When did "regular" start referring to Type 3 languages/grammars? In his 1959 paper, On Certain Formal Properties of Grammars, Chomsky defined a "regular" grammar as a specific form of a type 2 (context-free) grammar. (See Definition 8 of that paper.) He then goes on (in Section 6) to demonstrate that regular (type 2, context-free) grammars can generate more languages than type 3 (finite state) grammars.

However, in every other computer-science reference I've read, type 3 languages (those recognizable by a finite state machine) are called "regular". (For example, see the Wikipedia entry for Chomsky Hierarchy.)

My question is, when did "regular" shift from referring to type 2 languages to type 3 languages? And why? (Was it intentional, or did someone mis-quote Chomsky at some point and that stuck?)
I found an answer, at least a partial one, hinted at in Footnote 10 of Chomsky's paper where he refers to a 1956 paper by Kleene in which Kleene describes &quot;regular events&quot; -- a language recognized by finite state machines.
So it would seem the common usage of &quot;regular language&quot; today traces back to Kleene's &quot;regular events&quot; rather than to Chomsky's &quot;regular grammar&quot;.
Furthermore, as renierpost pointed out in a comment, what Chomsky called a &quot;regular&quot; grammar in 1959 is now known as Chomsky normal form.
Edit: I asked Professor Chomsky via email whether he intended his &quot;regular grammar&quot; to have a connection to Kleene's &quot;regular events&quot;. He replied: &quot;Long time ago and I don't recall. I suppose just a coincidence.&quot;