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

Back references

Back references provide a convenient way of matching a repeated character or repeated tokens in the input text. By using back references, the regular expression engine can match the exact same text as previously matched by a capturing group.

The syntax of a back reference is a backslash followed by a capturing group number, as shown in the following example:


The preceding example is a back reference of the third capturing group.

In Java regular expressions, there can be up to 99 back references, each number referencing a captured group number.

For example, if we need to match a two-digit number with the restriction that both digits must be the same, then we need to capture the first digit and then use a back reference for the first captured group, as follows:


Now, this regex will match any of these strings: 11, 22, 55, and 88.

We can also use quantifiers after back references as we use them with capturing or non-capturing groups.

For example, if we have to match...