3 perlfaq - frequently asked questions about Perl
7 The perlfaq comprises several documents that answer the most commonly
8 asked questions about Perl and Perl programming. It's divided by topic
9 into nine major sections outlined in this document.
11 =head2 Where to get the perlfaq
13 The perlfaq comes with the standard Perl distribution, so if you have Perl
14 you should have the perlfaq. You should also have the C<perldoc> tool
15 that lets you read the L<perlfaq>:
19 Besides your local system, you can find the perlfaq on the web, including
20 at http://perldoc.perl.org/ .
22 The perlfaq is an evolving document and you can read the latest version
23 at http://faq.perl.org/ . The perlfaq-workers periodically post extracts
24 of the latest perlfaq to comp.lang.perl.misc.
26 You can view the source tree at
27 https://github.com/briandfoy/perlfaq (which is outside of the
28 main Perl source tree). The git repository notes all changes to the FAQ
29 and holds the latest version of the working documents and may vary
30 significantly from the version distributed with the latest version of
31 Perl. Check the repository before sending your corrections.
33 =head2 How to contribute to the perlfaq
35 You can mail corrections, additions, and suggestions to
36 C<< <perlfaq-workers AT perl DOT org> >>. The perlfaq volunteers use this
37 address to coordinate their efforts and track the perlfaq development.
38 They appreciate your contributions to the FAQ but do not have time to
39 provide individual help, so don't use this address to ask FAQs.
41 The perlfaq server posts extracts of the perlfaq to that newsgroup every
42 6 hours (or so), and the community of volunteers reviews and updates the
43 answers. If you'd like to help review and update the answers, check out
46 You can also fork the git repository for the perlfaq and send a pull
47 request so the main repository can pull your changes. The repository
50 https://github.com/briandfoy/perlfaq
52 =head2 What will happen if you mail your Perl programming problems to the authors?
54 The perlfaq-workers like to keep all traffic on the perlfaq-workers list
55 so that everyone can see the work being done (and the work that needs to
56 be done). The mailing list serves as an official record. If you email the
57 authors or maintainers directly, you'll probably get a reply asking you
58 to post to the mailing list. If you don't get a reply, it probably means
59 that the person never saw the message or didn't have time to deal with
60 it. Posting to the list allows the volunteers with time to deal with it
63 If you have a question that isn't in the FAQ and you would like help with
64 it, try the resources in L<perlfaq2>.
68 Tom Christiansen wrote the original perlfaq then expanded it with the
69 help of Nat Torkington. The perlfaq-workers maintain current document
70 and the denizens of comp.lang.perl.misc regularly review and update the
71 FAQ. Several people have contributed answers, corrections, and comments,
72 and the perlfaq notes those contributions wherever appropriate.
74 =head1 AUTHOR AND COPYRIGHT
76 Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and
77 other authors as noted. All rights reserved.
79 Tom Christainsen wrote the original version of this document.
80 brian d foy C<< <bdfoy@cpan.org> >> wrote this version. See the
81 individual perlfaq documents for additional copyright information.
83 This document is available under the same terms as Perl itself. Code
84 examples in all the perlfaq documents are in the public domain. Use
85 them as you see fit and at your own risk with no warranty from anyone.
