Javascript: negative lookbehind equivalent?

Since 2018, Lookbehind Assertions are part of the ECMAScript language specification.

// positive lookbehind
(?<=...)
// negative lookbehind
(?<!...)

Answer pre-2018

As Javascript supports negative lookahead, one way to do it is:

1. reverse the input string

2. match with a reversed regex

3. reverse and reformat the matches

const reverse = s => s.split('').reverse().join('');

const test = (stringToTests, reversedRegexp) => stringToTests
  .map(reverse)
  .forEach((s,i) => {
    const match = reversedRegexp.test(s);
    console.log(stringToTests[i], match, 'token:', match ? reverse(reversedRegexp.exec(s)[0]) : 'Ø');
  });

Example 1:

Following @andrew-ensley's question:

test(['jim', 'm', 'jam'], /m(?!([abcdefg]))/)

Outputs:

jim true token: m
m true token: m
jam false token: Ø

Example 2:

Following @neaumusic comment (match max-height but not line-height, the token being height):

test(['max-height', 'line-height'], /thgieh(?!(-enil))/)

Outputs:

max-height true token: height
line-height false token: Ø

Lookbehind Assertions got accepted into the ECMAScript specification in 2018.

Positive lookbehind usage:

console.log(
  "$9.99  €8.47".match(/(?<=\$)\d+\.\d*/) // Matches "9.99"
);

Negative lookbehind usage:

console.log(
  "$9.99  €8.47".match(/(?<!\$)\d+\.\d*/) // Matches "8.47"
);

Platform support:

• ✔️ V8
  • ✔️ Google Chrome 62.0
  • ✔️ Microsoft Edge 79.0
  • ✔️ Node.js 6.0 behind a flag and 9.0 without a flag
  • ✔️ Deno (all versions)
• ✔️ SpiderMonkey
  • ✔️ Mozilla Firefox 78.0
• ️ JavaScriptCore: Apple is working on it
  • ️ Apple Safari
  • ️ iOS WebView (all browsers on iOS + iPadOS)
• ❌ Chakra: Microsoft was working on it but Chakra is now abandoned in favor of V8
  • ❌ Internet Explorer
  • ❌ Edge versions prior to 79 (the ones based on EdgeHTML+Chakra)

Let's suppose you want to find all int not preceded by unsigned:

With support for negative look-behind:

(?<!unsigned )int

Without support for negative look-behind:

((?!unsigned ).{9}|^.{0,8})int

Basically idea is to grab n preceding characters and exclude match with negative look-ahead, but also match the cases where there's no preceeding n characters. (where n is length of look-behind).

So the regex in question:

(?<!([abcdefg]))m

would translate to:

((?!([abcdefg])).|^)m

You might need to play with capturing groups to find exact spot of the string that interests you or you want to replace specific part with something else.