Book Image

Java 9 Regular Expressions

By : Anubhava Srivastava
Book Image

Java 9 Regular Expressions

By: Anubhava Srivastava

Overview of this book

Regular expressions are a powerful tool in the programmer's toolbox and allow pattern matching. They are also used for manipulating text and data. This book will provide you with the know-how (and practical examples) to solve real-world problems using regex in Java. You will begin by discovering what regular expressions are and how they work with Java. This easy-to-follow guide is a great place from which to familiarize yourself with the core concepts of regular expressions and to master its implementation with the features of Java 9. You will learn how to match, extract, and transform text by matching specific words, characters, and patterns. You will learn when and where to apply the methods for finding patterns in digits, letters, Unicode characters, and string literals. Going forward, you will learn to use zero-length assertions and lookarounds, parsing the source code, and processing the log files. Finally, you will master tips, tricks, and best practices in regex with Java.
Table of Contents (15 chapters)
Title page
Credits
About the Author
About the Reviewer
www.PacktPub.com
Customer Feedback
Preface
Free Chapter
1
Getting Started with Regular Expressions

Catastrophic or exponential backtracking


Regular expression engines can be broadly categorized into two types:

  1. The Non-deterministic Finite Automation (NFA) engine
  2. The Deterministic Finite Automation (DFA) engine

The DFA engines do not evaluate each character more than once to find a match. On the other hand, the NFA engines support backtracking, which means that each character in the input can be evaluated multiple times by the regex engine. The Java regular expression engine is an NFA engine.

Regex engines backtrack or give back one position at a time to make various attempts to match a given pattern when using the greedy quantifier or trying alternations. Similarly, when using lazy quantifiers, the regex engine moves forward one position at a time to attempt matching.

Regex engines usually take less time to find a positive match in the given input as compared to returning a failure for a non-match. The NFA regex engines need to evaluate all the possible permutations before returning a failure...