3 perlfaq - frequently asked questions about Perl ($Date: 2005/03/27 07:21:21 $)
7 The perlfaq is divided into several documents based on topics. A table
8 of contents is at the end of this document.
10 =head2 Where to get the perlfaq
12 Extracts of the perlfaq are posted regularly to
13 comp.lang.perl.misc. It is available on many web sites:
14 http://www.perldoc.com/ and http://faq.perl.org/
16 =head2 How to contribute to the perlfaq
18 You may mail corrections, additions, and suggestions to
19 perlfaq-workers@perl.org . This alias should not be used to
20 I<ask> FAQs. It's for fixing the current FAQ. Send
21 questions to the comp.lang.perl.misc newsgroup. You can
22 view the source tree at http://cvs.perl.org/cvsweb/perlfaq/
23 (which is outside of the main Perl source tree). The CVS
24 repository notes all changes to the FAQ.
26 =head2 What will happen if you mail your Perl programming problems to the authors
28 Your questions will probably go unread, unless they're
29 suggestions of new questions to add to the FAQ, in which
30 case they should have gone to the perlfaq-workers@perl.org
33 You should have read section 2 of this faq. There you would
34 have learned that comp.lang.perl.misc is the appropriate
35 place to go for free advice. If your question is really
36 important and you require a prompt and correct answer, you
37 should hire a consultant.
41 The original perlfaq was written by Tom Christiansen, then expanded
42 by collaboration between Tom and Nathan Torkington. The current
43 document is maintained by the perlfaq-workers (perlfaq-workers@perl.org).
44 Several people have contributed answers, corrections, and comments.
46 =head1 Author and Copyright Information
48 Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
49 other contributors noted in the answers.
53 =head2 Bundled Distributions
55 This documentation is free; you can redistribute it and/or modify it
56 under the same terms as Perl itself.
58 Irrespective of its distribution, all code examples in these files
59 are hereby placed into the public domain. You are permitted and
60 encouraged to use this code in your own programs for fun
61 or for profit as you see fit. A simple comment in the code giving
62 credit would be courteous but is not required.
66 This information is offered in good faith and in the hope that it may
67 be of use, but is not guaranteed to be correct, up to date, or suitable
68 for any particular purpose whatsoever. The authors accept no liability
69 in respect of this information or its use.
