How can I use a variable inside a RegExp?

Question

var string = "This is a string";
var key = "Th"; 
var patt = new RegExp(/\b key /, "i");

if(patt.test(string)){
    alert("true");
}

How can I escape \b so I can use the key variable?

Thanks in advance!

Wouldn´t indexOf be enough? (string.indexOf(key) > -1) ? See developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… or even developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Stefan
– Stefan, Commented Oct 21, 2013 at 9:29
possible duplicate of How do you pass a variable to a Regular Expression JavaScript? — GOTO 0
– GOTO 0, Commented Oct 21, 2013 at 9:30

Denys Séguret · Accepted Answer · 2013-10-21 09:23:23Z

2

Don't use a regular expression literal when using the RegExp constructor.

When you want a \ in a string literal, you have to escape it as \\.

Here's the code :

var patt = new RegExp("\\b " + key + " ", "i");

answered Oct 21, 2013 at 9:23

Denys Séguret

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2 Comments

I'm not positive, but don't you still need the delimiters?

If you mean the /, they're what defines a regular expression literal. So no, you can't use them when you want to use a variable.