacter in the pattern matches only at the
+end of the subject string. Without this option, a dollar also matches
+immediately before the final character if it is a newline (but not before any
+other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
+set. There is no equivalent to this option in Perl.
+
+  PCRE_DOTALL
+
+If this bit is set, a dot metacharater in the pattern matches all characters,
+including newlines. Without it, newlines are excluded. This option is
+equivalent to Perl's /s option. A negative class such as [^a] always matches a
+newline character, independent of the setting of this option.
+
+  PCRE_EXTENDED
+
+If this bit is set, whitespace data characters in the pattern are totally
+ignored except when escaped or inside a character class, and characters between
+an unescaped # outside a character class and the next newline character,
+inclusive, are also ignored. This is equivalent to Perl's /x option, and makes
+it possible to include comments inside complicated patterns. Note, however,
+that this applies only to data characters. Whitespace characters may never
+appear within special character sequences in a pattern, for example within the
+sequence (?( which introduces a conditional subpattern.
+
+  PCRE_EXTRA
+
+This option was invented in order to turn on additional functionality of PCRE
+that is incompatible with Perl, but it is currently of very little use. When
+set, any backslash in a pattern that is followed by a letter that has no
+special meaning causes an error, thus reserving these combinations for future
+expansion. By default, as in Perl, a backslash followed by a letter with no
+special meaning is treated as a literal. There are at present no other features
+controlled by this option. It can also be set by a (?X) option setting within a
+pattern.
+
+  PCRE_MULTILINE
+
+By default, PCRE treats the subject string as consisting of a single "line" of
+characters (even if it actually contains several newlines). The "start of line"
+metacharacter (^) matches only at the start of the string, while the "end of
+line" metacharacter ($) matches only at the end of the string, or before a
+terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as
+Perl.
+
+When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
+match immediately following or immediately before any newline in the subject
+string, respectively, as well as at the very start and end. This is equivalent
+to Perl's /m option. If there are no "\\n" characters in a subject string, or
+no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
+effect.
+
+  PCRE_UNGREEDY
+
+This option inverts the "greediness" of the quantifiers so that they are not
+greedy by default, but become greedy if followed by "?". It is not compatible
+with Perl. It can also be set by a (?U) option setting within the pattern.
+
+  PCRE_UTF8
+
+This option causes PCRE to regard both the pattern and the subject as strings
+of UTF-8 characters instead of just byte strings. However, it is available only
+if PCRE has been built to include UTF-8 support. If not, the use of this option
+provokes an error. Support for UTF-8 is new, experimental, and incomplete.
+Details of exactly what it entails are given below.
+
+
+.SH STUDYING A PATTERN
+When a pattern is going to be used several times, it is worth spending more
+time analyzing it in order to speed up the time taken for matching. The
+function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
+argument, and returns a pointer to a \fBpcre_extra\fR block (another typedef
+for a structure with hidden contents) containing additional information about
+the pattern; this can be passed to \fBpcre_exec()\fR. If no additional
+information is available, NULL is returned.
+
+The second argument contains option bits. At present, no options are defined
+for \fBpcre_study()\fR, and this argument should always be zero.
+
+The third argument for \fBpcre_study()\fR is a pointer to an error message. If
+studying succeeds (even if no data is returned), the variable it points to is
+set to NULL. Otherwise it points to a textual error message.
+
+This is a typical call to \fBpcre_study\fR():
+
+  pcre_extra *pe;
+  pe = pcre_study(
+    re,             /* result of pcre_compile() */
+    0,              /* no options exist */
+    &error);        /* set to NULL or points to a message */
+
+At present, studying a pattern is useful only for non-anchored patterns that do
+not have a single fixed starting character. A bitmap of possible starting
+characters is created.
+
+
+.SH LOCALE SUPPORT
+PCRE handles caseless matching, and determines whether characters are letters,
+digits, or whatever, by reference to a set of tables. The library contains a
+default set of tables which is created in the default C locale when PCRE is
+compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
+and is sufficient for many applications.
+
+An alternative set of tables can, however, be supplied. Such tables are built
+by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
+relevant locale. The result can then be passed to \fBpcre_compile()\fR as often
+as necessary. For example, to build and use tables that are appropriate for the
+French locale (where accented characters with codes greater than 128 are
+treated as letters), the following code could be used:
+
+  setlocale(LC_CTYPE, "fr");
+  tables = pcre_maketables();
+  re = pcre_compile(..., tables);
+
+The tables are built in memory that is obtained via \fBpcre_malloc\fR. The
+pointer that is passed to \fBpcre_compile\fR is saved with the compiled
+pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
+and \fBpcre_exec()\fR. Thus for any single pattern, compilation, studying and
+matching all happen in the same locale, but different patterns can be compiled
+in different locales. It is the caller's responsibility to ensure that the
+memory containing the tables remains available for as long as it is needed.
+
+
+.SH INFORMATION ABOUT A PATTERN
+The \fBpcre_fullinfo()\fR function returns information about a compiled
+pattern. It replaces the obsolete \fBpcre_info()\fR function, which is
+nevertheless retained for backwards compability (and is documented below).
+
+The first argument for \fBpcre_fullinfo()\fR is a pointer to the compiled
+pattern. The second argument is the result of \fBpcre_study()\fR, or NULL if
+the pattern was not studied. The third argument specifies which piece of
+information is required, while the fourth argument is a pointer to a variable
+to receive the data. The yield of the function is zero for success, or one of
+the following negative numbers:
+
+  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
+
+Here is a typical call of \fBpcre_fullinfo()\fR, to obtain the length of the
+compiled pattern:
+
+  int rc;
+  unsigned long int length;
+  rc = pcre_fullinfo(
+    re,               /* result of pcre_compile() */
+    pe,               /* result of pcre_study(), or NULL */
+    PCRE_INFO_SIZE,   /* what is required */
+    &length);         /* where to put the data */
+
+The possible values for the third argument are defined in \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 an \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.
+
+Here is an example of a simple call to \fBpcre_exec()\fR:
+
+  int rc;
+  int ovector[30];
+  rc = pcre_exec(
+    re,             /* result of pcre_compile() */
+    NULL,           /* we didn't study the pattern */
+    "some string",  /* the subject string */
+    11,             /* the length of the subject string */
+    0,              /* start at offset 0 in the subject */
+    0,              /* default options */
+    ovector,        /* vector for substring information */
+    30);            /* number of elements in the vector */
+
+The PCRE_ANCHORED option can be passed in the \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, the subject may contain binary zero characters. When the starting
+offset is zero, the search for a match starts at the beginning of the subject,
+and this is by far the most common case.
+
+A non-zero starting offset is useful when searching for another match in the
+same subject by calling \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 memory 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
+memory failed for \fBpcre_get_substring()\fR.
+
+  PCRE_ERROR_NOSUBSTRING    (-7)
+
+There is no substring whose number is \fIstringnumber\fR.
+
+The \fBpcre_get_substring_list()\fR function extracts all available substrings
+and builds a list of pointers to them. All this is done in a single block of
+memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
+is returned via \fIlistptr\fR, which is also the start of the list of string
+pointers. The end of the list is marked by a NULL pointer. The yield of the
+function is zero if all went well, or
+
+  PCRE_ERROR_NOMEMORY       (-6)
+
+if the attempt to get the memory block failed.
