basename(suffix = "") -> Pathname[permalink][rdoc]Pathname.new(File.basename(self.to_s, suffix)) と同じです。
例
require "pathname"
Pathname("ruby/ruby.c").basename          #=> #<Pathname:"ruby.c">
Pathname("ruby/ruby.c").basename(".c")    #=> #<Pathname:"ruby">
Pathname("ruby/ruby.c").basename(".*")    #=> #<Pathname:"ruby">
Pathname("ruby/ruby.exe").basename(".*")  #=> #<Pathname:"ruby">
Pathname("ruby/y.tab.c").basename(".*")   #=> #<Pathname:"y.tab">
[SEE_ALSO] File.basename