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Pattern Matching in JavaScript

Introduction

Pattern Matching in JavaScript is performed using Regular Expressions (RegEx). It is used to search, match, replace, and validate strings based on predefined patterns.

Pattern matching is commonly used in:

  • Email Validation
  • Password Validation
  • Mobile Number Validation
  • Search Operations
  • Replace Operations
  • Form Validation
  • Text Processing

Syntax

/Pattern/OptionalIdentifiers

General Syntax

var regex = /pattern/flags;

Example:

var regex = /hello/i;

Here:

  • hello → Pattern
  • i → Identifier (Flag)

Identifiers (Flags)

JavaScript provides several identifiers to modify the behavior of regular expressions.

Identifier Meaning
g Global Search
i Case Insensitive Search
m Multi-line Search
s Dot matches newline characters
u Unicode matching
y Sticky matching

1. g - Global Search

Searches for all occurrences of a pattern.

Example

var str = "hello hello hello";

var result = str.match(/hello/g);

console.log(result);

Output

["hello","hello","hello"]

2. i - Case Insensitive Search

Ignores uppercase and lowercase differences.

Example

var str = "HELLO";

console.log(/hello/i.test(str));

Output

true

3. m - Multi-line Search

Matches patterns across multiple lines.

Example

var str = `Java
Script`;

console.log(/^Script/m.test(str));

Output

true

4. s - Dot All Modifier

Allows dot (.) to match newline characters.

Example

var str = "Hello\nWorld";

console.log(/Hello.World/s.test(str));

Output

true

5. u - Unicode Flag

Treats the pattern as Unicode characters.

Example

var str = "😊";

console.log(/^.$/u.test(str));

Output

true

6. y - Sticky Flag

Matches only from the last matched position.

Example

var regex = /hello/y;

regex.lastIndex = 0;

console.log(regex.test("hello world"));

Output

true

Methods Used in Pattern Matching

There are three important methods:

  1. search()
  2. replace()
  3. match()

1. search() Method

The search() method searches for a pattern inside a string and returns the index position of the first match.

Syntax

string.search(regex)

Example

Common.html

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Regular Expressions in JavaScript</title>
</head>
<body>

    <p id="res"></p>

    <script>

        var str = "KodNest Technologies";

        var n = str.search(/T/i);

        document.getElementById("res").innerHTML = n;

    </script>

</body>
</html>

Output

8

Explanation

var str = "KodNest Technologies";

var n = str.search(/T/i);
  • /T/ searches for T
  • i ignores uppercase/lowercase
  • search() returns:
8

because T is found at index 8.

2. replace() Method

The replace() method replaces the matched text with a new value.

Syntax

string.replace(regex,newValue)

Example

<!DOCTYPE html>
<html lang="en">

<head>
    <title>Regular Expressions in JavaScript</title>
</head>

<body>

    <p id="res"></p>

    <script>

        var str = "KodNest Technologies";

        var newStr = str.replace(/T/i,"X");

        document.getElementById("res").innerHTML = newStr;

    </script>

</body>
</html>

Output

KodNest Xechnologies

Explanation

var newStr = str.replace(/T/i,"X");
  • Searches T
  • Replaces T with X

Result:

KodNest Xechnologies

3. match() Method

The match() method returns the matched value as an array.

Syntax

string.match(regex)

Example

<!DOCTYPE html>
<html lang="en">

<head>
    <title>Regular Expressions in JavaScript</title>
</head>

<body>

    <p id="res"></p>

    <script>

        var str = "KodNest Technologies";

        var result = str.match(/T/i);

        document.getElementById("res").innerHTML = result;

    </script>

</body>
</html>

Output

T

or

["T"]

Explanation

The match() method returns:

["T"]

because the character T is matched.

Difference Between search(), replace(), and match()

Method Purpose Return Type
search() Finds position Index Number
replace() Replaces matched text New String
match() Finds matched pattern Array

Example Using All Three Methods

var str = "KodNest Technologies";

console.log(str.search(/T/i));

console.log(str.replace(/T/i,"X"));

console.log(str.match(/T/i));

Output

8
KodNest Xechnologies
["T"]

Real Time Applications

Pattern Matching is used in:

  1. Email Validation
  2. Password Validation
  3. Mobile Number Validation
  4. PAN Card Validation
  5. Aadhaar Number Validation
  6. Search Engines
  7. Text Editors
  8. Find and Replace Features
  9. Form Validation
  10. Data Extraction

Frequently Asked Interview Questions

Q1. What is Pattern Matching in JavaScript?

Answer:

Pattern Matching is the process of searching and manipulating strings using Regular Expressions.

Q2. What is the syntax of Regular Expression?

Answer:

/pattern/flags

Example:

/hello/i

Q3. What does the i flag do?

Answer:

It performs case-insensitive matching.

Example:

/hello/i

Matches:

hello
HELLO
Hello

Q4. What does search() return?

Answer:

It returns the index position of the first matched character.

Q5. What does replace() return?

Answer:

It returns a new string after replacing the matched text.

Q6. What does match() return?

Answer:

It returns an array containing the matched values.

Summary

Pattern Matching is one of the most important applications of Regular Expressions in JavaScript. By using identifiers (flags) and methods such as search(), replace(), and match(), developers can efficiently search, validate, replace, and manipulate strings. These concepts are widely used in real-world applications such as form validations, search engines, data extraction, and text processing systems.