Throw descriptive exception when escape character '@' is at the end of a highlighting expression.

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/branches/3.0@3439 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/Libraries/ICSharpCode.TextEditor/Project/Src/Document/HighlightingStrategy/DefaultHighlightingStrategy.cs

@@ -786,6 +786,8 @@ namespace ICSharpCode.TextEditor.Document
 				switch (expr[i]) {
 					case '@': // "special" meaning
 						++i;
+						if (i == expr.Length)
+							throw new HighlightingDefinitionInvalidException("Unexpected end of @ sequence, use @@ to look for a single @.");
 						switch (expr[i]) {
 							case '!': // don't match the following expression
 								StringBuilder whatmatch = new StringBuilder();
@@ -819,7 +821,8 @@ namespace ICSharpCode.TextEditor.Document
 				switch (expr[i]) {
 					case '@': // "special" meaning
 						++i;
-						if (i < expr.Length) {
+						if (i == expr.Length)
+							throw new HighlightingDefinitionInvalidException("Unexpected end of @ sequence, use @@ to look for a single @.");
 						switch (expr[i]) {
 							case 'C': // match whitespace or punctuation
 								if (index + j == lineSegment.Offset || index + j >= lineSegment.Offset + lineSegment.Length) {
@@ -882,7 +885,6 @@ namespace ICSharpCode.TextEditor.Document
 								}
 								break;
 						}
-						}
 						break;
 					default:
 						{