Question

I’d like a regex (using Java) that captures three digits such as “876”, but not if they are buried inside a 4-digit sequence.

To capture “876” within “876” and “foo876″ and ” 876 ” and “876” and “food876” and “4foo876”.

But NOT within “88foo9876” or “9876” or “a8876” or “a8876foo”.

How do I do this?

I want to say something like X(\d\d\d)X, but in place of the first X in that to say “\D or ^ (start-string)” and in place of the second X in that to say “\D or $ (end-string)”.

Edit:

For answers, see Xanatos, also Code Jockey, and Tim Pietzcker.

Answer 1

well, then! for X(\d\d\d)X as you asked for, use

(?<=\D|^)(\d\d\d)(?=\D|$)

which is

(?<=\D|^)      # lookbehind for «\D or ^ (start-string)»
(\d\d\d)       # then match «three digits such as "876"»
(?=\D|$)       # lookahead for «\D or $ (end-string)»

and will

…capture “876” within “876” and “foo876″ and ” 876 ” and “876” and “food876”.

But NOT within “88foo9876” or “9876” or “a8876” or “a8876foo”.

as you specified 😀

Here it is shown below in RegexBuddy:

if you’re using a language without lookbehind (like ECMA/JavaScript) you’ll have to either use

(\D|^)(\d\d\d)(?=\D|$)     # and use the second capturing group -or-
                           # use
(?:\D|^)(\d\d\d)(?=\D|$)   # and use the first capturing group