I want to match a portion of a string using a regular expression and then access that parenthesized substring:

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

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

What am I doing wrong?

I've discovered that there was nothing wrong with the regular expression 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 it is not directly related to this question, so I've opened a new one, Why is a matched substring returning "undefined" in JavaScript?.

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.

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