VBScript If Statement - Trim X Letter after \

The script below captures the logged in user's domain and employee number.
This can be letters followed by a backslash \
e.g. ABC\12345
ABCD\123456
ABCDE\E539Z
etc.
The characters after the \ is the employee number.
The following line trims the domain and \, and keeps only the employee number:

UserID = Mid(UserID,instr(UserID,"\")+1)

Unfortunately there is an exception, and I need to add an if statement.
Some employee numbers have an X letter prefix, and I need to trim the X too.
For example if the employee number is X123456 I need only 123456 returned.
i.e if the logged in user is ABC\X123456 I need to trim ABC\X and leave only 123456

This is the only exception; for all the other logins I just need to trim the domain and \
like the script below shows.
Would you please help me out with an if statement that would do that for me?
Thank you.

<%
If Request.ServerVariables("LOGON_USER") = "" Then
Response.Status = "401 Access Denied"
Response.End
End If
%>
<%
set rs = Server.CreateObject("ADODB.recordset")
Dim UserID
UserID = Request.ServerVariables("LOGON_USER")

UserID = Mid(UserID,instr(UserID,"\")+1)



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

do until rs.EOF
Emp_Id = rs("Emp_Id")
First_Name = rs("First_Name")
Last_Name = rs("Last_Name")


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

Employee Number: <%=Emp_Id%>
<br />
First Name: <%=First_Name%>
<br />
Last Name: <%=Last_Name%>

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