Tips for searching

I have a document that has title: 33_FR_EIS.
When I enter 33_FR_ in the search bar,I can't find it. But if I enter _FR_ I do.

I know I have 35 documents with FR in the title, but if I type FR in the search bar I can only see 12.

Is there a sign to add before or after one or more words in the search bar to get a better result? 

Thanks for sharing tips and tricks.