.match() with a regular expression returns null

Question

I am trying to do something I thought would be pretty easy to do, which is to restrict a string to certain characters by matching a regular expression.

var v         


        

Answer 1

Regex must be surrounded with /, not ', so that JavaScript creates a variable of type regex, not of type string. So for instance, for your ALPHA case, you should have

regex = /^[a-zA-Z]+$/;

See MDN's page on .match for more information about using .match.

Answer 2

Your code looks like it's inside a function { ... }. Are you returning anything? If not, that's why you're getting null back...

Also, regexes are surrounded by slashes (/.../), not quotes.

Answer 3

You want RegExp.test, which tests a value for a match instead of retrieving the match. With your existing code, that would mean:

if(!new RegExp(regex).test(value)){
    alert('Your string was invalid.');
}

However, it would be preferable to use RegExp literals instead of strings, as they're much more efficient and clear, and less prone to error:

var value = 'FailureStr1ng';
var type = 'ALPHA';
var regex = null;

switch(type) {
    case 'ALPHA':
        regex = /^[a-zA-Z]+$/;
        break;
    case 'NUMERIC':
        regex = /^[0-9]+$/;
        break;
    case 'ALPHANUMERIC':
        regex = /^[a-zA-Z0-9]+$/;
        break;
}

if(!regex.test(value)) {
    alert('Your string was invalid.');
}

Even better, use a dictionary:

var expressions = {
    ALPHA: /^[a-zA-Z]+$/,
    NUMERIC: /^[0-9]+$/,
    ALPHANUMERIC: /^[a-zA-Z0-9]+$/
};

if(!expressions[type].test(value)) {
    alert('Your string was invalid.');
}