3 perlfaq - frequently asked questions about Perl ($Date: 2005/01/31 15:52:15 $)
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 generate simple menus without using CGI or Tk?
324 How can I make my Perl program run faster?
328 How can I make my Perl program take less memory?
332 Is it safe to return a reference to local or lexical data?
336 How can I free an array or hash so my program shrinks?
340 How can I make my CGI script more efficient?
344 How can I hide the source for my Perl program?
348 How can I compile my Perl program into byte code or C?
352 How can I compile Perl into Java?
356 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
360 Can I write useful Perl programs on the command line?
364 Why don't Perl one-liners work on my DOS/Mac/VMS system?
368 Where can I learn about CGI or Web programming in Perl?
372 Where can I learn about object-oriented Perl programming?
376 Where can I learn about linking C with Perl? [h2xs, xsubpp]
380 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?
384 When I tried to run my script, I got this message. What does it mean?
393 =head2 L<perlfaq4>: Data Manipulation
395 Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
401 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
409 Why isn't my octal data interpreted correctly?
413 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
417 How do I convert between numeric representations/bases/radixes?
421 Why doesn't & work the way I want it to?
425 How do I multiply matrices?
429 How do I perform an operation on a series of integers?
433 How can I output Roman numerals?
437 Why aren't my random numbers random?
441 How do I get a random number between X and Y?
445 How do I find the day or week of the year?
449 How do I find the current century or millennium?
453 How can I compare two dates and find the difference?
457 How can I take a string and turn it into epoch seconds?
461 How can I find the Julian Day?
465 How do I find yesterday's date?
469 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
473 How do I validate input?
477 How do I unescape a string?
481 How do I remove consecutive pairs of characters?
485 How do I expand function calls in a string?
489 How do I find matching/nesting anything?
493 How do I reverse a string?
497 How do I expand tabs in a string?
501 How do I reformat a paragraph?
505 How can I access or change N characters of a string?
509 How do I change the Nth occurrence of something?
513 How can I count the number of occurrences of a substring within a string?
517 How do I capitalize all the words on one line?
521 How can I split a [character] delimited string except when inside [character]?
525 How do I strip blank space from the beginning/end of a string?
529 How do I pad a string with blanks or pad a number with zeroes?
533 How do I extract selected columns from a string?
537 How do I find the soundex value of a string?
541 How can I expand variables in text strings?
545 What's wrong with always quoting "$vars"?
549 Why don't my E<lt>E<lt>HERE documents work?
553 What is the difference between a list and an array?
557 What is the difference between $array[1] and @array[1]?
561 How can I remove duplicate elements from a list or array?
565 How can I tell whether a certain element is contained in a list or array?
569 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
573 How do I test whether two arrays or hashes are equal?
577 How do I find the first array element for which a condition is true?
581 How do I handle linked lists?
585 How do I handle circular lists?
589 How do I shuffle an array randomly?
593 How do I process/modify each element of an array?
597 How do I select a random element from an array?
601 How do I permute N elements of a list?
605 How do I sort an array by (anything)?
609 How do I manipulate arrays of bits?
613 Why does defined() return true on empty arrays and hashes?
617 How do I process an entire hash?
621 What happens if I add or remove keys from a hash while iterating over it?
625 How do I look up a hash element by value?
629 How can I know how many entries are in a hash?
633 How do I sort a hash (optionally by value instead of key)?
637 How can I always keep my hash sorted?
641 What's the difference between "delete" and "undef" with hashes?
645 Why don't my tied hashes make the defined/exists distinction?
649 How do I reset an each() operation part-way through?
653 How can I get the unique keys from two hashes?
657 How can I store a multidimensional array in a DBM file?
661 How can I make my hash remember the order I put elements into it?
665 Why does passing a subroutine an undefined element in a hash create it?
669 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
673 How can I use a reference as a hash key?
677 How do I handle binary data correctly?
681 How do I determine whether a scalar is a number/whole/integer/float?
685 How do I keep persistent data across program calls?
689 How do I print out or copy a recursive data structure?
693 How do I define methods for every class/object?
697 How do I verify a credit card checksum?
701 How do I pack arrays of doubles or floats for XS code?
706 =head2 L<perlfaq5>: Files and Formats
708 I/O and the "f" issues: filehandles, flushing, formats, and footers.
714 How do I flush/unbuffer an output filehandle? Why must I do this?
718 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?
