Java RegEx Tester

Java Regex Tester

The Java Regex Tester by Qodex is a real-time tool built for developers to write, test, and debug regular expressions specifically for Java. Backed by the java.util.regex package, this tester helps you validate complex string patterns used for email, password, phone number, and text parsing operations.

It’s designed to simplify development by instantly showing match results, capturing groups, and syntax errors—making it easier to integrate regex confidently in your Java projects.

Getting Started

How to Use:

1. Enter your regex pattern in the “Regular Expression” input field.

2. Type a test string into the “Test String” box.

3. View the result instantly with highlights for matches and groups.

Need example inputs? Use our Email Generator, Phone Number Generator, or UUID Generator.

Core Java Regex Syntax

Java regex operates with a variety of constructs, each serving a specific purpose:

Meta characters

• .: Matches any single character except newline characters (\n).
  Commonly used to represent any wildcard character.

• ^: Anchors the match at the beginning of a line or string.
  Example: ^abc matches abc only if it’s at the start.

• $: Anchors the match at the end of a line or string.

  Example: xyz$ matches xyz only at the end.

• |: Acts as a logical OR operator.

  Example: cat|dog matches either cat or dog. Useful for alternatives.

Character Classes

• [abc]: Matches any one character: either a, b, or c.

• [^abc]: Matches any character except a, b, or c. The caret ^ negates the set.

• [a-zA-Z]: Specifies a range, matches any uppercase or lowercase letter from a to z.

Predefined Character Classes

• \\d: Matches any digit character; equivalent to [0-9].

• \\D: Matches any non-digit character; equivalent to [^0-9].

• \\s: Matches any whitespace character (space, tab, newline).

• \\S: Matches any non-whitespace character.

• \\w: Matches any word character (letters, digits, or underscore); equivalent to [a-zA-Z0-9_].

• \\W: Matches any character that is not a word character.

Quantifiers

• *: Matches the preceding element zero or more times.

  Example: lo*l matches ll, lol, lool, etc.+: One or more times.

• +: Matches one or more occurrences.

  Example: lo+l matches lol, lool, but not ll.

• ?: Matches zero or one occurrence. Also makes quantifiers lazy when placed after them

  Example: *?

• {n}: Matches exactly n occurrences.

  Example: a{3} matches aaa.

• {n,}: Matches n or more occurrences.

  Example: a{2,} matches aa, aaa, aaaa, etc.

• {n,m}: Matches between n and m occurrences.
  Example: a{2,4} matches aa, aaa, or aaaa.

Groups and Capturing

• (abc): Captures and groups the match for abc. This can be reused with backreferences.

• (?:abc): Groups abc without capturing it. Useful for applying quantifiers or alternation without creating references.

• (?i)abc: Enables case-insensitive matching for the group. Matches abc, ABC, AbC, etc.

• \\\\b: A word boundary.

• \\\\B: A non-word boundary.

• \1, \2, etc. : Backreferences to captured groups.

  Example: (\w+)\s\1 matches repeated words like hello hello.

Boundary Matchers

• \b : Matches a word boundary. Example: \bcat\b matches cat but not scatter.

• \B : Matches a non-word boundary. Opposite of \b.

• \A : Matches the beginning of the input string (unlike ^, which matches beginning of a line in multiline mode).

• \Z : Matches the end of the input string or before a final line break.

• \z : Matches only the absolute end of the input string (ignores trailing newline characters).

• ^ and $ : Also function as boundary anchors (start and end of line respectively).

  
  

Logical Assertions (Lookaheads & Lookbehinds)

• (?=...) : Positive lookahead. Ensures what follows the current position matches the pattern inside.

  Example: \d(?=px) matches a digit only if it’s followed by px.

  
  

• (?!...) : Negative lookahead. Ensures the following characters do not match the pattern.

  Example: foo(?!bar) matches foo only if it’s not followed by bar.

  
  

• (?<=...) : Positive lookbehind. Ensures that what precedes the current position matches the pattern.

  Example: (?<=@)\w+ matches the username after an @.

  
  

• (?<!...) : Negative lookbehind. Ensures that what precedes is not a match.

