gnus-button-valid-localpart-regexp

gnus-button-valid-localpart-regexp is a variable defined in `gnus-art.el'.
Its value is

"[a-z0-9$%(*-=?[_][^<>\")!;:,{}
	 @]*"

Documentation:
Regular expression that matches a localpart of mail addresses or MIDs.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.