RegularExpressionValidator Always Fails, But ValidationExpression Works In Testing?

I found the answer to this, but it's a bit of a gotcha so I wanted to share it here.

I have a regular expression that validates passwords. They should be 7 to 60 characters with at least one numeric and one alpha character. Pretty standard. I used positive lookaheads (the (?= operator) to implement it:

(?=^.{7,60}$)(?=.*[0-9].*)(?=.*[a-zA-Z].*)

I checked this expression in my unit tests using Regex.IsMatch(), and it worked fine. However, when I use it in a RegularExpressionValidator, it always fails.