path: root/winsup/cygwin/regexp/regexp.3

REGEXP(3)                          BSD Library Functions Manual                         REGEXP(3)

NAME
     regcomp, regexec, regsub, regerror -- regular expression handlers

SYNOPSIS
     #include <regexp.h>

     regexp *
     regcomp(const char *exp);

     int
     regexec(const regexp *prog, const char *string);

     void
     regsub(const regexp *prog, const char *source, char *dest);

DESCRIPTION
     This interface is made obsolete by regex(3).  It is available from the compatibility li-
     brary, libcompat.

     The regcomp(), regexec(), regsub(), and regerror() functions implement egrep(1)-style regu-
     lar expressions and supporting facilities.

     The regcomp() function compiles a regular expression into a structure of type regexp, and
     returns a pointer to it.  The space has been allocated using malloc(3) and may be released
     by free.

     The regexec() function matches a NUL-terminated string against the compiled regular expres-
     sion in prog.  It returns 1 for success and 0 for failure, and adjusts the contents of
     prog's startp and endp (see below) accordingly.

     The members of a regexp structure include at least the following (not necessarily in order):

           char *startp[NSUBEXP];
           char *endp[NSUBEXP];

     where NSUBEXP is defined (as 10) in the header file.  Once a successful regexec() has been
     done using the regexp(), each startp- endp pair describes one substring within the string,
     with the startp pointing to the first character of the substring and the endp pointing to
     the first character following the substring.  The 0th substring is the substring of string
     that matched the whole regular expression.  The others are those substrings that matched
     parenthesized expressions within the regular expression, with parenthesized expressions num-
     bered in left-to-right order of their opening parentheses.

     The regsub() function copies source to dest, making substitutions according to the most re-
     cent regexec() performed using prog.  Each instance of `&' in source is replaced by the sub-
     string indicated by startp[] and endp[].  Each instance of '\n', where n is a digit, is re-
     placed by the substring indicated by startp[n] and endp[n].  To get a literal `&' or '\n'
     into dest, prefix it with `\'; to get a literal `\' preceding `&' or '\n', prefix it with
     another `\'.

     The regerror() function is called whenever an error is detected in regcomp(), regexec(), or
     regsub().  The default regerror() writes the string msg, with a suitable indicator of ori-
     gin, on the standard error output and invokes exit(2).  The regerror() function can be re-
     placed by the user if other actions are desirable.

REGULAR EXPRESSION SYNTAX
     A regular expression is zero or more branches, separated by `|'.  It matches anything that
     matches one of the branches.

     A branch is zero 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 `*', `+', or `?'.  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 match of the atom, or the
     null string.

     An atom is a regular expression in parentheses (matching a match for the regular expres-
     sion), a range (see below), `.'  (matching any single character), `^' (matching the null
     string at the beginning of the input string), `$' (matching the null string at the end of
     the input string), a `\' followed by a single character (matching that character), or a sin-
     gle character with no other significance (matching that character).

     A range is a sequence of characters enclosed in `[]'.  It normally matches any single char-
     acter from the sequence.  If the sequence begins with `^', it matches any single character
     not from the rest of the sequence.  If two characters in the sequence are separated by `-',
     this is shorthand for the full list of ASCII characters between them (e.g. `[0-9]' matches
     any decimal digit).  To include a literal `]' in the sequence, make it the first character
     (following a possible `^').  To include a literal `-', make it the first or last character.

AMBIGUITY
     If a regular expression could match two different parts of the input string, it will match
     the one which begins earliest.  If both begin in the same place but match different lengths,
     or match the same length in different ways, life gets messier, as follows.

     In general, the possibilities in a list of branches are considered in left-to-right order,
     the possibilities for `*', `+', and `?' are considered longest-first, nested constructs are
     considered from the outermost in, and concatenated constructs are considered leftmost-first.
     The match that will be chosen is the one that uses the earliest possibility in the first
     choice that has to be made.  If there is more than one choice, the next will be made in the
     same manner (earliest possibility) subject to the decision on the first choice.  And so
     forth.

     For example, '(ab|a)b*c' could match `abc' in one of two ways.  The first choice is between
     `ab' and `a'; since `ab' is earlier, and does lead to a successful overall match, it is cho-
     sen.  Since the `b' is already spoken for, the `b*' must match its last possibility--the
     empty string--since it must respect the earlier choice.

     In the particular case where no `|'s are present and there is only one `*', `+', or `?', the
     net effect is that the longest possible match will be chosen.  So 'ab*', presented with
     `xabbbby', will match `abbbb'.  Note that if 'ab*', is tried against `xabyabbbz', it will
     match `ab' just after `x', due to the begins-earliest rule.  (In effect, the decision on
     where to start the match is the first choice to be made, hence subsequent choices must re-
     spect it even if this leads them to less-preferred alternatives.)

RETURN VALUES
     The regcomp() function returns NULL for a failure (regerror() permitting), where failures
     are syntax errors, exceeding implementation limits, or applying `+' or `*' to a possibly-
     null operand.

SEE ALSO
     ed(1), ex(1), expr(1), egrep(1), fgrep(1), grep(1), regex(3)

HISTORY
     Both code and manual page for regcomp(), regexec(), regsub(), and regerror() were written at
     the University of Toronto and appeared in 4.3BSD-Tahoe.  They are intended to be compatible
     with the Bell V8 regexp(3), but are not derived from Bell code.

BUGS
     Empty branches and empty regular expressions are not portable to V8.

     The restriction against applying `*' or `+' to a possibly-null operand is an artifact of the
     simplistic implementation.

     Does not support egrep's newline-separated branches; neither does the V8 regexp(3), though.

     Due to emphasis on compactness and simplicity, it's not strikingly fast.  It does give spe-
     cial attention to handling simple cases quickly.

BSD                                        June 4, 1993                                       BSD