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Below are examples of Fox custom filters that might prove useful.

Smileys

While you can eliminate the use of directives in Fox (indeed it is eliminated by default) by using $EnablePostDirectives = false; using smileys (as in :) or (:, in particular) will still break the page. You can avoid this by replacing these with special character codes.

  $FoxFilterFunctions['smiley'] = 'FoxSmileyFilter';

  function FoxSmileyFilter($pagename, $fields) {
    $fields = preg_replace('/\(:/', '&#40;:', $fields);
    $fields = preg_replace('/:\)/', ':&#41;', $fields);
    return $fields;
  }

URLs

When you have a Website field, you never know when people will type http:// or not. This filter will check and see, then add it if necessary.

  $FoxFilterFunctions['FoxFixUrl'] = 'FoxFixUrlFilter';

  function FoxFixUrlFilter($pagename, $fields) {
    if ($fields['website']!="") $matches = strpos("http://",$fields['website']);
    if ($matches==false && $fields['website'] != "") {
      $fields['website'] = "http://".$fields['website'];
    }
    return $fields;
  }

I don't know why, but the above doesn't work. It adds http:// even if it's already there. Until I figure out why (or someone who's not a PHP n00b explains that to me) I've added the following line above the "return $fields" line.

$fields['website'] = preg_replace('/http:\/\/http:\/\//', 'http:\/\/', $fields['website']);

E-mail

E-mails provide several potential pitfalls for a form submission. Here are a couple of examples of ways you can filter an e-mail field. First, if you just want to make sure something gets entered in the field, regardless of whether it's an e-mail or not, you can use this function (note you could use this kind of filter for any field that's necessary but not required):

  $FoxFilterFunctions['FoxVerifyEmail'] = 'FoxVerifyEmailFilter';

  function FoxVerifyEmailFilter($pagename, $fields) {
    if ($fields['email']=="") $fields['email']="noemail";
    return $fields;
  }

You may also want to validate that what was entered in the field was really an e-mail address and not something else. the following function will verify the most common email addresses:

  $FoxFilterFunctions['FoxVerifyEmail'] = 'FoxVerifyEmailFilter';

  function FoxVerifyEmailFilter($pagename, $fields) {
  if(!preg_match("/^[a-z0-9!#$%&'*+\^_`=\/{|}~-]+(?:\.[a-z0-9!#$%&'*+\^_`=\/{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+(?:[A-Z]{2,4}|museum|travel)\b$/i", $fields['email'])) {
       $fields['email'] = "";
       FoxAbort($pagename, "Please provide a valid email address!");
    }
    return $fields;
  }

Note that email addresses can contain quoted strings, but the regex function above will not catch these.
See more about regex patterns for e-mail: