rework the rules

Octokit/Helpers/StringExtensions.cs

@@ -102,11 +102,14 @@ namespace Octokit
             yield return new String(letters, wordStartIndex, letters.Length - wordStartIndex);
         }
 
-        static Regex nameWithOwner = new Regex("[a-zA-Z.]{1,}/[a-zA-Z.]{1,}"
+        // the rule:
+        // Username may only contain alphanumeric characters or single hyphens
+        // and cannot begin or end with a hyphen
+        static readonly Regex nameWithOwner = new Regex("[a-z0-9.-]{1,}/[a-z0-9.-]{1,}", 
 #if (!PORTABLE && !NETFX_CORE)
-, RegexOptions.Compiled
+            RegexOptions.Compiled | 
 #endif
-);
+            RegexOptions.IgnoreCase);
 
         internal static bool IsNameWithOwnerFormat(this string input)
         {