Parsing a function for its arguments - vba

Question

I have a UDF in written in VBA, which I call from my sheet. The function takes 3 arguments: Function CONCATIF(arg1 As Range, arg2 As Boolean, Optional arg3 As Range) As Variant
The UDF needs to know the formula of arg2, i.e. to intercept arg2 before it has been evaluated to TRUE or FALSE. To do this I use Application.Caller.Formula, which gives me (in its simplest form) "=CONCATIF(arg1, arg2, arg3)" (or in place of ,arg3) either ) or,))
I can then get arg 2 by simply using Split(Application.Caller.Formula, ",")
There are several problems I'd like to deal with though
Splitting at commas means that none of my arguments can contain commas, which they may have to
The formula may be nested, e.g =SUM(1,IF(CONCATIF(arg1, arg2, arg3)="a",1,0)), so I don't know which item of my split array is arg2. (I think this should be fairly easy to fix: find CONCATIF in the string and chop off the start, count the open/close brackets which follow it until open = close, and then chop off the end.
The arguments may be formulae themselves; arg1 could be a reference to a range, not an actual range.
Tricky: CONCATIF may come up multiple times in 1 formula, but with standard string searches I'll always pick up the first one (I may just have to return an error if there are multiple in 1 formula, as I can't think how to get around this at all)
So what I want: A generic way of obtaining the correct CONTCATIF() formula from the caller cell, and then parsing out the three arguments as 3 strings in an array. For reference, here's my code (sorry, naming is a little different from question)
Public Function CONCATIF(checkRange As Range, testFunction As Boolean, Optional concatRange As Range) As Variant
Dim concatArray() As Variant
Dim formulaText As String, formulaParts() As String, formulaTest As String
Dim topLeft As Range, subCell As Range
Dim newTest As String
Dim results() As Boolean, result As Boolean
Dim loopval As Long
'''
'input checking
'''
If concatRange Is Nothing Then
concatArray = checkRange
ElseIf Not (checkRange.Cells.Count = concatRange.Cells.Count And checkRange.Rows.Count = concatRange.Rows.Count And checkRange.Rows.Count = 1) Then
CONCATIF = CVErr(xlErrValue)
Exit Function
Else
concatArray = concatRange.Value2
End If
'''
'Extract test function
'''
formulaText = Application.Caller.Formula
formulaParts = Split(formulaText, ",") 'Assumes 1)no commas 2) formula isn't nested 3) formula doesn't contain nested functions
formulaTest = formulaParts(1) 'get the test function as a string to be evaluated
Set topLeft = checkRange.Cells(1, 1) 'This is the 'reference' cell - substitute each of the values in the check range for this to test each one
ReDim results(0)
On Error GoTo Err
'''
'Run test on each of the cells in checkRange
'''
For Each subCell In checkRange
newTest = Replace(formulaTest, topLeft.Address(0, 0), subCell.Address)
If Count(newTest, "(") < Count(newTest, ")") Then 'when optional parameter is missed out, sometimes you get a ,) and sometimes a ) after formulaTest, so we must check
newTest = Left(newTest, Len(newTest) - 1)
End If
result = (Evaluate(newTest))
skip:
results(UBound(results)) = result
ReDim Preserve results(UBound(results) + 1)
Next subCell
'''
'Then use array of Booleans for UDF function
'''
CONCATIF = "test"
Exit Function
Err:
result = False 'if the evaluate results in an error, it means the input was invalid, so probably won't meet the criteria, therefore can be ignored
loopval = loopval + 1
If loopval > checkRange.Cells.Count Then CONCATIF = CVErr(xlErrNA): Exit Function 'exit error loop gracefully if I've missed some edge case
Resume skip
End Function
And then referenced in my UDF is this:
Function Count(str As String, chr As String) As Long 'counts the number of instances of a character in a string
Count = Len(str) - Len(Replace(str, chr, ""))
End Function

Answer 1

If you had a proper formula parser you could solve all of these problems except the one of handling multiple calls to CONCATIF in a single formula: I don't know of a way of finding out 100% which instance of CONCATIF is currently being called.
There are various Formula parsers around you could maybe adapt: start here
http://ewbi.blogs.com/develops/2004/12/excel_formula_p.html