Chapter 12: Regular Expressions

Regular expressions (regex) are a powerful tool for pattern matching and text manipulation. They allow you to search for, match, and replace patterns in strings using a concise and flexible syntax. This chapter covers the basics of regular expressions, including pattern matching and validation techniques.

Introduction to Regular Expressions

A regular expression is a sequence of characters that forms a search pattern. Regular expressions can be used to perform various operations, such as searching, matching, and replacing text.

Basic Syntax

• Literal Characters: Matches the exact characters in the string.

  • Example: /hello/ matches "hello" in the string "hello world".

• Meta-Characters: Characters with special meanings.

  • . (Dot): Matches any single character except a newline.
  • ^ (Caret): Matches the beginning of a string.
  • $ (Dollar): Matches the end of a string.
  • * (Asterisk): Matches zero or more occurrences of the preceding element.
  • + (Plus): Matches one or more occurrences of the preceding element.
  • ? (Question Mark): Matches zero or one occurrence of the preceding element.
  • [] (Square Brackets): Matches any one of the enclosed characters.
  • | (Pipe): Acts as a logical OR between expressions.
  • () (Parentheses): Groups expressions and captures matched text.

Example:

let pattern = /a.b/;
console.log(pattern.test("a_b")); // Output: true

Flags

Flags modify the behavior of regular expressions:

• i: Case-insensitive matching.
• g: Global search (matches all occurrences).
• m: Multiline matching (affects ^ and $).

Example:

let pattern = /hello/i; // Case-insensitive match
console.log(pattern.test("Hello")); // Output: true

Pattern Matching and Validation

Regular expressions are commonly used for pattern matching and validation of input. Here are some common use cases:

Matching Patterns

1. Matching a Specific Word

Matches the exact word "hello" in a string.

Example:

let pattern = /hello/;
console.log(pattern.test("hello world")); // Output: true
console.log(pattern.test("hi there")); // Output: false

2. Matching Any Digit

Matches any single digit from 0 to 9.

Example:

let pattern = /\d/;
console.log(pattern.test("123abc")); // Output: true
console.log(pattern.test("abc")); // Output: false

3. Matching a Range of Characters

Matches any letter from 'a' to 'f'.

Example:

let pattern = /[a-f]/;
console.log(pattern.test("abcdef")); // Output: true
console.log(pattern.test("ghijkl")); // Output: false

Validation

Regular expressions are often used to validate input, such as email addresses, phone numbers, and passwords.

1. Validating an Email Address

A simple regex pattern to validate an email address.

Example:

let emailPattern = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
console.log(emailPattern.test("example@example.com")); // Output: true
console.log(emailPattern.test("invalid-email")); // Output: false

2. Validating a Phone Number

A pattern to validate a phone number with optional area code.

Example:

let phonePattern = /^(\+?\d{1,3})?(\d{10})$/;
console.log(phonePattern.test("+1234567890")); // Output: true
console.log(phonePattern.test("1234567890")); // Output: true
console.log(phonePattern.test("123-456-7890")); // Output: false

3. Validating a Password

A pattern to validate a password that must be at least 8 characters long and include at least one uppercase letter, one lowercase letter, one digit, and one special character.

Example:

let passwordPattern =
  /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/;
console.log(passwordPattern.test("Password1!")); // Output: true
console.log(passwordPattern.test("password")); // Output: false

Common Methods

Regular expressions in JavaScript can be used with several methods to search and manipulate strings:

1. test()

Tests if a pattern matches a string and returns true or false.

Syntax:

regex.test(string);

Example:

let pattern = /hello/;
console.log(pattern.test("hello world")); // Output: true

2. exec()

Executes a search for a match in a string and returns an array of information or null if no match is found.

Syntax:

regex.exec(string);

Example:

let pattern = /(\d+)/;
let result = pattern.exec("There are 123 apples");
console.log(result[0]); // Output: '123'
console.log(result[1]); // Output: '123'

3. match()

Searches a string for a match to a regular expression and returns an array of results or null if no match is found.

Syntax:

string.match(regex);

Example:

let text = "The price is $100";
let result = text.match(/\d+/);
console.log(result[0]); // Output: '100'

4. replace()

Searches a string for a match to a regular expression and replaces the matched substring with a new string.

Syntax:

string.replace(regex, replacement);

Example:

let text = "Hello John Doe";
let result = text.replace(/John/, "Jane");
console.log(result); // Output: 'Hello Jane Doe'

5. split()

Splits a string into an array of substrings based on a regular expression.

Syntax:

string.split(regex);

Example:

let text = "apple,banana,grape";
let result = text.split(/,/);
console.log(result); // Output: ['apple', 'banana', 'grape']