This article will show you step-by-step instructions to build your own NTP CentOS server. The network diagram shows the best practice to setup the NTP server farm for redundancy. This article also serves as your NTP server documentation.
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: