$FileExt =~/              # "file extension matches ...."
              \A            # the start of the variable
              \.            # match a literal dot
              (?:           # begin a set of options
                 foo        #    the option itself
              |             # OR
                 bar        #    another option
              )             # end of option group
              \z            # end of variable
             /x;            # end of pattern, enable comments in the regex