#!/usr/bin/perl
use strict;
use diagnostics;

use Data::Dumper;
use Regexp::Common qw(balanced);

my $re=$RE{balanced}{-parens=>'[]'};

# a valid string is: beginning, one or more [things] perhaps seperated by whitespace, end
foreach (
    'a [foo]  [bar][quux] grmbl42', #1  ok
    'a [foo]  [bar]bla[quux] grmbl42', #2 nok: has random crap instead of whitespace
    'a [foo]  [bar] grmbl42',    #3 ok
    'a [foo] grmbl42',  #4 ok
    'a [fo grmbl42',    #5 nok: unbalanced
    'a [fo]o] grmbl42',    #6 nok: unbalanced
    'a [fo[o] grmbl42',    #7 nok: unbalanced
    'a o] grmbl42',    #8 nok: unbalanced
    'a grmbl42', #9 nok: no [] at all
) {
    my $didit=/^
        a         # literal a
        \s*       # optional whitespace
        (?:
            $re
            \s*
        )+
        .*        # something
        42        # literal 42
    $/x;
    print Dumper $_, $didit;
};