Single Regex for filtering roman numerals from the text files

I am stuck in between of a problem where o开发者_开发知识库nly 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?

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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.

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I like this one:

\b(?=[MDCLXVI]+\b)M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})\b