722 How do I count the number of lines in a file?
726 How can I use Perl's C<-i> option from within a program?
730 How can I copy a file?
734 How do I make a temporary file name?
738 How can I manipulate fixed-record-length files?
742 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?
746 How can I use a filehandle indirectly?
750 How can I set up a footer format to be used with write()?
754 How can I write() into a string?
758 How can I output my numbers with commas added?
762 How can I translate tildes (~) in a filename?
766 How come when I open a file read-write it wipes it out?
770 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
774 Is there a leak/bug in glob()?
778 How can I open a file with a leading ">" or trailing blanks?
782 How can I reliably rename a file?
786 How can I lock a file?
790 Why can't I just open(FH, "E<gt>file.lock")?
794 I still don't get locking. I just want to increment the number in the file. How can I do this?
798 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?
802 How do I randomly update a binary file?
806 How do I get a file's timestamp in perl?
810 How do I set a file's timestamp in perl?
814 How do I print to more than one file at once?
818 How can I read in an entire file all at once?
822 How can I read in a file by paragraphs?
826 How can I read a single character from a file? From the keyboard?
830 How can I tell whether there's a character waiting on a filehandle?
834 How do I do a C<tail -f> in perl?
838 How do I dup() a filehandle in Perl?
842 How do I close a file descriptor by number?
846 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
850 Why doesn't glob("*.*") get all the files?
854 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
858 How do I select a random line from a file?
862 Why do I get weird spaces when I print an array of lines?
867 =head2 L<perlfaq6>: Regular Expressions
869 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).
875 How can I hope to use regular expressions without creating illegible and unmaintainable code?
879 I'm having trouble matching over more than one line. What's wrong?
883 How can I pull out lines between two patterns that are themselves on different lines?
887 I put a regular expression into $/ but it didn't work. What's wrong?
891 How do I substitute case insensitively on the LHS while preserving case on the RHS?
895 How can I make C<\w> match national character sets?
899 How can I match a locale-smart version of C</[a-zA-Z]/>?
903 How can I quote a variable to use in a regex?
907 What is C</o> really for?
911 How do I use a regular expression to strip C style comments from a file?
915 Can I use Perl regular expressions to match balanced text?
919 What does it mean that regexes are greedy? How can I get around it?
923 How do I process each word on each line?
927 How can I print out a word-frequency or line-frequency summary?
931 How can I do approximate matching?
935 How do I efficiently match many regular expressions at once?
939 Why don't word-boundary searches with C<\b> work for me?
943 Why does using $&, $`, or $' slow my program down?
947 What good is C<\G> in a regular expression?
951 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
955 What's wrong with using grep in a void context?
959 How can I match strings with multibyte characters?
963 How do I match a pattern that is supplied by the user?
968 =head2 L<perlfaq7>: General Perl Language Issues
970 General Perl language issues that don't clearly fit into any of the other sections.
976 Can I get a BNF/yacc/RE for the Perl language?
980 What are all these $@%&* punctuation signs, and how do I know when to use them?
984 Do I always/never have to quote my strings or use semicolons and commas?
988 How do I skip some return values?
992 How do I temporarily block warnings?
1000 Why do Perl operators have different precedence than C operators?
1004 How do I declare/create a structure?
1008 How do I create a module?
1012 How do I create a class?
1016 How can I tell if a variable is tainted?
1024 What is variable suicide and how can I prevent it?
1028 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1032 How do I create a static variable?
1036 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1040 How can I access a dynamic variable while a similarly named lexical is in scope?
