improve RegExp handling (#1959)

- remove `options.output.unescape_regexps`
- preserve original pattern whenever possible

fixes #54
fixes #1929

lib/parse.js

@@ -477,31 +477,33 @@ function tokenizer($TEXT, filename, html5_comments, shebang) {
         return name;
     };
 
-    var read_regexp = with_eof_error("Unterminated regular expression", function(regexp){
+    var read_regexp = with_eof_error("Unterminated regular expression", function(source) {
         var prev_backslash = false, ch, in_class = false;
         while ((ch = next(true))) if (NEWLINE_CHARS(ch)) {
             parse_error("Unexpected line terminator");
         } else if (prev_backslash) {
-            regexp += "\\" + ch;
+            source += "\\" + ch;
             prev_backslash = false;
         } else if (ch == "[") {
             in_class = true;
-            regexp += ch;
+            source += ch;
         } else if (ch == "]" && in_class) {
             in_class = false;
-            regexp += ch;
+            source += ch;
         } else if (ch == "/" && !in_class) {
             break;
         } else if (ch == "\\") {
             prev_backslash = true;
         } else {
-            regexp += ch;
+            source += ch;
         }
         var mods = read_name();
         try {
-          return token("regexp", new RegExp(regexp, mods));
+            var regexp = new RegExp(source, mods);
+            regexp.raw_source = source;
+            return token("regexp", regexp);
         } catch(e) {
-          parse_error(e.message);
+            parse_error(e.message);
         }
     });