- add boundary tests

Sample.HumanFriendlyId.UnitTest/HumanFriendlyIdTests.cs

@@ -17,17 +17,48 @@ public void Id_IsGenerated()
         Assert.That(actualId, Does.Match($"^[{allowedChars}]+$"));
     }
 
-    [Test]
-    public void DisplayId_IsGroupedBySpaces()
+    [TestCase(1, 1)]
+    [TestCase(4, 4)]
+    [TestCase(5, 6)]
+    [TestCase(8, 9)]
+    [TestCase(9, 11)]
+    [TestCase(12, 14)]
+    [TestCase(13, 16)]
+    public void DisplayId_IsGroupedBySpaces(int length, int expectedLength)
     {
-        var length = 16;
-        var expectedLength = 19;
-
         var sut = new HumanFriendlyId(length);
         var actualDisplayId = sut.DisplayId;
 
+        string regex = CreateRegex(length);
+
         Assert.That(actualDisplayId.Length, Is.EqualTo(expectedLength));
-        Assert.That(actualDisplayId, Does.Match($"^[{allowedChars}]{{4}} [{allowedChars}]{{4}} [{allowedChars}]{{4}} [{allowedChars}]{{4}}$"));
+        Assert.That(actualDisplayId, Does.Match(regex));
+    }
+
+    private static string CreateRegex(int length)
+    {
+        var rest = length % 4;
+        var expectedBlocks = length / 4;
+
+        const string regexTemplate = $"[{allowedChars}]{{4}}";
+        string lastRegex = rest > 0 ? $"[{allowedChars}]{{{rest}}}" : string.Empty;
+        string regex = $"^";
+
+        for (int i = 0; i < expectedBlocks; i++)
+        {
+            if (i != 0)
+            {
+                regex += " ";
+            }
+            regex += regexTemplate;
+        }
+
+        if (expectedBlocks > 0 && rest > 0)
+        {
+            regex += " ";
+        }
+        regex += lastRegex + "$";
+        return regex;
     }
 
     [TestCase("ABCDEFGHJKLM", "ABCDEFGHJKLM", TestName = nameof(Parse) + "_InputIsUpperCase")]