#!/usr/bin/perl
my $q;
$q = qr/(?:\[(?:(?>[^\[\]]+)|(??{$q}))*\])/;

while ( defined( $_ = <DATA> ) ) {
  while (/(\w{1,2})($q)/g) {
    print $1, $2, $/;
    #push(@{$hash{$1}},$2);
  }
}
__DATA__
T[[pP]attern]
X[]
X[xx[xxx[xx]]]