2
Agenda
Today Regular languages
Finite languages are regular Regular operations on languages
Union () Concatenation () Kleene star (*)
For next time: Read 1.3 and handout on minimization
Thursday, 9/20 (revised ): HW1 collected
3
Definition of Regular Language
Recall the definition of a regular language:DEF: The language accepted by an FA M
is the set of all strings which are accepted by M and is denoted by L (M).
Would like to understand what types of languages are regular. Languages of this type are amenable to super-fast recognition of their elements
Would be nice to know for example, which of the following are regular:
4
Language Examples
Unary prime numbers:{ 11, 111, 11111, 1111111, 11111111111, … }= {12, 13, 15, 17, 111, 113, … }= { 1p | p is a prime number }
Unary squares:{, 1, 14, 19, 116, 125, 136, … }= { 1n | n is a perfect square }
Palindromic bit strings:{, 0, 1, 00, 11, 000, 010, 101, 111, …} = {x {0,1}* | x = xR } o
Will explore whether or not these are regular in future.
5
Finite Languages
All the previous examples had the following property in common: infinite cardinality
NOTE: The strings which made up the language were finite (as they always will be in this course); however, the collection of such strings was infinite.
Before looking at infinite languages, should definitely look at finite languages.
6
Languages of Cardinality 1
Q: Is the singleton language containing one string regular? For example, is
{ banana }regular?
8
Languages of Cardinality 1
A: Nothing, really. This an example of a nondeterministic FA. This turns out to be the most concise way to encapsulate the language { banana }
But we will deal with nondeterminism in coming lectures. So:
Q: Is there a way of fixing this and making it deterministic?
9
Languages of Cardinality 1A: Yes, just add a fail state q7; I.e., put
a state that sucks in all strings different from “banana” for all eternity –unless they happen to be the “banana” prefixes {, b, ba, ban, bana, banan}.
13
Arbitrary Finite Number of Strings
Q1: How about more? For example{ banana, nab, ban, babba } ?
Q2: Or less (the empty set):Ø = {} ?
15
Arbitrary Finite Number of Strings: Empty Language
A2: Build a 1-state automaton whose accept states set F is empty!
16
Arbitrary Finite Number of Strings
THM: All finite languages are regular.Proof : Can always construct a tree whose
leaves are word-ending. In our example the tree is:
Now make word endings into accept states, add a fail sink-state and add links to the fail state to finish the construction. �
b
a a
b
a
n b
a
n
b
a
n
18
Infinite Cardinality
A: No! Many infinite languages are regular. Common Mistake 1: The strings of regular
languages are finite, therefore the regular languages must be finite.
Common Mistake 2: Regular languages are –by definition– accepted by finite automata, therefore regular languages are finite.
Q: Give an example of a infinite but regular language.
19
Infinite Cardinality bit strings with an even number of b’s
Simplest example is
many, many moreHome exercise: think of a criterion for
non-finiteness
20
Regular OperationsYou may have come across the regular
operations when doing advanced searches utilizing programs such as emacs, egrep, perl, python, etc. There are three basic operations we will work with:
1. Union2. Concatenation3. Kleene-starAnd a fourth definable in terms of the previous:4. Kleene-plus
21
Regular Operations – Summarizing Table
Operation
Symbol
UNIX version
Meaning
Union | match one of the patterns
Concatenation implicit in
UNIX
match patterns in sequence
Kleene-star
* *Match pattern
0 or more times
Kleene-plus
+ +Match pattern
1 or more times
22
Regular operations - Union
UNIX: to search for all lines containing vowels in a text one could use the command
egrep -i `a|e|i|o|u’
Here the pattern “vowel ” is matched by any line containing one of a, e, i, o or u.
Q: What is a string pattern?
23
String Patterns
A: A good way to define a pattern is as a set of strings, i.e. a language. The language for a given pattern is the set of all strings satisfying the predicate of the pattern.
EG: vowel-pattern = { the set of strings which
contain at least one of: a e i o u }
24
UNIX patterns vs. Computability patterns
In UNIX, a pattern is implicitly assumed to occur as a substring of the matched strings.
In our course, however, a pattern needs to specify the whole string, and not just a substring.
25
Regular operations - Union
Computability: union is exactly what we expect. If you have patterns
A = {aardvark}, B = {bobcat}, C = {chimpanzee}
union the patterns together to getAB C = {aardvark, bobcat,
chimpanzee}
26
Regular operations - Concatenation
UNIX: to search for all consecutive double occurrences of vowels, use:egrep -i `(a|e|i|o|u)(a|e|i|o|u)’
Here the pattern “vowel ” has been repeated. Parentheses have been introduced to specify where exactly in the pattern the concatenation is occurring.
27
Regular operations - Concatenation
Computability. Consider the previous result:
L = {aardvark, bobcat, chimpanzee}
Q: What language results when we concatenate L with itself obtaining
LL ?
28
Regular operations - Concatenation
A: LL = {aardvark, bobcat, chimpanzee}{aardvark, bobcat,
chimpanzee}
={aardvarkaardvark, aardvarkbobcat, aardvarkchimpanzee, bobcataardvark, bobcatbobcat, bobcatchimpanzee, chimpanzeeaardvark, chimpanzeebobcat,
chimpanzeechimpanzee}
Q1: What is L ?
Q2: What is LØ ?
29
Algebra of LanguagesA1: L = L. In general, is the identity in
the “algebra” of languages. I.e., if we think of concatenation as being like multiplication, acts like the number 1.
A2: LØ = Ø. Opposite to , Ø acts like the number zero obliterating everything it is concatenated with.
Note: We can carry on the analogy between numbers and languages. Addition becomes union, multiplication becomes concatenation. This forms a so-called “algebra”.
30
Regular operations – Kleene-*
UNIX: search for lines consisting purely of vowels (including the empty line):
egrep -i `^(a|e|i|o|u)*$’
NOTE: ^ and $ are special symbols in UNIX regular expressions which respectively anchor the pattern at the beginning and end of a line. The trick above can be used to convert any Computability regular expression into an equivalent UNIX form.
31
Regular operations – Kleene-*
Computability: Suppose we have a language
B = { ba, na }
Q: What is the language B * ?
32
Regular operations – Kleene-*
A:B * = { ba, na }*= { ,
ba, na, baba, bana, naba, nana, bababa, babana, banaba, banana, nababa, nabana, nanaba, nanana, babababa, bababana, … }
33
Regular operations – Kleene-+
Kleene-+ is just like Kleene-* except that the pattern is forced to occur at least once.
UNIX: search for lines consisting purely of vowels (not including the empty line):
egrep -i `^(a|e|i|o|u)+$’
Computability: B+ = { ba, na }+= { ba, na, baba, bana, naba, nana, bababa, babana, banaba, banana, nababa, nabana, nanaba, nanana, babababa, bababana, … }
34
Generating the Regular Languages
The real reason that regular languages are called regular is the following:
THM: The regular languages are all those languages which can be generated starting from the finite languages by applying the regular operations.
This will be proved in the coming lectures.Q: Can we start with even more basic
languages than arbitrary finite languages?
35
Generating the Regular LanguagesA: Yes. We can start with languages
consisting of single strings which are themselves just a single character. These are the “atomic” regular languages.
EG: To generate the finite language L = { banana, nab }
we can start with the atomic languages A = {a}, B = {b}, N = {n}.
Then we can express L as:
L = (B A N A N A) (N A B )