Can Negative Lookahead Mimic Regex Lookbehind in JavaScript?

Negative Lookahead in JavaScript: An Alternative to Regex Lookbehind

Question:

In JavaScript, which lacks Regex lookbehind, is there a way to match a specific pattern excluding a certain condition?

Answer:

Prior to ECMAScript 2018, JavaScript did not natively support negative lookbehind assertions. Here's an alternative approach:

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

Explanation:

This regex simulates lookbehind by explicitly checking each character of the string. If the lookbehind expression ("filename.js$"), followed by the rest of the regex (".js$"), does not match at the current character, the character is permitted.

^                 # Start of string
(?:               # Try to match the following:
 (?!              # First assert that we can't match the following:
  filename\.js    # filename.js 
  $               # and end-of-string
 )                # End of negative lookahead
 .                # Match any character
)*                # Repeat as needed
\.js              # Match .js
$                 # End of string

However, a simpler alternative has emerged since then:

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

This latter approach is more efficient as it does not check the lookahead at every character.

^                 # Start of string
(?!               # Assert that we can't match the following:
 .*               # any string, 
  filename\.js    # followed by filename.js
  $               # and end-of-string
)                 # End of negative lookahead
.*                # Match any string
\.js              # Match .js
$                 # End of string

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