Regular expressions/regular language

Question 1

What is a regular expression?

Answer 1

A notation used to describe a pattern of characters in an object or a set

Question 2

Using a and b, what is the notation used for concatenation?

Answer 2

• ab : meaning a string consisting of symbol a followed by symbol b
• ba : meaning a string consisting of symbol b followed by symbol a

Question 3

What does the notation a+ mean?

Answer 3

+ means pattern matches one or more “a”s going {a,aa,aaa,…}

Question 4

What does the notation a* mean?

Answer 4

• means pattern matches zero or more “a”s going {0,a,aa,aaa,…}

Question 5

What does the notation ab?c mean?

Answer 5

? means it matches the preceding character one or zero times, giving {ab,abc}

Question 6

What does the notation a | b mean?

Answer 6

means pattern matches the symbol “a” or the symbol “b”

Question 7

What is the series of rules to follow?

Answer 7

• operations + and * are highest priority and always carried out first
• Concatenation is carried out next
• operation | (or) is carried out last

Question 8

What are the string searching applications, regular expressions can be used in?

Answer 8

• 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

Question 9

What are the definitions of the terms used for the following string searching?

• Literal
• Metacharacter
• Escape character
• Search Expression

Answer 9

• 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

Question 10

What is a regular language?

Answer 10

• 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