Filter:
Regular expression for filtering the search results. It is possible to have several expressions each restricted to a specific data field, by preceding the expression with the target field's code prefixed with an ⟪@⟫ symbol.
For example, ⟪bird @eng_lem ^$⟫ will search for all entries containing ⟪bird⟫ in any field, and additionally that have an empty ⟪eng_lem⟫ (English keyword/lemma) field, ⟪^$⟫ being the regular expression for matching empty strings.