I have created a regular expression to validate dates in MM/DD/YYYY, MM/YYYY, and YYYY. "((0[1-9]|1[012])[/]((0[1-9]|[12][0-9]|3[01])[/])?)?(19|20)\d\d" . How can I make this expression work with a multi-line textbox, so that a user would be able to enter multiple dates (each on a new line) and still be validated by my expression.

Well, this seems to be the answer

(((0[1-9]|1[012])[/]((0[1-9]|[12][0-9]|3[01])[/])?)?(19|20)\d\d(\s+)?)+