InStr and Mid crap [resolved]

[R.a.B.B.i.T]

ok i was told how to get just part of a line from a text file using InStr and Mid, and ive looked all over for how to do that, because i have no idea how to use InStr and Mid at all. i see things like

InStr(strCheck, Mid(sInvalidChars, i, 1)

but i have no idea why its setup like that, or what it means (ill post the entire script of the program dealy that had it inside). thats the closest thing to instructions on what InStr and Mid are that ive seen

Dim bCK As Boolean
Dim strDomainType As String
Dim strDomainName As String
Const sInvalidChars As String = "!#$%^&*()=+{}[]|\;:'/?>,< "
Dim i As Integer

bCK = Not InStr(1, strCheck, Chr(34)) > 0 'Check to see if there is a double quote
If Not bCK Then GoTo ExitFunction

bCK = Not InStr(1, strCheck, "..") > 0 'Check to see if there are consecutive dots
If Not bCK Then GoTo ExitFunction

' Check for invalid characters.
If Len(strCheck) > Len(sInvalidChars) Then
    For i = 1 To Len(sInvalidChars)
        If InStr(strCheck, Mid(sInvalidChars, i, 1)) > 0 Then
            bCK = False
            GoTo ExitFunction
        End If
    Next
Else
    For i = 1 To Len(strCheck)
        If InStr(sInvalidChars, Mid(strCheck, i, 1)) > 0 Then
            bCK = False
            GoTo ExitFunction
        End If
    Next
End If

If InStr(1, strCheck, "@") > 1 Then 'Check for an @ symbol
    bCK = Len(Left(strCheck, InStr(1, strCheck, "@") - 1)) > 0
Else
    bCK = False
End If
If Not bCK Then GoTo ExitFunction

strCheck = Right(strCheck, Len(strCheck) - InStr(1, strCheck, "@"))
bCK = Not InStr(1, strCheck, "@") > 0 'Check to see if there are too many @'s
If Not bCK Then GoTo ExitFunction

strDomainType = Right(strCheck, Len(strCheck) - InStr(1, strCheck, "."))
bCK = Len(strDomainType) > 0 And InStr(1, strCheck, ".") < Len(strCheck)
If Not bCK Then GoTo ExitFunction

strCheck = Left(strCheck, Len(strCheck) - Len(strDomainType) - 1)
Do Until InStr(1, strCheck, ".") <= 1
    If Len(strCheck) >= InStr(1, strCheck, ".") Then
        strCheck = Left(strCheck, Len(strCheck) - (InStr(1, strCheck, ".") - 1))
    Else
        bCK = False
        GoTo ExitFunction
    End If
Loop
If strCheck = "." Or Len(strCheck) = 0 Then bCK = False

ExitFunction:
ValidEmail = bCK
End Function

PS: i dont have any help files oO
go figure...

PS: thats not my code...