How to find indices of groups in JavaScript regular expressions match?

Question

When I write a regular expression like:

var m = /(s+).*?(l)[^l]*?(o+)/.exec(\"this is hello to you\");
console.log(m);

I get a match object

Answer 1

Based on the ecma regular expression syntax I've written a parser respective an extension of the RegExp class which solves besides this problem (full indexed exec method) as well other limitations of the JavaScript RegExp implementation for example: Group based search & replace. You can test and download the implementation here (is as well available as NPM module).

The implementation works as follows (small example):

//Retrieve content and position of: opening-, closing tags and body content for: non-nested html-tags.
var pattern = '(<([^ >]+)[^>]*>)([^<]*)(<\\/\\2>)';
var str = 'first
second
';
var regex = new Regex(pattern, 'g');
var result = regex.exec(str);

console.log(5 === result.length);
console.log('first'=== result[0]);
console.log(''=== result[1]);
console.log('first'=== result[3]);
console.log(''=== result[4]);
console.log(5=== result.index.length);
console.log(6=== result.index[0]);
console.log(6=== result.index[1]);
console.log(31=== result.index[3]);
console.log(36=== result.index[4]);

I tried as well the implementation from @velop but the implementation seems buggy for example it does not handle backreferences correctly e.g. "/a(?: )bc(def(\1ghi)xyz)/g" - when adding paranthesis in front then the backreference \1 needs to be incremented accordingly (which is not the case in his implementation).