Numbers and A to Z in form

One little thing, why is this wrong if i also want a field for a-z, A-Z and 0-9.

if (/^[a-zA-Z0-9]*$/.test(document.mcform.frmParticipantPassword.value)) {

alert("Only number is allowed for quantity!");
document.mcform.frmParticipantPassword.focus();
return false;

it don't seems to work

<script language="JavaScript" type="text/javascript">
<!--
function MCValidation()
{


if (document.mcform.frmParticipantPassword.value == "") {

alert("Password is required!");
document.mcform.frmParticipantPassword.focus();
return false;

}

if (document.mcform.frmParticipantPassword.value.length < 6) {

alert("Password must contain at least six characters!");
document.mcform.frmParticipantPassword.focus();
return false;

}
if (/^[a-zA-Z0-9]*$/.test(document.mcform.frmParticipantPassword.value)) {

alert("Only number and a-z is allowed!");
document.mcform.frmParticipantPassword.focus();
return false;

} 

return true;

}

// -->
</script>

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