How do you access the matched groups in a javascript regex?

Question

I think I must just be really tired, because this should be really simple, but it's just not working for me. I want to match a portion of a string using a regex and then access that parenthesized substring.

var myString = "something format_abc"; // I want "abc"

var arr = /(?:^|\s)format_(.*?)(?:\s|$)/(myString);

console.log(arr);     // prints: [" format_abc", "abc"] .. so far so good.
console.log(arr[1]);  // prints: undefined  (???)
console.log(arr[0]);  // prints: format_undefined (!!!)

can anyone see what I'm doing wrong?

---

Update: I've discovered that there was nothing wrong with the regex code above: the actual string which I was testing against was this:

"date format_%A"

Reporting that "%A" is undefined seems a very strange behaviour, but not directly related to this question, so I've opened a new one here. Thanks to everyone who responded.

---

Update: The issue was that console.log takes its parameters like a printf statement, and since the string I was logging ("%A") had a special value, it was trying to find the value of the next parameter.

Since this was just a silly bug on my part, I'll now close this question.

Answer 1

You can access capturing groups like this:

var myString = "something format_abc";
var myRegexp = /(?:^|\s)format_(.*?)(?:\s|$)/g;
var match = myRegexp.exec(myString);
alert(match[1]);  // abc

And if you there are multiple matches you can iterate over them:

while (match != null) {
    match = myregexp.exec(myString);

    // matched text: match[0]
    // match start: match.index
    // capturing group n: match[n]
}

Answer 2

Using your code:

console.log(arr[1]);  // prints: abc
console.log(arr[0]);  // prints:  format_abc

Edit: Safari 3, if it matters.

Answer 3

var myString = "something format_abc";
var arr = myString.match(/\bformat_(.*?)\b/);
console.log(arr[0] + " " + arr[1]);

The \b isn't exactly the same thing (works on "--format_foo/", doesn't work on "format_a_b") though... But I wanted to show an alternative to your expression, which is fine. Of course, the match call is the important thing.

Answer 4

Your code works for me (FF3 on Mac) even if I agree with PhiLo that the regex should probably be:

/\bformat_(.*?)\b/

(But, of course, I'm not sure because I don't know the context of the regex.)

Answer 5

Your syntax probably isn't the best to keep. FF/Gecko defines RegExp as an extension of Function.
(FF2 went as far as typeof(/pattern/) == 'function')

It seems this is specific to FF -- IE, Opera, and Chrome all throw exceptions for it.

Instead, use either method previously mentioned by others: RegExp#exec or String#match.
They offer the same results:

var regex = /(?:^|\s)format_(.*?)(?:\s|$)/;
var input = "something format_abc";

regex(input);        //=> [" format_abc", "abc"]
regex.exec(input);   //=> [" format_abc", "abc"]
input.match(regex);  //=> [" format_abc", "abc"]