diff --git a/pathname_builtin.rb b/pathname_builtin.rb
index 4a0ecb8480..486e49d092 100644
--- a/pathname_builtin.rb
+++ b/pathname_builtin.rb
@@ -311,7 +311,9 @@ class Pathname
   end
 
   if File::ALT_SEPARATOR
+    # Separator list string.
     SEPARATOR_LIST = "#{Regexp.quote File::ALT_SEPARATOR}#{Regexp.quote File::SEPARATOR}"
+    # Regexp that matches a separator.
     SEPARATOR_PAT = /[#{SEPARATOR_LIST}]/
   else
     SEPARATOR_LIST = "#{Regexp.quote File::SEPARATOR}"
@@ -319,6 +321,7 @@ class Pathname
   end
 
   if File.dirname('A:') == 'A:.' # DOSish drive letter
+    # Regexp that matches an absoltute path.
     ABSOLUTE_PATH = /\A(?:[A-Za-z]:|#{SEPARATOR_PAT})/
   else
     ABSOLUTE_PATH = /\A#{SEPARATOR_PAT}/