user974061 user974061 - 17 days ago 6x
Javascript Question

Javascript conditional regular expression if-then-else

I'm trying to limit the entries to a specific format.

If the entry has 5500 or 5100 such as 01\01-5500-000-00 then I want to have this:


But if the entry has anything other than 5500 or 5100 I want to have this:


How can this be accomplished with the if then else idea?


Conditional regex syntax is not supported by JavaScript regex engine, but it can be worked around with a non-capturing group containing 2 alternatives:

  1. One with the positive look-ahead and

  2. The second with the reversed, negative look-ahead.

This regex meets your criteria and is JavaScript compatible:


See regex demo

Let me break it down:

  • ^ - Start of string
  • (?:
    • (?=.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}\-[0-9]{4}\-[0-9]{3}\- - First alternative with the (?=.*\b5[15]00\b) look-ahead that requires a whole word 5500 or 5100 inside the string, and the first pattern you have
    • | - alternation operator
    • (?!.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}\-[0-9]{4}\-[0-9]{3}\-[0-9]{2}) - Second alternative that is prepended with the (?!.*\b5[15]00\b) negative look-ahead that makes sure there is no 5100 or 5500 inside the string, and only then matches your second pattern.
  • $ - end of string.