# Closure Properties of regular expressions [Union Intersection Concatenation]

## Closure Properties of Regular Expressions

• Union
• Intersection
• concatenation
• Kleene closure
• Complement

## Union Of Regular Expression With Examples

If RE1 and RE2 are regular, then RE1 union RE2 is also a regular expression. This can be written as;

• RE1 | RE2
• RE1 U RE2
• RE1 + RE2

## Examples of Union Operation Of Regular Expression

Example 1

RE1= a(b)*

RE2 = b(a)*

RE1 + RE2= a(b)* + b(a)*

Result: Regular expressions are closed under union operation.

Example 2

Regular expression 1:  a

Regular expression 2:  b

Regular expression 1 + Regular expression 2 = a+b

## More examples of Union as a closure property Of Regular Expression

Regular expression 1:  a(a)

Regular expression 2:  b(a+b)

Regular expression 1 + Regular expression 2 = a(a)+ b(a+b)

Regular expression 1:  b(aa+b)

Regular expression 2:  b(aa+b)

Regular expression 1 + Regular expression 2 = b(aa+b) + b(aa+b)

Regular expression 1:  a(aa+bb)*

Regular expression 2:  b(aa)

Regular expression 1 + Regular expression 2 = a(aa+bb)*+ b(aa)

## Intersection Of Regular Expression With Examples

If RE1 and RE2 are regular, then

RE1 ∩  RE2

is also a regular expression.

## Example 1:

RE1 = b(b*)

RE2 = (bb)*

RE1 ∩ RE2=  bb(bb)*

Explanation:

L1 = { b,bb, bbb, bbbb, ….} (Strings of all possible lengths of b without Null)

L2 = { ε, bb, bbbb, bbbbbb,…….} (Strings of even length of b including Null)

L1 ∩ L2 = { bb, bbbb, bbbbbb,…….} (Strings of even length of b excluding Null)

# Concatenation Of Regular Expression With Examples

If RE1 and RE2 are regular, then

RE1.RE2

is also a regular expression.

## Example 1:

Regular Expression 1 = ( 00 + 10 )

strings are = {00, 10 only}

Regular Expression 2 = ( 0 + 1 )

strings are = {0, 1 only}

Regular Expression 1 ◦Regular Expression 2 = ( 00 + 10 )( 0 + 1 )

strings are = {000, 001, 100, 101}

## Example 2:

Regular Expression 1 = (0+1)*0

set of strings ending in 0

Regular Expression 2 = 01(0+1)*

set of strings beginning with 01

Regular Expression 1Regular Expression 2 = 01(0+1)* (0+1)*0 which can be written in more optimized way as;

01(0+1)*0

Regular Expression Basics and rules