Regular expressions for all strings with at least two a

Write a Regular expressions defined over {a, b} for all strings with at least two a’s.

(a + b)* a (a + b)* a (a + b)*

Valid strings: (must be accepted by our Regular Expression)

Strings of length 1: No valid strings

Strings of length 2: aa, No other valid strings

Strings of length 3: aaa, aab, baa, No other valid strings

 Strings of length 4: aaaa, baab, aaab, baaa,……. and many other similar strings.

Strings of length 5: aaaab, aaaba, aaabb, abba, aabaa, bbbaa,………and many other similar strings.

Note: Many other strings of lengths 6, 7, 8. 9, 10 11, ….etc that have at least two a. You can try it by yourself.

Invalid strings: (must be Rejected by our Regular Expression)

Strings of length 1: b, a No other invalid strings

Strings of length 2: bb, ab, ba,  No other invalid strings

Strings of length 3: abb, bab, bbb, bba,  bbb, No other invalid strings

Note: All other strings defined over {a, b) that not have at least two a’s are invalid strings. You can try it by yourself.

Write a Regular expressions defined over {a, b} for all strings with at least two b’s.

(a + b)* b (a + b)* b (a + b)*

Write a Regular expressions defined over {0, 1} for all strings with at least two 0’s.

(0 + 1)* 0 (0 + 1)* 0 (0 + 1)*

Write a Regular expressions defined over {0, 1} for all strings with at least two 1’s.

(0 + 1)* 1 (0 + 1)* 1 (0 + 1)*

More Examples of Regular Expression 

1. Regular Expression for no 0 or many triples of 0’s and many 1 in the strings.
2. RegExp for strings of one or many 11 or no 11.
3. Regular expressions for all non empty strings
4. Regular expressions over {a, b} for all non empty strings of even length
5. Regular expressions for all non empty strings of odd length
6. A regular expression for ending with abb
7. A regular expression for all strings having 010 or 101.
8. Regular expression for Even Length Strings defined over {a,b}
9. Regular Expression for strings having at least one double 0 or double 1.
10. Regular Expression of starting with 0 and having multiple even 1’s or no 1.
11. Regular Expression for an odd number of 0’s or an odd number of 1’s in the strings.
12. Regular Expression for having strings of multiple double 1’s or null.
13. Regular Expression (RE) for starting with 0 and ending with 1.
14. RE for ending with b and having zero or multiple sets of aa and bb.
15. A regular expression of the second last symbol is 1.
16. RE for starting with 1 having zero or multiple even 1’s.
17. Regular Expression for multiple a’s and multiple b’s.
18. RE for exactly single 1 many 0’s |exactly single a many b.
19. A regular expression for strings starting with aa and ending with ba.
20. A regular expression for the language of all consecutive even length a’s.
21. A regular expression for the language of all odd-length strings
22. A regular expression for the language of all even length strings but ends with aa.
23. A regular expression for the language of an odd number of 1s.
24. A regular expression for the language of even length strings starting with a and ending with b in theory of automata.
25. A regular expression for the language of all even length strings but starts with a.
26. A Regular Expression for the Language of all strings with an even number of 0’s or even number of 1’s.
27. A regular expression for the language of all those strings end with abb.
28. A regular expression for string having must 010 or 101.
29. 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.
30. A Regular Expression of all strings divisible by 4.
31. A Regular Expression Strings that does not contain substring 110.
32. Regular expressions for all strings with at least one a
33. Regular expressions for all strings with at least two a’s