How To Change Script To Trigger On Dynamic Form Name

I have this bit of code:

function frmsubmit( func )
{
if ( document.itemform )
{
frm = document.itemform;
frm.func.value = func;
frm.submit();
}}

This used to serve my needs, but now I have changed the php script such that the individual form names for each product are like:

<form method="post" action="mypage.php" name="itemform_125400" >...</form>

How can I modify my above script to trigger on any form with the name in the form of "itemform_nnnnnn" where "n" is always an arbitrary 6-digit number?