NAME
regcomp,
regexec,
regsub,
regerror —
obsolete 'regexp' regular
expression handlers
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);
void
regerror(
const
char *msg);
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(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
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.
The
regsub() function 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[] 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 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.)
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),
egrep(1),
ex(1),
expr(1),
fgrep(1),
grep(1),
regex(3),
re_format(7)
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(1)'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.