[Cap'n Steve]

I'm not sure if that's possible in a single regular expression. Here's 4, though:

numeric chars (min of 1)
/\d/
upper case letters (min of 1)
/[A-Z]/
lower case letters (min of 1)
/[a-z]/
The password must be at least 6 characters and only be made up of upper and lower case letters and numbers.
/[a-z0-9]{6,}/i