The two, \d and [0-9], are not equivalent unless you use the /a modifier. By someone's IMNSHO incorrect decision, \d was implemented to mean "anything that might be considered a digit in languages you will never hear of", not "number understood by Perl and usable in computation". While the likelihood that you end up with a string containing any such characters is pretty slim, you should play safe and either use [0-9] or the /a modifier!