Regex non capturing groups in javascript

Question

I'm a bit rusty on my regex and javascript. I have the following string var:

var subject = "javascript:loadNewsItemWithIndex(5, null);";

I want to extract 5 using a regex. This is my regex:

/(?:loadNewsItemWithIndex\()[0-9]+/)

Applied like so:

subject.match(/(?:loadNewsItemWithIndex\()[0-9]+/)

The result is:

loadNewsItemWithIndex(5

What is cleanest, most readable way to extract 5 as a one-liner? Is it possible to do this by excluding loadNewsItemWithIndex( from the match rather than matching 5 as a sub group?

mellamokb · Accepted Answer · 2011-09-21 21:16:29Z

8

The return value from String.match is an array of matches, so you can put parentheses around the number part and just retrieve that particular match index (where the first match is the entire matched result, and subsequent entries are for each capture group):

var subject = "javascript:loadNewsItemWithIndex(5, null);";
var result = subject.match(/loadNewsItemWithIndex\(([0-9]+)/);
                                    //             ^      ^  added parens
document.writeln(result[1]);
                  //    ^ retrieve second match (1 in 0-based indexing)

Sample code: http://jsfiddle.net/LT62w/

Edit: Thanks @Alan for the correction on how non-capturing matches work.

Actually, it's working perfectly. Text that's matched inside a non-capturing group is still consumed, the same as text that's matched outside of any group. A capturing group is like a non-capturing group with extra functionality: in addition to grouping, it allows you to extract whatever it matches independently of the overall match.

edited Sep 21, 2011 at 21:16

answered Sep 21, 2011 at 19:57

mellamokb

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3 Comments

Benedict Cohen Over a year ago

The non-capturing match apparently doesn't work properly. I knew there were some odd regex behaviours in javascript, but I though this problem was me being a bit rusty.

Alan Moore Over a year ago

Actually, it's working perfectly. Text that's matched inside a non-capturing group is still consumed, the same as text that's matched outside of any group. A capturing group is like a non-capturing group with extra functionality: in addition to grouping, it allows you to extract whatever it matches independently of the overall match.

FizxMike Over a year ago

...And the indexing on the result array matches regex "group" numbers (which start at 1).

Ryan Matthews · Accepted Answer · 2011-09-21 20:02:10Z

4

I believe the following regex should work for you:

loadNewsItemWithIndex\(([0-9]+).*$

var test = new RegExp(/loadNewsItemWithIndex\(([0-9]+).*$/);
test.exec('var subject = "javascript:loadNewsItemWithIndex(5, null);";');

The break down of this is

loadNewsItemWithIndex = exactly that

\( = open parentheses 

([0-9]+) = Capture the number
.* = Anything after that number

$ = end of the line

edited Sep 21, 2011 at 20:02

answered Sep 21, 2011 at 19:53

Ryan Matthews

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2 Comments

Ryan Matthews Over a year ago

You were correct. I added it when I was thinking it out in my head and never removed it.

davin Over a year ago

so is .*$ for the same reason

derp · Accepted Answer · 2011-09-21 20:02:30Z

3

This should suffice:

<script>
var subject = "javascript:loadNewsItemWithIndex(5, null);";
number = subject.match(/[0-9]+/);
alert(number);
</script>

answered Sep 21, 2011 at 20:02

derp

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1 Comment

Matteo Riva Over a year ago

Best answer with the context provided

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