<script type="text/javascript">
function req(field)
{
with (field)
{
if (value==null||value=="")
{
alert("Трябва да попълниш полето!");return false;
}
else
{
return true;
}
}
}
</script>
<form method="post" onsubmit="req(search)" action="">
<input type="text" name="search" size="35" />
<input type="submit" name="submit" value="Давай!"/>
</form>