<form method="post" name="registration" onsubmit="if (!check_reg_daten()) {return false;};" id="registration" action="">
</form>
if(name.lenght > 4) {bla bla}
и т д
myOnSubmitFunction
<form onsubmit="myOnSubmitFunction">
event
event.preventDefault()
function myOnSubmitFunction (event) {
if (some === any) {
event.preventDefault()
}
}