Module 10: Strings, Files, and Everyday Core APIs
Regex Basics for Validation and Search
Learn a small set of regex tools for validation, search, and simple text extraction.
Author
Java Learner Editorial Team
Reviewer
Technical review by Java Learner
Last reviewed
2026-04-17
Java version
Java 25 LTS
Learning goals
- Read and write small, practical regex patterns
- Validate common input formats
- Know when regex is the wrong tool
Regex is best for patterns, not for full parsing logic: Email-like checks, digit validation, and token searches are good fits. Entire languages and complex nested data usually are not.
Start small: Character classes, quantifiers, anchors, and groups cover most practical beginner-to-intermediate use cases.
Test patterns with real examples: One pattern is never “done” until it has been tried against valid and invalid inputs.
Keep readability in mind: A slightly longer but clearer pattern is often better than one compact unreadable expression.
Runnable examples
Match a basic three-digit code
String code = "245";
boolean valid = code.matches("\\d{3}");
System.out.println(valid);Expected output
true
Mini exercise
Write a regex that matches a username made of 4 to 12 lowercase letters only.
Summary
- Regex is useful for pattern matching and validation.
- Start with simple pieces and test against examples.
- Do not force regex onto problems that need real parsing logic.
Next step
After text handling, move to file paths and modern file operations with NIO.2.
Sources used