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

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

More notes:
 * "Regular expression" is 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.