Reporting Services column visibility expression includes substring column names

Reporting Services column visibility expression includes substring column
names

I have an SSRS 2012 report with columns that are to displayed or hidden
based on user parameter selections. Each column has a visibility
expression associated with it. For instance, the expression to show or
hide the "Cash Flow" column is:
=IIF(InStr(JOIN(Parameters!IF_Variables.Value,","),"CashFlow")>0,False,True)
The problem is that I also have a column called "AnnualCashFlow", with a
corresponding visibility expression. If the users selects it, the
"CashFlow" column will also be displayed, even if it wasn't chosen,
because it is a substring of the longer column name.
There are several other columns that are related in this way. How can I
formulate an expression that will only find the exact string requested?