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

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 

Answer 2

I wrote a simple (well the initialization got a bit bloated) javascript object to solve this problem on a project I've been working on recently. It works the same way as the accepted answer but generates the new regexp and pulls out the data you requested automatically.

var exp = new MultiRegExp(/(firstBit\w+)this text is ignored(optionalBit)?/i); var value = exp.exec("firstbitWithMorethis text is ignored");  value = {0: {index: 0, text: 'firstbitWithMore'},          1: null}; 

Git Repo: My MultiRegExp. Hope this helps someone out there.

edit Aug, 2015:

Try me: MultiRegExp Live.