+
+When any of these functions encounter a substring that is unset, which can
+happen when capturing subpattern number \fIn+1\fR matches some part of the
+subject, but subpattern \fIn\fR 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\fR, which is negative for unset
+substrings.
+
+The two convenience functions \fBpcre_free_substring()\fR and
+\fBpcre_free_substring_list()\fR can be used to free the memory returned by
+a previous call of \fBpcre_get_substring()\fR or
+\fBpcre_get_substring_list()\fR, respectively. They do nothing more than call
+the function pointed to by \fBpcre_free\fR, 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\fR directly; it is for these cases that the functions are
+provided.
+
+
+.SH LIMITATIONS
+There are some size limitations in PCRE but it is hoped that they will never in
+practice be relevant.
+The maximum length of a compiled pattern is 65539 (sic) bytes.
+All values in repeating quantifiers must be less than 65536.
+There maximum number of capturing subpatterns is 65535.
+There is no limit to the number of non-capturing subpatterns, but the maximum
+depth of nesting of all kinds of parenthesized subpattern, including capturing
+subpatterns, assertions, and other types of subpattern, is 200.
+
+The maximum length of a subject string is the largest positive number that an
+integer variable can hold. However, PCRE uses recursion to handle subpatterns
+and indefinite repetition. This means that the available stack space may limit
+the size of a subject string that can be processed by certain patterns.
+
+
+.SH DIFFERENCES FROM PERL
+The differences described here are with respect to Perl 5.005.
+
+1. By default, a whitespace character is any character that the C library
+function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
+alternative character type tables. Normally \fBisspace()\fR matches space,
+formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
+no longer includes vertical tab in its set of whitespace characters. The \\v
+escape that was in the Perl documentation for a long time was never in fact
+recognized. However, the character itself was treated as whitespace at least
+up to 5.002. In 5.004 and 5.005 it does not match \\s.
+
+2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
+them, but they do not mean what you might think. For example, (?!a){3} does
+not assert that the next three characters are not "a". It just asserts that the
+next character is not "a" three times.
+
+3. Capturing subpatterns that occur inside negative lookahead assertions are
+counted, but their entries in the offsets vector are never set. Perl sets its
+numerical variables from any such patterns that are matched before the
+assertion fails to match something (thereby succeeding), but only if the
+negative lookahead assertion contains just one branch.
+
+4. Though binary zero characters are supported in the subject string, they are
+not allowed in a pattern string because it is passed as a normal C string,
+terminated by zero. The escape sequence "\\0" can be used in the pattern to
+represent a binary zero.
+
+5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
+\\E, \\Q. In fact these are implemented by Perl's general string-handling and
+are not part of its pattern matching engine.
+
+6. The Perl \\G assertion is not supported as it is not relevant to single
+pattern matches.
+
+7. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
+constructions. However, there is some experimental support for recursive
+patterns using the non-Perl item (?R).
+
+8. There are at the time of writing some oddities in Perl 5.005_02 concerned
+with the settings of captured strings when part of a pattern is repeated. For
+example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
+"b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
+the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) are set.
+
+In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
+future Perl changes to a consistent state that is different, PCRE may change to
+follow.
+
+9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
+/^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
+However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
+
+10. PCRE provides some extensions to the Perl regular expression facilities:
+
+(a) Although lookbehind assertions must match fixed length strings, each
+alternative branch of a lookbehind assertion can match a different length of
+string. Perl 5.005 requires them all to have the same length.
+
+(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
+character matches only at the very end of the string.
+
+(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
+meaning is faulted.
+
+(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
+inverted, that is, by default they are not greedy, but if followed by a
+question mark they are.
+
+(e) PCRE_ANCHORED can be used to force a pattern to be tried only at the start
+of the subject.
+
+(f) The PCRE_NOTBOL, PCRE_NOTEOL, and PCRE_NOTEMPTY options for
+\fBpcre_exec()\fR have no Perl equivalents.
+
+(g) The (?R) construct allows for recursive pattern matching (Perl 5.6 can do
+this using the (?p{code}) construct, which PCRE cannot of course support.)
+
+
+.SH REGULAR EXPRESSION DETAILS
+The syntax and semantics of the regular expressions supported by PCRE are
+described below. Regular expressions are also described in the Perl
+documentation and in a number of other books, some of which have copious
+examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
+O'Reilly (ISBN 1-56592-257), covers them in great detail.
+
+The description here is intended as reference documentation. The basic
+operation of PCRE is on strings of bytes. However, there is the beginnings of
+some support for UTF-8 character strings. To use this support you must
+configure PCRE to include it, and then call \fBpcre_compile()\fR with the
+PCRE_UTF8 option. How this affects the pattern matching is described in the
+final section of this document.
+
+A regular expression is a pattern that is matched against a subject string from
+left to right. Most characters stand for themselves in a pattern, and match the
+corresponding characters in the subject. As a trivial example, the pattern
+
+  The quick brown fox
+
+matches a portion of a subject string that is identical to itself. The power of
+regular expressions comes from the ability to include alternatives and
+repetitions in the pattern. These are encoded in the pattern by the use of
+\fImeta-characters\fR, which do not stand for themselves but instead are
+interpreted in some special way.
+
+There are two different sets of meta-characters: those that are recognized
+anywhere in the pattern except within square brackets, and those that are
+recognized in square brackets. Outside square brackets, the meta-characters are
+as follows:
+
+  \\      general escape character with several uses
+  ^      assert start of subject (or line, in multiline mode)
+  $      assert end of subject (or line, in multiline mode)
+  .      match any character except newline (by default)
+  [      start character class definition
+  |      start of alternative branch
+  (      start subpattern
+  )      end subpattern
+  ?      extends the meaning of (
+         also 0 or 1 quantifier
+         also quantifier minimizer
+  *      0 or more quantifier
+  +      1 or more quantifier
+  {      start min/max quantifier
+
+Part of a pattern that is in square brackets is called a "character class". In
+a character class the only meta-characters are:
+
+  \\      general escape character
+  ^      negate the class, but only if the first character
+  -      indicates character range
+  ]      terminates the character class
+
+The following sections describe the use of each of the meta-characters.
+
+
+.SH BACKSLASH
+The backslash character has several uses. Firstly, if it is followed by a
+non-alphameric character, it takes away any special meaning that character may
+have. This use of backslash as an escape character applies both inside and
+outside character classes.
+
+For example, if you want to match a "*" character, you write "\\*" in the
+pattern. This applies whether or not the following character would otherwise be
+interpreted as a meta-character, so it is always safe to precede a
+non-alphameric with "\\" to specify that it stands for itself. In particular,
+if you want to match a backslash, you write "\\\\".
+
+If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
+pattern (other than in a character class) and characters between a "#" outside
+a character class and the next newline character are ignored. An escaping
+backslash can be used to include a whitespace or "#" character as part of the
+pattern.
