If a client wants fancy validation i mostly use
http://www.livevalidation.com/
Of course all user input is tainted so you still need to filter all input to make your application secure. It is best you use whitelist method for total control. You can check if input is alphanumeric with ctype_alnum(); you can also use regular expressions to filter everything out of a string so you only are left with alphanumeric data.
Example
PHP Code:
<?php
if(!ctype_alnum($_POST['name'])) {
$name = preg_replace('/[^A-Za-z0-9 ]/', '',$_POST['name']);
} else {
echo "WIN";
}
?>
Hope this was what you were looking for.
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