address review comments

lib/marked.js

@@ -1355,36 +1355,36 @@ function splitCells(tableRow, count) {
   return cells;
 }
 
-// Return str with all trailing {c | all but c} removed
-// allButC: Default false
-function rtrim(str, c, allButC) {
-  if (typeof allButC === 'undefined') {
-    allButC = false;
+// Remove trailing 'c's. Equivalent to str.replace(/c*$/, '').
+// /c*$/ is vulnerable to REDOS.
+// invert: Remove suffix of non-c chars instead. Default false.
+function rtrim(str, c, invert) {
+  if (typeof invert === 'undefined' || !invert) {
+    invert = false;
   } else {
-    allButC = true;
+    invert = true;
   }
-  var mustMatchC = !allButC;
 
   if (str.length === 0) {
     return '';
   }
 
-  // ix+1 of leftmost that fits description
-  // i.e. the length of the string we should return
-  var curr = str.length;
+  // Length of suffix matching the invert condition.
+  var suffLen = 0;
 
-  while (curr > 0) {
-    var currChar = str.charAt(curr - 1);
-    if (mustMatchC && currChar === c) {
-      curr--;
-    } else if (!mustMatchC && currChar !== c) {
-      curr--;
+  // Step left until we fail to match the invert condition.
+  while (suffLen < str.length) {
+    var currChar = str.charAt(str.length - suffLen - 1);
+    if (currChar === c && !invert) {
+      suffLen++;
+    } else if (currChar !== c && invert) {
+      suffLen++;
     } else {
       break;
     }
   }
 
-  return str.substr(0, curr);
+  return str.substr(0, str.length - suffLen);
 }
 
 /**