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How to explain $1,$2 in JavaScript when using regular expression?

rated 0 times [ 133] [ 1] / answers: 1 / hits: 88618 / 11 Years ago, wed, may 22, 2013, 12:00:00

A piece of JavaScript code is as follows:

num = "11222333";

re = /(d+)(d{3})/;

re.test(num);

num.replace(re, "$1,$2");

I could not understand the grammar of "$1,$2". The book from which this code comes says $1 means RegExp.$1, $2 means RegExp.$2. But these explanations lead to more questions:

It is known that in JavaScript, the name of variables should begin with letter or _, how can $1 be a valid name of member variable of RegExp here?

If I input $1, the command line says it is not defined; if I input "$1", the command line only echoes $1, not 11222. So, how does the replace method know what "$1,$2" mean?

Thank you.

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gageherberth

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3 Years ago

answered 11 Years ago keyonna · Accepted Answer

It's not a "variable" - it's a placeholder that is used in the .replace() call. $n represents the nth capture group of the regular expression.

var num = 11222333;



// This regex captures the last 3 digits as capture group #2

// and all preceding digits as capture group #1

var re = /(d+)(d{3})/;



console.log(re.test(num));



// This replace call replaces the match of the regex (which happens

// to match everything) with the first capture group ($1) followed by

// a comma, followed by the second capture group ($2)

console.log(num.replace(re, $1,$2));