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I have a huge text file with Roman numbers for the paragraphs and I want to substitute them:

text text ... text

I

text ...

II

and so on

My question is: what regular expression describes one Roman number per line?

In other hand this one /^\vM{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$ seems to be ok, except that it includes all blank lines. How refactor it in a way to exclude the empty lines?

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You can use a look-behind assertion to check the length of the matched string:

\v^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$.@<=

. means match any one character (i.e., zero characters will not work).

@<= means look behind and accept only if the preceding atom (i.e., .) matches text immediately before this position in text.

This assertion will fail if the line is empty.

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