## Regular Expression for strings having at least one double 0 or double 1

Let us see the Regular Expression for strings having at least one double 0 or double 1 defined over {0,1}.

**Rule**

All strings having at least one double 0 or double 1 must be accepted and all other strings must be rejected by our Regular Expression.

**Regular Expression = (0+1)* (00+11) (0+1)***

## DFA of Language having at least one double 0 or double 1

**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.

- 3 strings of length 1 = {1, 0, no more string}
- 3 strings of length 2 = {01, 10, no more string}
- 3 strings of length 3= {101,010, no more string}
- 3 strings of length 4 = {1010, 0101, no more string}
- 3 strings of length 7 = {0101010, 1010101, no more string}
- 3 strings of length 10 = {1010101010, 0101010101, no more string}
- 3 strings of length 15 = {101010101010101, 010101010101010, no more strings}
- 3 strings of length 20 = {01010101010101010101, 10101010101010101010, no more string}

3 strings of length 25 = {1010101010101010101010101, 0101010101010101010101010, no more string} - Many more similar strings.

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

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

- 3 strings of length 1 = {no string}
- 3 strings of length 2 = {00, 11, no more string}
- 3 strings of length 3= {000,111,001, and many more similar strings}
- 3 strings of length 4 = {0000, 1111, 0110, and many more similar strings}
- 3 strings of length 7 = {0010001, 1101110, 0100001, and many more similar strings}
- 3 strings of length 10 = {0011001010, 1010111000, 0111110010, and many more similar strings}
- 3 strings of length 15 = {000001111100110, 110011001011001, 111111111110010, and many more similar strings}
- 3 strings of length 20= {00001111001111000010, 11001111000001111001, 10101010101101101001, and many more similar strings}

3 strings of length 25= {0010101000110101110011000, 1111100000100100110100001, 0101010101010110110000001, and many more similar strings} - Many more similar strings.

## Regular Expression for strings having at least one double a or double b

Let us see the Regular Expression for strings having at least one double a or double b defined over {a,b}.

**Rule**

All strings having at least one double a or double b must be accepted and all other strings must be rejected by our Regular Expression.

**Regular Expression = (a+b)* (aa+bb) (a+b)***

**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.

- 3 strings of length 1 = {b, a, no more string}
- 3 strings of length 2 = {ab, ba, no more string}
- 3 strings of length 3= {bab,aba, no more string}
- 3 strings of length 4 = {baba, abab, no more string}
- 3 strings of length 7 = {abababa, bababab, no more string}
- 3 strings of length 10 = {bababababa, ababababab, no more string}
- 3 strings of length 15 = {bababababababab, abababababababa, no more strings}
- 3 strings of length 20 = {abababababababababab, babababababababababa, no more string}

3 strings of length 25 = {babababababababababababab, ababababababababababababa, no more string} - Many more similar strings.

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

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

- 3 strings of length 1 = {no string}
- 3 strings of length 2 = {aa, bb, no more string}
- 3 strings of length 3= {aaa,bbb,aab, and many more similar strings}
- 3 strings of length 4 = {aaaa, bbbb, abba, and many more similar strings}
- 3 strings of length 7 = {aabaaab, bbabbba, abaaaab, and many more similar strings}
- 3 strings of length 10 = {aabbaababa, bababbbaaa, abbbbbaaba, and many more similar strings}
- 3 strings of length 15 = {aaaaabbbbbaabba, bbaabbaababbaab, bbbbbbbbbbbaaba, and many more similar strings}
- 3 strings of length 20= {aaaabbbbaabbbbaaaaba, bbaabbbbaaaaabbbbaab, babababababbabbabaab, and many more similar strings}

3 strings of length 25= {aabababaaabbababbbaabbaaa, bbbbbaaaaabaabaabbabaaaab, abababababababbabbaaaaaab, and many more similar strings} - Many more 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.