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Lecture 04 Regular Expressions 1

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   Theory of Automata Regular Expressions Dr. Aftab Maroof  Adeel Ashraf CheemaFaryal Saud  10/6/2019Dr Aftab Maroof -Theory of Automata2 Contents   Regular Expressions   Language Defining Symbols   Alternation, Either/OR, Disjunction, Plus Sign   Defining Languages by Another New Method   Formal Definition of Regular Expressions   Product Set   Languages Associated with Regular Expressions   Finite Languages Are Regular    How Hard It Is to Understand a Regular Expression   Introducing EVEN-EVEN  10/6/2019Dr Aftab Maroof -Theory of Automata3 Regular Expressions   RE is the sequence of characters or symbols thatrepresent a finite or infinite set of text strings.   Pattern-matching   is the process of checking whether atext string conforms to a set of characteristics definedby patterns such as regular expressions.   A regular expression is a set of pattern matching rulesencoded in a string according to certain syntax rules. Although the syntax is somewhat complex it is verypowerful and allows much more useful pattern matchingthan say simple wildcards like ? and *.  10/6/2019Dr Aftab Maroof -Theory of Automata4 Regular Expression   A regular expression (sometimes abbreviated to regex ) is away for a computer user or programmer to express how acomputer program should look for a specified pattern in text andthen what the program is to do when each pattern match isfound.   For example, a regular expression could tell a program to search for  all text lines that contain the word Windows 95 and then to print out each line in which a match is found or substitute another textsequence (for example, just Windows ) where any match occurs.   The best known tool for specifying and handling the incidence of regular expressions is grep, a utility found in Unix-based operating systems and also offered as a separate utility program for Windows and other operating systems.
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