User Authentication with Classic ASP

Hi Experts,
With the attached code I can retrieve the logged in user's employee number if the Logon_User is in one of the following formats:
ABC\U123456
or
ABCDE\123456
or ABC\123456

123456 is the employee number that I need to capture. My problem is that sometimes the Logon_User is in the following format, and I cannot capture the employee number:
ABC\U123456.last_name.first_name

How could I change the code below in order the capture only 123456, and ignore everything starting with the dot

<%
If Request.ServerVariables("LOGON_USER") = "" Then
Response.Status = "401 Access Denied"
Response.End
End If
%>

<%
set rs = Server.CreateObject("ADODB.recordset")
Dim UserID
UserID = UCase(Request.ServerVariables("LOGON_USER"))
UserID = Replace(UCase(UserID), "\U", "\")
UserID = Mid( UserID, InStrRev( UserID, "\", -1, 1 ) + 1, Len( userID ) )
if InStr( UserID, "U" ) >= 0 then
    UserID = Replace( UserID, "U", "" )
end if

rs.Open "exec sp_Authentication'" & UserID & "'", Conn

do until rs.EOF
EMP_ID = rs("EMP_ID")

rs.MoveNext
loop
rs.Close
Set rs = Nothing
%>

<%=EMP_ID%>

<br />

<%=Request.ServerVariables("Logon_User")%>

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Thank you for your help