1. When you input keywords, the blank regards as the separator of the words and means a "and" relationship.
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.