Perl Compatible Regexes
<?php
    if (preg_match('/^\d{5}(-\d{4})?$/',
                   $_POST['zip'])) {  }
?>
<?php
    $text = "Hello, /Alice/. How are you?";
    print preg_replace('{/([^/]+)/}','<b>\\1</b>',
                       $text);
?>
Output
Hello, Alice. How are you?