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.)
)
function
returns
NULL
for a failure
(regerror(
)
permitting),
where failures are syntax errors, exceeding implementation limits,
or applying `+' or `*' to a possibly-null operand.
),
regexec(
),
regsub(
),
and
regerror(
)
were written at the University of Toronto
and appeared in
4.3BSDtahoe.
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.