# Regular Expression in compiler construction

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

## 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

## 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.
Prof.Fazal Rehman Shamil (Available for Professional Discussions)
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