Count common characters in two strings

For i = 1 to len(str1)
  c = mid(str1,i,1)
  if instr(str2, c) then
    count = count + 1
  end if
Next

' // Will count unique letters in the string
' || Will *not* count numbers or other characters
Function UniqueLetters(strInput As String) As Integer
    Dim HasLetter(0 To 25) As Boolean ' A flag for each of the 26 letters of the alphabet
    Dim n As Integer                  ' A loop counter
    Dim c As Integer                  ' To get the letter
  ' // Loop through the string and flag each letter
    For n = 1 To Len(strInput)
      ' // Get the ASCII value then convert to 0(A)-25(Z)
      ' || Convert to upper case count A and a only as 1
        c = Asc(Mid(UCase(strInput), n, 1)) - 65
      ' // If it's a letter record that letter as counted
        If c >= 0 And c <= 25 Then
            HasLetter(c) = True
        End If
    Next n
  ' // Count the letters used and return result
    For n = 0 To 25
        If HasLetter(n) Then
            UniqueLetters = UniqueLetters + 1
        End If
    Next n
End Function

Sub Examples()
    Dim strPass As String
    strPass = "303 Lee Street" ' note it won't count the space or numbers
    MsgBox strPass & " has " & UniqueLetters(strPass) & " unique letters"
    strPass = "The Quick Brown Fox Jumped Over The Lazy Dogs"
    MsgBox strPass & " has " & UniqueLetters(strPass) & " unique letters"
    strPass = "equinox"
    MsgBox strPass & " has " & UniqueLetters(strPass) & " unique letters"
End Sub