Pattern Match Username

I am trying to make a pattern match a username that has the following rules:

• No more than 15 characters
• Must have a full stop in between two strings
• Digits can only be at the end of the string
• Only 2 digits allowed

Example Fails

• .asdfa
• dadad.
• 1apple.pear
• apple.pear123

Example Success

• apple.pear
• ben.peach12