"string match" is an under-used gem of a command. I did some minor testing and that single string match is faster than doing a while loop from 0 to the length of the string (without anything in the loop body). I thought I could be cute and just iterate through the characters in the string but that's almost 3x slower just for the iteration than the string match. I think you'd be hard pressed to get a faster solution in straight TCL without writing a proc in native code.