python模拟网站注册要求用户名必须包含数字和字母_某网络服务平台要求新注册的用户名必须以字母开头，同时包含字母、数字和下划线，长度为6～18个字符。编写一个VB程序，判断用户名的合法性，具体功能如...

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实现上述功能的VB程序如下，请在划线处填入合适的代码

Private Sub Btn_Click ()

Dim s As String, ch

As String, n As Integer, i As Integer

Dim n1 As Integer,

n2 As Integer, n3 As Integer

n1 =0:n2=0:n3=0

s=Text1. Text

①

ch=Mid (s, 1, 1)

If n＜6 Or n＞18 Then Text2. Text=“不合法”：Exit Sub′退出当前子程序

If Not (ch＞=“a”And ch＜=“z”Or

ch＞=“A”And ch＜=“Z”)Then

Text2. Text=“不合法”：Exit Sub

End If

For i=2 To n

ch= Mid (s, I, 1)

If ch＞=“a”And ch＜=“z”Or ch＞=“A”And ch＜=“Z”Then

n1=n1+ 1

Elself ch＞=“0”And ch=“9”Then

n2=n2+1

ElseIf

ch=“_”Then

n3=n3+1

End If

Next i

If②Then

Text2. Text=“不合法”Else text2. Text=“合法”

End Sub

①________ ②________