regexp(3)

NAME

regcomp, regexec, regsub, regerror - regular expression han

dlers

LIBRARY

Compatibility Library (libcompat, -lcompat)

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).

The regcomp(), regexec(), regsub(), and regerror() functions

implement
egrep(1)-style regular expressions and supporting facili

ties.

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(3).

The regexec() function 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 fol

lowing (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 oth

ers are those
substrings that matched parenthesized expressions within the

regular
expression, with parenthesized expressions numbered in left

to-right
order of their opening parentheses.

The regsub() function copies source to dest, making substi

tutions according 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 re

placed by the
substring indicated by startp[n] and endp[n]. To get a lit

eral `&' or
`n' into dest, prefix it with `'; to get a literal `' pre

ceding `&' or
`n', prefix it with another `'.

The regerror() function is called whenever an error is de

tected 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 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 in

put string), a
`' followed by a single character (matching that character),

or a single
character with no other significance (matching that charac

ter).

A range is a sequence of characters enclosed in `[]'. It

normally
matches any single character from the sequence. If the se

quence 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 con

sidered 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 possi

bility 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 possi

ble 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-pre

ferred alternatives.)

RETURN VALUES

The regcomp() function returns NULL for a failure

(regerror() permitting), where failures are syntax errors, exceeding implemen

tation 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 ap

peared 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(1)'s newline-separated branches; nei

ther does the
V8 regexp(3), though.

Due to emphasis on compactness and simplicity, it is not

strikingly fast.
It does give special attention to handling simple cases

quickly.

BSD June 4, 1993

BSD