+
+A second use of backslash provides a way of encoding non-printing characters
+in patterns in a visible manner. There is no restriction on the appearance of
+non-printing characters, apart from the binary zero that terminates a pattern,
+but when a pattern is being prepared by text editing, it is usually easier to
+use one of the following escape sequences than the binary character it
+represents:
+
+  \\a     alarm, that is, the BEL character (hex 07)
+  \\cx    "control-x", where x is any character
+  \\e     escape (hex 1B)
+  \\f     formfeed (hex 0C)
+  \\n     newline (hex 0A)
+  \\r     carriage return (hex 0D)
+  \\t     tab (hex 09)
+  \\xhh   character with hex code hh
+  \\ddd   character with octal code ddd, or backreference
+
+The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
+is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
+Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
+7B.
+
+After "\\x", up to two hexadecimal digits are read (letters can be in upper or
+lower case).
+
+After "\\0" up to two further octal digits are read. In both cases, if there
+are fewer than two digits, just those that are present are used. Thus the
+sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
+Make sure you supply two digits after the initial zero if the character that
+follows is itself an octal digit.
+
+The handling of a backslash followed by a digit other than 0 is complicated.
+Outside a character class, PCRE reads it and any following digits as a decimal
+number. If the number is less than 10, or if there have been at least that many
+previous capturing left parentheses in the expression, the entire sequence is
+taken as a \fIback reference\fR. A description of how this works is given
+later, following the discussion of parenthesized subpatterns.
+
+Inside a character class, or if the decimal number is greater than 9 and there
+have not been that many capturing subpatterns, PCRE re-reads up to three octal
+digits following the backslash, and generates a single byte from the least
+significant 8 bits of the value. Any subsequent digits stand for themselves.
+For example:
+
+  \\040   is another way of writing a space
+  \\40    is the same, provided there are fewer than 40
+            previous capturing subpatterns
+  \\7     is always a back reference
+  \\11    might be a back reference, or another way of
+            writing a tab
+  \\011   is always a tab
+  \\0113  is a tab followed by the character "3"
+  \\113   is the character with octal code 113 (since there
+            can be no more than 99 back references)
+  \\377   is a byte consisting entirely of 1 bits
+  \\81    is either a back reference, or a binary zero
+            followed by the two characters "8" and "1"
+
+Note that octal values of 100 or greater must not be introduced by a leading
+zero, because no more than three octal digits are ever read.
+
+All the sequences that define a single byte value can be used both inside and
+outside character classes. In addition, inside a character class, the sequence
+"\\b" is interpreted as the backspace character (hex 08). Outside a character
+class it has a different meaning (see below).
+
+The third use of backslash is for specifying generic character types:
+
+  \\d     any decimal digit
+  \\D     any character that is not a decimal digit
+  \\s     any whitespace character
+  \\S     any character that is not a whitespace character
+  \\w     any "word" character
+  \\W     any "non-word" character
+
+Each pair of escape sequences partitions the complete set of characters into
+two disjoint sets. Any given character matches one, and only one, of each pair.
+
+A "word" character is any letter or digit or the underscore character, that is,
+any character which can be part of a Perl "word". The definition of letters and
+digits is controlled by PCRE's character tables, and may vary if locale-
+specific matching is taking place (see "Locale support" above). For example, in
+the "fr" (French) locale, some character codes greater than 128 are used for
+accented letters, and these are matched by \\w.
+
+These character type sequences can appear both inside and outside character
+classes. They each match one character of the appropriate type. If the current
+matching point is at the end of the subject string, all of them fail, since
+there is no character to match.
+
+The fourth use of backslash is for certain simple assertions. An assertion
+specifies a condition that has to be met at a particular point in a match,
+without consuming any characters from the subject string. The use of
+subpatterns for more complicated assertions is described below. The backslashed
+assertions are
+
+  \\b     word boundary
+  \\B     not a word boundary
+  \\A     start of subject (independent of multiline mode)
+  \\Z     end of subject or newline at end (independent of multiline mode)
+  \\z     end of subject (independent of multiline mode)
+
+These assertions may not appear in character classes (but note that "\\b" has a
+different meaning, namely the backspace character, inside a character class).
+
+A word boundary is a position in the subject string where the current character
+and the previous character do not both match \\w or \\W (i.e. one matches
+\\w and the other matches \\W), or the start or end of the string if the
+first or last character matches \\w, respectively.
+
+The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
+dollar (described below) in that they only ever match at the very start and end
+of the subject string, whatever options are set. They are not affected by the
+PCRE_NOTBOL or PCRE_NOTEOL options. If the \fIstartoffset\fR argument of
+\fBpcre_exec()\fR is non-zero, \\A can never match. The difference between \\Z
+and \\z is that \\Z matches before a newline that is the last character of the
+string as well as at the end of the string, whereas \\z matches only at the
+end.
+
+
+.SH CIRCUMFLEX AND DOLLAR
+Outside a character class, in the default matching mode, the circumflex
+character is an assertion which is true only if the current matching point is
+at the start of the subject string. If the \fIstartoffset\fR argument of
+\fBpcre_exec()\fR is non-zero, circumflex can never match. Inside a character
+class, circumflex has an entirely different meaning (see below).
+
+Circumflex need not be the first character of the pattern if a number of
+alternatives are involved, but it should be the first thing in each alternative
+in which it appears if the pattern is ever to match that branch. If all
+possible alternatives start with a circumflex, that is, if the pattern is
+constrained to match only at the start of the subject, it is said to be an
+"anchored" pattern. (There are also other constructs that can cause a pattern
+to be anchored.)
+
+A dollar character is an assertion which is true only if the current matching
+point is at the end of the subject string, or immediately before a newline
+character that is the last character in the string (by default). Dollar need
+not be the last character of the pattern if a number of alternatives are
+involved, but it should be the last item in any branch in which it appears.
+Dollar has no special meaning in a character class.
+
+The meaning of dollar can be changed so that it matches only at the very end of
+the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
+time. This does not affect the \\Z assertion.
+
+The meanings of the circumflex and dollar characters are changed if the
+PCRE_MULTILINE option is set. When this is the case, they match immediately
+after and immediately before an internal "\\n" character, respectively, in
+addition to matching at the start and end of the subject string. For example,
+the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
+but not otherwise. Consequently, patterns that are anchored in single line mode
+because all branches start with "^" are not anchored in multiline mode, and a
+match for circumflex is possible when the \fIstartoffset\fR argument of
+\fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
+PCRE_MULTILINE is set.
+
+Note that the sequences \\A, \\Z, and \\z can be used to match the start and
+end of the subject in both modes, and if all branches of a pattern start with
+\\A it is always anchored, whether PCRE_MULTILINE is set or not.
+
+
+.SH FULL STOP (PERIOD, DOT)
+Outside a character class, a dot in the pattern matches any one character in
+the subject, including a non-printing character, but not (by default) newline.
+If the PCRE_DOTALL option is set, dots match newlines as well. The handling of
+dot is entirely independent of the handling of circumflex and dollar, the only
+relationship being that they both involve newline characters. Dot has no
+special meaning in a character class.
