REGEX for any file extension

^\.[^.]+$

This means start with . and then anything other than dot (.)

You can also use this one if you want to have only aplhanumeric.:

^\.[a-zA-Z0-9]+$

Try this regex:

^\.[\w]+$

Matches a string starting with a ".", followed by one or more "word" character(s), until the end of the string.

Try this regex, which matches all strings starting with a dot followed by at least one other character:

^\.[^.]+$