how to validate a text field which can be A,B,C,D, Null, or groups of A,B,C,D (ex. ‘A,B,C’ with commas no space).

my quick suggestion:

a!localVariables(
  local!text,
  
  a!textField(
    value: local!text,
    saveInto: local!text,
    validations: {
      if(
        and(
          a!isNotNullOrEmpty(local!text),
          a!isNotNullOrEmpty(stripwith(local!text, "ABCD,"))
        ),
        "invalid characters entered",
        ""
      )
    }
  )
)