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) 1997-2003 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)?
405 Why isn't my octal data interpreted correctly?
409 Does Perl have a round() function? What about ceil() and 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 $array[1] and @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 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 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 write()?
750 How can I 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 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 open(FH, "E<gt>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 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 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 littered with answers involving regular expressions. For example, decoding a URL and checking whether something is a number are handled with regular expressions, but those answers are found elsewhere in this document (in L<perlfaq9>: ``How do I decode or create those %-encodings on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is a number/whole/integer/float'', to be precise).
871 How can I hope to use regular expressions without creating illegible and unmaintainable code?
875 I'm having trouble matching over more than one line. What's wrong?
879 How can I pull out lines between two patterns that are themselves on different lines?
883 I put a regular expression into $/ but it didn't work. What's wrong?
887 How do I substitute case insensitively on the LHS while preserving case on the RHS?
891 How can I make C<\w> match national character sets?
895 How can I match a locale-smart version of C</[a-zA-Z]/>?
899 How can I quote a variable to use in a regex?
903 What is C</o> really for?
907 How do I use a regular expression to strip C style comments from a file?
911 Can I use Perl regular expressions to match balanced text?
915 What does it mean that regexes are greedy? How can I get around it?
919 How do I process each word on each line?
923 How can I print out a word-frequency or line-frequency summary?
927 How can I do approximate matching?
931 How do I efficiently match many regular expressions at once?
935 Why don't word-boundary searches with C<\b> work for me?
939 Why does using $&, $`, or $' slow my program down?
943 What good is C<\G> in a regular expression?
947 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
951 What's wrong with using grep in a void context?
955 How can I match strings with multibyte characters?
959 How do I match a pattern that is supplied by the user?
964 =head2 L<perlfaq7>: General Perl Language Issues
966 General Perl language issues that don't clearly fit into any of the other sections.
972 Can I get a BNF/yacc/RE for the Perl language?
976 What are all these $@%&* punctuation signs, and how do I know when to use them?
980 Do I always/never have to quote my strings or use semicolons and commas?
984 How do I skip some return values?
988 How do I temporarily block warnings?
996 Why do Perl operators have different precedence than C operators?
1000 How do I declare/create a structure?
1004 How do I create a module?
1008 How do I create a class?
1012 How can I tell if a variable is tainted?
1020 What is variable suicide and how can I prevent it?
1024 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1028 How do I create a static variable?
1032 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1036 How can I access a dynamic variable while a similarly named lexical is in scope?
1040 What's the difference between deep and shallow binding?
1044 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
1048 How do I redefine a builtin function, operator, or method?
1052 What's the difference between calling a function as &foo and foo()?
1056 How do I create a switch or case statement?
1060 How can I catch accesses to undefined variables, functions, or methods?
1064 Why can't a method included in this same file be found?
1068 How can I find out my current package?
1072 How can I comment out a large block of perl code?
1076 How do I clear a package?
1080 How can I use a variable as a variable name?
1084 What does "bad interpreter" mean?
1089 =head2 L<perlfaq8>: System Interaction
1091 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.
1097 How do I find out which operating system I'm running under?
1101 How come exec() doesn't return?
1105 How do I do fancy stuff with the keyboard/screen/mouse?
1109 How do I print something out in color?
1113 How do I read just one key without waiting for a return key?
1117 How do I check whether input is ready on the keyboard?
1121 How do I clear the screen?
1125 How do I get the screen size?
1129 How do I ask the user for a password?
1133 How do I read and write the serial port?
1137 How do I decode encrypted password files?
1141 How do I start a process in the background?
1145 How do I trap control characters/signals?
1149 How do I modify the shadow password file on a Unix system?
1153 How do I set the time and date?
1157 How can I sleep() or alarm() for under a second?
1161 How can I measure time under a second?
1165 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
1169 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1173 How can I call my system's unique C functions from Perl?
1177 Where do I get the include files to do ioctl() or syscall()?
1181 Why do setuid perl scripts complain about kernel problems?
1185 How can I open a pipe both to and from a command?
1189 Why can't I get the output of a command with system()?
1193 How can I capture STDERR from an external command?
1197 Why doesn't open() return an error when a pipe open fails?
1201 What's wrong with using backticks in a void context?
1205 How can I call backticks without shell processing?
1209 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1213 How can I convert my shell script to perl?
1217 Can I use perl to run a telnet or ftp session?
1221 How can I write expect in Perl?
1225 Is there a way to hide perl's command line from programs such as "ps"?
1229 I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible?
1233 How do I close a process's filehandle without waiting for it to complete?
1237 How do I fork a daemon process?
1241 How do I find out if I'm running interactively or not?
1245 How do I timeout a slow event?
1249 How do I set CPU limits?
1253 How do I avoid zombies on a Unix system?
1257 How do I use an SQL database?
1261 How do I make a system() exit on control-C?
1265 How do I open a file without blocking?
1269 How do I tell the difference between errors from the shell and perl?
1273 How do I install a module from CPAN?
1277 What's the difference between require and use?
1281 How do I keep my own module/library directory?
1285 How do I add the directory my program lives in to the module/library search path?
1289 How do I add a directory to my include path (@INC) at runtime?
1293 What is socket.ph and where do I get it?
1298 =head2 L<perlfaq9>: Networking
1300 Networking, the internet, and a few on the web.
1306 What is the correct form of response from a CGI script?
1310 My CGI script runs from the command line but not the browser. (500 Server Error)
1314 How can I get better error messages from a CGI program?
1318 How do I remove HTML from a string?
1322 How do I extract URLs?
1326 How do I download a file from the user's machine? How do I open a file on another machine?
1330 How do I make a pop-up menu in HTML?
1334 How do I fetch an HTML file?
1338 How do I automate an HTML form submission?
1342 How do I decode or create those %-encodings on the web?
1346 How do I redirect to another page?
1350 How do I put a password on my web pages?
1354 How do I edit my .htpasswd and .htgroup files with Perl?
1358 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1362 How do I parse a mail header?
1366 How do I decode a CGI form?
1370 How do I check a valid mail address?
1374 How do I decode a MIME/BASE64 string?
1378 How do I return the user's mail address?
1386 How do I use MIME to make an attachment to a mail message?
1394 How do I find out my hostname/domainname/IP address?
1398 How do I fetch a news article or the active newsgroups?
1402 How do I fetch/put an FTP file?
1406 How can I do RPC in Perl?
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