Regular expressions/regular language
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What is a regular expression?
A notation used to describe a pattern of characters in an object or a set
Using a and b, what is the notation used for concatenation?
• ab : meaning a string consisting of symbol a followed by symbol b • ba : meaning a string consisting of symbol b followed by symbol a
What does the notation a+ mean?
+ means pattern matches one or more “a”s going {a,aa,aaa,…}
What does the notation a* mean?
• means pattern matches zero or more “a”s going {0,a,aa,aaa,…}
What does the notation ab?c mean?
? means it matches the preceding character one or zero times, giving {ab,abc}
What does the notation a | b mean?
means pattern matches the symbol “a” or the symbol “b”
What is the series of rules to follow?
• operations + and * are highest priority and always carried out first • Concatenation is carried out next • operation | (or) is carried out last
What are the string searching applications, regular expressions can be used in?
• Searching and locating files and folders • Find or Search and Replace text strings in a block of text • Parsing or analysing syntax in computer languages • Searching for identifiers in a computer program • Validation for entry fields in a database / online form • Pattern matching with commands in an operating system
What are the definitions of the terms used for the following string searching? • Literal • Metacharacter • Escape character • Search Expression
• Literal - any character that is used in a search expression • Metacharacter - these are special characters that have a unique meaning such as: ^ or * • Escape character - using the \ (backslash) in front of a metacharacter allows it to be used in a search expression as a literal, for example: \^ • Search Expression - The regular expression that is used to find the target string
What is a regular language?
• A formal language that can be represented by regular expressions. • A language that is acceptable for use by a finite state machine • Regular languages have to be constructed from a finite number of states