Because you could do something with substrings
or, I think, mod might be able to do it.
But don't quote me on that.
( SuperMatt, Wed 17 Jun 2009, 17:51, archived)

Not possible using a math approach.
Plus if the string is an even number of numbers - problem.
( lion-cake Bad lion-cake, bad., Wed 17 Jun 2009, 17:55, archived)