REGEXP(3)

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

SYNOPSIS
#include <regexp.h>
regexp *regcomp(exp)
char *exp;
int regexec(prog, string)
regexp *prog;
char *string;
regsub(prog, source, dest)
regexp *prog;
char *source;
char *dest;
regerror(msg)
char *msg;

DESCRIPTION
These functions implement egrep(1)-style regular expressions and supporting facilities.

Regcomp 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.

Regexec matches a NUL-terminated string against the compiled 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 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 numbered in left-to-right order of their opening parentheses.

Regsub copies source to dest, making substitutions according to the most recent regexec performed using prog. Each instance of `&' in source is replaced by the substring indicated by startp[0] and endp[0]. 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 `\'.

Regerror 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(2). Regerror 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 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 expression), 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 single character with no other significance (matching that character).

A range is a sequence of characters enclosed in `[]'. It normally matches 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 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 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 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 respect it even if this leads them to less-preferred alternatives.)

SEE ALSO
egrep(1), expr(1)

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

HISTORY
Both code and manual page were written at U of T. 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.