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
About the Author
About the Reviewer
Customer Feedback
Free Chapter
Getting Started with Regular Expressions

Zero-width assertions

Zero-width or zero-length assertion in regular expressions means that there is a zero-length match that does not change the current position of the pointer in the input string. These assertions do not consume characters in the string but only assert whether a match is possible or not, giving us a binary true or false match result. Although many zero-width assertions are denoted inside parentheses, like groups, we will soon see that they do not capture any text. Zero-width assertions have no real meaning in back-references or in replacements.


We have already discussed a few zero-width assertions in the previous chapters, such as anchors and boundary assertions.

The Java regular expression engine allows many predefined zero-width assertions, including the ones we have discussed already, such as start, end anchors, and word boundaries.

Predefined zero-width assertions

Zero-width Assertion



Asserts a word boundary


Asserts anywhere except at a word boundary