How to Use Regex in Java: Syntax, Examples, and Tips

In Java, you use regex by importing java.util.regex.Pattern and java.util.regex.Matcher. You create a Pattern object with your regex, then use a Matcher to find matches in text.

Syntax

To use regex in Java, you first create a Pattern object with your regex string. Then, you create a Matcher object by calling pattern.matcher(inputString). Use matcher.find() or matcher.matches() to check for matches.

• Pattern.compile(regex): Compiles the regex into a pattern.
• pattern.matcher(text): Creates a matcher for the text.
• matcher.find(): Finds the next match.
• matcher.matches(): Checks if the entire text matches the pattern.

java

import java.util.regex.Pattern;
import java.util.regex.Matcher;

Pattern pattern = Pattern.compile("your-regex");
Matcher matcher = pattern.matcher("input text");

if (matcher.find()) {
    // match found
}

if (matcher.matches()) {
    // entire input matches
}

Example

This example shows how to check if a string contains a number using regex. It prints all numbers found in the text.

java

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class RegexExample {
    public static void main(String[] args) {
        String text = "I have 2 apples and 10 bananas.";
        Pattern pattern = Pattern.compile("\\d+"); // regex for one or more digits
        Matcher matcher = pattern.matcher(text);

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

Output

Found number: 2
Found number: 10

Common Pitfalls

Common mistakes include forgetting to escape special characters in regex strings, confusing matches() with find(), and not using raw strings properly.

Wrong: Using matches() when you want to find a substring match.

Right: Use find() to search for a pattern anywhere in the text.

java

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class RegexPitfall {
    public static void main(String[] args) {
        String text = "abc123def";
        Pattern pattern = Pattern.compile("\\d+");
        Matcher matcher = pattern.matcher(text);

        // Wrong: matches() checks whole string
        System.out.println("Using matches(): " + matcher.matches()); // false

        // Right: find() checks for substring match
        System.out.println("Using find(): " + matcher.find()); // true
    }
}

Output

Using matches(): false
Using find(): true

Quick Reference

Method	Description
Pattern.compile(regex)	Creates a regex pattern object
pattern.matcher(text)	Creates a matcher for the text
matcher.find()	Finds next substring matching the pattern
matcher.matches()	Checks if entire text matches pattern
matcher.group()	Returns the matched substring
matcher.start()	Returns start index of match
matcher.end()	Returns end index of match

Key Takeaways

Use Pattern.compile() to create a regex pattern in Java.

Use Matcher to search text with methods like find() and matches().

Escape special regex characters properly in Java strings.

Use find() to locate substrings, matches() to check full string match.

Use matcher.group() to get the matched text after a successful find().