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.\" @(#)regexp.3 5.2 (Berkeley) 4/20/91
.Nd regular expression handlers
.Fn regcomp "const char *exp"
.Fn regexec "const regexp *prog" "const char *string"
.Fn regsub "const regexp *prog" "const char *source" "char *dest"
regular expressions and supporting facilities.
compiles a regular expression into a structure of type
and returns a pointer to it.
The space has been allocated using
against the compiled regular expression
It returns 1 for success and 0 for failure, and adjusts the contents of
structure include at least the following (not necessarily in order):
.Bd -literal -offset indent
is defined (as 10) in the header file.
pair describes one substring
pointing to the first character of the substring and
pointing to the first character following the substring.
The 0th substring is the substring of
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.
making substitutions according to the
is replaced by the substring
is a digit, is replaced by
the substring indicated by
to get a literal `\e' preceding `&' or
is called whenever an error is detected in
with a suitable indicator of origin,
can be replaced by the user if other actions are desirable.
.Sh REGULAR EXPRESSION SYNTAX
A regular expression is zero or more
It matches anything that matches one of the branches.
It matches a match for the first, followed by a match for the second, etc.
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
(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).
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
from the rest of the sequence.
If two characters in the sequence are separated by `\-', this is shorthand
(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.
`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.
In the particular case where no `|'s are present and there is only one
`*', `+', or `?', the net effect is that the longest possible
presented with `xabbbby', will match `abbbb'.
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.)
where failures are syntax errors, exceeding implementation limits,
or applying `+' or `*' to a possibly-null operand.
Both code and manual page for
were written at the University of Toronto
They are intended to be compatible with the Bell V8
but are not derived from Bell code.
Empty branches and empty regular expressions are not portable to V8.
applying `*' or `+' to a possibly-null operand is an artifact of the
simplistic implementation.
newline-separated branches;
compactness and simplicity,
it's not strikingly fast.
It does give special attention to handling simple cases quickly.