javascript regex - look behind alternative?

Question

Here is a regex that works fine in most regex implementations:

(?<!filename).js$

This matches .js for a string which ends with .js except for filename.js

Javascript doesn't have regex lookbehind. Is anyone able put together an alternative regex which achieve the same result and works in javascript?

Here are some thoughts, but needs helper functions. I was hoping to achieve it just with a regex: http://blog.stevenlevithan.com/archives/mimic-lookbehind-javascript

Answer 1

EDIT: From ECMAScript 2018 onwards, lookbehind assertions (even unbounded) are supported natively.

In previous versions, you can do this:

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

This does explicitly what the lookbehind expression is doing implicitly: check each character of the string if the lookbehind expression plus the regex after it will not match, and only then allow that character to match.

^                 # 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

Another edit:

It pains me to say (especially since this answer has been upvoted so much) that there is a far easier way to accomplish this goal. There is no need to check the lookahead at every character:

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

works just as well:

^                 # 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