Match optional pattern with find command

With a GNU find, you can use

find * -regextype posix-extended -regex '.*\.[Ff][0-9Oo]?[0-9Rr]?'

The pattern matches a string that fully matches:

• .*\. - any text up to the rightmost . including it
• [Ff] - F or f
• [0-9Oo]? - an optional digit, O or o
• [0-9Rr]? - an optional digit, R or r.

You can also use a non-regex approach using mere glob patterns:

find * ( -name "*.[Ff][0-9oO][0-9rR]" -o -name "*.[Ff]" )

Note that here, the * glob pattern matches any text. In the previous solution, it was a quantifier, an operator that makes the preceding pattern match zero or more times. Also, in glob patterns, . matches a literal dot, no need to escape it.