Regular Expressions Basics

Part of the Logic & Flow section of Coddy's Java journey — lesson 46 of 59.

Regular Expressions (regex) are patterns used to search and validate strings. In Java, we use the matches() method to check if a string matches a pattern.

Check if a string contains only digits:

String text = "12345";
boolean isMatch = text.matches("[0-9]+");

After executing the above code, isMatch contains:

true

Check if a string contains only letters:

String text = "Hello";
boolean isMatch = text.matches("[a-zA-Z]+");

After executing the above code, isMatch contains:

true

Challenge

Easy

Create a method named validateInput that takes two arguments:

A String (text) to validate
A String (type) that specifies the validation type

The method should validate the text based on these types:

"number": must contain only digits (0-9)
"word": must contain only letters (a-z or A-Z)
"email": must contain @ and at least one character before it
"phone": must contain exactly 10 digits

Return messages should be:

If text is null: return "Invalid input"
If type is invalid: return "Invalid type"
If validation passes: return "Valid"
If validation fails: return "Invalid"

Cheat sheet

Regular Expressions (regex) are patterns used to search and validate strings. In Java, use the matches() method to check if a string matches a pattern.

Check if a string contains only digits:

String text = "12345";
boolean isMatch = text.matches("[0-9]+");

Check if a string contains only letters:

String text = "Hello";
boolean isMatch = text.matches("[a-zA-Z]+");

Common regex patterns:

[0-9]+ - one or more digits
[a-zA-Z]+ - one or more letters
[0-9]{10} - exactly 10 digits
.+@.+ - contains @ with at least one character before it

Try it yourself

import java.util.Scanner;

public class Main {
    public static String validateInput(String text, String type) {
        // Write your code here
    }
    
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String text = scanner.nextLine();
        String type = scanner.nextLine();
        
        if (text.equals("null")) text = null;
        System.out.println(validateInput(text, type));
    }
}