71 =head1 Table of Contents
75 =item perlfaq - this document
77 =item perlfaq1 - General Questions About Perl
79 =item perlfaq2 - Obtaining and Learning about Perl
81 =item perlfaq3 - Programming Tools
83 =item perlfaq4 - Data Manipulation
85 =item perlfaq5 - Files and Formats
87 =item perlfaq6 - Regular Expressions
89 =item perlfaq7 - General Perl Language Issues
91 =item perlfaq8 - System Interaction
93 =item perlfaq9 - Networking
102 =head2 L<perlfaq1>: General Questions About Perl
104 Very general, high-level questions about Perl.
114 Who supports Perl? Who develops it? Why is it free?
118 Which version of Perl should I use?
122 What are perl4, perl5, or perl6?
138 Is Perl difficult to learn?
142 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
146 Can I do [task] in Perl?
150 When shouldn't I program in Perl?
154 What's the difference between "perl" and "Perl"?
158 Is it a Perl program or a Perl script?
166 Where can I get a list of Larry Wall witticisms?
170 How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language?
175 =head2 L<perlfaq2>: Obtaining and Learning about Perl
177 Where to find source and documentation for Perl, support, and related matters.
183 What machines support Perl? Where do I get it?
187 How can I get a binary version of Perl?
191 I don't have a C compiler on my system. How can I compile perl?
195 I copied the Perl binary from one machine to another, but scripts don't work.
199 I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
203 What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
207 Is there an ISO or ANSI certified version of Perl?
211 Where can I get information on Perl?
215 What are the Perl newsgroups on Usenet? Where do I post questions?
219 Where should I post source code?
231 Perl on the Net: FTP and WWW Access
235 What mailing lists are there for Perl?
239 Archives of comp.lang.perl.misc
243 Where can I buy a commercial version of Perl?
247 Where do I send bug reports?
251 What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org?
256 =head2 L<perlfaq3>: Programming Tools
258 Programmer tools and programming support.
264 How do I do (anything)?
268 How can I use Perl interactively?
272 Is there a Perl shell?
276 How do I find which modules are installed on my system?
280 How do I debug my Perl programs?
284 How do I profile my Perl programs?
288 How do I cross-reference my Perl programs?
292 Is there a pretty-printer (formatter) for Perl?
296 Is there a ctags for Perl?
300 Is there an IDE or Windows Perl Editor?
304 Where can I get Perl macros for vi?
308 Where can I get perl-mode for emacs?
312 How can I use curses with Perl?
316 How can I use X or Tk with Perl?
320 How can I make my Perl program run faster?
324 How can I make my Perl program take less memory?
328 Is it safe to return a reference to local or lexical data?
332 How can I free an array or hash so my program shrinks?
336 How can I make my CGI script more efficient?
340 How can I hide the source for my Perl program?
344 How can I compile my Perl program into byte code or C?
348 How can I compile Perl into Java?
352 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
356 Can I write useful Perl programs on the command line?
360 Why don't Perl one-liners work on my DOS/Mac/VMS system?
364 Where can I learn about CGI or Web programming in Perl?
368 Where can I learn about object-oriented Perl programming?
372 Where can I learn about linking C with Perl? [h2xs, xsubpp]
376 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?
