Approaching a programming problem can be like balancing Columbus's egg. If no one shows you how, you may sit forever perplexed, watching the egg—and your program—fall over again and again, no closer to the Indies than when you began. This is especially true in a language as idiomatic as Perl.
This book isn't meant to be a complete reference book for Perl. Keeping a copy of Programming Perl handy will let you look up exact definitions of operators, keywords, functions, pragmata, or modules. Alternatively, every Perl installation comes with a voluminous collection of searchable, online reference materials. If those aren't where you can easily get at them, see your system administrator if you have one, or consult the documentation section at http://www.perl.com.
Neither is this book meant to be a bare-bones introduction for programmers who have never seen Perl before. That's what Learning Perl, a kinder and gentler introduction to Perl, is designed for. (If you're on a Microsoft system, you might prefer the Learning Perl for Win32 Systems version.)
Instead, this is a book for learning more Perl. Neither a reference book nor a tutorial book, Perl Cookbook serves as a companion book to both. It's for people who already know the basics but are wondering how to mix all those ingredients together into a complete program. Spread across 22 chapters and more than 400 focused topic areas affectionately called recipes, this task-oriented book contains thousands of solutions to everyday challenges encountered by novice and journeyman alike.
We tried hard to make this book useful for both random and sequential access. Each recipe is self-contained, but has a list of references at the end should you need further information on the topic. We've tried to put the simpler, more common recipes toward the front of each chapter and the simpler chapters toward the front of the book. Perl novices should find that these recipes about Perl's basic data types and operators are just what they're looking for. We gradually work our way through topic areas and solutions more geared toward the journeyman Perl programmer. Now and then we include material that should inspire even the master Perl programmer.
Each chapter begins with an overview of that chapter's topic. This introduction is followed by the main body of each chapter, its recipes. In the spirit of the Perl slogan of TMTOWTDI, There's more than one way to do it, most recipes show several different techniques for solving the same or closely related problems. These recipes range from short-but-sweet solutions to in-depth mini-tutorials. Where more than one technique is given, we often show costs and benefits of each approach.
As with a traditional cookbook, we expect you to access this book more or less at random. When you want to learn how to do something, you'll look up its recipe. Even if the exact solutions presented don't fit your problem exactly, they'll give you ideas about possible approaches.
Each chapter concludes with one or more complete programs. Although some recipes already include small programs, these longer applications highlight the chapter's principal focus and combine techniques from other chapters, just as any real-world program would. All are useful, and many are used on a daily basis. Some even helped us put this book together.
0.1. What's in This Book
Spread over five chapters, the first portion of the book addresses Perl's basic data types. Chapter 1, covers matters like accessing substrings, expanding function calls in strings, and parsing comma-separated data; it also covers Unicode strings. Chapter 2, tackles oddities of floating-point representation, placing commas in numbers, and pseudo-random numbers. Chapter 3, demonstrates conversions between numeric and string date formats and using timers. Chapter 4, covers everything relating to list and array manipulation, including finding unique elements in a list, efficiently sorting lists, and randomizing them. Chapter 5, concludes the basics with a demonstration of the most useful data type, the associative array. The chapter shows how to access a hash in insertion order, how to sort a hash by value, how to have multiple values per key, and how to have an immutable hash.
Chapter 6, includes recipes for converting a shell wildcard into a pattern, matching letters or words, matching multiple lines, avoiding greediness, matching nested or recursive patterns, and matching strings that are close to but not exactly what you're looking for. Although this chapter is one of the longest in the book, it could easily have been longer still—every chapter contains uses of regular expressions. It's part of what makes Perl Perl.
The next three chapters cover the filesystem. Chapter 7, shows opening files, locking them for concurrent access, modifying them in place, and storing filehandles in variables. Chapter 8, discusses storing filehandles in variables, managing temporary files, watching the end of a growing file, reading a particular line from a file, handling alternative character encodings like Unicode and Microsoft character sets, and random access binary I/O. Finally, in Chapter 9, we show techniques to copy, move, or delete a file, manipulate a file's timestamps, and recursively process all files in a directory.
Chapter 10 through Chapter 13 focus on making your program flexible and powerful. Chapter 10, includes recipes on creating persistent local variables, passing parameters by reference, calling functions indirectly, crafting a switch statement, and handling exceptions. Chapter 11, is about data structures; basic manipulation of references to data and functions are demonstrated. Later recipes show how to create elaborate data structures and how to save and restore these structures from permanent storage. Chapter 12, concerns breaking up your program into separate files; we discuss how to make variables and functions private to a module, customize warnings for modules, replace built-ins, trap errors loading missing modules, and use the h2ph and h2xs tools to interact with C and C++ code. Lastly, Chapter 13, covers the fundamentals of building your own object-based module to create user-defined types, complete with constructors, destructors, and inheritance. Other recipes show examples of circular data structures, operator overloading, and tied data types.
The next two chapters are about interfaces: one to databases, the other to users. Chapter 14, includes techniques for manipulating DBM files and querying and updating databases with SQL and the DBI module. Chapter 15, covers topics such as clearing the screen, processing command-line switches, single-character input, moving the cursor using termcap and curses, thumbnailing images, and graphing data.
The last portion of the book is devoted to interacting with other programs and services. Chapter 16, is about running other programs and collecting their output, handling zombie processes, named pipes, signal management, and sharing variables between running programs. Chapter 17, shows how to establish stream connections or use datagrams to create low-level networking applications for client-server programming. Chapter 18, is about higher-level protocols such as mail, FTP, Usenet news, XML-RPC, and SOAP. Chapter 19, contains recipes for processing web forms, trapping their errors, avoiding shell escapes for security, managing cookies, shopping cart techniques, and saving forms to files or pipes. Chapter 20, covers non-interactive uses of the Web, such as fetching web pages, automating form submissions in a script, extracting URLs from a web page, removing HTML tags, finding fresh or stale links, and parsing HTML. Chapter 21, introduces mod_perl, the Perl interpreter embedded in Apache. It covers fetching form parameters, issuing redirections, customizing Apache's logging, handling authentication, and advanced templating with Mason and the Template Toolkit. Finally, Chapter 22 is about the ubiquitous data format XML and includes recipes such as validating XML, parsing XML into events and trees, and transforming XML into other formats.
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