support for regular expressions in coercion

Readme.md

@@ -185,6 +185,12 @@ program.prompt('Birthdate: ', Date, function(date){
 });
 ```
 
+```js
+program.prompt('Email: ', /^.+@.+\..+$/, function(email){
+  console.log('email: %j', email);
+});
+```
+
 ## .password(msg[, mask], fn)
 
 Prompt for password without echoing:

index.js

@@ -811,6 +811,25 @@ Command.prototype.promptForDate = function(str, fn){
   });
 };
 
+
+/**
+ * Prompt for a `Regular Expression`.
+ *
+ * @param {String} str
+ * @param {Object} pattern regular expression object to test
+ * @param {Function} fn
+ * @api private
+ */
+
+Command.prototype.promptForRegexp = function(str, pattern, fn){
+  var self = this;
+  this.promptSingleLine(str, function parseRegexp(val){
+    if(!pattern.test(val)) return self.promptSingleLine(str + '(regular expression mismatch) ', parseRegexp);
+    fn(val);
+  });
+};
+
+
 /**
  * Single-line prompt.
  *
@@ -820,7 +839,10 @@ Command.prototype.promptForDate = function(str, fn){
  */
 
 Command.prototype.promptSingleLine = function(str, fn){
-  if ('function' == typeof arguments[2]) {
+  // determine if the 2nd argument is a regular expression
+  if (arguments[1].global !== undefined && arguments[1].multiline !== undefined) {
+    return this.promptForRegexp(str, arguments[1], arguments[2]);
+  } else if ('function' == typeof arguments[2]) {
     return this['promptFor' + (fn.name || fn)](str, arguments[2]);
   }
 
@@ -879,7 +901,6 @@ Command.prototype.promptMultiLine = function(str, fn){
 
 Command.prototype.prompt = function(str, fn){
   var self = this;
-
   if ('string' == typeof str) {
     if (/ $/.test(str)) return this.promptSingleLine.apply(this, arguments);
     this.promptMultiLine(str, fn);