Regular Expression

Does anybody knows what the regular expresion pattern for a variable string (includes '_' but does not starts with '_' and does not include ?/#@!%^&* and spaces).

Thanks

Code:

---

Private Sub Text1_KeyPress(KeyAscii As Integer)

'allow only alpha and numerical and _ and no other keys
If KeyAscii = 95 Or KeyAscii >= 65 And _
  KeyAscii <= 90 Or KeyAscii >= 97 And KeyAscii <= 122 _
      Or KeyAscii >= 48 And KeyAscii <= 57 Then
   
        KeyAscii = KeyAscii
    Else
        KeyAscii = 0
End If
   
End Sub

---