How to match a number if it is not in between two _ characters like _1234_

Question

Given this string the symbol for four is 4 like in 34. _1234_ . How can regex match the individual 4 and the 4 in 34 but not the one between the two _ characters in _1234_ .

Answer 1

(?<!_\d*)4(?!\d*_)

It makes sure the 4 is neither followed nor follows any count of digits and then an underscore.
Edit: I'm sorry, this won't work because JS does not support negative lookbehinds.

Answer 2

Your requirements are not very specific, so sorry if this will not work in practice for the actual application.

/(?:(?:^|\b)[^\_][a-zA-Z0-9]*)(4)(?:[a-zA-Z0-9\.]*[^\_](?:\b|$))/g

It checks every word that it does not begin and end with underscores for a 4.

Answer 3

Look for _\\d+_|(4) , and check for a match with the (4) group. Since the alternatives in | are tried left-to-right, the (4) group will only match 4s that aren't in numbers surrounded by underscores