Cleaning input enough?

Input validation is a known issue, but writing some PHP-code today let me write the following and I’m wondering if I forgot something. It is only to make sure no cleansed variable will enter a switch statement for example.

if (isset(_POST['action']))
    if (strlen(preg_replace("/[^a-zA-Z0-9-]/i","",_POST['action'])) == 0)
        page_action =_POST['action'];
    else
        page_action = '';
else
    page_action = '';

switch ($page_action) {
...

For now, I need to check the code that no $_POST variable is entering the code unchecked before I put the code online. This also includes variables for SQL-statements to eliminate SQL-injections.