3.9. Filter
Filters specify on what data elements a rule should operate, by evaluating expressions. The expression language supports simple comparisons, logic, math, and regular expression matching.
There are also two special filters that do not evaluate expresions but depend on whether other previous filters in the style found a match:
<ElseFilter> matches if no other filter in a style matched so far, so it can be used as a fallback default.
<AlsoFilter> only matches if at least one previous filter in a style matched.
The <Filter> tag has no tag attributes.