+
+
+.SH SQUARE BRACKETS
+An opening square bracket introduces a character class, terminated by a closing
+square bracket. A closing square bracket on its own is not special. If a
+closing square bracket is required as a member of the class, it should be the
+first data character in the class (after an initial circumflex, if present) or
+escaped with a backslash.
+
+A character class matches a single character in the subject; the character must
+be in the set of characters defined by the class, unless the first character in
+the class is a circumflex, in which case the subject character must not be in
+the set defined by the class. If a circumflex is actually required as a member
+of the class, ensure it is not the first character, or escape it with a
+backslash.
+
+For example, the character class [aeiou] matches any lower case vowel, while
+[^aeiou] matches any character that is not a lower case vowel. Note that a
+circumflex is just a convenient notation for specifying the characters which
+are in the class by enumerating those that are not. It is not an assertion: it
+still consumes a character from the subject string, and fails if the current
+pointer is at the end of the string.
+
+When caseless matching is set, any letters in a class represent both their
+upper case and lower case versions, so for example, a caseless [aeiou] matches
+"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
+caseful version would.
+
+The newline character is never treated in any special way in character classes,
+whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
+such as [^a] will always match a newline.
+
+The minus (hyphen) character can be used to specify a range of characters in a
+character class. For example, [d-m] matches any letter between d and m,
+inclusive. If a minus character is required in a class, it must be escaped with
+a backslash or appear in a position where it cannot be interpreted as
+indicating a range, typically as the first or last character in the class.
+
+It is not possible to have the literal character "]" as the end character of a
+range. A pattern such as [W-]46] is interpreted as a class of two characters
+("W" and "-") followed by a literal string "46]", so it would match "W46]" or
+"-46]". However, if the "]" is escaped with a backslash it is interpreted as
+the end of range, so [W-\\]46] is interpreted as a single class containing a
+range followed by two separate characters. The octal or hexadecimal
+representation of "]" can also be used to end a range.
+
+Ranges operate in ASCII collating sequence. They can also be used for
+characters specified numerically, for example [\\000-\\037]. If a range that
+includes letters is used when caseless matching is set, it matches the letters
+in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
+caselessly, and if character tables for the "fr" locale are in use,
+[\\xc8-\\xcb] matches accented E characters in both cases.
+
+The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
+character class, and add the characters that they match to the class. For
+example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
+conveniently be used with the upper case character types to specify a more
+restricted set of characters than the matching lower case type. For example,
+the class [^\\W_] matches any letter or digit, but not underscore.
+
+All non-alphameric characters other than \\, -, ^ (at the start) and the
+terminating ] are non-special in character classes, but it does no harm if they
+are escaped.
+
+
+.SH POSIX CHARACTER CLASSES
+Perl 5.6 (not yet released at the time of writing) is going to support the
+POSIX notation for character classes, which uses names enclosed by [: and :]
+within the enclosing square brackets. PCRE supports this notation. For example,
+
+  [01[:alpha:]%]
+
+matches "0", "1", any alphabetic character, or "%". The supported class names
+are
+
+  alnum    letters and digits
+  alpha    letters
+  ascii    character codes 0 - 127
+  cntrl    control characters
+  digit    decimal digits (same as \\d)
+  graph    printing characters, excluding space
+  lower    lower case letters
+  print    printing characters, including space
+  punct    printing characters, excluding letters and digits
+  space    white space (same as \\s)
+  upper    upper case letters
+  word     "word" characters (same as \\w)
+  xdigit   hexadecimal digits
+
+The names "ascii" and "word" are Perl extensions. Another Perl extension is
+negation, which is indicated by a ^ character after the colon. For example,
+
+  [12[:^digit:]]
+
+matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
+syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
+supported, and an error is given if they are encountered.
+
+
+.SH VERTICAL BAR
+Vertical bar characters are used to separate alternative patterns. For example,
+the pattern
+
+  gilbert|sullivan
+
+matches either "gilbert" or "sullivan". Any number of alternatives may appear,
+and an empty alternative is permitted (matching the empty string).
+The matching process tries each alternative in turn, from left to right,
+and the first one that succeeds is used. If the alternatives are within a
+subpattern (defined below), "succeeds" means matching the rest of the main
+pattern as well as the alternative in the subpattern.
+
+
+.SH INTERNAL OPTION SETTING
+The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
+can be changed from within the pattern by a sequence of Perl option letters
+enclosed between "(?" and ")". The option letters are
+
+  i  for PCRE_CASELESS
+  m  for PCRE_MULTILINE
+  s  for PCRE_DOTALL
+  x  for PCRE_EXTENDED
+
+For example, (?im) sets caseless, multiline matching. It is also possible to
+unset these options by preceding the letter with a hyphen, and a combined
+setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
+PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
+permitted. If a letter appears both before and after the hyphen, the option is
+unset.
+
+The scope of these option changes depends on where in the pattern the setting
+occurs. For settings that are outside any subpattern (defined below), the
+effect is the same as if the options were set or unset at the start of
+matching. The following patterns all behave in exactly the same way:
+
+  (?i)abc
+  a(?i)bc
+  ab(?i)c
+  abc(?i)
+
+which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
+In other words, such "top level" settings apply to the whole pattern (unless
+there are other changes inside subpatterns). If there is more than one setting
+of the same option at top level, the rightmost setting is used.
+
+If an option change occurs inside a subpattern, the effect is different. This
+is a change of behaviour in Perl 5.005. An option change inside a subpattern
+affects only that part of the subpattern that follows it, so
+
+  (a(?i)b)c
+
+matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
+By this means, options can be made to have different settings in different
+parts of the pattern. Any changes made in one alternative do carry on
+into subsequent branches within the same subpattern. For example,
+
+  (a(?i)b|c)
+
+matches "ab", "aB", "c", and "C", even though when matching "C" the first
+branch is abandoned before the option setting. This is because the effects of
+option settings happen at compile time. There would be some very weird
+behaviour otherwise.
+
+The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
+same way as the Perl-compatible options by using the characters U and X
+respectively. The (?X) flag setting is special in that it must always occur
+earlier in the pattern than any of the additional features it turns on, even
+when it is at top level. It is best put at the start.
+
+
+.SH SUBPATTERNS
+Subpatterns are delimited by parentheses (round brackets), which can be nested.
+Marking part of a pattern as a subpattern does two things:
+
+1. It localizes a set of alternatives. For example, the pattern
+
+  cat(aract|erpillar|)
+
+matches one of the words "cat", "cataract", or "caterpillar". Without the
+parentheses, it would match "cataract", "erpillar" or the empty string.
+
+2. It sets up the subpattern as a capturing subpattern (as defined above).
+When the whole pattern matches, that portion of the subject string that matched
+the subpattern is passed back to the caller via the \fIovector\fR argument of
+\fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
+from 1) to obtain the numbers of the capturing subpatterns.
+
+For example, if the string "the red king" is matched against the pattern
+
+  the ((red|white) (king|queen))
+
+the captured substrings are "red king", "red", and "king", and are numbered 1,
+2, and 3, respectively.
+
+The fact that plain parentheses fulfil two functions is not always helpful.
