Concatenate strings and remove excess comma - regex

Question

I'm trying to concatenate multiple strings and separate them by comma,
and then subsequently to remove excess, leading and trailing commata.
For example, with an input of TEST("", "b", "c", "", ""), I would like to get
b, c
However, my regex ,$| ,+|^, does not really take repeated commas into account:
Function TEST(a, b, c, d, e)
res = a & ", " & b & ", " & c & ", " & d & ", " & e
Debug.Print (res)
Dim regex As Object, str, result As String
Set regex = CreateObject("VBScript.RegExp")
With regex
.Pattern = ",$| ,+|^,"
End With
Dim ReplacePattern As String
ReplacePattern = ""
res = regex.Replace(res, ReplacePattern)
TEST = res
End Function
How can I do this?

Answer 1

Most elegant is #ScottCraner's suggestion of TEXTJOIN (will remove this part of answer, if he wishes to post this as his own)
Private Function nonEmptyFields(ParamArray strings() As Variant) As String
nonEmptyFields = WorksheetFunction.TextJoin(",", True, Array(strings))
End Function
Note: This will only work for Office 365+, but you can always create your own version of
TEXTJOIN
Another option would be to loop over the ParamArray of strings and add them together, depending on their content (whether they are populated or empty)
Private Function nonEmptyFields(ParamArray strings() As Variant) As String
Dim result As String
Dim i As Byte
For i = LBound(strings) To UBound(strings)
If Len(strings(i)) <> 0 Then
If result = vbNullString Then
result = strings(i)
Else
result = result & "," & strings(i)
End If
End If
Next i
nonEmptyFields = result
End Function
Both would yield desired result with set up of
Debug.Print nonEmptyFields(a, b, c, d, e, f) ' "", "b", "c", "", "", ""

Answer 2

My ugly solution maintaining the same parameters:
Function TEST(a, b, c, d, e)
If a <> "" Then res = a
If b <> "" Then
If res <> "" Then
res = res & ", " & b
Else
res = b
End If
End If
If c <> "" Then
If res <> "" Then
res = res & ", " & c
Else
res = c
End If
End If
If d <> "" Then
If res <> "" Then
res = res & ", " & d
Else
res = d
End If
End If
If e <> "" Then
If res <> "" Then
res = res & ", " & e
Else
res = e
End If
End If
TEST = res
End Function