Well, look at the following line out the code you are using:
if (!String.fromCharCode(event.keyCode).match(regAllow)) event.returnValue=false;
It uses the event.keyCode, which ofcourse you can check or print directly as well. For example, you could insert an alert in front of the if statement reading:
alert ("Code for this key is: " + event.keyCode);
Once you know the code for the enter button (probably 13, but you can check with the above function), you can then put that in an if statement as well. Possible constructs would be to first check if the keyCode = 13, if it is, just let it pass through, and else, check with the regular expression.
Ofcourse you can make both expressions into one IF-statement as well, it's all about what you want to do with it.