How do you use a variable in a regular expression?

javascript regex
JC Grubbs picture JC Grubbs · Jan 30, 2009 · Viewed 839.1k times · Source

I would like to create a String.replaceAll() method in JavaScript and I'm thinking that using a regex would be most terse way to do it. However, I can't figure out how to pass a variable in to a regex. I can do this already which will replace all the instances of "B" with "A".

"ABABAB".replace(/B/g, "A");

But I want to do something like this:

String.prototype.replaceAll = function(replaceThis, withThis) {
    this.replace(/replaceThis/g, withThis);
};

But obviously this will only replace the text "replaceThis"...so how do I pass this variable in to my regex string?

Answer

Eric Wendelin picture Eric Wendelin · Jan 30, 2009

Instead of using the /regex/g syntax, you can construct a new RegExp object:

var replace = "regex";
var re = new RegExp(replace,"g");

You can dynamically create regex objects this way. Then you will do:

"mystring".replace(re, "newstring");

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