ref: d7f37646965ee26214da0e1d97f3c1d54349ae2d
dir: /testdir/t.re4/
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 }