| Operator | Function |
| + | A leading plus sign indicates that this word must be present in every document
returned. |
| - | A leading minus sign indicates that this word must not be present in any document
returned. |
| -blank- |
By default (when neither plus nor minus is specified) the word is optional, but
the document that contain it will be rated higher. |
| < > | These two operators are used to change a word's contribution to the relevance
value that is assigned to a document. The < operator decreases the contribution and
the > operator increases it. |
| ( ) | Parentheses are used to group words into subexpressions. |
| ~ | A leading tilde acts as a negation operator, causing the word's contribution
to the document relevance to be negative. It's useful for marking noise words. A
document that contains such a word will be rated lower than others, but will not be
excluded altogether, as it would be with the - operator. |