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

SYNOPSIS
#include <regexp.h>

regexp *regcomp(exp)
const char *exp;

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

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

void regerror(msg)
const char *msg;

DESCRIPTION
Regcomp, regexec, regsub, and regerror 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 ‘0, where n is a digit, is replaced by
the substring indicated by startp[n] and endp[n]. To get a literal
‘&’ or ‘0 into dest, prefix it with ‘´; to get a literal ‘´
preceding ‘&’ or ‘0, 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.)

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 for regcomp, regexec, regsub, and
regerror were written at the University of Toronto. 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.

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