Hi guys I am new to this forum and I really need someones help here with this, as I am trying to validate this form and really struggle with it. I only started here with first field validation and I tested it for a length of input although I need this to validate only numbers from 1-99999.
I would be very grateful if someone can point me to the right direction. Here is my html code:

<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-type" content="text/html; charset=iso-8859-1"/>
<title>Submit runner time</title>

<script type="text/javascript">
/* <![CDATA[ */
function validateRunnerID()
{
var RunnerID = document.getElementById("RunnerID").value;
if (RunnerID.length == 0)
{
return "Error username is length zero\n";
}
else
{
alert("enter the number");
return false;
}
}
</script>


</head>
<body>
<hr/>
<h1>Submit runner time</h1>
<hr/>
Note: all fields marked '*' are mandatory.
<p/>
<form name="RunnerID" onsubmit="return validate(); method="post" action="http://test.php">

<table>
<tr><td>Runner ID*</td>
<td><input type="text" name="RunnerID" id="RunnerID" size="5" maxlength="5"/></td>
</tr>
<tr><td>Event ID*</td>
<td><input type="text" name="EventID" size="5" maxlength="5"/></td>
</tr>
<tr><td>Date (YYYY-MM-DD)*</td>
<td><input type="text" name="Date" size="10" maxlength="10"/></td>
</tr>
<tr><td>Finish time (HH:MM:SS)*</td>
<td><input type="text" name="FinishTime" size="8" maxlength="8"/></td>
</tr>
<tr><td>Position*</td>
<td><input type="text" name="Position" size="5" maxlength="5"/></td>
</tr>
<tr><td>Category ID*</td>
<td><input type="text" name="CategoryID" size="2" maxlength="2"/></td>
</tr>
<tr><td>Age grade*</td>
<td><input type="text" name="AgeGrade" size="5" maxlength="5"/></td>
</tr>
<tr><td>Personal best</td>
<td><input type="text" name="PB" size="1" maxlength="1"/></td>
</tr>
</table>
<input type="submit" name="submitrunnertime" value="submit"/>
<hr/>
</form>
</body>
</html>