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Learning to Program in Perl

by Graham J Ellis

Languages of the Web

Learning to Program in Perl version 1.7

Written by Graham Ellis [email protected]

Design by Lisa Ellis Well House Consultants, Ltd. 404, The Spa, Melksham, Wiltshire SN12 6QL England

+44 (0) 1225 708 225 (phone) +44 (0) 1225 707 126 (fax)

Find us on the World Wide Web at: http://www.wellho.net

Or contact us at: [email protected]

Copyright © 2003 by Well House Consultants, Ltd. Printed in Great Britain.

Printing History

  • May 1999
  • 1.0

1.1 1.2 1.3 1.4
First Edition
February 2000 June 2000
Minor additions Compliation of modules Name change, revisions Added modules Added modules Updated modules Updated modules
October 2000 April 2002 September 2002 1.5 January 2003 February 2003
1.6 1.7
This manual was printed on 21 May 2003.

Notice of Rights

All rights reserved. No part of this manual, including interior design, may be reproduced or translated into any language in any form, or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise, without prior written permission of Well House Consultants except in the case of brief quotations embodied in critical articles and reviews. For more information on getting permission for reprints and excerpts, contact Graham Ellis at Well House Consultants.

This manual is subject to the condition that it shall not, by way of trade or otherwise, be lent, sold, hired out or otherwise circulated without the publisher's prior consent, incomplete nor in any form of binding or cover other than in which it is published and without a similar condition including this condition being imposed on the subsequent receiver.

Notice of Liability

All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. Well House Consultants is not associated with any product or vendor in this book and where such designations appear in this book, and Well House Consultants was aware of a trademark claim, the designations have been appropriately capitalised. Well House Consultants cannot attest to the accuracy of this information and use of a term should not be regarded as affecting the validity of any trademark or service mark.

The information in this book is distributed on an "as is" basis, without warranty. Whilst every precaution has been taken in the preparation of this manual, neither the author nor Well House Consultants assumes responsibility for errors or omissions, nor shall be liable to any person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the instructions contained in this book or by the computer software and hardware products described herein.

Table of Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.1 What is Perl? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 Perl Platforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.3 Perl Versions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.4 Examples of Perl in use . . . . . . . . . . . . . . . . . . . . 15

2 Hello Perl World. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1 Let's study a first program. . . . . . . . . . . . . . . . . . . 17 2.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   22

3 Variables and Operations . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1 Reading from the user . . . . . . . . . . . . . . . . . . . . . 23 3.2 More about variables . . . . . . . . . . . . . . . . . . . . . . 24 3.3 How do I do calculations?. . . . . . . . . . . . . . . . . . . 27 3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   30

4 Perl Fundamentals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.1 First Perl program. . . . . . . . . . . . . . . . . . . . . . . . . 31 4.2 Comments and documentation. . . . . . . . . . . . . . . 32 4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   38

4.4 Reading data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   42

5 More about the Perl Environment . . . . . . . . . . . . . . . . . 43

5.1 Integrating your program with your computer. . . . 43 5.2 Unix and Linux systems . . . . . . . . . . . . . . . . . . . . 43 5.3 Windows 98, 2000 and Windows NT systems . . . 45 5.4 MS-DOS users . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 5.5 Macintosh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 5.6 The compiler and the interpreter. . . . . . . . . . . . . . 47 5.7 Some questions on compilers and interpreters . . 48 5.8 Debugging tools . . . . . . . . . . . . . . . . . . . . . . . . . . 49 5.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   52

6 Conditionals and Loops . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.1 The "if" statement . . . . . . . . . . . . . . . . . . . . . . . . . 53 6.2 The "while" statement. . . . . . . . . . . . . . . . . . . . . . 54 6.3 Shorthand operators. . . . . . . . . . . . . . . . . . . . . . . 56 6.4 Ways of writing numbers . . . . . . . . . . . . . . . . . . . 59 6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   60

  • Learning to Program in Perl
  • 3

Contents

7 Analysing a Programming Task. . . . . . . . . . . . . . . . . . . 61

7.1 A small job . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   67

7.2 As a job gets larger. . . . . . . . . . . . . . . . . . . . . . . . 67 7.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   72

8 Initial String Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

8.1 String handling functions . . . . . . . . . . . . . . . . . . . 73 8.2 String handling operators . . . . . . . . . . . . . . . . . . . 74 8.3 Comparing strings exactly . . . . . . . . . . . . . . . . . . 76 8.4 Comparing strings to regular expressions . . . . . . 77 8.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   84

9 More Loops and Conditionals . . . . . . . . . . . . . . . . . . . . 85

9.1 The variety that is Perl . . . . . . . . . . . . . . . . . . . . . 85 9.2 More conditional statements. . . . . . . . . . . . . . . . . 85 9.3 More loop statements . . . . . . . . . . . . . . . . . . . . . . 89 9.4 Breaking a loop. . . . . . . . . . . . . . . . . . . . . . . . . . . 90 9.5 Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 9.6 The goto statement. . . . . . . . . . . . . . . . . . . . . . . . 92 9.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   94

