Regular expression

Regular expression

A regular expression is a sequence of pattern that defines a string.

The language accepted by finite automata can be easily described by simple expressions called Regular Expressions.

Operators of Regular expression

The definition of regular expression includes three basic operators: 

  1. Union
  2. Concatenation
  3. Closure
1. Union: If p and q are regular expressions, then p+q is a regular expression denoting the union of L(p) and L(q), that is, L(p+q) = L(p) U L(q)

2. Concatenation: If p and q are regular expressions, then p.q is a regular expression denoting the concatenation of L(p) and L(q), that is, L(pq) = L(p) L(q).

3. Closure: If p is regular expression, then so is p*, denoting the closure of L(p), that is L(p*) = (L(p))*.

Some regular expressions and its language

Regular expression


r = a

L(r) = {a}

r = ab

L(r) = {ab}

r = a+b

L(r) = {a, b}

r = a*

L(r) = {, a, aa, aaa, …}

r = ab*

L(r) = {a, ab, abb, abbb,….}

r = (ab)*

L(r) = {∈, ab, abab, ababab, …}

r = a(a+b)

L(r) = {aa, ab}

a* VS a+
a= a power * means, a may not exist or may exist 
a= a power + means, a exist atleast once. 

Characterstics of regular expression

1. Regular expression is language defining notation in terms of algebraic description.
2. The languages accepted by finite automata, regular language, is easily described by simple 3. expressions called as regular expression. 
4. It is more precise and formal as compared to any natural language. 
5. In contrast to the transition graph, regular expressions can be conveniently written out in a line from left to right.
6. Main two areas of application of regular expression are: Lexical analysis (compilers) and text editors.
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Previous years solved papers:
A list of Video lectures References:
  1. Sebesta,”Concept of programming Language”, Pearson Edu 
  2. Louden, “Programming Languages: Principles & Practices” , Cengage Learning 
  3. Tucker, “Programming Languages: Principles and paradigms “, Tata McGraw –Hill. 
  4. E Horowitz, "Programming Languages", 2nd Edition, Addison Wesley

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    1. Structure of desktop computers
    2. Logic gates
    3. Register organization
    4. Bus structure
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    7. Direct mapping numericals
    8. Register in Assembly Language Programming
    9. Arrays in Assembly Language Programming


    1. William stalling ,“Computer Architecture and Organization” PHI
    2. Morris Mano , “Computer System Organization ”PHI

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    2. Framing
    3. Byte count framing method
    4. Flag bytes with byte stuffing framing method
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    7. Error control in data link layer
    8. Stop and Wait scheme
    9. Sliding Window Protocol
    10. One bit sliding window protocol
    11. A protocol Using Go-Back-N
    12. Selective repeat protocol
    13. Application layer
    1. Andrew S. Tanenbaum, David J. Wetherall, “Computer Networks” Pearson Education.
    2. Douglas E Comer, “Internetworking with TCP/IP Principles, Protocols, And Architecture",Pearson Education
    3. KavehPahlavan, Prashant Krishnamurthy, “Networking Fundamentals”, Wiley Publication.
    4. Ying-Dar Lin, Ren-Hung Hwang, Fred Baker, “Computer Networks: An Open Source Approach”, McGraw Hill.