BEGIN {	r1 = "xxx"
	r2 = "xxx" ":"
	r3 = ":" r2
	r4 = "a"
}

$0 ~ r1	{ print $0 " matches " r1 }
$0 ~ r2	{ print $0 " matches " r2 }
$0 ~ r3	{ print $0 " matches " r3 }
$0 ~ r4	{ print $0 " matches " r4 }