Could be worse :)
#!/usr/bin/perl
# http://perlmonks.org/?node_id=1189605
use strict;
use warnings;
$| = 1;
sub err { die "ERROR: ", s/\G/<@_>/r, "\n" }
sub crossmap # this is clear, isn't it?
{
my ($left, $right) = @_;
[ map { my $prefix = $_ ; map $prefix.$_, @$right } @$left ];
}
sub expr
{
my $answer = [ '' ];
while( /\G (
\| (?{ 'ALTERNATION' })
| \d+ (?{ 'STRING' })
| \[ \d+ \] (?{ 'CHARACTERCLASS' })
| \( (?{ 'PARENTHESIS' })
) /gcx )
{
goto $^R;
ALTERNATION:
$answer = [ @$answer, @{ expr() } ];
next;
STRING:
$answer = crossmap $answer, [ $1 ];
next;
CHARACTERCLASS:
$answer = crossmap $answer, [ $1 =~ /\d/g ];
next;
PARENTHESIS:
$answer = crossmap $answer, expr();
/\G\)/gc or err "missing ')'";
}
return $answer;
}
while(<DATA>)
{
chomp;
print "$_ => ";
my $answer = expr();
/\G\z/gc or err "incomplete parse";
local $" = ',';
print "@$answer\n";
}
__DATA__
(65|70)
(3[678]|4[1678])
5[45]
(6[4569]|7[01])