380 When I tried to run my script, I got this message. What does it mean?
389 =head2 L<perlfaq4>: Data Manipulation
391 Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
397 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
401 Why is C<int()> broken?
405 Why isn't my octal data interpreted correctly?
409 Does Perl have a C<round()> function? What about C<ceil()> and C<floor()>? Trig functions?
413 How do I convert between numeric representations/bases/radixes?
417 Why doesn't & work the way I want it to?
421 How do I multiply matrices?
425 How do I perform an operation on a series of integers?
429 How can I output Roman numerals?
433 Why aren't my random numbers random?
437 How do I get a random number between X and Y?
441 How do I find the day or week of the year?
445 How do I find the current century or millennium?
449 How can I compare two dates and find the difference?
453 How can I take a string and turn it into epoch seconds?
457 How can I find the Julian Day?
461 How do I find yesterday's date?
465 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
469 How do I validate input?
473 How do I unescape a string?
477 How do I remove consecutive pairs of characters?
481 How do I expand function calls in a string?
485 How do I find matching/nesting anything?
489 How do I reverse a string?
493 How do I expand tabs in a string?
497 How do I reformat a paragraph?
501 How can I access or change N characters of a string?
505 How do I change the Nth occurrence of something?
509 How can I count the number of occurrences of a substring within a string?
513 How do I capitalize all the words on one line?
517 How can I split a [character] delimited string except when inside [character]?
521 How do I strip blank space from the beginning/end of a string?
525 How do I pad a string with blanks or pad a number with zeroes?
529 How do I extract selected columns from a string?
533 How do I find the soundex value of a string?
537 How can I expand variables in text strings?
541 What's wrong with always quoting "$vars"?
545 Why don't my E<lt>E<lt>HERE documents work?
549 What is the difference between a list and an array?
553 What is the difference between C<$array[1]> and C<@array[1]>?
557 How can I remove duplicate elements from a list or array?
561 How can I tell whether a certain element is contained in a list or array?
565 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
569 How do I test whether two arrays or hashes are equal?
573 How do I find the first array element for which a condition is true?
577 How do I handle linked lists?
581 How do I handle circular lists?
585 How do I shuffle an array randomly?
589 How do I process/modify each element of an array?
593 How do I select a random element from an array?
597 How do I permute N elements of a list?
601 How do I sort an array by (anything)?
605 How do I manipulate arrays of bits?
609 Why does C<defined()> return true on empty arrays and hashes?
613 How do I process an entire hash?
617 What happens if I add or remove keys from a hash while iterating over it?
621 How do I look up a hash element by value?
625 How can I know how many entries are in a hash?
629 How do I sort a hash (optionally by value instead of key)?
633 How can I always keep my hash sorted?
637 What's the difference between "delete" and "undef" with hashes?
641 Why don't my tied hashes make the defined/exists distinction?
645 How do I reset an C<each()> operation part-way through?
649 How can I get the unique keys from two hashes?
653 How can I store a multidimensional array in a DBM file?
657 How can I make my hash remember the order I put elements into it?
661 Why does passing a subroutine an undefined element in a hash create it?
665 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
669 How can I use a reference as a hash key?
673 How do I handle binary data correctly?
677 How do I determine whether a scalar is a number/whole/integer/float?
681 How do I keep persistent data across program calls?
685 How do I print out or copy a recursive data structure?
689 How do I define methods for every class/object?
693 How do I verify a credit card checksum?
697 How do I pack arrays of doubles or floats for XS code?
702 =head2 L<perlfaq5>: Files and Formats
704 I/O and the "f" issues: filehandles, flushing, formats, and footers.
710 How do I flush/unbuffer an output filehandle? Why must I do this?
714 How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
718 How do I count the number of lines in a file?
722 How can I use Perl's C<-i> option from within a program?
726 How can I copy a file?
730 How do I make a temporary file name?
734 How can I manipulate fixed-record-length files?
738 How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
742 How can I use a filehandle indirectly?
746 How can I set up a footer format to be used with C<write()>?
750 How can I C<write()> into a string?
754 How can I output my numbers with commas added?
758 How can I translate tildes (~) in a filename?
762 How come when I open a file read-write it wipes it out?
766 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
770 Is there a leak/bug in C<glob()>?
774 How can I open a file with a leading ">" or trailing blanks?
778 How can I reliably rename a file?
782 How can I lock a file?
786 Why can't I just C<< open(FH, ">file.lock") >>?
790 I still don't get locking. I just want to increment the number in the file. How can I do this?
794 All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?
798 How do I randomly update a binary file?
802 How do I get a file's timestamp in perl?
806 How do I set a file's timestamp in perl?
810 How do I print to more than one file at once?
814 How can I read in an entire file all at once?
818 How can I read in a file by paragraphs?
822 How can I read a single character from a file? From the keyboard?
826 How can I tell whether there's a character waiting on a filehandle?
830 How do I do a C<tail -f> in perl?
834 How do I C<dup()> a filehandle in Perl?
838 How do I close a file descriptor by number?
842 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
846 Why doesn't C<glob("*.*")> get all the files?
850 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
854 How do I select a random line from a file?
858 Why do I get weird spaces when I print an array of lines?
863 =head2 L<perlfaq6>: Regular Expressions
865 This section is surprisingly small because the rest of the FAQ is
866 littered with answers involving regular expressions. For example,
867 decoding a URL and checking whether something is a number are handled
868 with regular expressions, but those answers are found elsewhere in this
869 document (in L<perlfaq9>: "How do I decode or create those %-encodings
870 on the web" and L<perlfaq4>: "How do I determine whether a scalar is
871 a number/whole/integer/float", to be precise).
877 How can I hope to use regular expressions without creating illegible and unmaintainable code?
881 I'm having trouble matching over more than one line. What's wrong?
885 How can I pull out lines between two patterns that are themselves on different lines?
889 I put a regular expression into C<$/> but it didn't work. What's wrong?
893 How do I substitute case insensitively on the LHS while preserving case on the RHS?
897 How can I make C<\w> match national character sets?
901 How can I match a locale-smart version of C</[a-zA-Z]/>?
905 How can I quote a variable to use in a regex?
909 What is C</o> really for?
913 How do I use a regular expression to strip C style comments from a file?
917 Can I use Perl regular expressions to match balanced text?
921 What does it mean that regexes are greedy? How can I get around it?
925 How do I process each word on each line?
929 How can I print out a word-frequency or line-frequency summary?
933 How can I do approximate matching?
937 How do I efficiently match many regular expressions at once?
941 Why don't word-boundary searches with C<\b> work for me?
945 Why does using C<$&>, C<$`>, or C<$'> slow my program down?
949 What good is C<\G> in a regular expression?
953 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
957 What's wrong with using grep in a void context?
961 How can I match strings with multibyte characters?
965 How do I match a pattern that is supplied by the user?
970 =head2 L<perlfaq7>: General Perl Language Issues
972 General Perl language issues that don't clearly fit into any of the other sections.
978 Can I get a BNF/yacc/RE for the Perl language?
982 What are all these $@%&* punctuation signs, and how do I know when to use them?
986 Do I always/never have to quote my strings or use semicolons and commas?
990 How do I skip some return values?
994 How do I temporarily block warnings?
1002 Why do Perl operators have different precedence than C operators?
1006 How do I declare/create a structure?
1010 How do I create a module?
1014 How do I create a class?
1018 How can I tell if a variable is tainted?
1026 What is variable suicide and how can I prevent it?
1030 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1034 How do I create a static variable?
1038 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1042 How can I access a dynamic variable while a similarly named lexical is in scope?
