The "\.?" part was pointless. It says that the string "nanorc"
*might* be preceded with a period. Sure, but if the period is
absent, then anything else may be there too, so the optional
period gives zero restriction.
The restriction should be: either the filename has the extension
".nanorc" OR the full filename is "nanorc". So the "nanorc" part
must be preceded by either a period or a slash.
(However, in the times of commits 3dc0e23e and dc9c40a5, fifteen
and eighteen years ago, it was still the user-provided filename
that was matched against the regex, not the full-path filename.
The latter method arrived in commit ec8d51be, six years ago.)
e52c81f5db