Optional replacement in a Regular Expression - regex

Question

I am creating a regular expression, in VBA that uses the JS flavor of RegEx. Here is the issue I have ran into:
Current RegEx:
(^6)(?:a|ab)?
I have a 6 followed by either nothing, an 'a' or 'ab'.
In the case of a 6 followed by nothing I want to return just the 6 using $1
In the case of a 6 followed by an 'a' or 'ab' I want to return 6B
So I need that 'B' to be optional, contingent on there being an 'a' or 'ab'.
Something to the effect of : $1B?
That of course does not work. I only want the B if the 'a' or 'ab' is present, otherwise just the $1.
Is this possible to do in a single regex pattern? I could just have 2 separate patterns, one looking for only a 6 and the other for 6'a'or'ab'... but my actual regex patterns are much more complicated and I might need several patterns to cover some of them...
Thanks for looking.

Answer 1

I don't think your question is clearly defined--for example, I don't know why you need a replace--but from what I can infer, something like the following may work for you:
target = "6ab"
result = ""
With New RegExp
.Pattern = "^(6)(?:a(b?))?"
Set matches = .Execute(target)
If Not matches Is Nothing Then
Set mat = matches(0)
result = mat.SubMatches(0)
If mat.SubMatches.Count > 1 Then
result = result & UCase(mat.SubMatches(1))
End If
End If
Debug.Print result
End With
You basically inspect the capture groups to determine whether or not there was a hit on the b capture. Whereas you used a|ab, I think and optional b (b?) is more to the point. It's probably stylistic more than anything.

Answer 2

As I mentioned in my comment, there is no way to tell a regex engine to choose between literal alternatives in the replacement string. Thus, all you can do is to access Submatches to check for values that you get there, and return appropriate values.
Note that a regex you have should have 2 capturing groups, or at least a capturing group where you do not know the exact text (the (ab?)).
Here is my idea in code:
Function RxCondReplace(ByVal str As String) As String
RxCondReplace = ""
Set objRegExp = CreateObject("VBScript.RegExp")
objRegExp.Pattern = "^6(ab?)?"
Set objMatches = objRegExp.Execute(str)
Set objMatch = objMatches.Item(0) ' Only 1 match as .Global=False
If objMatch.SubMatches.Item(0) = "a" Or _ ' check if 1st group equals "a"
objMatch.SubMatches.Item(0) = "ab" Then ' check if 1st group equals "ab"
RxCondReplace = "6B"
ElseIf objMatch.SubMatches.Item(1) = "" Then ' check if 2nd group is empty
RxCondReplace = "6"
End If
End Function
' Calling the function above
Sub CallConditionalReplace()
Debug.Print RxCondReplace("6") ' => 6
Debug.Print RxCondReplace("6a") ' => 6B
Debug.Print RxCondReplace("6ab") ' => 6B
End Sub