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.TH REGEXP 3 local

.DA 30 Nov 1985

.SH NAME

regcomp, regexec, regsub, regerror \- regular expression handler

.SH SYNOPSIS

.ft B

.nf

#include 

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;

.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.

.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].

.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 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.)

.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

Both code and manual page were

written at U of T.

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 V8.

.PP

The restriction against

applying `*' or `+' to a possibly-null operand is an artifact of the

simplistic implementation.

.PP

Does not support \fIegrep\fR's newline-separated branches;

neither does the V8 \fIregexp\fR(3), though.

.PP

Due to emphasis on

compactness and simplicity,

it's not strikingly fast.

It does give special attention to handling simple cases quickly.