I need a regular expression for a password field that:

* Must have 1 number
* Must have 1 letter (uppercase)
* Must have 1 letter (lowercase)
* Must be at least 8 characters in length
* Must only contain alpha and numeric characters

So far I have:

```
((?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{8,})

```

This meets most of my conditions above. But how can I limit this to only allow alpha numeric characters.