How to Implement Lookbehind Assertions in JavaScript Regex?

Javascript Regex: Alternative to Lookbehind Assertions

Regex implementations in Javascript lack the concept of lookbehind assertions. This poses a challenge in constructing regular expressions that require this functionality. Fortunately, there are alternative methods to achieve similar results.

Consider the following regex:

(?<!filename)\.js$

This regex matches ".js" at the end of a string, excluding "filename.js." However, without lookbehind support in Javascript, we need an alternative.

One such alternative is to expand the lookbehind assertion into a series of explicit checks against each character in the string:

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

This regex breaks down as follows:

• ^: Start of string
• (?:...)*: Loop through the following expression for every character
• (?!filename.js$): Negative lookahead to ensure the next characters do not match "filename.js"
• .: Match a period
• *: Repeat the period match as needed
• .js: Match ".js"
• $: End of string

Alternatively, a simpler solution is available:

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

This regex essentially checks that the entire string does not contain "filename.js" before matching ".js."

By leveraging these alternatives, we can implement regex functionality akin to lookbehind assertions in Javascript.

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