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" @(#)regexp.3 5.2 (Berkeley) 4/20/91
"
April 20, 1991 REGEXP 3
NAME
regcomp ,
regexec ,
regsub ,
regerror
regular expression handlers
SYNOPSIS
#include
regexp *regcomp "const char *exp"
int regexec "const regexp *prog" "const char *string"
void regsub "const regexp *prog" "const char *source" "char *dest"
DESCRIPTION
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 .
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 2 . 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 ,
ex 1 ,
expr 1 ,
egrep 1 ,
fgrep 1 ,
grep 1 ,
regex 3
HISTORY
Both code and manual page for regcomp , regexec , regsub , and
regerror were written at the University of Toronto and appeared
in 4.3 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'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.