Regular Expressions in JavaScript

1. Creating Regular Expressions

In JavaScript, a regular expression can be created in two ways:

• Using a regular expression literal:

let regex = /pattern/;

• Using the RegExp constructor:

let regex = new RegExp('pattern');

Both methods do the same thing, but regular expression literals are more common and easier to use.

let regexLiteral = /hello/;  // Regular Expression literal
                        let regexConstructor = new RegExp('hello');  // Regular Expression constructor
                        
                        console.log(regexLiteral.test('hello world'));  // Output: true
                        console.log(regexConstructor.test('hello world'));  // Output: true
                            

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2. Regular Expression Patterns and Flags

A regular expression consists of a pattern and optional flags.

• Patterns: The pattern defines what characters to match. For example:
• ^: Matches the start of the string.
• $: Matches the end of the string.
• \d: Matches any digit.
• \s: Matches any whitespace character.
• Flags: Flags modify how the regular expression is matched.
• g: Global search (finds all matches).
• i: Case-insensitive search.
• m: Multiline matching (affects ^ and $).

let regex = /hello/i;  // Case-insensitive search
                        
                        console.log(regex.test('HELLO'));  // Output: true
                        console.log(regex.test('hello'));  // Output: true
                            

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3. String Methods for Regular Expressions

JavaScript provides several string methods that work with regular expressions:

• match(): Returns an array of all matches found in the string, or null if no match is found.
• search(): Returns the index of the first match, or -1 if no match is found.
• replace(): Replaces a match in the string with a new substring.
• split(): Splits the string into an array of substrings based on the pattern.

Example for match()

let str = 'The quick brown fox jumps over the lazy dog';
                        let regex = /fox/;
                        
                        let result = str.match(regex);
                        console.log(result);  // Output: ['fox']
                            

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Example for search()

let str = 'The quick brown fox jumps over the lazy dog';
                        let regex = /fox/;
                        
                        let index = str.search(regex);
                        console.log(index);  // Output: 16
                            

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Example for replace()

let str = 'The quick brown fox jumps over the lazy dog';
                        let regex = /fox/;
                        
                        let newStr = str.replace(regex, 'cat');
                        console.log(newStr);  // Output: 'The quick brown cat jumps over the lazy dog'
                            

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Example for split()

let str = 'The quick brown fox jumps over the lazy dog';
                        let regex = /\s+/;
                        
                        let result = str.split(regex);
                        console.log(result);  // Output: ['The', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'lazy', 'dog']
                            

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4. Example Summary

let str = 'The quick brown fox jumps over the lazy dog';
                        
                        let regexMatch = /fox/;
                        let matchResult = str.match(regexMatch);
                        console.log(matchResult);  // Output: ['fox']
                        
                        let regexSearch = /fox/;
                        let searchResult = str.search(regexSearch);
                        console.log(searchResult);  // Output: 16
                        
                        let regexReplace = /fox/;
                        let replaceResult = str.replace(regexReplace, 'cat');
                        console.log(replaceResult);  // Output: The quick brown cat jumps over the lazy dog
                        
                        let regexSplit = /\s+/;
                        let splitResult = str.split(regexSplit);
                        console.log(splitResult);  // Output: ['The', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'lazy', 'dog']
                            

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