Learning Perl: Making Easy Things Easy and Hard Things Possible

398 Pages·2021·5.479 MB·English

by  Randal L. Schwartz, brian d foy, Tom Phoenix

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Learning 5.
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Making Easy Things Easy
and Hard Things Possible
Randal L. Schwartz,
brian d foy &
Tom Phoenix

Praise for Learning Perl
Still the best way to learn Perl: friendly, accurate, and encouraging.
—Nathan Torkington, Coauthor of the Perl Cookbook
I consider the Llama the de facto standard when it comes to an introductory book
for the Perl language. It’s coherent, easy to approach, and it broadly covers the language
from legacy information to the bleeding edge.
—Grzegorz Szpetkowski, Software Engineer,
Intel Technology Poland
Learning Perl is an investment that will help unlock the full potential of this powerful
programming language. The authors are insightful as they methodically tackle a maze
of new and exciting concepts that should be part of everyone’s journey towards
Perl proficiency.
—André Philipp, Freelance software engineer
Learning Perl [4th ed.] should rightly be regarded as one of the classic texts for Perl
programmers to read through at least once in their Perl careers. The book is chock-full of
useful information, and even experienced Perl coders would do well to at least leaf
through the pages of this book for paradigms to help their coding.
—Craig Maloney, Slashdot reviewer

EIGHTH EDITION
Learning Perl
Making Easy Things Easy
and Hard Things Possible
Randal L. Schwartz, brian d foy,
and Tom Phoenix
BBeeiijjiinngg BBoossttoonn FFaarrnnhhaamm SSeebbaassttooppooll TTookkyyoo
Learning Perl
by Randal L. Schwartz, brian d foy, and Tom Phoenix
Copyright © 2021 Enhydra Services, LLC. All rights reserved.
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July 2021: Eighth Edition
Revision History for the Eighth Edition
2021-06-29: First Release
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978-1-492-09495-1
[GP]
Table of Contents
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Questions and Answers 1
Is This the Right Book for You? 1
What About the Exercises and Their Answers? 2
What If I’m a Perl Course Instructor? 3
What Does “Perl” Stand For? 4
Why Did Larry Create Perl? 4
Why Didn’t Larry Just Use Some Other Language? 4
Is Perl Easy or Hard? 5
How Did Perl Get to Be So Popular? 6
What’s Happening with Perl Now? 7
What’s Perl Really Good For? 7
What Is Perl Not Good For? 8
How Can I Get Perl? 8
What Is CPAN? 9
Is There Any Kind of Support? 9
What If I Find a Bug in Perl? 10
How Do I Make a Perl Program? 10
A Simple Program 11
What’s Inside That Program? 13
How Do I Compile My Perl Program? 15
A Whirlwind Tour of Perl 15
Exercises 17
v
2. Scalar Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Numbers 19
All Numbers Have the Same Format Internally 20
Integer Literals 20
Nondecimal Integer Literals 20
Floating-Point Literals 21
Numeric Operators 22
Strings 23
Single-Quoted String Literals 23
Double-Quoted String Literals 24
String Operators 25
Automatic Conversion Between Numbers and Strings 26
Perl’s Built-in Warnings 27
Interpreting Nondecimal Numerals 28
Scalar Variables 29
Choosing Good Variable Names 30
Scalar Assignment 31
Compound Assignment Operators 31
Output with print 32
Interpolation of Scalar Variables into Strings 32
Creating Characters by Code Point 34
Operator Precedence and Associativity 34
Comparison Operators 36
The if Control Structure 37
Boolean Values 37
Getting User Input 38
The chomp Operator 39
The while Control Structure 40
The undef Value 40
The defined Function 41
Exercises 42
3. Lists and Arrays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Accessing Elements of an Array 44
Special Array Indices 45
List Literals 45
The qw Shortcut 46
List Assignment 47
The pop and push Operators 49
The shift and unshift Operators 49
The splice Operator 50
vi | Table of Contents
Interpolating Arrays into Strings 51
The foreach Control Structure 52
Perl’s Favorite Default: $_ 53
The reverse Operator 53
The sort Operator 54
The each Operator 54
Scalar and List Context 55
Using List-Producing Expressions in Scalar Context 56
Using Scalar-Producing Expressions in List Context 57
Forcing Scalar Context 58
<STDIN> in List Context 58
Exercises 59
4. Subroutines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Defining a Subroutine 61
Invoking a Subroutine 62
Return Values 62
Arguments 64
Private Variables in Subroutines 66
Variable-Length Parameter Lists 67
A Better &max Routine 67
Empty Parameter Lists 68
Notes on Lexical (my) Variables 69
The use strict Pragma 70
The return Operator 71
Omitting the Ampersand 72
Nonscalar Return Values 74
Persistent, Private Variables 74
Subroutine Signatures 76
Prototypes 78
Exercises 79
5. Input and Output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Input from Standard Input 81
Input from the Diamond Operator 83
The Double Diamond 85
The Invocation Arguments 85
Output to Standard Output 86
Formatted Output with printf 89
Arrays and printf 91
Filehandles 91
Table of Contents | vii
Opening a Filehandle 93
Binmoding Filehandles 96
Bad Filehandles 96
Closing a Filehandle 97
Fatal Errors with die 97
Warning Messages with warn 99
Automatically die-ing 99
Using Filehandles 100
Changing the Default Output Filehandle 100
Reopening a Standard Filehandle 101
Output with say 101
Filehandles in a Scalar 102
Exercises 104
6. Hashes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
What Is a Hash? 105
Why Use a Hash? 107
Hash Element Access 108
The Hash as a Whole 109
Hash Assignment 110
The Big Arrow 111
Hash Functions 112
The keys and values Functions 112
The each Function 113
Typical Use of a Hash 115
The exists Function 115
The delete Function 115
Hash Element Interpolation 116
The %ENV Hash 116
Exercises 117
7. Regular Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Sequences 119
Practice Some Patterns 121
The Wildcard 123
Quantifiers 125
Grouping in Patterns 129
Alternation 132
Character Classes 134
Character Class Shortcuts 135
Negating the Shortcuts 136
viii | Table of Contents