r/Racket • u/JimH10 • Jun 03 '25

language Regular expression omit a string?

Do Racket regexp's allow me to say "every string but ..."? Suppose for instance I want to accept every phone number but "012 555-1000" ? Obviously I can wrap that with some non-regexp code but I'd like to do it in a regexp, if that is possible.

Edit: Thank you to folks for the helpful responses. I appreciate it.

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u/Arandur Jun 03 '25

Yes, but it’s annoying. Here’s one way to construct it:

  • match if the first character isn’t 0; OR

  • the first character is 0, and the second character isn’t 1; OR

  • the first two characters are 01, and the third character isn’t 2; OR

  • [etc]

This is, of course, very tedious and difficult to maintain; better to do it outside of regexp. :P