Text Processing with JavaScript

Recipe 48

Testing a Pattern with the Negative Lookahead

Task

Suppose you want to match any two-digit number except 41, 66, and 77 because these are retired jersey numbers, so you want to make sure they don’t get reassigned to a new player.

You could use several character classes matching a range between 00 to 40, 42 to 65, 67 to 76, and 78 to 99. But you’d end up with a very long and complicated pattern. Fortunately, there’s a more compact way to get the same result with lookahead assertions.

Solution

Use a negative lookahead assertion denoted by (?! ... ):

part_2/negative_lookahead/negative_lookahead_ex1.js

const re = /#(?!41|66|77)\d{2}/

re.test("#00");    // → true
re.test("#39");    // → true
re.test("#41");    // → false ...