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

## 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.

## 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.