Single Regex for filtering roman numerals from the text files.

Question

I am stuck in between of a problem where only one pass of regular expression is allowed( some old hard code). I need the regex for roman numerals. I have tried the standard one i.e. ^(?i)M*(D?C{0,3}|C[DM])(L?X{0,3}|X[LC])(V?I{0,3}|I[VX])$, but the problem is it allows null( '') values also. Is there any way around to check is problem?

Answer 1

To require that at least one character must be present, you can use a lookahead (?=.) at the start of your regular expression:

^(?=.)(?i)M*(D?C{0,3}|C[DM])(L?X{0,3}|X[LC])(V?I{0,3}|I[VX])$

Another solution is to separately test that your string is not the empty string.