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 str in bytes.
+ *
+ * "\x80\u3042".bytesize #=> 4
+ * "hello".bytesize #=> 5
*/
static VALUE
@@ -1163,6 +1166,7 @@ rb_str_bytesize(VALUE str)
* Returns true
if str 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 String
containing integer copies of
- * the receiver.
+ * the receiver. integer 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 str with all nonprinting characters replaced by
* \nnn
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 integer is greater than the length of str, returns a new
* String
of length integer with str centered and