+There are often times when a grouping subpattern is required without a
+capturing requirement. If an opening parenthesis is followed by "?:", the
+subpattern does not do any capturing, and is not counted when computing the
+number of any subsequent capturing subpatterns. For example, if the string "the
+white queen" is matched against the pattern
+
+  the ((?:red|white) (king|queen))
+
+the captured substrings are "white queen" and "queen", and are numbered 1 and
+2. The maximum number of captured substrings is 99, and the maximum number of
+all subpatterns, both capturing and non-capturing, is 200.
+
+As a convenient shorthand, if any option settings are required at the start of
+a non-capturing subpattern, the option letters may appear between the "?" and
+the ":". Thus the two patterns
+
+  (?i:saturday|sunday)
+  (?:(?i)saturday|sunday)
+
+match exactly the same set of strings. Because alternative branches are tried
+from left to right, and options are not reset until the end of the subpattern
+is reached, an option setting in one branch does affect subsequent branches, so
+the above patterns match "SUNDAY" as well as "Saturday".
+
+
+.SH REPETITION
+Repetition is specified by quantifiers, which can follow any of the following
+items:
+
+  a single character, possibly escaped
+  the . metacharacter
+  a character class
+  a back reference (see next section)
+  a parenthesized subpattern (unless it is an assertion - see below)
+
+The general repetition quantifier specifies a minimum and maximum number of
+permitted matches, by giving the two numbers in curly brackets (braces),
+separated by a comma. The numbers must be less than 65536, and the first must
+be less than or equal to the second. For example:
+
+  z{2,4}
+
+matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
+character. If the second number is omitted, but the comma is present, there is
+no upper limit; if the second number and the comma are both omitted, the
+quantifier specifies an exact number of required matches. Thus
+
+  [aeiou]{3,}
+
+matches at least 3 successive vowels, but may match many more, while
+
+  \\d{8}
+
+matches exactly 8 digits. An opening curly bracket that appears in a position
+where a quantifier is not allowed, or one that does not match the syntax of a
+quantifier, is taken as a literal character. For example, {,6} is not a
+quantifier, but a literal string of four characters.
+
+The quantifier {0} is permitted, causing the expression to behave as if the
+previous item and the quantifier were not present.
+
+For convenience (and historical compatibility) the three most common
+quantifiers have single-character abbreviations:
+
+  *    is equivalent to {0,}
+  +    is equivalent to {1,}
+  ?    is equivalent to {0,1}
+
+It is possible to construct infinite loops by following a subpattern that can
+match no characters with a quantifier that has no upper limit, for example:
+
+  (a?)*
+
+Earlier versions of Perl and PCRE used to give an error at compile time for
+such patterns. However, because there are cases where this can be useful, such
+patterns are now accepted, but if any repetition of the subpattern does in fact
+match no characters, the loop is forcibly broken.
+
+By default, the quantifiers are "greedy", that is, they match as much as
+possible (up to the maximum number of permitted times), without causing the
+rest of the pattern to fail. The classic example of where this gives problems
+is in trying to match comments in C programs. These appear between the
+sequences /* and */ and within the sequence, individual * and / characters may
+appear. An attempt to match C comments by applying the pattern
+
+  /\\*.*\\*/
+
+to the string
+
+  /* first command */  not comment  /* second comment */
+
+fails, because it matches the entire string owing to the greediness of the .*
+item.
+
+However, if a quantifier is followed by a question mark, it ceases to be
+greedy, and instead matches the minimum number of times possible, so the
+pattern
+
+  /\\*.*?\\*/
+
+does the right thing with the C comments. The meaning of the various
+quantifiers is not otherwise changed, just the preferred number of matches.
+Do not confuse this use of question mark with its use as a quantifier in its
+own right. Because it has two uses, it can sometimes appear doubled, as in
+
+  \\d??\\d
+
+which matches one digit by preference, but can match two if that is the only
+way the rest of the pattern matches.
+
+If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
+the quantifiers are not greedy by default, but individual ones can be made
+greedy by following them with a question mark. In other words, it inverts the
+default behaviour.
+
+When a parenthesized subpattern is quantified with a minimum repeat count that
+is greater than 1 or with a limited maximum, more store is required for the
+compiled pattern, in proportion to the size of the minimum or maximum.
+
+If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
+to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
+implicitly anchored, because whatever follows will be tried against every
+character position in the subject string, so there is no point in retrying the
+overall match at any position after the first. PCRE treats such a pattern as
+though it were preceded by \\A. In cases where it is known that the subject
+string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
+begins with .* in order to obtain this optimization, or alternatively using ^
+to indicate anchoring explicitly.
+
+When a capturing subpattern is repeated, the value captured is the substring
+that matched the final iteration. For example, after
+
+  (tweedle[dume]{3}\\s*)+
+
+has matched "tweedledum tweedledee" the value of the captured substring is
+"tweedledee". However, if there are nested capturing subpatterns, the
+corresponding captured values may have been set in previous iterations. For
+example, after
+
+  /(a|(b))+/
+
+matches "aba" the value of the second captured substring is "b".
+
+
+.SH BACK REFERENCES
+Outside a character class, a backslash followed by a digit greater than 0 (and
+possibly further digits) is a back reference to a capturing subpattern earlier
+(i.e. to its left) in the pattern, provided there have been that many previous
+capturing left parentheses.
+
+However, if the decimal number following the backslash is less than 10, it is
+always taken as a back reference, and causes an error only if there are not
+that many capturing left parentheses in the entire pattern. In other words, the
+parentheses that are referenced need not be to the left of the reference for
+numbers less than 10. See the section entitled "Backslash" above for further
+details of the handling of digits following a backslash.
+
+A back reference matches whatever actually matched the capturing subpattern in
+the current subject string, rather than anything matching the subpattern
+itself. So the pattern
+
+  (sens|respons)e and \\1ibility
+
+matches "sense and sensibility" and "response and responsibility", but not
+"sense and responsibility". If caseful matching is in force at the time of the
+back reference, the case of letters is relevant. For example,
+
+  ((?i)rah)\\s+\\1
+
+matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
+capturing subpattern is matched caselessly.
+
+There may be more than one back reference to the same subpattern. If a
+subpattern has not actually been used in a particular match, any back
+references to it always fail. For example, the pattern
+
+  (a|(bc))\\2
+
+always fails if it starts to match "a" rather than "bc". Because there may be
+up to 99 back references, all digits following the backslash are taken
+as part of a potential back reference number. If the pattern continues with a
+digit character, some delimiter must be used to terminate the back reference.
+If the PCRE_EXTENDED option is set, this can be whitespace. Otherwise an empty
+comment can be used.
+
+A back reference that occurs inside the parentheses to which it refers fails
+when the subpattern is first used, so, for example, (a\\1) never matches.
+However, such references can be useful inside repeated subpatterns. For
+example, the pattern
+
+  (a|b\\1)+
+
+matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
+the subpattern, the back reference matches the character string corresponding
+to the previous iteration. In order for this to work, the pattern must be such
+that the first iteration does not need to match the back reference. This can be
+done using alternation, as in the example above, or by a quantifier with a
+minimum of zero.
