regexp(3) LPC Library Functions regexp(3)
NAME
regexp() - regular expression handler
SYNOPSIS
string *regexp( string *lines, string pattern, void | int
flag );
DESCRIPTION
Typically when presented with an array of lines of text and
a regular expression, regexp(3) returns an array containing
those lines which match the pattern specified by the regular
expression. If the flag (default 0) has bit 2 set, then
non-matches will be returned instead of matches. If the flag
has bit 1 set, the array returned will be of the form ({
index1 + 1, match1, ..., indexn + 1, matchn }) where index1
is the index of 1st match/non match in the array lines.
REGULAR EXPRESSION SYNTAX
A regular expression is zero or more _b_r_a_n_c_h_e_s, separated by
`|'. It matches anything that matches one of the branches.
A branch is zero or more _p_i_e_c_e_s, concatenated. It matches a
match for the first, followed by a match for the second,
etc.
A piece is an _a_t_o_m 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 _r_a_n_g_e (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 `\' fol-
lowed by a single character (matching that character), or a
single character with no other significance (matching that
character).
A _r_a_n_g_e 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 charac-
ter _n_o_t 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
MudOS Last change: 5 Sep 1994 1
regexp(3) LPC Library Functions regexp(3)
character.
AMBIGUITY
If a regular expression could match two different parts of
the input string, it will match the one which begins earli-
est. 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 con-
sidered in left-to-right order, the possibilities for `*',
`+', and `?' are considered longest-first, nested constructs
are considered from the outermost in, and concatenated con-
structs 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 (earli-
est 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 deci-
sion 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.)
SEE ALSO
sscanf(3), explode(3), strsrch(3), ed(3)
MudOS Last change: 5 Sep 1994 2