3 Lecture - Regular Expression

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3 Lecture

CS402

Midterm & Final Term Short Notes

Regular Expression

A regular expression, or regex for short, is a sequence of characters that defines a search pattern. It is a powerful tool for text processing, used to match and manipulate strings of text based on certain rules or patterns. Regular expressions


Important Mcq's
Midterm & Finalterm Prepration
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  1. What is a regular expression? a) A sequence of numbers b) A sequence of characters that defines a search pattern c) A mathematical function d) None of the above Answer: b Which programming language uses regular expressions extensively? a) Java b) C++ c) Python d) All of the above Answer: d What is the basic syntax of a regular expression? a) ( ) b) { } c) [ ] d) / / Answer: d Which of the following characters is used to match any single character? a) * b) . c) + d) ? Answer: b What is the purpose of the pipe symbol (|) in a regular expression? a) To match the beginning of a line b) To match the end of a line c) To match either one pattern or another d) To match any character Answer: c Which quantifier is used to match zero or one occurrence of the preceding character? a) * b) + c) ? d) { Answer: c Which of the following regular expressions matches any digit? a) \w b) \d c) \s d) \W Answer: b Which character class matches any whitespace character? a) \w b) \d c) \s d) \W Answer: c Which of the following regular expressions matches the end of a line? a) $ b) ^ c) | d) . Answer: a What is the purpose of the lookahead assertion in a regular expression? a) To match a pattern only if it is followed by another pattern b) To match a pattern only if it is not followed by another pattern c) To match a pattern at the beginning of a line d) None of the above Answer: a



Subjective Short Notes
Midterm & Finalterm Prepration
Past papers included

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  1. What is a regular expression and what is it used for? Answer: A regular expression is a sequence of characters that defines a search pattern. It is used to match and manipulate strings of text based on certain rules or patterns. What is the difference between a basic and extended regular expression? Answer: Basic regular expressions use fewer metacharacters and have simpler syntax compared to extended regular expressions, which provide more functionality and are more expressive. What is the purpose of the dot (.) character in a regular expression? Answer: The dot character matches any single character in a regular expression. How is alternation expressed in a regular expression? Answer: Alternation is expressed in a regular expression using the pipe (|) symbol, which indicates a choice between two or more patterns. What is a character class in a regular expression? Answer: A character class in a regular expression is a set of characters that match any one character from the set. It is represented by enclosing the set in square brackets. What is the purpose of the caret (^) character in a regular expression? Answer: The caret character is used to match the beginning of a line in a regular expression. What is the difference between a greedy and non-greedy quantifier in a regular expression? Answer: A greedy quantifier matches as many characters as possible, while a non-greedy quantifier matches as few characters as possible. What is a backreference in a regular expression? Answer: A backreference is a reference to a previously matched group in a regular expression. What is the purpose of the lookahead assertion in a regular expression? Answer: The lookahead assertion is used to match a pattern only if it is followed by another pattern. How can regular expressions be used for input validation in web development? Answer: Regular expressions can be used to ensure that user input on a website conforms to a specific format, such as a valid email address or phone number.

A regular expression, or regex for short, is a powerful tool used for pattern matching in text processing. It is a sequence of characters that defines a search pattern and is commonly used in programming languages, search engines, and text editors. Regular expressions provide a flexible way to match, replace, and manipulate text based on certain rules or patterns. They can be used to match specific strings or patterns within larger text, validate input in web forms, and extract data from text files. The basic syntax of a regular expression involves using a combination of ordinary characters and special characters known as metacharacters. For example, the dot character (.) matches any single character, while the asterisk (*) matches zero or more occurrences of the preceding character. Character classes can also be used in regular expressions to match a specific set of characters. For example, the \d character class matches any digit character, while \w matches any word character (letters, digits, or underscore). Quantifiers in regular expressions allow for matching of repeated occurrences of characters. For example, the plus (+) quantifier matches one or more occurrences of the preceding character, while the question mark (?) matches zero or one occurrence. Regular expressions also allow for grouping of characters and the use of alternation, which allows for matching of multiple patterns. Backreferences can be used to refer back to previously matched groups within the regular expression. In web development, regular expressions are commonly used for input validation to ensure that user input conforms to a specific format, such as a valid email address or phone number. They can also be used to extract data from web pages or search engine results. Overall, regular expressions provide a powerful and flexible way to match, replace, and manipulate text based on specific patterns or rules. They are an essential tool in many programming and text processing applications.

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