regex - Javascript: negative lookbehind equivalent?

Question

Is there a way to achieve the equivalent of a negative lookbehind in javascript regular expressions? I need to match a string that does not start with a specific set of characters.

It seems I am unable to find a regex that does this without failing if the matched part is found at the beginning of the string. Negative lookbehinds seem to be the only answer, but javascript doesn't have one.

EDIT: This is the regex that I would like to work, but it doesn't:

(?<!([abcdefg]))m

So it would match the 'm' in 'jim' or 'm', but not 'jam'

Answer 1

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]) : '?');
  });

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: ?

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: ?