Use "[]" & "[^]" for empty character classes, not "(?!)" & "[\\S\\s]"

See #441.

lib/compiler/passes/generate-bytecode.js

@@ -569,43 +569,31 @@ function generateBytecode(ast) {
     },
 
     "class": function(node) {
-      var regexp, parts, regexpIndex, expectedIndex;
-
-      if (node.parts.length > 0) {
-        regexp = '/^['
-          + (node.inverted ? '^' : '')
-          + node.parts.map(function(part) {
-              return Array.isArray(part)
-                ? js.regexpClassEscape(part[0])
-                  + '-'
-                  + js.regexpClassEscape(part[1])
-                : js.regexpClassEscape(part);
-            }).join('')
-          + ']/' + (node.ignoreCase ? 'i' : '');
-      } else {
-        /*
-         * IE considers regexps /[]/ and /[^]/ as syntactically invalid, so we
-         * translate them into equivalents it can handle.
-         */
-        regexp = node.inverted ? '/^[\\S\\s]/' : '/^(?!)/';
-      }
-
-      parts = '['
-        + node.parts.map(function(part) {
-            return Array.isArray(part)
-              ? '["' + js.stringEscape(part[0]) + '", "' + js.stringEscape(part[1]) + '"]'
-              : '"' + js.stringEscape(part) + '"';
-          }).join(', ')
-        + ']';
-
-      regexpIndex   = addConst(regexp);
-      expectedIndex = addConst(
-        'peg$classExpectation('
-          + parts + ', '
-          + node.inverted + ', '
-          + node.ignoreCase
-          + ')'
-      );
+      var regexp        = '/^['
+            + (node.inverted ? '^' : '')
+            + node.parts.map(function(part) {
+                return Array.isArray(part)
+                  ? js.regexpClassEscape(part[0])
+                    + '-'
+                    + js.regexpClassEscape(part[1])
+                  : js.regexpClassEscape(part);
+              }).join('')
+            + ']/' + (node.ignoreCase ? 'i' : ''),
+          parts         = '['
+            + node.parts.map(function(part) {
+                return Array.isArray(part)
+                  ? '["' + js.stringEscape(part[0]) + '", "' + js.stringEscape(part[1]) + '"]'
+                  : '"' + js.stringEscape(part) + '"';
+              }).join(', ')
+            + ']',
+          regexpIndex   = addConst(regexp),
+          expectedIndex = addConst(
+            'peg$classExpectation('
+              + parts + ', '
+              + node.inverted + ', '
+              + node.ignoreCase
+              + ')'
+          );
 
       return buildCondition(
         [op.MATCH_REGEXP, regexpIndex],

spec/unit/compiler/passes/generate-bytecode.spec.js

@@ -594,7 +594,7 @@ describe("compiler pass |generateBytecode|", function() {
       ]));
     });
 
-    describe("non-empty non-inverted case-sensitive", function() {
+    describe("non-inverted case-sensitive", function() {
       it("defines correct constants", function() {
         expect(pass).toChangeAST('start = [a]', constsDetails([
           '/^[a]/',
@@ -603,7 +603,7 @@ describe("compiler pass |generateBytecode|", function() {
       });
     });
 
-    describe("non-empty inverted case-sensitive", function() {
+    describe("inverted case-sensitive", function() {
       it("defines correct constants", function() {
         expect(pass).toChangeAST('start = [^a]', constsDetails([
           '/^[^a]/',
@@ -612,7 +612,7 @@ describe("compiler pass |generateBytecode|", function() {
       });
     });
 
-    describe("non-empty non-inverted case-insensitive", function() {
+    describe("non-inverted case-insensitive", function() {
       it("defines correct constants", function() {
         expect(pass).toChangeAST('start = [a]i', constsDetails([
           '/^[a]/i',
@@ -621,7 +621,7 @@ describe("compiler pass |generateBytecode|", function() {
       });
     });
 
-    describe("non-empty complex", function() {
+    describe("complex", function() {
       it("defines correct constants", function() {
         expect(pass).toChangeAST('start = [ab-def-hij-l]', constsDetails([
           '/^[ab-def-hij-l]/',
@@ -629,24 +629,6 @@ describe("compiler pass |generateBytecode|", function() {
         ]));
       });
     });
-
-    describe("empty non-inverted", function() {
-      it("defines correct constants", function() {
-        expect(pass).toChangeAST('start = []', constsDetails([
-          '/^(?!)/',
-          'peg$classExpectation([], false, false)'
-        ]));
-      });
-    });
-
-    describe("empty inverted", function() {
-      it("defines correct constants", function() {
-        expect(pass).toChangeAST('start = [^]', constsDetails([
-          '/^[\\S\\s]/',
-          'peg$classExpectation([], true, false)'
-        ]));
-      });
-    });
   });
 
   describe("for any", function() {