Type Token
    tokenType As Integer    '1=lbr, 2=rbr, 3=string, 4=number, 5=symbol
    value As String
    numValue As Double
End Type

Type TokenList
    tokens(1000) As Token
    cnt As Integer
End Type

Function isNumber(word As String) As Boolean
    For i As Integer = 1 To Len(word)
        If (Mid(word, i, 1) < "0" Or Mid(word, i, 1) > "9") And Mid(word, i, 1) <> "." Then Return False
    Next
    Return True
End Function

Function ParseSExpr(cad As String) As TokenList
    Dim As TokenList tlist
    Dim As Integer state = 1 '1=token_start, 2=read_quoted_string, 3=read_string_or_number
    Dim As String word = ""

    For i As Integer = 1 To Len(cad)
        Select Case state
        Case 1 'token_start
            Select Case Mid(cad, i, 1)
            Case "("
                tlist.tokens(tlist.cnt).tokenType = 1
                tlist.cnt += 1
            Case ")"
                tlist.tokens(tlist.cnt).tokenType = 2
                tlist.cnt += 1
            Case " ", Chr(9), Chr(10), Chr(13)
            Case """"
                state = 2 : word = ""
            Case Else
                state = 3 : word = Mid(cad, i, 1)
            End Select

        Case 2 'read_quoted_string
            If Mid(cad, i, 1) = """" Then
                tlist.tokens(tlist.cnt).tokenType = 3
                tlist.tokens(tlist.cnt).value = word
                tlist.cnt += 1
                state = 1
            Else
                word &= Mid(cad, i, 1)
            End If

        Case 3 'read_string_or_number
            If Instr(" " & Chr(9) & Chr(10) & Chr(13) & ")", Mid(cad, i, 1)) Then
                tlist.tokens(tlist.cnt).tokenType = Iif(isNumber(word), 4, 5)
                If isNumber(word) Then
                    tlist.tokens(tlist.cnt).numValue = Val(word)
                Else
                    tlist.tokens(tlist.cnt).value = word
                End If
                tlist.cnt += 1
                If Mid(cad, i, 1) = ")" Then
                    tlist.tokens(tlist.cnt).tokenType = 2
                    tlist.cnt += 1
                End If
                state = 1
            Else
                word &= Mid(cad, i, 1)
            End If
        End Select
    Next
    Return tlist
End Function

Function ArrayToString(tlist As TokenList, startIdx As Integer, Byref endIdx As Integer) As String
    Dim As String result = ""
    Dim As Integer i = startIdx

    If startIdx = 0 Then result = "["

    While i < tlist.cnt
        Select Case tlist.tokens(i).tokenType
        Case 1 'lbr
            result &= "["
            i += 1
            result &= ArrayToString(tlist, i, i)
        Case 2 'rbr
            endIdx = i
            Return result & "]"
        Case 3 'string
            result &= """" & tlist.tokens(i).value & """"
        Case 4 'number
            result &= Rtrim(Str(tlist.tokens(i).numValue))
        Case 5 'symbol
            result &= ":" & tlist.tokens(i).value
        End Select
        If i < tlist.cnt - 1 Andalso tlist.tokens(i+1).tokenType <> 2 Then result &= ", "
        i += 1
    Wend
    Return result
End Function

Function ToSExpr(tlist As TokenList) As String
    Dim As String result = ""
    For i As Integer = 0 To tlist.cnt - 1
        Select Case tlist.tokens(i).tokenType
            Case 1: result &= "("
            Case 2: result &= ")"
            Case 3: result &= """" & tlist.tokens(i).value & """"
            Case 4: result &= Rtrim(Str(tlist.tokens(i).numValue))
            Case 5: result &= tlist.tokens(i).value
        End Select
        If i < tlist.cnt - 1 Andalso tlist.tokens(i+1).tokenType <> 2 Andalso tlist.tokens(i).tokenType <> 1 Then result &= " "
    Next
    Return result
End Function

' Main program
Dim As String inputString = _
"((data ""quoted data"" 123 4.5)" & Chr(10) & _
" (data (!@# (4.5) ""(more"" ""data)"")))"

Print "Original S-Expression:"
Print inputString
Dim As TokenList tokens = ParseSExpr(inputString)

Print !"\nNative Structure:"
Dim As Integer dummy
Print ArrayToString(tokens, 0, dummy)

Print !"\nand back to S-Expression:"
Print ToSExpr(tokens)

Sleep