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regcomp
=======

Syntax
------

     #include <sys/types.h>
     #include <regex.h>
     
     int regcomp(regex_t *preg, const char *pattern, int cflags);

Description
-----------

This function is part of the implementation of POSIX 1003.2 regular
expressions ("RE"s).

`regcomp' compiles the regular expression contained in the PATTERN
string, subject to the flags in CFLAGS, and places the results in the
`regex_t' structure pointed to by PREG.  (The regular expression
syntax, as defined by POSIX 1003.2, is described below.)

The parameter CFLAGS is the bitwise OR of zero or more of the following
flags:

`REG_EXTENDED'
     Compile modern ("extended") REs, rather than the obsolete
     ("basic") REs that are the default.

`REG_BASIC'
     This is a synonym for 0, provided as a counterpart to
     `REG_EXTENDED' to improve readability.

`REG_NOSPEC'
     Compile with recognition of all special characters turned off.  All
     characters are thus considered ordinary, so the RE in PATTERN is a
     literal string.  This is an extension, compatible with but not
     specified by POSIX 1003.2, and should be used with caution in
     software intended to be portable to other systems.  `REG_EXTENDED'
     and `REG_NOSPEC' may not be used in the same call to `regcomp'.

`REG_ICASE'
     Compile for matching that ignores upper/lower case distinctions.
     See the description of regular expressions below for details of
     case-independent matching.

`REG_NOSUB'
     Compile for matching that need only report success or failure, not
     what was matched.

`REG_NEWLINE'
     Compile for newline-sensitive matching.  By default, newline is a
     completely ordinary character with no special meaning in either
     REs or strings.  With this flag, `[' bracket expressions and `.'
     never match newline, a `' anchor matches the null string after any
     newline in the string in addition to its normal function, and the
     `$' anchor matches the null string before any newline in the string
     in addition to its normal function.

`REG_PEND'
     The regular expression ends, not at the first NUL, but just before
     the character pointed to by the `re_endp' member of the structure
     pointed to by PREG.  The `re_endp' member is of type `const char
     *'.  This flag permits inclusion of NULs in the RE; they are
     considered ordinary characters.  This is an extension, compatible
     with but not specified by POSIX 1003.2, and should be used with
     caution in software intended to be portable to other systems.

When successful, `regcomp' returns 0 and fills in the structure pointed
to by PREG.  One member of that structure (other than `re_endp') is
publicized: `re_nsub', of type `size_t', contains the number of
parenthesized subexpressions within the RE (except that the value of
this member is undefined if the `REG_NOSUB' flag was used).

Note that the length of the RE does matter; in particular, there is a
strong speed bonus for keeping RE length under about 30 characters,
with most special characters counting roughly double.

Return Value
------------

If `regcomp' succeeds, it returns zero; if it fails, it returns a
non-zero error code, which is one of these:

`REG_BADPAT'
     invalid regular expression

`REG_ECOLLATE'
     invalid collating element

`REG_ECTYPE'
     invalid character class

`REG_EESCAPE'
     `\' applied to unescapable character

`REG_ESUBREG'
     invalid backreference number (e.g., larger than the number of
     parenthesized subexpressions in the RE)

`REG_EBRACK'
     brackets [ ] not balanced

`REG_EPAREN'
     parentheses ( ) not balanced

`REG_EBRACE'
     braces { } not balanced

`REG_BADBR'
     invalid repetition count(s) in { }

`REG_ERANGE'
     invalid character range in [ ]

`REG_ESPACE'
     ran out of memory (an RE like, say,
     `((((a{1,100}){1,100}){1,100}){1,100}){1,100}'' will eventually
     run almost any existing machine out of swap space)

`REG_BADRPT'
     ?, *, or + operand invalid

`REG_EMPTY'
     empty (sub)expression

`REG_ASSERT'
     "can't happen" (you found a bug in `regcomp')

`REG_INVARG'
     invalid argument (e.g. a negative-length string)

Regular Expressions' Syntax
---------------------------

Regular expressions ("RE"s), as defined in POSIX 1003.2, come in two
forms: modern REs (roughly those of `egrep'; 1003.2 calls these
*extended* REs) and obsolete REs (roughly those of `ed'; 1003.2 *basic*
REs).  Obsolete REs mostly exist for backward compatibility in some old
programs; they will be discussed at the end.  1003.2 leaves some
aspects of RE syntax and semantics open; `(*)' marks decisions on these
aspects that may not be fully portable to other 1003.2 implementations.

