A <<.def "filter operator">> is a predefined keyword attached to an individual [[step|Filter Step]] of a [[filter|Filters]]. It defines the particular action of that step.
''Important:'' Each first [[step|Filter Step]] of a [[filter run|Filter Run]] not given any input titles receives the output of <$link to="all Operator">[all[tiddlers]]</$link> as its input.
The following table lists all core operators, the most common ones marked ✓. The last column indicates whether an operator allows ''negation'' using the <$link to="Filter Step"><code>!</code> prefix</$link>. For specifics as to each operator's negated output please refer to its documentation.
Most steps process the [[selection of titles|Title Selection]] that are supplied as their input, but a few [[construct an entirely new selection|Selection Constructors]] instead. For the exact rules, see [[Filter Syntax]].