A simple PHP script to evaluate regular expressions

<?php
$mode = $_GET['mode'];

$data = "";
$regex = "";
$replace = "";

if($mode == "exec") {
    $data = stripslashes($_POST['data']);
    // Workaround to replace an e which could cause php to execute a command
    $regex = stripslashes(str_replace("e", "x", $_POST['regex']));
    $replace = stripslashes($_POST['replace']);

    setcookie("data", addslashes($data));
    setcookie("regex", addslashes($regex));
    setcookie("replace", addslashes($replace));

    $_COOKIE["data"] = $data;
    $_COOKIE["regex"] = $regex;
    $_COOKIE["replace"] = $replace;
}

echo('<?xml version="1.0" encoding="UTF-8"?>'."\n");
?>
<html>
<head>
<title>Regex Ausgabe</title>
</head>
<body>
<h2>Regex Ausgabe</h2>
<p>
<?php
if($mode == "exec") {
    $output = preg_replace($regex, $replace, $data);

    echo(nl2br(htmlspecialchars($output)));
?>
</p>
<hr />
<?php
    echo('<h2>Eingegebene Daten</h2>');
    echo("\n<p>\n");
    echo("<b>Regex : </b>".htmlspecialchars($regex)."<br />\n");
    echo("<b>Replace : </b>".htmlspecialchars($replace)."<br />\n");
    echo("<b>Daten : </b><br />\n".nl2br(htmlspecialchars($data)));
    echo("\n</p>\n");
}
?>
<hr />
<h2>Eingabe</h2>
<p>
<form method="post" action="index.php?mode=exec">
<label for="data">Daten</label>
<br />
<textarea name="data" id="data" rows="20" cols="80">
<?php echo($_COOKIE['data']); ?>
</textarea>
<br />
<label for="regex">Regulärer Ausdruck</label>
<br />
<input type="text" name="regex" id="regex" size="40" value="<?php echo(htmlspecialchars($_COOKIE['regex'])); ?>" />
<br />
<label for="replace">Ersetzungstext</label>
<br />
<input type="text" name="replace" id="replace" size="40" value="<?php echo(htmlspecialchars($_COOKIE['replace'])); ?>" />
<br />
<input type="submit" value="Regulären Ausdruck anwenden" />
</form>
</p>
<hr />
</body>
</html>