Error:
I was getting this error when validating email addresses from forms...

"Value cannot be null. Parameter name: pattern"

Solution:
Try dirtying Admin > User Accounts > User Settings > Email Address Validation by entering a random character (say X) and clicking Update, then remove the X and updating. This resolved the problem for me...

Oh and here is the validation string for emails:
\b[a-zA-Z0-9._%\-+']+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,4}\b