How does grouping work in Javascript regular expressions?

Question

Given this Javascript:

String.method('deentityify', function () {
    // The entity table.  It maps entity names to
    // characters.
    var entity = {
        quot: '"',
        lt: '<',
        gt: '>'
    };

    // Return the deentityify method
    return function () {

        return this.replace(/&([^&;]+);/g,
            function (a, b) {
                var r = entity[b];
                return typeof r === 'string' ? r : a;
            }
        );
    };
}());

document.writeln('&lt;&quot;&gt;'.deentityify());

What goes into the last function as a and b, and why?

I get that we're splitting up the string I'm passing in into 3 groups, but I don't understand why < is going into a and why just lt is going into b.

Have a look at the MDN documentation.

Felix Kling
– Felix Kling

2012-03-14 17:17:19 +00:00
Commented Mar 14, 2012 at 17:17 — Felix Kling
– Felix Kling, Commented Mar 14, 2012 at 17:17

Rob W · Accepted Answer · 2012-03-14 17:15:11Z

2

The first argument contains the whole match, all consecutive arguments are the matched groups. The last two arguments are the offset and full input string.

var input = '&lt;&quot;&gt;'
input.replace( /&([^&;]+);/g, function (a, b)

The pattern matches all occurrences of & + every non-& + ;.

a      b
&lt;   lt 
&quot; quot
&gt;   gt

See also: MDN: String.replace with a function

answered Mar 14, 2012 at 17:15

Rob W

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5 Comments

Tad Donaghe Over a year ago

So, the first match is < right? Where does the lt come from? Sorry - I know I'm being dense.

Rob W Over a year ago

lt are all non-& characters between the & and the ;. It is captured by the parens.

Tad Donaghe Over a year ago

Argh. I'm still not getting it. I don't understand why we're passing two things per match. It seems to me that we're only matching 3 things: < " > Are we really capturing 6 things?

Rob W Over a year ago

@TerryDonaghe The parens create a new group. a contains the full match and b the first group. The pattern is found three time, and since you've added the g (global) flag to the RegEx, the function is called three times, with a and b as specified in my answer.

Tad Donaghe Over a year ago

Thanks @Rob W! I think I've finally got it. I looked at the doc link you provided and that helps. My brain hurts.

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