diff --git a/ChangeLog b/ChangeLog
index 99bc01acc9..fbdcb2b439 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Mar 10 09:12:51 2013  Tadayoshi Funaba  <tadf@dotrb.org>
+
+	* complex.c: described syntax of string form.
+	* rational.c: ditto.
+
 Sat Mar  9 11:58:39 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* marshal.c (w_extended): check for prepended object.
diff --git a/complex.c b/complex.c
index edbe2965f0..5445b490ec 100644
--- a/complex.c
+++ b/complex.c
@@ -483,6 +483,24 @@ f_complex_new2(VALUE klass, VALUE x, VALUE y)
  *
  *    Complex(1, 2)    #=> (1+2i)
  *    Complex('1+2i')  #=> (1+2i)
+ *
+ * Syntax of string form:
+ *
+ *   complex = real part | [ sign ] , imaginary part
+ *           | real part , sign , imaginary part
+ *           | rational , "@" , rational ;
+ *   real part = rational ;
+ *   imaginary part = imaginary unit | unsigned rational , imaginary unit ;
+ *   rational = [ sign ] , unsigned rational ;
+ *   unsigned rational = numerator | numerator , "/" , denominator ;
+ *   numerator = integer part | fractional part | integer part , fractional part ;
+ *   denominator = digits ;
+ *   integer part = digits ;
+ *   fractional part = "." , digits , [ ( "e" | "E" ) , [ sign ] , digits ] ;
+ *   imaginary unit = "i" | "I" | "j" | "J" ;
+ *   sign = "-" | "+" ;
+ *   digits = digit , [ { digit | "_" , digit } ] ;
+ *   digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
  */
 static VALUE
 nucomp_f_complex(int argc, VALUE *argv, VALUE klass)
diff --git a/rational.c b/rational.c
index a78b3202ec..e5992ed76f 100644
--- a/rational.c
+++ b/rational.c
@@ -571,6 +571,18 @@ f_rational_new_no_reduce2(VALUE klass, VALUE x, VALUE y)
  *
  *    Rational(1, 2)   #=> (1/2)
  *    Rational('1/2')  #=> (1/2)
+ *
+ * Syntax of string form:
+ *
+ *   rational = [ sign ] , unsigned rational ;
+ *   unsigned rational = numerator | numerator , "/" , denominator ;
+ *   numerator = integer part | fractional part | integer part , fractional part ;
+ *   denominator = digits ;
+ *   integer part = digits ;
+ *   fractional part = "." , digits , [ ( "e" | "E" ) , [ sign ] , digits ] ;
+ *   sign = "-" | "+" ;
+ *   digits = digit , [ { digit | "_" , digit } ] ;
+ *   digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
  */
 static VALUE
 nurat_f_rational(int argc, VALUE *argv, VALUE klass)