<p>A quick check of the sources reveals that <a href="http://www.diku.dk/hjemmesider/ansatte/henglein/papers/chomsky1959.pdf">Chomsky</a> called the levels of his hierarchy just “type 0, type 1, type 2, type 3”. He mentions in a footnote that his type 3 corresponds to “regular events” of <a href="http://www.rand.org/pubs/research_memoranda/2008/RM704.pdf">Kleene</a>. Kleene wrote there: <em>We shall presently describe a class of events which we will call "regular events." (We would welcome any suggestions as to a more descriptive term.)</em></p>

<p>It would thus appear that the term is a historical accident, and in any case has no bearing on the relation of regular languages to context-free languages.</p>

CC BY-SA 3.0
2011-10-26T17:06:14.420Z
The Kleene's paper (an awesome paper, thank you!) contrasts "regular" events with "indefinite" events (that depend on another event that may have happened in an infinite past) and "definite" events (that depend on at most $p-l-1$ past moments). "Regular" events are those who depend on "the value" of the last moment $p$.
gioele
CC BY-SA 3.0
2011-10-26T17:07:53.403Z
The paper also shows that we must thank Kleen if regular languages are called "regular" and not "prehensible".
gioele
CC BY-SA 3.0
2011-10-26T16:49:07.047Z
2011-10-26T16:49:07.047Z
Emil Jeřábek
<p>Why are regular languages (and from that regular expressions) called "regular"? There is lot of <em>regularity</em> also in context-free languages other types of languages.</p>

<p>I suppose that, in the beginning, the adjective "regular" has been used to differentiate that type of languages from other "non-regular" or somehow abnormal languages. If so, what where these other types, and what was their <em>non-regularity</em>?</p>

CC BY-SA 3.0
2011-10-27T01:05:13.517Z
no better reason than the reason why normal subgroups are called "normal"?
Sasho Nikolov
CC BY-SA 3.0
2016-03-18T17:20:23.900Z
@SashoNikolov: http://math.stackexchange.com/questions/898977/why-are-normal-subgroups-called-normal
Joshua Grochow
<regular-expressions><regular-language><ho.history-overview>
Why are regular languages called "regular"?
gioele
# Why are regular languages called "regular"?

(Asked by: username_0 on 2011-10-26)

Why are regular languages (and from that regular expressions) called "regular"? There is lot of *regularity* also in context-free languages other types of languages.

I suppose that, in the beginning, the adjective "regular" has been used to differentiate that type of languages from other "non-regular" or somehow abnormal languages. If so, what where these other types, and what was their *non-regularity*?

**Tags:** regular-expressions, regular-language, ho.history-overview

> **Comments:**
> **username_1**: no better reason than the reason why normal subgroups are called "normal"?  
> **username_2**: @SashoNikolov: http://math.stackexchange.com/questions/898977/why-are-normal-subgroups-called-normal  

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## 1 answers

### Answer by username_3 on 2011-10-26. Score: 41 | Accepted answer

A quick check of the sources reveals that [Chomsky](http://www.diku.dk/hjemmesider/ansatte/henglein/papers/chomsky1959.pdf) called the levels of his hierarchy just “type 0, type 1, type 2, type 3”. He mentions in a footnote that his type 3 corresponds to “regular events” of [Kleene](http://www.rand.org/pubs/research_memoranda/2008/RM704.pdf). Kleene wrote there: *We shall presently describe a class of events which we will call "regular events." (We would welcome any suggestions as to a more descriptive term.)*

It would thus appear that the term is a historical accident, and in any case has no bearing on the relation of regular languages to context-free languages.

> **Comments:**
> **username_0**: The Kleene's paper (an awesome paper, thank you!) contrasts "regular" events with "indefinite" events (that depend on another event that may have happened in an infinite past) and "definite" events (that depend on at most $p-l-1$ past moments). "Regular" events are those who depend on "the value" of the last moment $p$.  
> **username_0**: The paper also shows that we must thank Kleen if regular languages are called "regular" and not "prehensible".  

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