/

^(0|1)$       # simple cases of 1 or 0,  with zero 01|10
   |
^1(0|1)*1$    # 1 .. anything .. 1
   |
^0(0|1)*0$    # 0 .. anything .. 0

/x

##</code><code>##

my @tests = qw(
	       0 1 
	       01 10 00 11 
	       000 111 001 011 101 010 100 110 
	       1001 1101 0101 1010
	      );

foreach (@tests) {
  print "$_ ";
  if (/^(0|1)$|^1(0|1)*1$|^0(0|1)*0$/) {
    print "passed\n";
  }
  else {
    print "failed\n";
  }
}