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.