Javascript conditional regular expression if-then-else

Question

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:

^[0-9]{2,}\         


        

Answer 1

Replace this part of the pattern [0-9]{4} with the literal values for 5100/5500:

/^\d{2,}\\\d{2}-(?:5[15]00-\d{3}-|(?!5[15]00)\d{4}-\d{3}-\d{2})$/

• [0-9] is the same thing as \d in JavaScript
• For the else part, use the negative lookahead (?!5[15]00)\d{4} to guarantee it's not 5100/5500.

Answer 2

Use conditionals, Eg:

(?(?=regex)then|else): [0-9]{2,}\\[0-9]{2}\-(?(?=5[15]00)[0-9]{4}\-[0-9]{3}\-|[0-9]{4}\-[0-9]{3}\-[0-9]{2})

Regex conditionals example: conditionals@regex101.com

In case that you're using a regex engine that is not PCRE based, you can be able to mimic the functionality by doing the following:

((?=positive-regex-statement)then|(?!negavite-regex-statement)then)

Eg.

^[0-9]{2,}\\[0-9]{2}\-((?=5[15]00)[0-9]{4}\-[0-9]{3}\-|[0-9]{4}\-|(?!5[15]00)[0-9]{4}\-[0-9]{3}\-[0-9]{2})$

Mimic regex conditionals example: mimic@regex101.com

Answer 3

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:

^(?:(?=.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}-[0-9]{4}-[0-9]{3}-|(?!.*\b5[15]00\b)[0-9]{2,}\\[0-9]{2}-[0-9]{4}-[0-9]{3}-[0-9]{2})$

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.