Help with vb regular expressions

I have 2 files csv files that I read.  I get the info from the first file and use that to update the second.  The first just contains a userid and ldap status and looks like:

userid1,Pilot
userid2,Non-Pilot

The second contains 7 columns and looks like

UK Branch 10000,userid1,12340,FA,DUNC,Joe Blow,Non-pilot
UK Branch 20000,userid2,12341,OAG,NBUSKIR,Jan Blow,Pilot

I need to read the first cvs and then update the second csv to look like

UK Branch 10000,userid1,password1,FA,DUNC,Joe Blow,Pilot
UK Branch 20000,userid2,password2,OAG,NBUSKIR,Jan Blow,Non-Pilot

Below is the code I am using, but it is changing the password instead of the last column.  What am I doing wrong?
'read pilotstatus
        Dim srset As New StreamReader("\\windows_scripts\Easypass\pilotstatus.txt")   'set stream to read passwords to reset file

        'read passwords.txt
        Dim sr As New System.IO.StreamReader("\\windows_scripts\Easypass\passwords.txt")   'set stream to read passwords file
        Dim input As String = sr.ReadToEnd()
        sr.Close()

       
        While Not srset.EndOfStream()
            Dim line() As String = srset.ReadLine().Split(","c)
            Dim strID As String = line(0)
            Dim strPilot As String = line(1)

            ' Set up a Regex pattern to locate the data to be changed in the file
            Dim Pattern As String = "(?<N1>^.*?,\s*" & strID.Trim & "\s*,)(?<N2>.*?)(?<N3>,.*$)"
            ' Locate and replace the data
            input = System.Text.RegularExpressions.Regex.Replace(input, _
                Pattern, "${N1}" & strPilot.Trim & "${N3}", _
                System.Text.RegularExpressions.RegexOptions.Multiline Or _
                System.Text.RegularExpressions.RegexOptions.Singleline)

        End While

        srset.Close()   'close stream on passwordstoreset file

        MsgBox(input)