MediaWiki:Wikieditor-toolbar-tool-replace-invalidregex/qqq

Parameters:
 * $1 - a browser-supplied error message, probably in English, possibly in the browser language.

A "regular expression" is a syntax to find strings based on patterns, for more see the Wikipedia article.

Regular expressions are mostly a programmer tool, which is sometimes exposed to normal users too.

It's almost as a programming language, but it is used to describe different kinds of strings (sometimes also called patterns and pattern matching), and is often used to change (parts of) the strings too.