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Introduction to Regular Expressions

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Optional Quantifiers in Regular Expressions

In Regular expressions, optional quantifiers are denoted by a question mark ?. It indicates that a character can appear either 0 or 1 time. For example, the regular expression humou?r will match the text humour as well as the text humor.

Character Sets in Regular Expressions

Regular expression character sets denoted by a pair of brackets [] will match any of the characters included within the brackets. For example, the regular expression con[sc]en[sc]us will match any of the spellings consensus, concensus, consencus, and concencus.

Literals in Regular Expressions

In Regular expression, the literals are the simplest characters that will match the exact text of the literals. For example, the regex monkey will completely match the text monkey but will also match monkey in text The monkeys like to eat bananas.

Fixed Quantifiers in Regular Expressions

In Regular expressions, fixed quantifiers are denoted by curly braces {}. It contains either the exact quantity or the quantity range of characters to be matched. For example, the regular expression roa{3}r will match the text roaaar, while the regular expression roa{3,6}r will match roaaar, roaaaar, roaaaaar, or roaaaaaar.

Alternation in Regular Expressions

Alternation indicated by the pipe symbol |, allows for the matching of either of two subexpressions. For example, the regex baboons|gorillas will match the text baboons as well as the text gorillas.

Anchors in Regular Expressions

Anchors (hat ^ and dollar sign $) are used in regular expressions to match text at the start and end of a string, respectively. For example, the regex ^Monkeys: my mortal enemy$ will completely match the text Monkeys: my mortal enemy but not match Spider Monkeys: my mortal enemy or Monkeys: my mortal enemy in the wild. The ^ ensures that the matched text begins with Monkeys, and the $ ensures the matched text ends with enemy.

Regular Expressions

Regular expressions are sequence of characters defining a pattern of text that needs to be found. They can be used for parsing the text files for specific pattern, verifying test results, and finding keywords in emails or webpages.

Wildcards in Regular expressions

In Regular expression, wildcards are denoted with the period . and it can match any single character (letter, number, symbol or whitespace) in a piece of text. For example, the regular expression ......... will match the text orangutan, marsupial, or any other 9-character text.

Regular Expression Ranges

Regular expression ranges are used to specify a range of characters that can be matched. Common regular expression ranges include: [A-Z]. : match any uppercase letter [a-z]. : match any lowercase letter [0-9]. : match any digit [A-Za-z] : match any uppercase or lowercase letter.

Shorthand Character Classes in Regular Expressions

Shorthand character classes simplify writing regular expressions. For example, \w represents the regex range [A-Za-z0-9_], \d represents [0-9], \W represents [^A-Za-z0-9_] matching any character not included by \w, \D represents [^0-9] matching any character not included by \d.

Kleene Star & Kleene Plus in Regular Expressions

In Regular expressions, the Kleene star(*) indicates that the preceding character can occur 0 or more times. For example, meo*w will match mew, meow, meooow, and meoooooooooooow. The Kleene plus(+) indicates that the preceding character can occur 1 or more times. For example, meo+w will match meow, meooow, and meoooooooooooow, but not match mew.

Grouping in Regular Expressions

In Regular expressions, grouping is accomplished by open ( and close parenthesis ). Thus the regular expression I love (baboons|gorillas) will match the text I love baboons as well as I love gorillas, as the grouping limits the reach of the | to the text within the parentheses.