diff --git a/string.c b/string.c
index e275e577ea601e..ce1f456a4407cf 100644
--- a/string.c
+++ b/string.c
@@ -1148,6 +1148,9 @@ rb_str_length(VALUE str)
  *     str.bytesize  -> integer
  *
  *  Returns the length of <i>str</i> in bytes.
+ *
+ *    "\x80\u3042".bytesize  #=> 4
+ *    "hello".bytesize       #=> 5
  */
 
 static VALUE
@@ -1163,6 +1166,7 @@ rb_str_bytesize(VALUE str)
  *  Returns <code>true</code> if <i>str</i> has a length of zero.
  *
  *     "hello".empty?   #=> false
+ *     " ".empty?       #=> false
  *     "".empty?        #=> true
  */
 
@@ -1210,9 +1214,10 @@ rb_str_plus(VALUE str1, VALUE str2)
  *     str * integer   -> new_str
  *
  *  Copy---Returns a new <code>String</code> containing <i>integer</i> copies of
- *  the receiver.
+ *  the receiver. <i>integer</i> must be greater than or equal to 0.
  *
  *     "Ho! " * 3   #=> "Ho! Ho! Ho! "
+ *     "Ho! " * 0   #=> ""
  */
 
 VALUE
@@ -4534,6 +4539,8 @@ rb_str_inspect(VALUE str)
  *
  *  Produces a version of <i>str</i> with all nonprinting characters replaced by
  *  <code>\nnn</code> notation and all special characters escaped.
+ *
+ *    "hello \n ''".dump  #=> "\"hello \\n ''\"
  */
 
 VALUE
@@ -7126,7 +7133,7 @@ rb_str_rjust(int argc, VALUE *argv, VALUE str)
 
 /*
  *  call-seq:
- *     str.center(integer, padstr)   -> new_str
+ *     str.center(integer, padstr='')   -> new_str
  *
  *  If <i>integer</i> is greater than the length of <i>str</i>, returns a new
  *  <code>String</code> of length <i>integer</i> with <i>str</i> centered and