docs: improve 'grep()' and clarify docs (#4714)

Author: juergba

docs/index.md

@@ -1138,17 +1138,20 @@ Cause Mocha to only run tests matching the given `regexp`, which is internally c
 
 Suppose, for example, you have "api" related tests, as well as "app" related tests, as shown in the following snippet; One could use `--grep api` or `--grep app` to run one or the other. The same goes for any other part of a suite or test-case title, `--grep users` would be valid as well, or even `--grep GET`.
 
+And another option with double quotes: `--grep "groupA|groupB"`.<br>
+And for more complex criterias: `--grep "/get/i"`. Some shells as e.g. Git-Bash-for-Windows may require: `--grep "'/get/i'"`. Using flags other than the `ignoreCase /i` (especially `/g` and `/y`) may lead to unpredictable results.
+
 ```js
 describe('api', function() {
-  describe('GET /api/users', function() {
+  describe('GET /api/users groupA', function() {
     it('respond with an array of users', function() {
       // ...
     });
   });
 });
 
 describe('app', function() {
-  describe('GET /users', function() {
+  describe('GET /users groupB', function() {
     it('respond with an array of users', function() {
       // ...
     });

example/config/.mocharc.js

@@ -1,7 +1,8 @@
 'use strict';
 
 // This is a JavaScript-based config file containing every Mocha option plus others.
-// If you need conditional logic, you might want to use this type of config.
+// If you need conditional logic, you might want to use this type of config,
+// e.g. set options via environment variables 'process.env'.
 // Otherwise, JSON or YAML is recommended.
 
 module.exports = {

lib/mocha.js

@@ -581,7 +581,7 @@ Mocha.prototype.fgrep = function(str) {
 Mocha.prototype.grep = function(re) {
   if (utils.isString(re)) {
     // extract args if it's regex-like, i.e: [string, pattern, flag]
-    var arg = re.match(/^\/(.*)\/(g|i|)$|.*/);
+    var arg = re.match(/^\/(.*)\/([gimy]{0,4})$|.*/);
     this.options.grep = new RegExp(arg[1] || arg[0], arg[2]);
   } else {
     this.options.grep = re;