Javascript regex (negative) lookbehind not working in firefox

Lookbehind assertions are part of ES2018. They are not yet supported by firefox, that's why you're getting an error.

Chrome supports them since version 62, and you can use them in Node.js >= 6.4 with the harmony flag, or in >= 9 without any flag.

You can check the proposal here & browser support here

July 1, 2020 Update

Starting with the FireFox 78 version, RegExp finally supports lookbehinds, dotAll s flag, Unicode escape sequences and named captures, see the Release Notes:

New RegExp engine in SpiderMonkey, adding support for the dotAll flag, Unicode escape sequences, lookbehind references, and named captures.

Thank you very much, FireFox developers!!! 👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍👍

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Lookbehinds are only available in browsers supporting ECMA2018 standard, and that means, only the latest versions of Chrome can handle them.

To support the majority of browsers, convert your pattern to only use lookaheads.

The (?<!=) negative lookbehind makes sure there is no = immediately to the left of the current location. [^"] is the atom that matches that character (note that ? quantifier makes it optional, but " that is before [^"] can't be = and there is no need restricting that position).

So, you may use

content = content.replace(/(?![^<]>)"([^"=]?)"(?!>)/g, '„$1"');
                                      ^^^^^

Note that (?:[\"]) is equal to ". [^"=]? matches 1 or 0 occurrences of a char other than " and =.

See the regex demo.

The exact equivalent of your regex (?![^<]*>)"([^"]*?)(?<!=)"(?!>)

without the lookbehind assertion is:

(?![^<]*>)"((?:[^"=]+|=(?!"))*)"(?!>)

Readable version

 (?! [^<]* > )
 "
 (                             # (1 start)
      (?:
           [^"=]+ 
        |  
           = 
           (?! " )
      )*
 )                             # (1 end)
 "
 (?! > )

Note this is not like your chosen answer, which is not an equivalent .