The first one to examine : the freeware WanyWord.
I put a big folder. And is working for a while.
I think this program covers all possibilities. Inside the files or search inside the file names. But the concept of text strings seems strange to me. I am not sure now that the program search for parallel strings at the same time. Only for a set of strings considering a word, couple, or a all sentence. But not two sentences separated by a , or ; or : , etc. So I think is only for a chain.