Validar letras y números con expresión regular

Public Function letras_y_numeros(ByVal s As String) As Boolean
        Dim match As Boolean = False
        Dim rgx As New Regex("^[a-zA-Z0-9]+$")
        match = rgx.IsMatch(s)
        Return match
    End Function

fuente: http://stackoverflow.com/questions/1181419/verifying-that-a-string-contains-only-letters-in-c-sharp