Regular expressions are used to generate patterns of strings. A regular expression describes patterns using operators like union, concatenation, and Kleene closure. Some examples of regular expressions include: 1) Describing Canadian postal codes as /^[A-Z][0-9][A-Z] [0-9][A-Z][0-9]$/ 2) Matching strings containing the word "main" as <letter>* main <letter>* 3) Matching strings of even length as (<letter><letter>)*

1. REGULAR EXPRESSIONS
Junaid Lodhi
Roll # 131357
Air University Pakistan

2. WHY TO STUDY REGULAR
EXPRESSION?

3. WHAT IS REGULAR EXPRESSION?
regular expressions are used to generate patterns of strings. A
regular expression is an algebraic formula whose value is a
pattern consisting of a set of strings, called the language of the
expression.
This regular expression describes Canadian postal codes
/^[A-Z][0-9][A-Z] [0-9][A-Z][0-9]$/
Any language accepted by the Finite Automata can be
represented by Regular Expression

4. OPERATORS USED IN REGULAR
EXPRESSIONS
I. Union: If R1 and R2 are regular expressions, then R1 | R2 (also
written as R1 U R2 or R1 + R2) is also a regular
expression.L(R1|R2) = L(R1) U L(R2).
II. Concatenation: If R1 and R2 are regular expressions, then R1R2
(also written as R1.R2) is also a regular expression.L(R1R2) = L(R1)
concatenated with L(R2).
III. Kleene closure: If R1 is a regular expression, then R1* (the Kleene
closure of R1) is also a regular expression.L(R1*) = epsilon U L(R1)
U L(R1R1) U L(R1R1R1) U ...
NOTE:
Closure has the highest precedence, followed by concatenation,

5. EXAMPLES OF REGULAR
EXPRESSION
The set of strings over {A..Z,a..z} that contain the word "main".
Let <letter> = A | B | ... | Z | a | b | ... | z <letter>* main <letter>*

6. EXAMPLES CONTINUED
Set of EVEN Length String
Regular expression can be made as

7. EXAMPLES CONTINUED
Set of ODD Length String
Regular expression can be made as

8. EXAMPLE CONTINUED
Set of Strings Divisible by 3
Regular expression can be made as

9. PRACTICE QUESTION
Congruent to 2 mod 3
What can be the Regular Expression?