use strict; use warnings; use feature 'say'; my $s = substr join('', map { int rand 2 } 0..40), 0, int rand 41; say "'$s'"; my $a =()= $s =~ /01/g; say "01 x $a"; my $b =()= $s =~ /10/g; say "10 x $b"; say $s =~ /^(|1+(0+1+)*|0+(1+0+)*)$/ ? 'yes' : 'no';