diff --git a/projects/Four4Sample/Four4.Test/NumberTest.cs b/projects/Four4Sample/Four4.Test/NumberTest.cs
index 1a011c0..ab57301 100644
--- a/projects/Four4Sample/Four4.Test/NumberTest.cs
+++ b/projects/Four4Sample/Four4.Test/NumberTest.cs
@@ -16,7 +16,7 @@ public sealed class NumberTest
         [InlineData("4444")]
         public void WholeNumbers(string input)
         {
-            Number.Parse(input).IsWhole.Should().BeTrue();
+            Parse(input).IsWhole.Should().BeTrue();
         }
 
         [Theory]
@@ -24,7 +24,7 @@ public void WholeNumbers(string input)
         [InlineData(".4_")]
         public void Fractions(string input)
         {
-            Number.Parse(input).IsWhole.Should().BeFalse();
+            Parse(input).IsWhole.Should().BeFalse();
         }
 
         [Theory]
@@ -43,9 +43,17 @@ public void NonPositiveNumbersAreNotWhole(string input)
         [InlineData("4444", 4444)]
         public void CastToInt32(string input, int result)
         {
-            int value = (int)Number.Parse(input);
+            int value = (int)Parse(input);
 
             value.Should().Be(result);
         }
+
+        private static Number Parse(string input)
+        {
+            Number number;
+            bool valid = Number.TryParse(input, out number, out int d);
+            valid.Should().BeTrue();
+            return number;
+        }
     }
 }
diff --git a/projects/Four4Sample/Four4/Expression.cs b/projects/Four4Sample/Four4/Expression.cs
index 6ff077d..2b781cc 100644
--- a/projects/Four4Sample/Four4/Expression.cs
+++ b/projects/Four4Sample/Four4/Expression.cs
@@ -62,11 +62,11 @@ public Expression Append(string token)
                 case "R":
                     return this.Unary(token, x => x.SquareRoot());
                 default:
+                    Number number;
                     int add;
-                    if (IsNumeric(token, out add))
+                    if (Number.TryParse(token, out number, out add))
                     {
-                        Number number = Number.Parse(token);
-                        return this.Push(add, token, Number.Parse(token));
+                        return this.Push(add, token, number);
                     }
                     else
                     {
@@ -75,41 +75,6 @@ public Expression Append(string token)
             }
         }
 
-        private static bool IsNumeric(string token, out int add)
-        {
-            add = 0;
-            int n = token.Length;
-            for (int i = 0; i < n; ++i)
-            {
-                char d = token[i];
-                switch (d)
-                {
-                    case '4':
-                        ++add;
-                        break;
-                    case '.':
-                        if (i > 0)
-                        {
-                            return false;
-                        }
-
-                        break;
-                    case '_':
-                        if (i < (n - 1))
-                        {
-                            return false;
-                        }
-
-                        break;
-
-                    default:
-                        return false;
-                }
-            }
-
-            return add > 0;
-        }
-
         private Expression Push(int add, string token, Number number)
         {
             return new Expression(this.count + add, this.Join(token), this.operands.Push(number));
diff --git a/projects/Four4Sample/Four4/Number.cs b/projects/Four4Sample/Four4/Number.cs
index 8e40454..c2c1cd6 100644
--- a/projects/Four4Sample/Four4/Number.cs
+++ b/projects/Four4Sample/Four4/Number.cs
@@ -71,17 +71,55 @@ public Number(int num, int denom)
             return left * new Number(right.denom, right.num);
         }
 
-        public static Number Parse(string s)
+        public static bool TryParse(string s, out Number number, out int digits)
         {
-            switch (s)
+            number = NaN;
+            digits = 0;
+            int a = 0;
+            int b = s.Length;
+            int nr = 0;
+            int dr = 0;
+            int f = 1;
+
+            if (b == 0)
+            {
+                return false;
+            }
+
+            if (s[0] == '.')
+            {
+                a = 1;
+                f = 10;
+            }
+
+            if (s[b - 1] == '_')
+            {
+                --b;
+                f = 9;
+            }
+
+            for (int i = a; i < b; ++i)
             {
-                case ".4":
-                    return new Number(2, 5);
-                case ".4_":
-                    return new Number(4, 9);
-                default:
-                    return new Number(int.Parse(s), 1);
+                int d = s[i] - '0';
+                if (d != 4)
+                {
+                    return false;
+                }
+
+                nr = (10 * nr) + d;
+                ++digits;
+                if (f != 1)
+                {
+                    dr = (dr * 10) + f;
+                }
+                else
+                {
+                    dr = 1;
+                }
             }
+
+            number = new Number(nr, dr);
+            return true;
         }
 
         public Number Factorial()