A (modern) RE is one(*) or more non-empty(*) *branches*, separated by
`|'.  It matches anything that matches one of the branches.

A branch is one(*) or more *pieces*, concatenated.  It matches a match
for the first, followed by a match for the second, etc.

A piece is an *atom* possibly followed by a single(*) `*', `+', `?', or
*bound*.  An atom followed by `*' matches a sequence of 0 or more
matches of the atom.  An atom followed by `+' matches a sequence of 1
or more matches of the atom.  An atom followed by `?' matches a
sequence of 0 or 1 matches of the atom.

A *bound* is `{' followed by an unsigned decimal integer, possibly
followed by `,' possibly followed by another unsigned decimal integer,
always followed by `}'.  The integers must lie between 0 and
`RE_DUP_MAX' (255(*)) inclusive, and if there are two of them, the
first may not exceed the second.  An atom followed by a bound containing
one integer `i' and no comma matches a sequence of exactly `i' matches
of the atom.  An atom followed by a bound containing one integer `i'
and a comma matches a sequence of `i' or more matches of the atom.  An
atom followed by a bound containing two integers `i' and `j' matches a
sequence of `i' through `j' (inclusive) matches of the atom.

An atom is a regular expression enclosed in `()' (matching a match for
the regular expression), an empty set of `()' (matching the null
string(*)), a *bracket expression* (see below), `.' (matching any single
character), `' (matching the null string at the beginning of a line),
`$' (matching the null string at the end of a line), a `\\' followed by
one of the characters `.[$()|*+?{\\' (matching that character taken as
an ordinary character), a `\\' followed by any other character(*)
(matching that character taken as an ordinary character, as if the `\\'
had not been present(*)), or a single character with no other
significance (matching that character).  A `{' followed by a character
other than a digit is an ordinary character, not the beginning of a
bound(*).  It is illegal to end an RE with `\\'.

A *bracket expression* is a list of characters enclosed in `[]'.  It
normally matches any single character from the list (but see below).
If the list begins with `', it matches any single character (but see
below) *not* from the rest of the list.  If two characters in the list
are separated by `-', this is shorthand for the full *range* of
characters between those two (inclusive) in the collating sequence,
e.g. `[0-9]' in ASCII matches any decimal digit.  It is illegal(*) for
two ranges to share an endpoint, e.g. `a-c-e'.  Ranges are very
collating-sequence-dependent, and portable programs should avoid relying
on them.

To include a literal `]' in the list, make it the first character
(following a possible `').  To include a literal `-', make it the
first or last character, or the second endpoint of a range.  To use a
literal `-' as the first endpoint of a range, enclose it in `[.' and
`.]' to make it a collating element (see below).  With the exception of
these and some combinations using `[' (see next paragraphs), all other
special characters, including `\\', lose their special significance
within a bracket expression.

Within a bracket expression, a collating element (a character, a
multi-character sequence that collates as if it were a single character,
or a collating-sequence name for either) enclosed in `[.' and `.]'
stands for the sequence of characters of that collating element.  The
sequence is a single element of the bracket expression's list.  A
bracket expression containing a multi-character collating element can
thus match more than one character, e.g. if the collating sequence
includes a `ch' collating element, then the RE `[[.ch.]]*c' matches the
first five characters of "chchcc".

Within a bracket expression, a collating element enclosed in `[=' and
`=]' is an equivalence class, standing for the sequences of characters
of all collating elements equivalent to that one, including itself.
(If there are no other equivalent collating elements, the treatment is
as if the enclosing delimiters were `[.' and `.]'.)  For example, if o
and \o'o' are the members of an equivalence class, then `[[=o=]]',
`[[=\o'o'=]]', and `[o\o'o']' are all synonymous.  An equivalence
class may not\(dg be an endpoint of a range.

