2. The sign "-" with a word like "-word" means you want to exclude the word from the search result.

3. The sign "+" with a word like "+word" means you maybe need to include the word in the search result.

4. The sign "-" and "+" in the front of a search string just as itself. In other word, they don't have the function as above.

5. You need to exclude ' and " in the search string.