What have I to insert into the debugger Watch Expression to be warned?

I’d like to be warned by debugger when whatever variable assumes a value containing a certain substring, name it for example “books”. In other words, if whatever variable contained a “superstring” of “books”, for instance, just to make an example, “Original books released”, I should receive a warning.
What have I to introduce into the field “watch expression”?
Can you help please?
Thanks in advance for any hint.