1044 What's the difference between deep and shallow binding?
1048 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
1052 How do I redefine a builtin function, operator, or method?
1056 What's the difference between calling a function as &foo and foo()?
1060 How do I create a switch or case statement?
1064 How can I catch accesses to undefined variables, functions, or methods?
1068 Why can't a method included in this same file be found?
1072 How can I find out my current package?
1076 How can I comment out a large block of perl code?
1080 How do I clear a package?
1084 How can I use a variable as a variable name?
1088 What does "bad interpreter" mean?
1093 =head2 L<perlfaq8>: System Interaction
1095 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.
1101 How do I find out which operating system I'm running under?
1105 How come exec() doesn't return?
1109 How do I do fancy stuff with the keyboard/screen/mouse?
1113 How do I print something out in color?
1117 How do I read just one key without waiting for a return key?
1121 How do I check whether input is ready on the keyboard?
1125 How do I clear the screen?
1129 How do I get the screen size?
1133 How do I ask the user for a password?
1137 How do I read and write the serial port?
1141 How do I decode encrypted password files?
1145 How do I start a process in the background?
1149 How do I trap control characters/signals?
1153 How do I modify the shadow password file on a Unix system?
1157 How do I set the time and date?
1161 How can I sleep() or alarm() for under a second?
1165 How can I measure time under a second?
1169 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
1173 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1177 How can I call my system's unique C functions from Perl?
1181 Where do I get the include files to do ioctl() or syscall()?
1185 Why do setuid perl scripts complain about kernel problems?
1189 How can I open a pipe both to and from a command?
1193 Why can't I get the output of a command with system()?
1197 How can I capture STDERR from an external command?
1201 Why doesn't open() return an error when a pipe open fails?
1205 What's wrong with using backticks in a void context?
1209 How can I call backticks without shell processing?
1213 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1217 How can I convert my shell script to perl?
1221 Can I use perl to run a telnet or ftp session?
1225 How can I write expect in Perl?
1229 Is there a way to hide perl's command line from programs such as "ps"?
1233 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?
1237 How do I close a process's filehandle without waiting for it to complete?
1241 How do I fork a daemon process?
1245 How do I find out if I'm running interactively or not?
1249 How do I timeout a slow event?
1253 How do I set CPU limits?
1257 How do I avoid zombies on a Unix system?
1261 How do I use an SQL database?
1265 How do I make a system() exit on control-C?
1269 How do I open a file without blocking?
1273 How do I tell the difference between errors from the shell and perl?
1277 How do I install a module from CPAN?
1281 What's the difference between require and use?
1285 How do I keep my own module/library directory?
1289 How do I add the directory my program lives in to the module/library search path?
1293 How do I add a directory to my include path (@INC) at runtime?
1297 What is socket.ph and where do I get it?
1302 =head2 L<perlfaq9>: Networking
1304 Networking, the internet, and a few on the web.
1310 What is the correct form of response from a CGI script?
1314 My CGI script runs from the command line but not the browser. (500 Server Error)
1318 How can I get better error messages from a CGI program?
1322 How do I remove HTML from a string?
1326 How do I extract URLs?
1330 How do I download a file from the user's machine? How do I open a file on another machine?
1334 How do I make a pop-up menu in HTML?
1338 How do I fetch an HTML file?
1342 How do I automate an HTML form submission?
1346 How do I decode or create those %-encodings on the web?
1350 How do I redirect to another page?
1354 How do I put a password on my web pages?
1358 How do I edit my .htpasswd and .htgroup files with Perl?
1362 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1366 How do I parse a mail header?
1370 How do I decode a CGI form?
1374 How do I check a valid mail address?
1378 How do I decode a MIME/BASE64 string?
1382 How do I return the user's mail address?
1390 How do I use MIME to make an attachment to a mail message?
1398 How do I find out my hostname/domainname/IP address?
1402 How do I fetch a news article or the active newsgroups?
1406 How do I fetch/put an FTP file?
1410 How can I do RPC in Perl?