Answer by user2023861 for Matching any certain selection of inputs

I'd suggest that you simplify. Imagine you're a junior programmer and you are asked to edit this abomination (or the one in the accepted answer) in order to add support for inputs of the type 10cm and -10cm:

-[0-9]+px$|^[0-9]+px$|^[0-9]+em$|^-[0-9]+em$|^[0-9]+\%$|^auto$|^0$

Would you be able to do that without introducing false positives or false negatives? How many test cases would you need to pass to be sure? Why not try this approach:

public bool IsLength(string arg) {    return IsLengthPixel(arg) || IsLengthEm(arg) || IsLengthPercent(arg) ...;}public bool IsLengthPixel(string arg) {    return IsMatch(arg, "^-?[0-9]+px$");}

In this way you can add IsLengthCm(string) without screwing up any of the existing length checks. It's much easier to read this code and, with good naming conventions, it documents itself.