Why are regular expressions called “regular” expressions?

Question

Why are regular expressions called regular expressions?

Answer 1

Why are they called "regular expressions?"

Regular expressions trace back to the work of an American mathematician by the name of Stephen Kleene (one of the most influential figures in the development of theoretical computer science) who developed regular expressions as a notation for describing what he called "the algebra of regular sets." His work eventually found its way into some early efforts with computational search algorithms, and from there to some of the earliest text-manipulation tools on the Unix platform (including ed and grep). In the context of computer searches, the "*" is formally known as a "Kleene star."

From here.