Java Regular Expressions (Regex)

Regular Expressions (Regex) are patterns used to match character combinations in strings.
In Java, regex is supported via the java.util.regex package.

Main classes: Pattern and Matcher
Used for validation, search, and text processing

1. Key Classes

Class	Description
`Pattern`	Compiles a regex pattern
`Matcher`	Performs match operations on a string
`PatternSyntaxException`	Exception for invalid regex patterns

2. Basic Syntax

Symbol	Meaning
`.`	Any character
`\d`	Digit (0-9)
`\D`	Non-digit
`\w`	Word character (a-z, A-Z, 0-9, _)
`\W`	Non-word character
`\s`	Whitespace
`\S`	Non-whitespace
`^`	Start of string
`$`	End of string
`*`	Zero or more occurrences
`+`	One or more occurrences
`?`	Zero or one occurrence
`{n}`	Exactly n occurrences
`{n,}`	n or more occurrences
`{n,m}`	Between n and m occurrences
`[]`	Character class

3. Example: Simple Match

import java.util.regex.Pattern;
import java.util.regex.Matcher;public class RegexExample {
public static void main(String[] args) {
String text = "Hello123";
String regex = "\\w+"; // Matches word characters

Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(text);

if (matcher.matches()) {
System.out.println("Text matches the pattern!");
} else {
System.out.println("No match found.");
}
}
}

import java.util.regex.Pattern;

import java.util.regex.Matcher;public class RegexExample {

public static void main(String[] args) {

String text = "Hello123";

String regex = "\\w+"; // Matches word characters

Pattern pattern = Pattern.compile(regex);

Matcher matcher = pattern.matcher(text);

if (matcher.matches()) {

System.out.println("Text matches the pattern!");

} else {

System.out.println("No match found.");

}

matches() checks entire string against pattern

4. Example: Find All Matches

import java.util.regex.Pattern;
import java.util.regex.Matcher;public class RegexFindExample {
public static void main(String[] args) {
String text = "My numbers: 123, 456, 789";
String regex = "\\d+"; // Matches digits

Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(text);

while (matcher.find()) {
System.out.println("Found number: " + matcher.group());
}
}
}

import java.util.regex.Pattern;

import java.util.regex.Matcher;public class RegexFindExample {

public static void main(String[] args) {

String text = "My numbers: 123, 456, 789";

String regex = "\\d+"; // Matches digits

Pattern pattern = Pattern.compile(regex);

Matcher matcher = pattern.matcher(text);

while (matcher.find()) {

System.out.println("Found number: " + matcher.group());

}

find() searches all occurrences in the string
group() returns the matched substring

5. Example: Replace Using Regex

public class RegexReplaceExample {
public static void main(String[] args) {
String text = "Hello 123 World 456";
String regex = "\\d+"; // Matches digitsString replaced = text.replaceAll(regex, "#");
System.out.println(replaced); // Output: Hello # World #
}
}

public class RegexReplaceExample {

public static void main(String[] args) {

String text = "Hello 123 World 456";

String regex = "\\d+"; // Matches digitsString replaced = text.replaceAll(regex, "#");

System.out.println(replaced); // Output: Hello # World #

}

replaceAll() replaces all matches with specified string

6. Example: Validate Email

public class EmailValidation {
public static void main(String[] args) {
String email = "test@example.com";
String regex = "^[\\w.-]+@[\\w.-]+\\.\\w{2,}$";if (email.matches(regex)) {
System.out.println("Valid email!");
} else {
System.out.println("Invalid email!");
}
}
}

public class EmailValidation {

public static void main(String[] args) {

String email = "test@example.com";

String regex = "^[\\w.-]+@[\\w.-]+\\.\\w{2,}$";if (email.matches(regex)) {

System.out.println("Valid email!");

} else {

System.out.println("Invalid email!");

}

matches() validates the entire string against pattern

7. Common Regex Patterns

Pattern	Description
`\\d{10}`	10-digit number
`\\w+`	One or more word characters
`[a-zA-Z]+`	Alphabetic characters only
`\\s+`	One or more whitespace characters
`^[A-Z]`	Starts with uppercase letter
`\\S+@\\S+\\.\\S+`	Simple email validation

Best Practices

Use Pattern.compile() when using the same regex multiple times.
Escape special characters with \\ in Java strings.
Use matches() for full string validation, find() for searching within text.
Consider regex readability for complex patterns.

Summary

Java supports regex using Pattern and Matcher
Use regex for validation, searching, replacing, and splitting
Supports quantifiers, character classes, anchors, and groups

Java Regular Expressions (Regex)

Java Regular Expressions (Regex)

1. Key Classes

2. Basic Syntax

3. Example: Simple Match

4. Example: Find All Matches

5. Example: Replace Using Regex

6. Example: Validate Email

7. Common Regex Patterns

Best Practices

Summary

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