Regular Expressions in Compiler Design: A Comprehensive Guide, Schemes and Mind Maps of Computer science

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Regular Expression

Course Name: Compiler Design Course Code: CSE Level:3, Term: Department of Computer Science and Engineering Daffodil International University

A Regular expression (sometimes called a rational expression) is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. “find and replace”-like operations.

Regular expression is a notation for defining the set of tokens that

normally occur in programming languages.

Regular Expression

• Letters or alphabets and digits are the most important elements of language.
• Let L be the set of alphabets {A, B,…Z, a, b,….z} and D be the set of digits { 0 , 1 , …., 9 }
• L could be in form of upper case and lower case.
• Examples: L U D is the set of letters and digits. LD is the set of strings consisting of a letter followed by a digit.
• LLLL = L 4 is the set of all four-letter strings.

Operations of Language

• L

is the set of all strings of letters, including ε, the empty string

• L(L U D)
  • is the set of all strings of letters and digits beginning with a letter.
• D
  • is the set of all strings of one or more digits.

Operations of Language

• (a|b)
  • or (a * |b * ) *
• Denotes the set of all strings containing zero or more instances of an a or b, that is, the set of all strings of a’s and b’s.
• a | a
  • b
• Denotes the set containing the string a and all strings consisting of zero or more a’s followed by a b

Examples

Examples:

• “Set of all strings having at least one ab”  (ab)
• “Set of all strings having even number of aa”  (aa)
• “Set of all strings having odd number of bb”  b(bb)
• “Set of all strings having even number of aa and even number of bb”  (aa)
  • (bb) * Language to Regular Expressions

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