This is in regards to registering on phpBB forums.

Selecting a username has the following text:

Length must be between 3 characters and 20 characters.

A shorter and better way of putting it, while saying the same thing, would be:

Length must be between 3 and 20 characters.

Selecting a password has the following text:

Must be between 6 characters and 100 characters.

A shorter and better way of putting it, while saying the same thing, would be:

Must be between 6 and 100 characters.

This is trivial and not overly important but it will reduce the amount of unnecessary text being read. It will also remove the extra unnecessary line used in each case.