regexp.3

.TH REGEXP 3 "5 Sept 1996"
.SH NAME
regcomp, regexec, regsub, regerror \- regular expression handler
.SH SYNOPSIS
.ft B
.nf
#include <regexp.h>

regexp *regcomp(exp)
const char *exp;

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

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

void regerror(msg)
char *msg;
.SH DESCRIPTION
These functions implement
.IR egrep (1)-style
regular expressions and supporting facilities.
.PP
.I Regcomp
compiles a regular expression into a structure of type
.IR regexp ,
and returns a pointer to it.
The space has been allocated using
.IR malloc (3)
and may be released by
.IR free .
.PP
.I Regexec
matches a NUL-terminated \fIstring\fR against the compiled regular expression
in \fIprog\fR.
It returns 1 for success and 0 for failure, and adjusts the contents of
\fIprog\fR's \fIstartp\fR and \fIendp\fR (see below) accordingly.
.PP
The members of a
.I regexp
structure include at least the following (not necessarily in order):
.PP
.RS
char *startp[NSUBEXP];
.br
char *endp[NSUBEXP];
.RE
.PP
where
.I NSUBEXP
is defined (as 10) in the header file.
Once a successful \fIregexec\fR has been done using the \fIregexp\fR,
each \fIstartp\fR-\fIendp\fR pair describes one substring
within the \fIstring\fR,
with the \fIstartp\fR pointing to the first character of the substring and
the \fIendp\fR pointing to the first character following the substring.
The 0th substring is the substring of \fIstring\fR 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.
If a parenthesized expression does not participate in the match at all,
its \fIstartp\fR and \fIendp\fR are NULL.
.PP
.I Regsub
copies \fIsource\fR to \fIdest\fR, making substitutions according to the
most recent \fIregexec\fR performed using \fIprog\fR.
Each instance of `&' in \fIsource\fR is replaced by the substring
indicated by \fIstartp\fR[\fI0\fR] and
\fIendp\fR[\fI0\fR].
Each instance of `\e\fIn\fR', where \fIn\fR is a digit, is replaced by
the substring indicated by
\fIstartp\fR[\fIn\fR] and
\fIendp\fR[\fIn\fR].
To get a literal `&' or `\e\fIn\fR' into \fIdest\fR, prefix it with `\e';
to get a literal `\e' preceding `&' or `\e\fIn\fR', prefix it with
another `\e'.
.PP
.I Regerror
is called whenever an error is detected in \fIregcomp\fR, \fIregexec\fR,
or \fIregsub\fR.
The default \fIregerror\fR writes the string \fImsg\fR,
with a suitable indicator of origin,
on the standard
error output
and invokes \fIexit\fR(2).
.I Regerror
can be replaced by the user if other actions are desirable.
.SH "REGULAR EXPRESSION SYNTAX"
A regular expression is zero or more \fIbranches\fR, separated by `|'.
It matches anything that matches one of the branches.
.PP
A branch is zero or more \fIpieces\fR, concatenated.
It matches a match for the first, followed by a match for the second, etc.
.PP
A piece is an \fIatom\fR 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.
.PP
An atom is a regular expression in parentheses (matching a match for the
regular expression), a \fIrange\fR (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 `\e' followed by a single character (matching
that character), or a single character with no other significance
(matching that character).
.PP
A \fIrange\fR 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 \fInot\fR 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.
.SH 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.
.PP
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.
.PP
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\(emthe empty string\(emsince
it must respect the earlier choice.
.PP
In the particular case where the regular expression does not use `|'
and does not apply `*', `+', or `?' to parenthesized subexpressions,
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.)
.SH SEE ALSO
egrep(1), expr(1)
.SH DIAGNOSTICS
\fIRegcomp\fR returns NULL for a failure
(\fIregerror\fR permitting),
where failures are syntax errors, exceeding implementation limits,
or applying `+' or `*' to a possibly-null operand.
.SH HISTORY
This is a revised version.
Both code and manual page were
originally written by Henry Spencer at University of Toronto.
They are intended to be compatible with the Bell V8 \fIregexp\fR(3),
but are not derived from Bell code.
.SH BUGS
Empty branches and empty regular expressions are not portable
to other, otherwise-similar, implementations.
.PP
The ban on
applying `*' or `+' to a possibly-null operand is an artifact of the
simplistic implementation.
.PP
The match-choice rules are complex.
A simple ``longest match'' rule would be preferable,
but is harder to implement.
.PP
Although there is a general similarity to POSIX.2 ``extended'' regular
expressions, neither the regular-expression syntax nor the programming
interface is an exact match.
.PP
Due to emphasis on
compactness and simplicity,
it's not strikingly fast.
It does give some attention to handling simple cases quickly.