Modifying a RegEx pattern to work whether or not there is a space between two elements.

I have a regex pattern that picks up on strings containing a number followed by a letter or letters, but I need it to pick up on strings where there is a space between the number and the letters that come after it. My current pattern is this: s*\d{1,3}[A-M]. It will pick up 18ABC but not 18 ABC. I haven't used RegEx for a while and I can't remember how to write that.  I'm working in Excel if that makes a difference.

Thanks!

John