PEG.js grammar: Fix line continuation handling in character classes

Before this commit, line continuations in character classes contributed
an empty string to the list of characters and character ranges matched
by a class. While this didn't lead to a buggy behavior with the current
code generator, the AST was wrong and the problem could have caused bugs
later.

This commit fixes the problem.

lib/parser.js

@@ -182,7 +182,7 @@ module.exports = (function() {
         peg$c103 = function(inverted, parts, ignoreCase) {
               return {
                 type:       "class",
-                parts:      parts,
+                parts:      filterEmptyStrings(parts),
                 inverted:   inverted !== null,
                 ignoreCase: ignoreCase !== null,
                 rawText:    text()
@@ -4743,6 +4743,18 @@ module.exports = (function() {
         "!": "semantic_not"
       };
 
+      function filterEmptyStrings(array) {
+        var result = [], i;
+
+        for (i = 0; i < array.length; i++) {
+          if (array[i] !== "") {
+            result.push(array[i]);
+          }
+        }
+
+        return result;
+      }
+
       function extractOptional(optional, index) {
         return optional ? optional[index] : null;
       }

spec/parser.spec.js

@@ -485,6 +485,10 @@ describe("PEG.js grammar parser", function() {
     expect('start = [a-d]i').toParseAs(
       classGrammar([["a", "d"]], false, true, "[a-d]i")
     );
+
+    expect('start = [\\\n]').toParseAs(
+      classGrammar([], false, false, "[\\\n]")
+    );
   });
 
   /* Canonical ClassCharacterRange is "a-d". */
@@ -510,7 +514,7 @@ describe("PEG.js grammar parser", function() {
       classGrammar(["\n"], false, false, "[\\n]")
     );
     expect('start = [\\\n]').toParseAs(
-      classGrammar([''], false, false, "[\\\n]")
+      classGrammar([], false, false, "[\\\n]")
     );
 
     expect('start = []]' ).toFailToParse();

src/parser.pegjs

@@ -41,6 +41,18 @@
     "!": "semantic_not"
   };
 
+  function filterEmptyStrings(array) {
+    var result = [], i;
+
+    for (i = 0; i < array.length; i++) {
+      if (array[i] !== "") {
+        result.push(array[i]);
+      }
+    }
+
+    return result;
+  }
+
   function extractOptional(optional, index) {
     return optional ? optional[index] : null;
   }
@@ -323,7 +335,7 @@ CharacterClassMatcher "character class"
     {
       return {
         type:       "class",
-        parts:      parts,
+        parts:      filterEmptyStrings(parts),
         inverted:   inverted !== null,
         ignoreCase: ignoreCase !== null,
         rawText:    text()