  Example: (?<!https:)// matches // not preceded by https:.
  
  

Greedy vs Lazy Quantifiers

• *? : Matches as few as possible, zero or more times.

• +? : Matches as few as possible, one or more times.

• ?? : Matches as few as possible, zero or one time.

• {n}? : Matches exactly n times, but tries to do it minimally.

• {n,}? : Matches at least n times, but lazily.

• {n,m}? : Matches between n and m times, choosing the smallest match possible.
  
  

Lazy quantifiers are useful when you’re trying to avoid consuming too much text—especially useful when working with nested tags or repeated delimiters.

Unicode Support

• \p{L} : Matches any kind of letter from any language.

• \p{N} : Matches any kind of numeric character.

• \p{IsGreek} : Matches any character in the Greek Unicode block.

• \P{...} : The inverse of \p{...}. For example, \P{L} matches any non-letter character.

  
  

Unicode properties allow your regex to work globally—great for international applications.

Java RegEx Examples

Example 1: Java Email Validation

Try this pattern with the Email Regex Java Validator.

import java.util.regex.*;

public class RegexExample {
    public static void main(String[] args) {
        String email = "user@example.com";
        Pattern pattern = Pattern.compile("^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\\\.[a-zA-Z]{2,}$");
        Matcher matcher = pattern.matcher(email);
        System.out.println("Email Valid: " + matcher.matches());
    }
}

Example 2: Password Strength Check

Use the Password Generator to test variations.

public class RegexExample {
    public static void main(String[] args) {
        String password = "Aa123456!";
        Pattern pattern = Pattern.compile("(?=.*[a-z])(?=.*[A-Z])(?=.*\\d)(?=.*[@$!%*?&])[A-Za-z\\d@$!%*?&]{8,}");
        Matcher matcher = pattern.matcher(password);
        System.out.println("Password Strong: " + matcher.matches());
    }
}

Example 3: Extracting Words from a String

import java.util.regex.*;

public class RegexExample {
    public static void main(String[] args) {
        String text = "Regex is #1 at pattern matching!";
        Pattern wordPattern = Pattern.compile("\\b\\w+\\b");
        Matcher matcher = wordPattern.matcher(text);

        while (matcher.find()) {
            System.out.println(matcher.group());
        }
    }
}

Practical Tips for Using Java Regex

1. Always escape characters twice in Java strings.

   For example, to match a dot (.), use \\. in code, which becomes \\\\. in Java string literals. You can test how these escape sequences behave using the Java Regex Tester.

2. Use Pattern.compile() to pre-compile regex for better performance, especially when validating inputs like emails or passwords repeatedly.

3. Leverage Matcher.groupCount() to understand how many capturing groups your regex contains - very useful when extracting structured data like UUIDs or date formats.

4. Use non-capturing groups (?:...) to improve performance when you don’t need to reference matches later.

5. Avoid overly greedy patterns like .* unless absolutely necessary. Consider using lazy quantifiers like .*? for better precision. This is especially helpful when testing data like URLs.

6. Use lookaheads (e.g., (?=...)) to enforce complex validation rules such as strong password requirements. You can test password logic in the Password Regex Java Validator.

7. Break down complex expressions using capturing groups or inline comments to keep your regex readable and easier to maintain.

8. Test your regex against realistic data. Use tools like the Email Generator, Phone Number Generator, or Password Generator to simulate real input cases.

9. Don’t use regex to parse structured data such as XML, JSON, or HTML. Always use dedicated parsers for these use cases.

10. Enable flags like (?i) (case-insensitive) or (?s) (dot-all) directly within your pattern instead of changing Java code. These flags can be previewed live in the Java Regex Tester.

Generate Sample Inputs:

• Email Generator

• UUID Generator

• Password Generator

• Phone Number Generator

Try More Java Regex Validators:

• Email Regex Java Validator

• Password Regex Java Validator

• Phone Number Regex Java Validator

• UUID Regex Java Validator

• Credit Card Regex Java Validator

• Date Regex Java Validator
  
  

Need to test in another language?

• JavaScript Regex Tester

• Go Regex Tester