How to pass a regular expression as a function parameter

Question

Why this returns false instead of true.

function doit(expression) {

    var regex = new RegExp(expression, 'g');

    alert(regex.test('[email protected]'));
}

doit("/^\w+([-+.\']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*/");
​

http://jsfiddle.net/hAV8Q/

Answer 1

Either format your expression properly:

function doit(expression) {
    var regex = new RegExp(expression, 'g');
    alert(regex.test('[email protected]'));
}

doit("^\\w+([-+.\\']\\w+)*@\\w+([-.]\\w+)*\\.\\w+([-.]\\w+)*");
// no / here, escape \

or pass the expression directly:

function doit(expression) {
    alert(expression.test('[email protected]'));
}

doit(/^\w+([-+.\']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*/g);

​

---

The slashes (/) are not part of the expression, they denote a regex literal. If you use a string containing the expression, you have to omit them and escape every backslash since the backslash is the escape character in strings as well.

Answer 2

Because when creating a regex with new RegExp(), you don't use the delimiters. Remove the / from before and after the string.

Alternatively, pass the regex itself by removing the quotes before and after, and leave out the new RegExp() call.