No other job is as rewarding and demanding as building an iPhone app is. It is not really in the hands of the developer for the success of an iPhone app. Many factors operate jointly for every iOS application's success in the market.
Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions.
Consider email address RFCs:
Look at HTML5 form input element (with type=email) regex pattern: