How to Achieve Look Behind Assertion in JavaScript?

JavaScript Regex: Alternative to Look Behind Assertion

In JavaScript, the look behind assertion (?) is not supported. This assertion is useful for matching a pattern that is preceded by a specific condition. However, there are alternative ways to achieve the same result in JavaScript.

One alternative is to use the ^ and (?!) pattern. The ^ symbol matches the beginning of a string, while the (?!) checks whether the following expression will not match the current position. For example, the following regex matches any string that ends with .js, except for filename.js:

^(?:(?!filename\.js$).)*\.js$

This regex explicitly checks each character of the string to ensure that it does not satisfy the look behind condition.

Another simpler alternative introduced in ECMAScript 2018 is to use the following regex:

^(?!.*filename\.js$).*\.js$

This regex uses the . expression to match any string, and the (?!) assertion to check that the following expression (.filename.js$) will not match the current position. This alternative is more efficient than the first one, as it does not require checking the condition at every character.

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