Let us see the Regular Expression in compiler construction and automata theory.

**Regular Expression for the Language of all strings with an even number of 0’s or even number of 1’s**

**Regular Expression : ****(1*01*01*)* + (0*10*10*)***

**Reject able strings (not part of the language)**

These strings are not part of the given language and must be rejected by our Regular Expression.

- strings of length 1 = {0, 1, no more strings}
- strings of length 2 = {10, 01, no more strings}
- strings of length 3 = {000,111, no more strings}
- strings of length 4 = {0001, 1000, 0111,…….. and many similar strings}
- strings of length 7 = {0000000, 1111111, …….. and many similar strings }
- strings of length 10 = {0000011111, 1111100000, 1010101010,…….. and many similar strings }
- strings of length 15 = {000000000000000, 111111111111111, ,…….. and many similar strings }
- strings of length 20 = {00000000000001111111, 00000011110000011111, 11111111111111100000,…….. and many similar strings }
- strings of length 25 = {0000000000000000000000000, 1111111111111111111111111, ,…….. and many similar strings }
- And many similar strings

**Acceptable strings (part of the language)**

These strings are part of the given language and must be accepted by our Regular Expression.

- strings of length 1 = {no string}
- strings of length 2 = {00, 11, no more strings}
- strings of length 3 = {100,110,010}
- strings of length 4 = {0000, 1111, 1010,…….. and many similar strings }
- strings of length 7 = {1010100, 0101011, 0010000,…….. and many similar strings }
- strings of length 10 = {0000001111, 1111000000, 0011111111,…….. and many similar strings }
- strings of length 15 = {000001111100000, 111110000011111, 101010101010101,…….. and many similar strings }
- strings of length 20 = {00000000001111111111, 00000111100000111111, 11111100000000000000,…….. and many similar strings }
- strings of length 25 = {0000000000000000000011111, 1111111111000000000000000, 0000011111000001111111111,…….. and many similar strings }
- And many similar strings

## Regular Expression for the Language of all strings with an even number of a’s or even number of b’s

**Regular Expression : ****(b*ab*ab*)* + (a*ba*ba*)***

**Reject able strings (not part of the language)**

These strings are not part of the given language and must be rejected by our Regular Expression.

- strings of length 1 = {a, b, no more strings}
- strings of length 2 = {ba, ab, no more strings}
- strings of length 3 = {aaa,bbb, no more strings}
- strings of length 4 = {aaab, baaa, abbb,…….. and many similar strings}
- strings of length 7 = {aaaaaaa, bbbbbbb, …….. and many similar strings }
- strings of length 10 = {aaaaabbbbb, bbbbbaaaaa, bababababa,…….. and many similar strings }
- strings of length 15 = {aaaaaaaaaaaaaaa, bbbbbbbbbbbbbbb, ,…….. and many similar strings }
- strings of length 20 = {aaaaaaaaaaaaabbbbbbb, aaaaaabbbbaaaaabbbbb, bbbbbbbbbbbbbbbaaaaa,…….. and many similar strings }
- strings of length 25 = {aaaaaaaaaaaaaaaaaaaaaaaaa, bbbbbbbbbbbbbbbbbbbbbbbbb, ,…….. and many similar strings }
- And many similar strings

**Acceptable strings (part of the language)**

These strings are part of the given language and must be accepted by our Regular Expression.

- strings of length 1 = {no string}
- strings of length 2 = {aa, bb, no more strings}
- strings of length 3 = {baa,bba,aba}
- strings of length 4 = {aaaa, bbbb, baba,…….. and many similar strings }
- strings of length 7 = {bababaa, abababb, aabaaaa,…….. and many similar strings }
- strings of length 10 = {aaaaaabbbb, bbbbaaaaaa, aabbbbbbbb,…….. and many similar strings }
- strings of length 15 = {aaaaabbbbbaaaaa, bbbbbaaaaabbbbb, bababababababab,…….. and many similar strings }
- strings of length 20 = {aaaaaaaaaabbbbbbbbbb, aaaaabbbbaaaaabbbbbb, bbbbbbaaaaaaaaaaaaaa,…….. and many similar strings }
- strings of length 25 = {aaaaaaaaaaaaaaaaaaaabbbbb, bbbbbbbbbbaaaaaaaaaaaaaaa, aaaaabbbbbaaaaabbbbbbbbbb,…….. and many similar strings }
- And many similar strings

## More Examples of Regular Expression

- Regular Expression for no 0 or many triples of 0’s and many 1 in the strings.
- RegExp for strings of one or many 11 or no 11.
- A regular expression for ending with abb
- A regular expression for all strings having 010 or 101.
- Regular expression for Even Length Strings defined over {a,b}
- Regular Expression for strings having at least one double 0 or double 1.
- Regular Expression of starting with 0 and having multiple even 1’s or no 1.
- Regular Expression for an odd number of 0’s or an odd number of 1’s in the strings.
- Regular Expression for having strings of multiple double 1’s or null.
- Regular Expression (RE) for starting with 0 and ending with 1.
- RE for ending with b and having zero or multiple sets of aa and bb.
- A regular expression of the second last symbol is 1.
- RE for starting with 1 having zero or multiple even 1’s.
- Regular Expression for multiple a’s and multiple b’s.
- RE for exactly single 1 many 0’s |exactly single a many b.
- A regular expression for strings starting with aa and ending with ba.
- A regular expression for the language of all consecutive even length a’s.
- A regular expression for the language of all odd-length strings
- A regular expression for the language of all even length strings but ends with aa.
- A regular expression for the language of an odd number of 1s.
- A regular expression for the language of even length strings starting with a and ending with b in theory of automata.
- A regular expression for the language of all even length strings but starts with a.
- A Regular Expression for the Language of all strings with an even number of 0’s or even number of 1’s.
- A regular expression for the language of all those strings end with abb.
- A regular expression for string having must 010 or 101.
- Regular expression of strings begin with 110

Regular expression of strings begin and end with 110

Regular expression of strings containing exactly three consecutive 1’s. - A Regular Expression of all strings divisible by 4.
- A Regular Expression Strings that does not contain substring 110.

## Tutorial: Regular Expression

A detailed tutorial of the regular expression is here in the link of regular expression tutorial. This page contains the practice questions of regular expressions with solutions.

**Tutorial covering the topics**

- Give a regular expression.
- Describe the strings of the regular expression.
- write a regular expression.
- create all strings from regular expression.
- Generate all strings from regular expression.
- Extract all strings from regular expression.
- Find all strings from regular expression.
- Examples of regular expression.