10 File Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

10.1 File input and output . . . . . . . . . . . . . . . . . . . . . . . 95 10.2 File testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 10.3 Formatted printing. . . . . . . . . . . . . . . . . . . . . . . . 100 10.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   104

11 Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

11.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 11.2 The length of a list . . . . . . . . . . . . . . . . . . . . . . . 107 11.3 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 11.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   111

11.5 Functions that operate on lists . . . . . . . . . . . . . . 112 11.6 Iterating through a list . . . . . . . . . . . . . . . . . . . . . 114 11.7 List slices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 11.8 Anonymous lists . . . . . . . . . . . . . . . . . . . . . . . . . 117 11.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   118

12 Subroutines in Perl . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

12.1 What are subroutines and why? . . . . . . . . . . . . . 119 12.2 Calling a subroutine . . . . . . . . . . . . . . . . . . . . . . 121 12.3 Writing your own subroutine . . . . . . . . . . . . . . . . 123 12.4 Writing subroutines in a separate file . . . . . . . . . 125 12.5 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   130

12.6 packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 12.7 Calling objects . . . . . . . . . . . . . . . . . . . . . . . . . . 134

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12.8 Writing a class – an introduction. . . . . . . . . . . . . 136

13 Special Variables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

13.1 The Command line . . . . . . . . . . . . . . . . . . . . . . . 137 13.2 Information variables . . . . . . . . . . . . . . . . . . . . . 138 13.3 Behaviour changing variables. . . . . . . . . . . . . . . 139 13.4 The default input and pattern match space . . . . 141 13.5 More options on Perl's command line. . . . . . . . . 143 13.6 Others. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 13.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   144

14 Hashes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

14.1 Setting up a hash . . . . . . . . . . . . . . . . . . . . . . . . 145 14.2 Accessing a hash . . . . . . . . . . . . . . . . . . . . . . . . 146 14.3 Processing every element of a hash. . . . . . . . . . 147 14.4 Ordering a hash (sorting) . . . . . . . . . . . . . . . . . . 149 14.5 Programming techniques . . . . . . . . . . . . . . . . . . 153 14.6 Special hashes . . . . . . . . . . . . . . . . . . . . . . . . . . 156 14.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   160

15 More on Character Strings . . . . . . . . . . . . . . . . . . . . . . 161

15.1 Summary to date . . . . . . . . . . . . . . . . . . . . . . . . 161 15.2 Extracting information from a match. . . . . . . . . . 162 15.3 More about regular expressions . . . . . . . . . . . . . 164 15.4 Match modifiers . . . . . . . . . . . . . . . . . . . . . . . . . 166 15.5 Alternative delimiters . . . . . . . . . . . . . . . . . . . . . 168 15.6 Some favourite regular expressions . . . . . . . . . . 168

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   172

15.7 Substitutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 15.8 Regular expression efficiency. . . . . . . . . . . . . . . 175 15.9 tr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 15.10 Handling binary text . . . . . . . . . . . . . . . . . . . . . . 176 15.11 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   180

16 HTML – Quick Reminder . . . . . . . . . . . . . . . . . . . . . . . . 181

16.1 Tags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 16.2 Structure of a page . . . . . . . . . . . . . . . . . . . . . . . 181 16.3 Special Characters and new lines . . . . . . . . . . . 181 16.4 Some common tags . . . . . . . . . . . . . . . . . . . . . . 182 16.5 Lists, Tables, Frames, Forms, Images etc . . . . . 183 16.6 Which HTML standard? . . . . . . . . . . . . . . . . . . . 184

17 Perl on the Web . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

17.1 The HTML form. . . . . . . . . . . . . . . . . . . . . . . . . . 185 17.2 Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 17.3 Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 17.4 All together!. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 17.5 The power of using Perl on the Web . . . . . . . . . 188 17.6 A real example of Perl on the Web. . . . . . . . . . . 188 17.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   196

  • Learning to Program in Perll
  • 5

Contents

18 System Dependencies. . . . . . . . . . . . . . . . . . . . . . . . . . 197

18.1 The Philosophy. . . . . . . . . . . . . . . . . . . . . . . . . . 197 18.2 Shell access . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 18.3 System database enquiries . . . . . . . . . . . . . . . . 200 18.4 How Perl helps on crossplatform requirements . 201 18.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

19 More than Simple Lists and Hashes!. . . . . . . . . . . . . . 205

19.1 Multidimensional arrays . . . . . . . . . . . . . . . . . . . 205 19.2 Something more complex. . . . . . . . . . . . . . . . . . 207 19.3 Grouping in Perl . . . . . . . . . . . . . . . . . . . . . . . . . 209 19.4 Interpreting a complex reference to a variable . . 210 19.5 Design MATTERS . . . . . . . . . . . . . . . . . . . . . . . 211 19.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   212

