We want to find out the first position $n at which strings $x and $y differ. If $x eq $y, then $n is undef. If $x ne $y but $x is a prefix (aka initial segment) of $y, then $n should be equal to length $x. The operation must preserve both $x and $y (i.e. destructive operations are disallowed).
By way of baseline, here's an utterly uninspired solution:
$n=0;while(($c=substr($x,$n,1)||undef $n) && $c eq substr($y,$n++,1)){;}
Second attempt:
$n=$x eq$y?$z:do{1while$x=~/./sg&$y=~/\G$&/g;$-[0]}
I think this is OK.
-- fxn
