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regular-expressions.md

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⚑ Regular Expressions:

A regular expression, often abbreviated as regex. It is a sequence of characters that defines a search pattern. It's used to match, find, replace, or manipulate text based on given patterns.

☴ Overview:

  1. Creating regular expressions
  2. Common regular expression patterns
  3. Matching patterns in strings

✦ Creating regular expressions:

Regular expressions can be created in two ways:

  • Literal notation: 
    let pattern = /pattern/;
  • Constructor: 
    let pattern = new RegExp("pattern");

Flags: Regular expressions can have flags that modify their behavior:

  • i: Case-insensitive matching.
  • g: Global matching, finding all matches instead of just the first.
  • m: Multiline matching, allowing ^ and $ to match the beginning and end of lines, respectively.

✦ Common regular expression patterns:

  • .: Matches any character except a newline.
  • ^: Matches the beginning of a string or line.
  • $: Matches the end of a string or line.
  • *: Matches zero or more occurrences of the preceding character or group.
  • +: Matches one or more occurrences of the preceding character or group.
  • ?: Matches zero or one occurrence of the preceding character or group.  
  • {n}: Matches exactly n occurrences of the preceding character or group.
  • {n,}: Matches at least n occurrences of the preceding character or group.  
  • {n,m}: Matches between n and m occurrences of the preceding character or group.  
  • |: Matches either the expression before or after it.
  • (): Groups a part of the regular expression.
  • [ ]: Matches any character within the brackets.
  • [^ ]: Matches any character not within the brackets

✦ Matching patterns in strings:

let text = "The quick brown fox jumps over the lazy dog.";
let pattern = /fox/;

let match = text.match(pattern);
console.log(match); // Output: ["fox"]

let index = text.search(pattern);
console.log(index); // Output: 16

let replacedText = text.replace(pattern, "cat");
console.log(replacedText); // Output: "The quick brown cat jumps over the lazy dog."

Other Methods: There are several methods to work with regular expressions:

  • match(): Finds all matches of a regular expression in a string and returns an array.
  • search(): Finds the index of the first match of a regular expression in a string.
  • replace(): Replaces all matches of a regular expression in a string with a new string.
  • test(): Checks if a string matches a regular expression and returns a boolean.
  • split(): Splits a string into an array using a regular expression as a separator.

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