1046 What's the difference between deep and shallow binding?
1050 Why doesn't C<< my($foo) = <FILE>; >> work right?
1054 How do I redefine a builtin function, operator, or method?
1058 What's the difference between calling a function as C<&foo> and C<foo()>?
1062 How do I create a switch or case statement?
1066 How can I catch accesses to undefined variables, functions, or methods?
1070 Why can't a method included in this same file be found?
1074 How can I find out my current package?
1078 How can I comment out a large block of perl code?
1082 How do I clear a package?
1086 How can I use a variable as a variable name?
1090 What does "bad interpreter" mean?
1095 =head2 L<perlfaq8>: System Interaction
1097 This section of the Perl FAQ covers questions involving operating system interaction. Topics include interprocess communication (IPC), control over the user-interface (keyboard, screen and pointing devices), and most anything else not related to data manipulation. Read the FAQs and documentation specific to the port of perl to your operating system (eg, L<perlvms>, L<perlplan9>, ...). These should contain more detailed information on the vagaries of your perl.
1103 How do I find out which operating system I'm running under?
1107 How come C<exec()> doesn't return?
1111 How do I do fancy stuff with the keyboard/screen/mouse?
1115 How do I print something out in color?
1119 How do I read just one key without waiting for a return key?
1123 How do I check whether input is ready on the keyboard?
1127 How do I clear the screen?
1131 How do I get the screen size?
1135 How do I ask the user for a password?
1139 How do I read and write the serial port?
1143 How do I decode encrypted password files?
1147 How do I start a process in the background?
1151 How do I trap control characters/signals?
1155 How do I modify the shadow password file on a Unix system?
1159 How do I set the time and date?
1163 How can I C<sleep()> or C<alarm()> for under a second?
1167 How can I measure time under a second?
1171 How can I do an C<atexit()> or C<setjmp()>/C<longjmp()>? (Exception handling)
1175 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1179 How can I call my system's unique C functions from Perl?
1183 Where do I get the include files to do C<ioctl()> or C<syscall()>?
1187 Why do setuid perl scripts complain about kernel problems?
1191 How can I open a pipe both to and from a command?
1195 Why can't I get the output of a command with C<system()>?
1199 How can I capture STDERR from an external command?
1203 Why doesn't C<open()> return an error when a pipe open fails?
1207 What's wrong with using backticks in a void context?
1211 How can I call backticks without shell processing?
1215 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1219 How can I convert my shell script to perl?
1223 Can I use perl to run a telnet or ftp session?
1227 How can I write expect in Perl?
1231 Is there a way to hide perl's command line from programs such as "ps"?
1235 I {changed directory, modified my environment} in a perl script. How come
1236 the change disappeared when I exited the script? How do I get my changes
1241 How do I close a process's filehandle without waiting for it to complete?
1245 How do I fork a daemon process?
1249 How do I find out if I'm running interactively or not?
1253 How do I timeout a slow event?
1257 How do I set CPU limits?
1261 How do I avoid zombies on a Unix system?
1265 How do I use an SQL database?
1269 How do I make a C<system()> exit on control-C?
1273 How do I open a file without blocking?
1277 How do I tell the difference between errors from the shell and perl?
1281 How do I install a module from CPAN?
1285 What's the difference between require and use?
1289 How do I keep my own module/library directory?
1293 How do I add the directory my program lives in to the module/library search path?
1297 How do I add a directory to my include path (C<@INC>) at runtime?
1301 What is F<socket.ph> and where do I get it?
1306 =head2 L<perlfaq9>: Networking
1308 Networking, the internet, and a few on the web.
1314 What is the correct form of response from a CGI script?
1318 My CGI script runs from the command line but not the browser. (500 Server Error)
1322 How can I get better error messages from a CGI program?
1326 How do I remove HTML from a string?
1330 How do I extract URLs?
1334 How do I download a file from the user's machine? How do I open a file on another machine?
1338 How do I make a pop-up menu in HTML?
1342 How do I fetch an HTML file?
1346 How do I automate an HTML form submission?
1350 How do I decode or create those %-encodings on the web?
1354 How do I redirect to another page?
1358 How do I put a password on my web pages?
1362 How do I edit my F<.htpasswd> and F<.htgroup> files with Perl?
1366 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1370 How do I parse a mail header?
1374 How do I decode a CGI form?
1378 How do I check a valid mail address?
1382 How do I decode a MIME/BASE64 string?
1386 How do I return the user's mail address?
1394 How do I use MIME to make an attachment to a mail message?
1402 How do I find out my hostname/domainname/IP address?
1406 How do I fetch a news article or the active newsgroups?
1410 How do I fetch/put an FTP file?
1414 How can I do RPC in Perl?
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