extract method for reuse as extension

Octokit/Helpers/StringExtensions.cs

@@ -101,5 +101,16 @@ namespace Octokit
             //We need to have the last word.
             yield return new String(letters, wordStartIndex, letters.Length - wordStartIndex);
         }
+
+        static Regex nameWithOwner = new Regex("[a-zA-Z.]{1,}/[a-zA-Z.]{1,}"
+#if (!PORTABLE && !NETFX_CORE)
+, RegexOptions.Compiled
+#endif
+);
+
+        internal static bool IsNameWithOwnerFormat(this string input)
+        {
+            return nameWithOwner.IsMatch(input);
+        }
     }
 }

Octokit/Models/Request/SearchIssuesRequest.cs

@@ -267,15 +267,10 @@ namespace Octokit
 
             if (Repos.Any())
             {
-                var invalidFormatRepos = Repos.Where(x => !IsNameWithOwnerFormat(x));
+                var invalidFormatRepos = Repos.Where(x => !x.IsNameWithOwnerFormat());
                 if (invalidFormatRepos.Any())
                 {
-                    var parameterList = string.Join(", ", invalidFormatRepos);
-                    var message = string.Format(
-                        CultureInfo.InvariantCulture,
-                        "The list of repositories must be formatted as 'owner/name' - these values don't match this rule: {0}",
-                        parameterList);
-                    throw new ArgumentException(message);
+                    throw new RepositoryFormatException(invalidFormatRepos);
                 }
 
                 parameters.Add(
@@ -285,19 +280,6 @@ namespace Octokit
             return new ReadOnlyCollection<string>(parameters);
         }
 
-        // what rules do we define here?
-
-        static Regex nameWithOwner = new Regex("[a-zA-Z.]{1,}/[a-zA-Z.]{1,}"
-#if (!PORTABLE && !NETFX_CORE)
-            , RegexOptions.Compiled
-#endif
-        );
-
-        static bool IsNameWithOwnerFormat(string input)
-        {
-            return nameWithOwner.IsMatch(input);
-        }
-
         internal string DebuggerDisplay
         {
             get