Regular Expressions in Java

Pattern Matching in Text

A regular expression is a miniature language for describing text patterns. Java supports it through the classes Pattern and Matcher, and through convenience methods on String.

String Methods That Accept Patterns

String email = "user@example.com";
System.out.println(email.matches(".+@.+\\.com"));   // true

String phone = "+91-9876543210";
String cleaned = phone.replaceAll("[^0-9]", "");
System.out.println(cleaned); // 919876543210

boolean hasDigit = "Room 42".matches(".*\\d.*");   // true

The Core Pattern Elements

• . any single character.
• \d a digit, \w a word character, \s whitespace.
• [abc] a, b or c; [a-z] any lowercase letter.
• + one or more, * zero or more, ? optional.
• {3} exactly three, {2,4} two to four.
• ^ start of string, $ end of string.

Validation Example

boolean validEmail(String email) {
    return email.matches("[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}");
}

Capturing Groups

import java.util.regex.*;

Pattern p = Pattern.compile("(\\d+)-(\\d+)");  // two groups
Matcher m = p.matcher("Call 100-200 today");
if (m.find()) {
    System.out.println(m.group(1)); // 100
    System.out.println(m.group(2)); // 200
}

Careful With Backslashes

In Java source, a regex backslash must be doubled: \d in the pattern is written as "\d" in code.

Key Points

• String.matches/replaceAll cover many cases without Pattern directly.
• Groups capture parts of a match for later use.
• Remember to double every backslash in Java string literals.