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regcomp, regexec, regsub, regerror - regular expression handler
int regexec(prog, string)
regsub(prog, source, dest)
These functions implement egrep(1)-style regular expressions and
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):
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 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 ‘\’.
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
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.
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
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.)
Regcomp returns NULL for a failure (regerror permitting), where
failures are syntax errors, exceeding implementation limits, or
applying ‘+’ or ‘*’ to a possibly-null operand.
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
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.