+
+
+.SH ASSERTIONS
+An assertion is a test on the characters following or preceding the current
+matching point that does not actually consume any characters. The simple
+assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
+complicated assertions are coded as subpatterns. There are two kinds: those
+that look ahead of the current position in the subject string, and those that
+look behind it.
+
+An assertion subpattern is matched in the normal way, except that it does not
+cause the current matching position to be changed. Lookahead assertions start
+with (?= for positive assertions and (?! for negative assertions. For example,
+
+  \\w+(?=;)
+
+matches a word followed by a semicolon, but does not include the semicolon in
+the match, and
+
+  foo(?!bar)
+
+matches any occurrence of "foo" that is not followed by "bar". Note that the
+apparently similar pattern
+
+  (?!foo)bar
+
+does not find an occurrence of "bar" that is preceded by something other than
+"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
+(?!foo) is always true when the next three characters are "bar". A
+lookbehind assertion is needed to achieve this effect.
+
+Lookbehind assertions start with (?<= for positive assertions and (?<! for
+negative assertions. For example,
+
+  (?<!foo)bar
+
+does find an occurrence of "bar" that is not preceded by "foo". The contents of
+a lookbehind assertion are restricted such that all the strings it matches must
+have a fixed length. However, if there are several alternatives, they do not
+all have to have the same fixed length. Thus
+
+  (?<=bullock|donkey)
+
+is permitted, but
+
+  (?<!dogs?|cats?)
+
+causes an error at compile time. Branches that match different length strings
+are permitted only at the top level of a lookbehind assertion. This is an
+extension compared with Perl 5.005, which requires all branches to match the
+same length of string. An assertion such as
+
+  (?<=ab(c|de))
+
+is not permitted, because its single top-level branch can match two different
+lengths, but it is acceptable if rewritten to use two top-level branches:
+
+  (?<=abc|abde)
+
+The implementation of lookbehind assertions is, for each alternative, to
+temporarily move the current position back by the fixed width and then try to
+match. If there are insufficient characters before the current position, the
+match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
+can be particularly useful for matching at the ends of strings; an example is
+given at the end of the section on once-only subpatterns.
+
+Several assertions (of any sort) may occur in succession. For example,
+
+  (?<=\\d{3})(?<!999)foo
+
+matches "foo" preceded by three digits that are not "999". Notice that each of
+the assertions is applied independently at the same point in the subject
+string. First there is a check that the previous three characters are all
+digits, and then there is a check that the same three characters are not "999".
+This pattern does \fInot\fR match "foo" preceded by six characters, the first
+of which are digits and the last three of which are not "999". For example, it
+doesn't match "123abcfoo". A pattern to do that is
+
+  (?<=\\d{3}...)(?<!999)foo
+
+This time the first assertion looks at the preceding six characters, checking
+that the first three are digits, and then the second assertion checks that the
+preceding three characters are not "999".
+
+Assertions can be nested in any combination. For example,
+
+  (?<=(?<!foo)bar)baz
+
+matches an occurrence of "baz" that is preceded by "bar" which in turn is not
+preceded by "foo", while
+
+  (?<=\\d{3}(?!999)...)foo
+
+is another pattern which matches "foo" preceded by three digits and any three
+characters that are not "999".
+
+Assertion subpatterns are not capturing subpatterns, and may not be repeated,
+because it makes no sense to assert the same thing several times. If any kind
+of assertion contains capturing subpatterns within it, these are counted for
+the purposes of numbering the capturing subpatterns in the whole pattern.
+However, substring capturing is carried out only for positive assertions,
+because it does not make sense for negative assertions.
+
+Assertions count towards the maximum of 200 parenthesized subpatterns.
+
+
+.SH ONCE-ONLY SUBPATTERNS
+With both maximizing and minimizing repetition, failure of what follows
+normally causes the repeated item to be re-evaluated to see if a different
+number of repeats allows the rest of the pattern to match. Sometimes it is
+useful to prevent this, either to change the nature of the match, or to cause
+it fail earlier than it otherwise might, when the author of the pattern knows
+there is no point in carrying on.
+
+Consider, for example, the pattern \\d+foo when applied to the subject line
+
+  123456bar
+
+After matching all 6 digits and then failing to match "foo", the normal
+action of the matcher is to try again with only 5 digits matching the \\d+
+item, and then with 4, and so on, before ultimately failing. Once-only
+subpatterns provide the means for specifying that once a portion of the pattern
+has matched, it is not to be re-evaluated in this way, so the matcher would
+give up immediately on failing to match "foo" the first time. The notation is
+another kind of special parenthesis, starting with (?> as in this example:
+
+  (?>\\d+)bar
+
+This kind of parenthesis "locks up" the  part of the pattern it contains once
+it has matched, and a failure further into the pattern is prevented from
+backtracking into it. Backtracking past it to previous items, however, works as
+normal.
+
+An alternative description is that a subpattern of this type matches the string
+of characters that an identical standalone pattern would match, if anchored at
+the current point in the subject string.
+
+Once-only subpatterns are not capturing subpatterns. Simple cases such as the
+above example can be thought of as a maximizing repeat that must swallow
+everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
+number of digits they match in order to make the rest of the pattern match,
+(?>\\d+) can only match an entire sequence of digits.
+
+This construction can of course contain arbitrarily complicated subpatterns,
+and it can be nested.
+
+Once-only subpatterns can be used in conjunction with lookbehind assertions to
+specify efficient matching at the end of the subject string. Consider a simple
+pattern such as
+
+  abcd$
+
+when applied to a long string which does not match. Because matching proceeds
+from left to right, PCRE will look for each "a" in the subject and then see if
+what follows matches the rest of the pattern. If the pattern is specified as
+
+  ^.*abcd$
+
+the initial .* matches the entire string at first, but when this fails (because
+there is no following "a"), it backtracks to match all but the last character,
+then all but the last two characters, and so on. Once again the search for "a"
+covers the entire string, from right to left, so we are no better off. However,
+if the pattern is written as
+
+  ^(?>.*)(?<=abcd)
+
+there can be no backtracking for the .* item; it can match only the entire
+string. The subsequent lookbehind assertion does a single test on the last four
+characters. If it fails, the match fails immediately. For long strings, this
+approach makes a significant difference to the processing time.
+
+When a pattern contains an unlimited repeat inside a subpattern that can itself
+be repeated an unlimited number of times, the use of a once-only subpattern is
+the only way to avoid some failing matches taking a very long time indeed.
+The pattern
+
+  (\\D+|<\\d+>)*[!?]
+
+matches an unlimited number of substrings that either consist of non-digits, or
+digits enclosed in <>, followed by either ! or ?. When it matches, it runs
+quickly. However, if it is applied to
+
+  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+
+it takes a long time before reporting failure. This is because the string can
+be divided between the two repeats in a large number of ways, and all have to
+be tried. (The example used [!?] rather than a single character at the end,
+because both PCRE and Perl have an optimization that allows for fast failure
+when a single character is used. They remember the last single character that
+is required for a match, and fail early if it is not present in the string.)
