REGEXP(3) | Library Functions Manual | REGEXP(3) |
regcomp
, regexec
,
regsub
, regerror
—
#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);
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.
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.
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.)
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.
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.
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.
June 4, 1993 | NetBSD 9.2 |