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

rated 0 times [ 0] [ 5] / answers: 1 / hits: 19758 / 15 Years ago, thu, december 31, 2009, 12:00:00

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?

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irvingjamieb

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3 Years ago

answered 15 Years ago austynp · Accepted Answer

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