+If the pattern is changed to
+
+  ((?>\\D+)|<\\d+>)*[!?]
+
+sequences of non-digits cannot be broken, and failure happens quickly.
+
+
+.SH CONDITIONAL SUBPATTERNS
+It is possible to cause the matching process to obey a subpattern
+conditionally or to choose between two alternative subpatterns, depending on
+the result of an assertion, or whether a previous capturing subpattern matched
+or not. The two possible forms of conditional subpattern are
+
+  (?(condition)yes-pattern)
+  (?(condition)yes-pattern|no-pattern)
+
+If the condition is satisfied, the yes-pattern is used; otherwise the
+no-pattern (if present) is used. If there are more than two alternatives in the
+subpattern, a compile-time error occurs.
+
+There are two kinds of condition. If the text between the parentheses consists
+of a sequence of digits, the condition is satisfied if the capturing subpattern
+of that number has previously matched. The number must be greater than zero.
+Consider the following pattern, which contains non-significant white space to
+make it more readable (assume the PCRE_EXTENDED option) and to divide it into
+three parts for ease of discussion:
+
+  ( \\( )?    [^()]+    (?(1) \\) )
+
+The first part matches an optional opening parenthesis, and if that
+character is present, sets it as the first captured substring. The second part
+matches one or more characters that are not parentheses. The third part is a
+conditional subpattern that tests whether the first set of parentheses matched
+or not. If they did, that is, if subject started with an opening parenthesis,
+the condition is true, and so the yes-pattern is executed and a closing
+parenthesis is required. Otherwise, since no-pattern is not present, the
+subpattern matches nothing. In other words, this pattern matches a sequence of
+non-parentheses, optionally enclosed in parentheses.
+
+If the condition is not a sequence of digits, it must be an assertion. This may
+be a positive or negative lookahead or lookbehind assertion. Consider this
+pattern, again containing non-significant white space, and with the two
+alternatives on the second line:
+
+  (?(?=[^a-z]*[a-z])
+  \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )
+
+The condition is a positive lookahead assertion that matches an optional
+sequence of non-letters followed by a letter. In other words, it tests for the
+presence of at least one letter in the subject. If a letter is found, the
+subject is matched against the first alternative; otherwise it is matched
+against the second. This pattern matches strings in one of the two forms
+dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
+
+
+.SH COMMENTS
+The sequence (?# marks the start of a comment which continues up to the next
+closing parenthesis. Nested parentheses are not permitted. The characters
+that make up a comment play no part in the pattern matching at all.
+
+If the PCRE_EXTENDED option is set, an unescaped # character outside a
+character class introduces a comment that continues up to the next newline
+character in the pattern.
+
+
+.SH RECURSIVE PATTERNS
+Consider the problem of matching a string in parentheses, allowing for
+unlimited nested parentheses. Without the use of recursion, the best that can
+be done is to use a pattern that matches up to some fixed depth of nesting. It
+is not possible to handle an arbitrary nesting depth. Perl 5.6 has provided an
+experimental facility that allows regular expressions to recurse (amongst other
+things). It does this by interpolating Perl code in the expression at run time,
+and the code can refer to the expression itself. A Perl pattern to solve the
+parentheses problem can be created like this:
+
+  $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;
+
+The (?p{...}) item interpolates Perl code at run time, and in this case refers
+recursively to the pattern in which it appears. Obviously, PCRE cannot support
+the interpolation of Perl code. Instead, the special item (?R) is provided for
+the specific case of recursion. This PCRE pattern solves the parentheses
+problem (assume the PCRE_EXTENDED option is set so that white space is
+ignored):
+
+  \\( ( (?>[^()]+) | (?R) )* \\)
+
+First it matches an opening parenthesis. Then it matches any number of
+substrings which can either be a sequence of non-parentheses, or a recursive
+match of the pattern itself (i.e. a correctly parenthesized substring). Finally
+there is a closing parenthesis.
+
+This particular example pattern contains nested unlimited repeats, and so the
+use of a once-only subpattern for matching strings of non-parentheses is
+important when applying the pattern to strings that do not match. For example,
+when it is applied to
+
+  (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
+
+it yields "no match" quickly. However, if a once-only subpattern is not used,
+the match runs for a very long time indeed because there are so many different
+ways the + and * repeats can carve up the subject, and all have to be tested
+before failure can be reported.
+
+The values set for any capturing subpatterns are those from the outermost level
+of the recursion at which the subpattern value is set. If the pattern above is
+matched against
+
+  (ab(cd)ef)
+
+the value for the capturing parentheses is "ef", which is the last value taken
+on at the top level. If additional parentheses are added, giving
+
+  \\( ( ( (?>[^()]+) | (?R) )* ) \\)
+     ^                        ^
+     ^                        ^
+the string they capture is "ab(cd)ef", the contents of the top level
+parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
+has to obtain extra memory to store data during a recursion, which it does by
+using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no
+memory can be obtained, it saves data for the first 15 capturing parentheses
+only, as there is no way to give an out-of-memory error from within a
+recursion.
+
+
+.SH PERFORMANCE
+Certain items that may appear in patterns are more efficient than others. It is
+more efficient to use a character class like [aeiou] than a set of alternatives
+such as (a|e|i|o|u). In general, the simplest construction that provides the
+required behaviour is usually the most efficient. Jeffrey Friedl's book
+contains a lot of discussion about optimizing regular expressions for efficient
+performance.
+
+When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is
+implicitly anchored by PCRE, since it can match only at the start of a subject
+string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization,
+because the . metacharacter does not then match a newline, and if the subject
+string contains newlines, the pattern may match from the character immediately
+following one of them instead of from the very start. For example, the pattern
+
+  (.*) second
+
+matches the subject "first\\nand second" (where \\n stands for a newline
+character) with the first captured substring being "and". In order to do this,
+PCRE has to retry the match starting after every newline in the subject.
+
+If you are using such a pattern with subject strings that do not contain
+newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
+the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
+having to scan along the subject looking for a newline to restart at.
+
+Beware of patterns that contain nested indefinite repeats. These can take a
+long time to run when applied to a string that does not match. Consider the
+pattern fragment
+
+  (a+)*
+
+This can match "aaaa" in 33 different ways, and this number increases very
+rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
+times, and for each of those cases other than 0, the + repeats can match
+different numbers of times.) When the remainder of the pattern is such that the
+entire match is going to fail, PCRE has in principle to try every possible
+variation, and this can take an extremely long time.
+
+An optimization catches some of the more simple cases such as
+
+  (a+)*b
+
+where a literal character follows. Before embarking on the standard matching
+procedure, PCRE checks that there is a "b" later in the subject string, and if
+there is not, it fails the match immediately. However, when there is no
+following literal this optimization cannot be used. You can see the difference
+by comparing the behaviour of
+
+  (a+)*\\d
+
+with the pattern above. The former gives a failure almost instantly when
+applied to a whole line of "a" characters, whereas the latter takes an
+appreciable time with strings longer than about 20 characters.
