我们最经常遇到的验证，就是电子邮件地址验证。网站上常见。各种网页脚本也都常用“正则表达式”（regular expression）对我们输入的电子邮件地址进行验证，判断是否合法。有的还能分解出用户名和域名。现在用PHP语言实现一下电子邮件地址验证程序，用的是PHP正则表达式库。
源代码如下：
<?php header ( "Content-Type: text/html; charset=UTF-8" ); $reply = ""; if ( isset($_POST["email_address"]) ) { $email_address = $_POST["email_address"]; $pattern = "/^([0-9A-Za-z\\-_\\.]+)@([0-9a-z]+\\.[a-z]{2,3}(\\.[a-z]{2})?)$/i"; if ( preg_match( $pattern, $email_address ) ) { $reply = "您输入的电子邮件地址合法<br /><br />\n"; $user_name = preg_replace( $pattern ,"$1", $email_address ); $domain_name = preg_replace( $pattern ,"$2", $email_address ); $reply .= "用户名：".$user_name."<br />\n"; $reply .= "域名：".$domain_name."<br />\n\n"; } else { $reply = "您输入的电子邮件地址不合法"; } } ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="zh" xml:lang="zh"> <head> <title>电子邮件地址验证程序</title> </head> <body style="text-align: center;"> <h1>电子邮件地址验证程序</h1> <form action="#" method="post"> 请输入电子邮件地址：<input name="email_address" type="text" style="width: 300px;" /><br /> <input type="submit" value="验证电子邮件地址" /> </form> <?php echo $reply; ?> </body> </html>