What does "Regex" mean?

Regex:

From Wikipedia: "A regular expression [regex] is a sequence of characters that specifies a search pattern. Usually such patterns are used by string-searching algorithms for 'Find' or 'Find and replace' operations on strings [of text], or for [form] input validation. It is a technique developed in theoretical computer science and formal language theory." In my experience it was very useful to learn regex because my duties included the management of the content and code of large websites. A simple example of using Regex is when you search for all files ending in ".html", by specifying *.html in a search box. The asterisk means "all files" in that instance. Regex very quickly becomes a lot more complicated than that. It is like learning a whole new search language.

 Wikipedia: Regex

 W3 Schools: Regex Examples