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 containing the following:

{
  0: "s is hello",
  1: "s",
  2: "l",
  3: "o",
  index: 3,
  input: "this is hello to you"
}

I know the index of the entire match from the index property, but I also need to know the start and end of the groups matched. Using a simple search won’t work. In this example it will find the first ‘l’ instead of the one found in the group.

Is there any way to get the offset of a matched group?

Answer 1

You can’t directly get the index of a match group. What you have to do is first put every character in a match group, even the ones you don’t care about:

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

Now you’ve got the whole match in parts:

['s is hello', 's', ' is hel', 'l', '', 'o']

So you can add up the lengths of the strings before your group to get the offset from the match index to the group index:

function indexOfGroup(match, n) {
    var ix= match.index;
    for (var i= 1; i<n; i++)
        ix+= match[i].length;
    return ix;
}

console.log(indexOfGroup(m, 3)); // 11