in this use case where there is only one number ("15") in the text:
That replaces everything but digits.
If the format is always the same, e.g = followed by the number you can also use:
@comment field.split("= ").pop().split(" ").shift()
Both ways return 15 - but it's technically a string (text) not a number, you can't calculate with it, e.g. that 15 plus 1 will return 151, not 16. You must parse it to a number first:
Number(@comment field.replace(/[^0-9]/g,"")) || 0