Test A Field For Valid Characters Using REGEX?

I'm writing a form validation function and would like to verify that a field only contains alphanumeric characters. How should I structure that statement? the following seems logical but doesn't work;

Code JavaScript:
if(document.getElementById('RegisForm_Username').value == /[^a-zA-Z 0-9]/g)
{

your username must be alphanumeric (consisting of the letters A-Z or numbers 0-9) and cannot have any special characters.");return false;}