This text is a reference guide to regular expressions, which are a language for parsing and manipulating text. The guide begins with an introduction to the basic concepts of regular expressions and how they are used for pattern matching. The rest of the guide provides detailed explanations of the syntax, features, and usage of regular expressions in several popular programming languages and tools, including Perl, Java, .NET, Ruby, Python, PHP, Apache, vi, JavaScript, and shell tools. Each language or tool has its own unique implementation of regular expressions, so the guide explains the specific metacharacters, functions, and methods available in each.