Function selectstr(ByVal str As String, ByVal selstr As String, ByVal index As Integer, ByVal place As Integer) As String

       Dim i As Integer
        Dim a As String = str
        If Mid(a, 1, selstr.Length) <> selstr Then '判断前几位是否和selstr相同不相同则使其相同
            a = selstr & str
        End If

 If Mid(a, a.Length - selstr.Length, selstr.Length) <> selstr Then '判断后几位是否和selstr相同不相同则使其相同

            a = a & selstr
        End If
        Select Case place
            Case Is = 0
                For i = 1 To index
                    str = Mid(a, selstr.Length + 1, InStr(Mid(a, selstr.Length + 1, a.Length), selstr)) '从和selstr长度相同的位置开始查找到selstr的位置
                    a = Mid(a, selstr.Length + 1, a.Length)
                    a = selstr & Mid(a, InStr(a, selstr) + 1, a.Length)
                Next
            Case Is = 1
                Try
                    For i = 1 To index
                        str = Mid(a, selstr.Length + 1, InStr(Mid(a, selstr.Length + 1, a.Length), selstr) - 1) '从和selstr长度相同的位置开始查找到selstr前面的位置
                        a = Mid(a, selstr.Length + 1, a.Length)
                        a = selstr & Mid(a, InStr(a, selstr) + 1, a.Length)
                    Next
                Catch ex As Exception
                    MsgBox("无法查找该位置!", MsgBoxStyle.OkOnly, "出错")
                    Return ""
                    Exit Function
                End Try
            Case Is = 2
                For i = 1 To index
                    str = Mid(a, selstr.Length + 1, InStr(Mid(a, selstr.Length + 1, a.Length), selstr) + 1) '从和selstr长度相同的位置开始查找到selstr后面的位置
                    a = Mid(a, selstr.Length + 1, a.Length)
                    a = selstr & Mid(a, InStr(a, selstr) + 1, a.Length)
                Next
            Case Else
                Return ""
                Exit Function
        End Select
        Return str
         End Function

  'select str(str,selstr,index,place)
        'str:被截取的字符串
        'selstr:唯一标示,要截取的字符的临界点(比如aa/dd/ff/gg这里的"/"就是唯一标示)
        'index:第几个匹配的位置
        'place:该参数有3个值0,1,2
        '0:截取到唯一标示的位置
        '1:截取到唯一标示的前一个位置
        '2:截取到唯一标示的后一个位置
        '例子:找出字符串aa/dd/ff/gg中第3个"/"前的字符
        'selectstr("aa/dd/ff/gg","/",3,1)