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

Lookbehind assertions

Positive and negative lookbehind assertions are zero-width assertions that allow for certain regex-based checks to be performed on the text that precedes (or is on the left-hand side) of the current position. The regex engine holds on to the current position after evaluating the lookbehind pattern. We can chain multiple lookbehind expressions one after another, but the regex engine does not move the control after checking all the lookbehind assertions. Lookbehind assertions can also help solve some complex problems that are not possible or are very difficult to solve without lookbehind support. Up to Java version 8, the Java regular expression engine didn't allow the use of variable-length quantifiers such as * and + in lookbehind patterns. With Java 9, the Java regex engine now allows the use of these quantifiers in lookbehind assertions.

There are two types of lookbehind assertions: positive lookbehind and negative lookbehind.

Positive lookbehind

A positive lookbehind...