+
+
+.SH UTF-8 SUPPORT
+Starting at release 3.3, PCRE has some support for character strings encoded
+in the UTF-8 format. This is incomplete, and is regarded as experimental. In
+order to use it, you must configure PCRE to include UTF-8 support in the code,
+and, in addition, you must call \fBpcre_compile()\fR with the PCRE_UTF8 option
+flag. When you do this, both the pattern and any subject strings that are
+matched against it are treated as UTF-8 strings instead of just strings of
+bytes, but only in the cases that are mentioned below.
+
+If you compile PCRE with UTF-8 support, but do not use it at run time, the
+library will be a bit bigger, but the additional run time overhead is limited
+to testing the PCRE_UTF8 flag in several places, so should not be very large.
+
+PCRE assumes that the strings it is given contain valid UTF-8 codes. It does
+not diagnose invalid UTF-8 strings. If you pass invalid UTF-8 strings to PCRE,
+the results are undefined.
+
+Running with PCRE_UTF8 set causes these changes in the way PCRE works:
+
+1. In a pattern, the escape sequence \\x{...}, where the contents of the braces
+is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
+code number is the given hexadecimal number, for example: \\x{1234}. This
+inserts from one to six literal bytes into the pattern, using the UTF-8
+encoding. If a non-hexadecimal digit appears between the braces, the item is
+not recognized.
+
+2. The original hexadecimal escape sequence, \\xhh, generates a two-byte UTF-8
+character if its value is greater than 127.
+
+3. Repeat quantifiers are NOT correctly handled if they follow a multibyte
+character. For example, \\x{100}* and \\xc3+ do not work. If you want to
+repeat such characters, you must enclose them in non-capturing parentheses,
+for example (?:\\x{100}), at present.
+
+4. The dot metacharacter matches one UTF-8 character instead of a single byte.
+
+5. Unlike literal UTF-8 characters, the dot metacharacter followed by a
+repeat quantifier does operate correctly on UTF-8 characters instead of
+single bytes.
+
+4. Although the \\x{...} escape is permitted in a character class, characters
+whose values are greater than 255 cannot be included in a class.
+
+5. A class is matched against a UTF-8 character instead of just a single byte,
+but it can match only characters whose values are less than 256. Characters
+with greater values always fail to match a class.
+
+6. Repeated classes work correctly on multiple characters.
+
+7. Classes containing just a single character whose value is greater than 127
+(but less than 256), for example, [\\x80] or [^\\x{93}], do not work because
+these are optimized into single byte matches. In the first case, of course,
+the class brackets are just redundant.
+
+8. Lookbehind assertions move backwards in the subject by a fixed number of
+characters instead of a fixed number of bytes. Simple cases have been tested
+to work correctly, but there may be hidden gotchas herein.
+
+9. The character types such as \\d and \\w do not work correctly with UTF-8
+characters. They continue to test a single byte.
+
+10. Anything not explicitly mentioned here continues to work in bytes rather
+than in characters.
+
+The following UTF-8 features of Perl 5.6 are not implemented:
+
+1. The escape sequence \\C to match a single byte.
+
+2. The use of Unicode tables and properties and escapes \\p, \\P, and \\X.
+
+
+.SH SAMPLE PROGRAM
+The code below is a simple, complete demonstration program, to get you started
+with using PCRE. This code is also supplied in the file \fIpcredemo.c\fR in the
+PCRE distribution.
+
+The program compiles the regular expression that is its first argument, and
+matches it against the subject string in its second argument. No options are
+set, and default character tables are used. If matching succeeds, the program
+outputs the portion of the subject that matched, together with the contents of
+any captured substrings.
+
+On a Unix system that has PCRE installed in \fI/usr/local\fR, you can compile
+the demonstration program using a command like this:
+
+  gcc -o pcredemo pcredemo.c -I/usr/local/include -L/usr/local/lib -lpcre
+
+Then you can run simple tests like this:
+
+  ./pcredemo 'cat|dog' 'the cat sat on the mat'
+
+Note that there is a much more comprehensive test program, called
+\fBpcretest\fR, which supports many more facilities for testing regular
+expressions. The \fBpcredemo\fR program is provided as a simple coding example.
+
+On some operating systems (e.g. Solaris) you may get an error like this when
+you try to run \fBpcredemo\fR:
+
+  ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or directory
+
+This is caused by the way shared library support works on those systems. You
+need to add
+
+  -R/usr/local/lib
+
+to the compile command to get round this problem. Here's the code:
+
+  #include <stdio.h>
+  #include <string.h>
+  #include <pcre.h>
+
+  #define OVECCOUNT 30    /* should be a multiple of 3 */
+
+  int main(int argc, char **argv)
+  {
+  pcre *re;
+  const char *error;
+  int erroffset;
+  int ovector[OVECCOUNT];
+  int rc, i;
+
+  if (argc != 3)
+    {
+    printf("Two arguments required: a regex and a "
+      "subject string\\n");
+    return 1;
+    }
+
+  /* Compile the regular expression in the first argument */
+
+  re = pcre_compile(
+    argv[1],     /* the pattern */
+    0,           /* default options */
+    &error,      /* for error message */
+    &erroffset,  /* for error offset */
+    NULL);       /* use default character tables */
+
+  /* Compilation failed: print the error message and exit */
+
+  if (re == NULL)
+    {
+    printf("PCRE compilation failed at offset %d: %s\\n",
+      erroffset, error);
+    return 1;
+    }
+
+  /* Compilation succeeded: match the subject in the second
+     argument */
+
+  rc = pcre_exec(
+    re,          /* the compiled pattern */
+    NULL,        /* we didn't study the pattern */
+    argv[2],     /* the subject string */
+    (int)strlen(argv[2]), /* the length of the subject */
+    0,           /* start at offset 0 in the subject */
+    0,           /* default options */
+    ovector,     /* vector for substring information */
+    OVECCOUNT);  /* number of elements in the vector */
+
+  /* Matching failed: handle error cases */
+
+  if (rc < 0)
+    {
+    switch(rc)
+      {
+      case PCRE_ERROR_NOMATCH: printf("No match\\n"); break;
+      /*
+      Handle other special cases if you like
+      */
+      default: printf("Matching error %d\\n", rc); break;
+      }
+    return 1;
+    }
+
+  /* Match succeded */
+
+  printf("Match succeeded\\n");
+
+  /* The output vector wasn't big enough */
+
+  if (rc == 0)
+    {
+    rc = OVECCOUNT/3;
+    printf("ovector only has room for %d captured "
+      substrings\\n", rc - 1);
+    }
+
+  /* Show substrings stored in the output vector */
+
+  for (i = 0; i < rc; i++)
+    {
+    char *substring_start = argv[2] + ovector[2*i];
+    int substring_length = ovector[2*i+1] - ovector[2*i];
+    printf("%2d: %.*s\\n", i, substring_length,
+      substring_start);
+    }
+
+  return 0;
+  }
+
+
+.SH AUTHOR
+Philip Hazel <ph10@cam.ac.uk>
+.br
+University Computing Service,
+.br
+New Museums Site,
+.br
+Cambridge CB2 3QG, England.
+.br
+Phone: +44 1223 334714
+
+Last updated: 15 August 2001
+.br
+Copyright (c) 1997-2001 University of Cambridge.