valida formulario linha 19 php

<!DOCTYPE html>
<html>
<head>
    <title>Validaçao Formulario</title>
</head>
<body>
<fieldset size="300">
    <legend>Cadastro</legend>
    <form action="valida_cadastro.php" method="POST">
        Nome: <input type="text" size="30" name="nome"><br>
        Email: <input type="text" size="30" name="email"><br>
        Twitter: <input type="text" size="30" name="twitter"><br>
        <button>Enviar</button>
    </form>
</fieldset>
</body>
</html>

<?php
$nome = trim($_POST['nome']);
$email = trim($_POST['email']);
$twiter =strtolower(trim($_POST['twitter']));

if(!preg_match('/^[^0-9]{2,80}$/i',$nome)){
    exit('Formato incorreto');
}
$emailregex=<<<REGEXEMAIL
/^[^0-9][a-z0-9_]+([.][a-z0-9_]+)*[@][a-z0-9-_]+([.][a-z0-9]{2,3})*[.][a-z]{2,3}$/
REGEXEMAIL;

if(!preg_match($emailregex, $email)){
    exit('Email invalido');
}
if(!preg_match('/^@[a-z_]{1,15}/', $twiter)){
    exit('TwitterInvalido');
}