Check out:

First a letter: combination of left and ischaracter: http://www.techonthenet.com/excel/formulas/left.php and http://techniclee.wordpress.com/2010...ction-for-vba/

The latter will also help you finding a capital letter and the special character using the ASCII-table. Shouldn't be too complicated.



Len: http://www.techonthenet.com/excel/formulas/len.php

For the number, I'd check if this is greater 0:
Function onlyDigits(s As String) As String
    Dim retval As String
    Dim i As Integer 

    retval = ""
                                  '
    For i = 1 To Len(s)
        If Mid(s, i, 1) >= "0" And Mid(s, i, 1) <= "9" Then
            retval = retval + Mid(s, i, 1)
        End If
    Next

    onlyDigits = retval
End Function

Hope that helped to push you forward.