A regular expression pattern can be used as a variable and passed to a method, e.g. match or replace.

Using literal

Here's an example - a varaible re - a literal regex pattern for parsing RGB color is passed to a match method.

var re = /^rgb\((\d{1,3}),\s*(\d{1,3}),\s*(\d{1,3})\)$/;
var ma = "rgb(233,23,23)".match(re);

This is the easist way.

Using Regexp Object

Of course, you have another way - using RegExp object. This way the regular expression pattern must be written in a string because RegExp object only accepts string.

var re = new RegExp( '^rgb\\((\\d{1,3}),\\s*(\\d{1,3}),\\s*(\\d{1,3})\\)$');
var ma = "rgb(233,23,23)".match(re);

Someone could have noticed I added another backslash before original backslash in the string of the regular expression. The 1st backslash is used to insert a special character (here a backslash) into the string so that a backslash is still in the pattern of the regular expression.

Variable in regexp

This method can be used when you need to pass a variable as part of a regular expression pattern to a function.

var txt = "(\\d{1,3}),\\s*(\\d{1,3}),\\s*(\\d{1,3})";

function checkrgb(txt){
var re = new RegExp( '^rgb\\('+txt+'\\)$');
var ma = "rgb(233,23,23)".match(re);
}