diff --git a/bin/_mocha b/bin/_mocha
index 5b3c0c5b29..a3b2daf452 100755
--- a/bin/_mocha
+++ b/bin/_mocha
@@ -254,11 +254,11 @@ mocha.suite.bail(program.bail);
 
 // --grep
 
-if (program.grep) mocha.grep(new RegExp(program.grep));
+if (program.grep) mocha.grep(program.grep);
 
 // --fgrep
 
-if (program.fgrep) mocha.grep(program.fgrep);
+if (program.fgrep) mocha.fgrep(program.fgrep);
 
 // --invert
 
diff --git a/lib/mocha.js b/lib/mocha.js
index 8a11124cb1..47d175ca9d 100644
--- a/lib/mocha.js
+++ b/lib/mocha.js
@@ -78,7 +78,7 @@ function Mocha(options) {
   this.files = [];
   this.options = options;
   if (options.grep) this.grep(new RegExp(options.grep));
-  if (options.fgrep) this.grep(options.fgrep);
+  if (options.fgrep) this.fgrep(options.fgrep);
   this.suite = new exports.Suite('', new exports.Context);
   this.ui(options.ui);
   this.bail(options.bail);
@@ -220,17 +220,33 @@ Mocha.prototype._growl = function(runner, reporter) {
 };
 
 /**
- * Add regexp to grep, if `re` is a string it is escaped.
+ * Escape string and add it to grep as a regexp.
+ *
+ * @param {String} str
+ * @return {Mocha}
+ * @api public
+ */
+
+Mocha.prototype.fgrep = function(str) {
+  return this.grep(new RegExp(escapeRe(str)));
+};
+
+/**
+ * Add regexp to grep.
  *
  * @param {RegExp|String} re
  * @return {Mocha}
  * @api public
  */
 
-Mocha.prototype.grep = function(re){
-  this.options.grep = 'string' == typeof re
-    ? new RegExp(escapeRe(re))
-    : re;
+Mocha.prototype.grep = function(re) {
+  if('string' == typeof re) {
+    // extract args if it's regex-like, i.e: [string, pattern, flag]
+    var arg = re.match(/^\/(.*)\/(g|i|)$|.*/);
+    this.options.grep = new RegExp(arg[1] || arg[0], arg[2]);
+  } else {
+    this.options.grep = re;
+  }
   return this;
 };
 
diff --git a/support/tail.js b/support/tail.js
index 030177aad7..38589c719e 100644
--- a/support/tail.js
+++ b/support/tail.js
@@ -139,8 +139,8 @@ mocha.run = function(fn){
   mocha.globals('location');
 
   var query = Mocha.utils.parseQuery(global.location.search || '');
-  if (query.grep) mocha.grep(new RegExp(query.grep));
-  if (query.fgrep) mocha.grep(query.fgrep);
+  if (query.grep) mocha.grep(query.grep);
+  if (query.fgrep) mocha.fgrep(query.fgrep);
   if (query.invert) mocha.invert();
 
   return Mocha.prototype.run.call(mocha, function(err){
diff --git a/test/grep.js b/test/grep.js
index 633aab4e63..e8ebeb11ed 100644
--- a/test/grep.js
+++ b/test/grep.js
@@ -20,18 +20,35 @@ describe('Mocha', function(){
     })
   })
 
-  describe('.grep()', function(){
-    it('should add a RegExp to the mocha.options object', function(){
-      var mocha = new Mocha;
-      mocha.grep(/foo/);
-      mocha.options.grep.toString().should.equal('/foo/');
-    })
+  describe('.grep()', function() {
+    // Test helper
+    function testGrep(mocha) {
+      return function testGrep(grep, expected) {
+        mocha.grep(grep);
+        mocha.options.grep.toString().should.equal(expected);
+      }
+    }
 
-    it('should convert grep string to a RegExp', function(){
-      var mocha = new Mocha;
-      mocha.grep('foo');
-      mocha.options.grep.toString().should.equal('/foo/');
-    })
+    it('should add a RegExp to the mocha.options object', function() {
+      var test = testGrep(new Mocha);
+      test(/foo/, '/foo/');
+    });
+
+    it('should convert grep string to a RegExp', function() {
+      var test = testGrep(new Mocha);
+      test('foo', '/foo/');
+      test('^foo.*bar$', '/^foo.*bar$/');
+      test('^@.*(?=\\(\\)$)', '/^@.*(?=\\(\\)$)/');
+    });
+
+    it('should covert grep regex-like string to a RegExp', function() {
+      var test = testGrep(new Mocha);
+      test('/foo/', '/foo/');
+      // Keep the flags
+      test('/baz/i', '/baz/i');
+      test('/bar/g', '/bar/g');
+      test('/^foo(.*)bar/g', '/^foo(.*)bar/g');
+    });
 
     it('should return it\'s parent Mocha object for chainability', function(){
       var mocha = new Mocha;