It is an old trick. Transform both sides of the comparison to a canonical form and the whole problems becomes much easier to solve.
in reply to Re^2: Regex question once-only use of chars in a charset
in thread Regex question once-only use of chars in a charset
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James