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#### Presentation Transcript

**1. **Lecture 9 NFA Subset Construction & Epsilon Transitions Topics:
Thompson Construction
Examples Thompson Construction
Test 1

**2. **Last Time: Readings 2.3
Mutual Induction Proof revisited
Languages denoted by regular expressions
Examples
Ruby Regular Expressions
TEST 1 – September 29th
New: Readings section 2.2.3-2.4
Sample Test 1new
Theorem 2.12 Proof
Author’s Website Solutions Online
e-NFA ? NFA
e-NFA ? DFA subset construction
Regular Expressions
Relations Machines and Regular expressions
Ruby: regular expressions
Ruby Pickaxe Book
Ruby: showmatch.rb

**3. **Regular Expressions & Ken Thompson http://en.wikipedia.org/wiki/Regular_expression
http://en.wikipedia.org/wiki/Ken_Thompson

**4. **Grep Unix utility
man grep
man –k regexp

**5. **Thompson Construction Based on recursive (inductive) definition of regular expressions
We describe NFAs (with epsilon moves) that recognize the base cases.
Then assuming we have NFAs for smaller expressions r and s we construct NFAs for
r + s
rs
r*

**6. **Recall Recursive Definition of Reg Expr Definition of regular expressions over an alphabet S
Base cases:
if a e S then A is a regular expression and denotes L(a) = { a }
e is a regular expression and denotes L(e) = { e }
Recursive definition
If r and s are regular expressions denoting the languages L(r) and L(s) then
rs is a regular expression denoting L(r)L(s)
r+s is a regular expression denoting L(r) L(s)
r* is a regular expression denoting [L(r)]* =

**7. **Thompson Construction Base cases

**8. **Thompson Construction Recursive cases r + s rs

**9. **Thompson Construction Recursive cases r*

**10. **Thompson Construction Examples

**13. **Sample test 1 outline Proof Techniques
Inductive proof
mutual induction proof
Given DFA
Transition diagram
Input “abaa”
L(M)
Give DFA for L NFA
NFA for L
NFA ? DFA (Subset)
eNFA
eNFA for L
e-closure (ECLOSE in text)
eNFA ? DFA (Subset)
Ruby regular expressions

**14. **Design DFA that accepts Language L Example
For a DFA D how do you prove L(D) = L ?

**15. **Mutual Induction Proof Define three statements for a mutual induction proof that could help in proving that L(M) = L ={x e {0, 1}* | that x has a number of zeroes divisible by 3 }

**18. **HW solutions 2.5.1

**19. **2.5.3 a,b The set of strings consisting of zero or more a’s followed by zero or more b’s followed by zero or more c’s
The set of strings consisting of either 01 repeated one or more times or 010 repeated one of more times

**20. **2.3.2 Subset NFA without e NFA DFA

**21. **2.3.4 Done before The set of strings over {0,1, … 9} such that the final digit has appeared before
The set of strings over {0,1, … 9} such that the final digit has not appeared before
The set of strings of 0’s and 1’s such that there are two 0’s separated by a number of positions that is a multiple of 4. Note 0 is allowable multiple.

**22. **Tenth symbol from the right is a ‘1’

**23. **Given L(M1) and L(M2) define DFA for intersection

**25. **Pop Quiz Induction proof

**28. **Homework: 2.5.1
2.5.3 a,b
Extra Credit (20pts): Modify dfa1.rb to print all strings of length<8 that would be accepted by that DFA.