Within a bracket expression, the name of a *character class* enclosed
in `[:' and `:]' stands for the list of all characters belonging to
that class.  Standard character class names are:

     alnum       digit       punct
     alpha       graph       space
     blank       lower       upper
     cntrl       print       xdigit

These stand for the character classes defined by `isalnum' (*note
isalnum::.), `isdigit' (isdigit:.), `ispunct' (*note   
ispunct::.), `isalpha' (isalpha:.), `isgraph' (*note   
isgraph::.), `isspace' (isspace:.) (`blank' is the same as   
`space'), `islower' (islower:.), `isupper' (isupper:.),      
`iscntrl' (iscntrl:.), `isprint' (isprint:.), and      
`isxdigit' (isxdigit:.), respectively.  A locale may provide   
others.  A character class may not be used as an endpoint of a range.

There are two special cases(*) of bracket expressions: the bracket
expressions `[[:<:]]' and `[[:>:]]' match the null string at the
beginning and end of a word respectively.  A word is defined as a
sequence of word characters which is neither preceded nor followed by
word characters.  A word character is an `alnum' character (as defined
by `isalnum' library function) or an underscore.  This is an extension,
compatible with but not specified by POSIX 1003.2, and should be used
with caution in software intended to be portable to other systems.

In the event that an RE could match more than one substring of a given
string, the RE matches the one starting earliest in the string.  If the
RE could match more than one substring starting at that point, it
matches the longest.  Subexpressions also match the longest possible
substrings, subject to the constraint that the whole match be as long as
possible, with subexpressions starting earlier in the RE taking priority
over ones starting later.  Note that higher-level subexpressions thus
take priority over their lower-level component subexpressions.

Match lengths are measured in characters, not collating elements.  A
null string is considered longer than no match at all.  For example,
`bb*' matches the three middle characters of `abbbc',
`(wee|week)(knights|nights)' matches all ten characters of
`weeknights', when `(.*).*' is matched against `abc' the parenthesized
subexpression matches all three characters, and when `(a*)*' is matched
against `bc' both the whole RE and the parenthesized subexpression
match the null string.

If case-independent matching is specified, the effect is much as if all
case distinctions had vanished from the alphabet.  When an alphabetic
that exists in multiple cases appears as an ordinary character outside a
bracket expression, it is effectively transformed into a bracket
expression containing both cases, e.g. `x' becomes `[xX]'.  When it
appears inside a bracket expression, all case counterparts of it are
added to the bracket expression, so that (e.g.) `[x]' becomes `[xX]' and
`[x]' becomes `[xX]'.

No particular limit is imposed on the length of REs(*).  Programs
intended to be portable should not employ REs longer than 256 bytes, as
an implementation can refuse to accept such REs and remain
POSIX-compliant.

Obsolete (*basic*) regular expressions differ in several respects.
`|', `+', and `?' are ordinary characters and there is no equivalent
for their functionality.  The delimiters for bounds are `\\{' and
`\\}', with `{' and `}' by themselves ordinary characters.  The
parentheses for nested subexpressions are `\(' and `\)', with `(' and
`)' by themselves ordinary characters.  `' is an ordinary character
except at the beginning of the RE or(*) the beginning of a parenthesized
subexpression, `$' is an ordinary character except at the end of the RE
or(*) the end of a parenthesized subexpression, and `*' is an ordinary
character if it appears at the beginning of the RE or the beginning of a
parenthesized subexpression (after a possible leading `').  Finally,
there is one new type of atom, a *back reference*: `\\' followed by a
non-zero decimal digit *d* matches the same sequence of characters
matched by the *d*th parenthesized subexpression (numbering
subexpressions by the positions of their opening parentheses, left to
right), so that (e.g.) `\\([bc]\\)\\1' matches `bb' or `cc' but not
`bc'.