Bail on failure.

src/Workspaces/Core/Portable/EmbeddedLanguages/RegularExpressions/LanguageServices/RegexPatternDetector.cs

@@ -160,7 +160,10 @@ public static bool IsDefinitelyNotPattern(SyntaxToken token, ISyntaxFactsService
                         }
                         else
                         {
-                            break;
+                            // hit something we don't understand.  bail out.  that will help ensure
+                            // users don't have weird behavior just because they misspelled something.
+                            // instead, they will know they need to fix it up.
+                            return false;
                         }
                     }