REGEXP(3)                 OpenBSD Programmer's 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 library, libcompat.

     The regcomp(), regexec(), regsub(), and regerror() functions implement
     egrep(1)-style  regular 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 com-
     piled regular expression 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 de-
     scribes one substring within the string, with the startp pointing to the
     first character of the substring and the endp pointing to the first char-
     acter 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 ex-
     pression, with parenthesized expressions numbered in left-to-right order
     of their opening parentheses.

     The regsub() function copies source to dest, making substitutions accord-
     ing to the most recent regexec() performed using prog. Each instance of
     `&' in source is replaced by the substring indicated by startp[] and
     endp[]. Each instance of `\n', where n is a digit, is replaced 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 origin, on the standard error
     output and invokes exit(3).  The regerror() function can be replaced 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 fol-
     lowed 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 expression), a range (see below), `.'  (matching any single char-
     acter), `^' (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 single
     character with no other significance (matching that character).

     A range is a sequence of characters enclosed in `[]'.  It normally match-
     es any single character 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 consid-
     ered 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 chosen.  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 af-
     ter `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 respect it even if this leads them to less-preferred alter-
     natives.)

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

SEE ALSO
     ed(1),  egrep(1),  ex(1),  expr(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 special attention to handling simple cases quickly.

OpenBSD 2.6                      June 4, 1993                                3