'a:b:c' -> ('a', 'b:c')
 'a\:b:c' -> ('a\:b', 'c')
 'a\\:b:c' -> ('a\\', 'b:c')
 'a\\\:b:c' -> ('a\\\:b', 'c')
 'a:' -> ('a', '')
 ':b' -> ('', 'b')