diff --git a/mrbgems/mruby-string-ext/mrblib/string.rb b/mrbgems/mruby-string-ext/mrblib/string.rb
index 45c631b94..18a7a62d3 100644
--- a/mrbgems/mruby-string-ext/mrblib/string.rb
+++ b/mrbgems/mruby-string-ext/mrblib/string.rb
@@ -1,4 +1,14 @@
 class String
+  ##
+  # call-seq:
+  #    str.lstrip   -> new_str
+  #
+  # Returns a copy of <i>str</i> with leading whitespace removed. See also
+  # <code>String#rstrip</code> and <code>String#strip</code>.
+  #
+  #    "  hello  ".lstrip   #=> "hello  "
+  #    "hello".lstrip       #=> "hello"
+  #
   def lstrip
     a = 0
     z = self.size - 1
@@ -6,6 +16,16 @@ class String
     (z >= 0) ? self[a..z] : ""
   end
 
+  ##
+  # call-seq:
+  #    str.rstrip   -> new_str
+  #
+  # Returns a copy of <i>str</i> with trailing whitespace removed. See also
+  # <code>String#lstrip</code> and <code>String#strip</code>.
+  #
+  #    "  hello  ".rstrip   #=> "  hello"
+  #    "hello".rstrip       #=> "hello"
+  #
   def rstrip
     a = 0
     z = self.size - 1
@@ -13,6 +33,15 @@ class String
     (z >= 0) ? self[a..z] : ""
   end
 
+  ##
+  # call-seq:
+  #    str.strip   -> new_str
+  #
+  # Returns a copy of <i>str</i> with leading and trailing whitespace removed.
+  #
+  #    "    hello    ".strip   #=> "hello"
+  #    "\tgoodbye\r\n".strip   #=> "goodbye"
+  #
   def strip
     a = 0
     z = self.size - 1
@@ -21,16 +50,45 @@ class String
     (z >= 0) ? self[a..z] : ""
   end
 
+  ##
+  # call-seq:
+  #    str.lstrip!   -> self or nil
+  #
+  # Removes leading whitespace from <i>str</i>, returning <code>nil</code> if no
+  # change was made. See also <code>String#rstrip!</code> and
+  # <code>String#strip!</code>.
+  #
+  #    "  hello  ".lstrip   #=> "hello  "
+  #    "hello".lstrip!      #=> nil
+  #
   def lstrip!
     s = self.lstrip
     (s == self) ? nil : self.replace(s)
   end
 
+  ##
+  # call-seq:
+  #    str.rstrip!   -> self or nil
+  #
+  # Removes trailing whitespace from <i>str</i>, returning <code>nil</code> if
+  # no change was made. See also <code>String#lstrip!</code> and
+  # <code>String#strip!</code>.
+  #
+  #    "  hello  ".rstrip   #=> "  hello"
+  #    "hello".rstrip!      #=> nil
+  #
   def rstrip!
     s = self.rstrip
     (s == self) ? nil : self.replace(s)
   end
 
+  ##
+  #  call-seq:
+  #     str.strip!   -> str or nil
+  #
+  #  Removes leading and trailing whitespace from <i>str</i>. Returns
+  #  <code>nil</code> if <i>str</i> was not altered.
+  #
   def strip!
     s = self.strip
     (s == self) ? nil : self.replace(s)