20 Handling Dates and Time . . . . . . . . . . . . . . . . . . . . . . . 213

20.1 So far . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 20.2 How Perl handles dates and times . . . . . . . . . . . 214 20.3 Convertors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 20.4 Handling centuries . . . . . . . . . . . . . . . . . . . . . . . 217 20.5 Elapsed time sleep . . . . . . . . . . . . . . . . . . . . . . . 217 20.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   220

21 Practical Example – Perl in use . . . . . . . . . . . . . . . . . . 221

21.1 The requirement . . . . . . . . . . . . . . . . . . . . . . . . . 221 21.2 Plain Old Documentation (POD). . . . . . . . . . . . . 231 21.3 Possible enhancements . . . . . . . . . . . . . . . . . . . 231

22 Libraries and Resources. . . . . . . . . . . . . . . . . . . . . . . . 233

22.1 Standard Perl modules . . . . . . . . . . . . . . . . . . . . 233 22.2 The CPAN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 22.3 Utility programs. . . . . . . . . . . . . . . . . . . . . . . . . . 237 22.4 Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . 238 22.5 Web resources . . . . . . . . . . . . . . . . . . . . . . . . . . 239 22.6 Newsgroups . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 22.7 Chat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 22.8 Books . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 22.9 Meeting users, getting local support and training 242

23 Perl 6 Look Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

23.1 Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 23.2 Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 23.3 Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 23.4 Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 23.5 Conditionals and loops . . . . . . . . . . . . . . . . . . . . 248 23.6 Exception handling . . . . . . . . . . . . . . . . . . . . . . . 250 23.7 Rules and grammar . . . . . . . . . . . . . . . . . . . . . . 251 23.8 Under the bonnet . . . . . . . . . . . . . . . . . . . . . . . . 253 23.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

24 A Quick Look Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

24.1 Fundamental and advanced topics. . . . . . . . . . . 255

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24.2 Other facilities in the Perl language . . . . . . . . . . 256 24.3 Other facilities in Perl – further modules. . . . . . . 260 24.4 Perl in other guises. . . . . . . . . . . . . . . . . . . . . . . 263 24.5 And also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

Appendix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

Exercise sample answers . . . . . . . . . . . . . . . . . 265 CPAN sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 Common Errors in Perl . . . . . . . . . . . . . . . . . . . 273

WHC Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

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    Practical Extraction and Report Language « Perl is a language of getting your job done » « There is more than one way to do it » Larry Wall VI, October 2006 Page 1 Perl Outline : History Structure of a simple Perl script Perl variables scalar ($) array (@) hash (%) Operators (numeric, string & logical) Statement modifiers (if/elsif/else, for/foreach, while) VI, October 2006 Page 2 Practical Extraction and Report Language http://perl.oreilly.com " Perl is both a programming language and an application on your computer that runs those programs " VI, October 2006 Page 3 Perl history A few dates: 1969 UNIX was born at Bell Labs. 1970 Brian Kernighan suggested the name "Unix" and the operating system we know today was born. 1972 The programming language C is born at the Bell Labs (C is one of Perl's ancestors). 1973 “grep” is introduced by Ken Thompson as an external utility: Global REgular expression Print. 1976 Steven Jobs and Steven Wozniak found Apple Computer (1 April). 1977 The computer language awk is designed by Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan (awk is one of Perl's ancestors). VI, October 2006 Page 4 Perl history 1987 Perl 1.000 is unleashed upon the world NAME perl | Practical Extraction and Report Language SYNOPSIS perl [options] filename args DESCRIPTION Perl is a interpreted language optimized for scanning arbitrary text files, extracting information from those text files, and printing reports based on that information. It's also a good language for many system management tasks. The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal).
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    SECOND EDITION Intermediate Perl Randal L. Schwartz, brian d foy, and Tom Phoenix Beijing • Cambridge • Farnham • Köln • Sebastopol • Tokyo Intermediate Perl, Second Edition by Randal L. Schwartz, brian d foy, and Tom Phoenix Copyright © 2012 Randal Schwartz, brian d foy, Tom Phoenix. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles (http://my.safaribooksonline.com). For more information, contact our corporate/institutional sales department: 800-998-9938 or [email protected]. Editors: Simon St. Laurent and Shawn Wallace Indexer: Lucie Haskins Production Editor: Kristen Borg Cover Designer: Karen Montgomery Copyeditor: Absolute Service, Inc. Interior Designer: David Futato Proofreader: Absolute Service, Inc. Illustrator: Rebecca Demarest March 2006: First Edition. August 2012: Second Edition. Revision History for the Second Edition: 2012-07-20 First release See http://oreilly.com/catalog/errata.csp?isbn=9781449393090 for release details. Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc. Intermediate Perl, the image of an alpaca, and related trade dress are trademarks of O’Reilly Media, Inc. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and O’Reilly Media, Inc., was aware of a trademark claim, the designations have been printed in caps or initial caps. While every precaution has been taken in the preparation of this book, the publisher and authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information con- tained herein.