Question

I have the following code at JSfiddle:

http://jsfiddle.net/ACG2D/

$(document).ready(function() {
    $('.testThis').click(function() {
        $('.regexValidation').each(function() {
            if ($(this).val() != "" || $(this).val() != null) {
                // Check the regex
                var expression = new RegExp('/^[0-9a-z_]+$/i');
                var myVal = $(this).val();
                if (expression.test(myVal) == true) {
                    // All is okay
                    alert("OK");
                } else {
                    alert("Error");
                }
            }
        });
    });    
});

The intended plan is to only let alphanumeric and underscores through. Disallowing spaces and punctuation etc.

I can’t figure out why this is going wrong, but it always returns false for the test.

Answer 1

Your syntax is wrong.

Change it to var expression = /^[0-9a-z_]+$/i;

Unlike PHP, Javascript supports regex literals the syntax /.../ creates a RegExp object.

The RegExp constructor takes a regex as a string, without separators.
Therefore, you could also write new RegExp('^[0-9a-z_]+$', 'i')