<<.def "Regular expressions">> are concise strings of characters that denote patterns of text to search for. The format used in ~TiddlyWiki is fully defined in [[this Mozilla reference|https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions]].
The [[filter syntax|Filter Run]] makes it impossible to directly specify a regular expression that contains square brackets. The solution is to store the expression in a [[variable|Variables]]. See the <<.operator-examples "regexp" "examples">>.