87 =head1 Table of Contents
91 =item perlfaq - this document
93 =item perlfaq1 - General Questions About Perl
95 =item perlfaq2 - Obtaining and Learning about Perl
97 =item perlfaq3 - Programming Tools
99 =item perlfaq4 - Data Manipulation
101 =item perlfaq5 - Files and Formats
103 =item perlfaq6 - Regular Expressions
105 =item perlfaq7 - General Perl Language Issues
107 =item perlfaq8 - System Interaction
109 =item perlfaq9 - Networking
115 =head2 L<perlfaq1>: General Questions About Perl
117 Very general, high-level questions about Perl.
127 Who supports Perl? Who develops it? Why is it free?
131 Which version of Perl should I use?
135 What are Perl 4, Perl 5, or Perl 6?
151 Is Perl difficult to learn?
155 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
159 Can I do [task] in Perl?
163 When shouldn't I program in Perl?
167 What's the difference between "perl" and "Perl"?
171 Is it a Perl program or a Perl script?
179 Where can I get a list of Larry Wall witticisms?
183 How can I convince others to use Perl?
188 =head2 L<perlfaq2>: Obtaining and Learning about Perl
190 Where to find source and documentation for Perl, support, and related matters.
196 What machines support perl? Where do I get it?
200 How can I get a binary version of perl?
204 I don't have a C compiler. How can I build my own Perl interpreter?
208 I copied the perl binary from one machine to another, but scripts don't work.
212 I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
216 What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
220 Is there an ISO or ANSI certified version of Perl?
224 Where can I get information on Perl?
228 What are the Perl newsgroups on Usenet? Where do I post questions?
232 Where should I post source code?
240 Which magazines have Perl content?
244 What mailing lists are there for Perl?
248 Where are the archives for comp.lang.perl.misc?
252 Where can I buy a commercial version of perl?
256 Where do I send bug reports?
260 What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org?
265 =head2 L<perlfaq3>: Programming Tools
267 Programmer tools and programming support.
273 How do I do (anything)?
277 How can I use Perl interactively?
281 Is there a Perl shell?
285 How do I find which modules are installed on my system?
289 How do I debug my Perl programs?
293 How do I profile my Perl programs?
297 How do I cross-reference my Perl programs?
301 Is there a pretty-printer (formatter) for Perl?
305 Is there a ctags for Perl?
309 Is there an IDE or Windows Perl Editor?
313 Where can I get Perl macros for vi?
317 Where can I get perl-mode for emacs?
321 How can I use curses with Perl?
325 How can I write a GUI (X, Tk, Gtk, etc.) in Perl?
329 How can I make my Perl program run faster?
333 How can I make my Perl program take less memory?
337 Is it safe to return a reference to local or lexical data?
341 How can I free an array or hash so my program shrinks?
345 How can I make my CGI script more efficient?
349 How can I hide the source for my Perl program?
353 How can I compile my Perl program into byte code or C?
357 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
361 Can I write useful Perl programs on the command line?
365 Why don't Perl one-liners work on my DOS/Mac/VMS system?
369 Where can I learn about CGI or Web programming in Perl?
373 Where can I learn about object-oriented Perl programming?
377 Where can I learn about linking C with Perl?
381 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?
385 When I tried to run my script, I got this message. What does it mean?
394 =head2 L<perlfaq4>: Data Manipulation
396 Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
402 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
410 Why isn't my octal data interpreted correctly?
414 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
418 How do I convert between numeric representations/bases/radixes?
422 Why doesn't & work the way I want it to?
426 How do I multiply matrices?
430 How do I perform an operation on a series of integers?
434 How can I output Roman numerals?
438 Why aren't my random numbers random?
442 How do I get a random number between X and Y?
446 How do I find the day or week of the year?
450 How do I find the current century or millennium?
454 How can I compare two dates and find the difference?
458 How can I take a string and turn it into epoch seconds?
462 How can I find the Julian Day?
466 How do I find yesterday's date?
470 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
474 How do I validate input?
478 How do I unescape a string?
482 How do I remove consecutive pairs of characters?
486 How do I expand function calls in a string?
490 How do I find matching/nesting anything?
494 How do I reverse a string?
498 How do I expand tabs in a string?
502 How do I reformat a paragraph?
506 How can I access or change N characters of a string?
510 How do I change the Nth occurrence of something?
514 How can I count the number of occurrences of a substring within a string?
518 How do I capitalize all the words on one line?
522 How can I split a [character] delimited string except when inside [character]?
526 How do I strip blank space from the beginning/end of a string?
530 How do I pad a string with blanks or pad a number with zeroes?
534 How do I extract selected columns from a string?
538 How do I find the soundex value of a string?
542 How can I expand variables in text strings?
546 What's wrong with always quoting "$vars"?
550 Why don't my E<lt>E<lt>HERE documents work?
554 What is the difference between a list and an array?
558 What is the difference between $array[1] and @array[1]?
562 How can I remove duplicate elements from a list or array?
566 How can I tell whether a certain element is contained in a list or array?
570 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
574 How do I test whether two arrays or hashes are equal?
578 How do I find the first array element for which a condition is true?
582 How do I handle linked lists?
586 How do I handle circular lists?
590 How do I shuffle an array randomly?
594 How do I process/modify each element of an array?
598 How do I select a random element from an array?
602 How do I permute N elements of a list?
606 How do I sort an array by (anything)?
610 How do I manipulate arrays of bits?
614 Why does defined() return true on empty arrays and hashes?
618 How do I process an entire hash?
622 How do I merge two hashes?
626 What happens if I add or remove keys from a hash while iterating over it?
630 How do I look up a hash element by value?
634 How can I know how many entries are in a hash?
638 How do I sort a hash (optionally by value instead of key)?
642 How can I always keep my hash sorted?
646 What's the difference between "delete" and "undef" with hashes?
650 Why don't my tied hashes make the defined/exists distinction?
654 How do I reset an each() operation part-way through?
658 How can I get the unique keys from two hashes?
662 How can I store a multidimensional array in a DBM file?
666 How can I make my hash remember the order I put elements into it?
670 Why does passing a subroutine an undefined element in a hash create it?
674 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
678 How can I use a reference as a hash key?
682 How do I handle binary data correctly?
686 How do I determine whether a scalar is a number/whole/integer/float?
690 How do I keep persistent data across program calls?
694 How do I print out or copy a recursive data structure?
698 How do I define methods for every class/object?
702 How do I verify a credit card checksum?
706 How do I pack arrays of doubles or floats for XS code?
711 =head2 L<perlfaq5>: Files and Formats
713 I/O and the "f" issues: filehandles, flushing, formats, and footers.
719 How do I flush/unbuffer an output filehandle? Why must I do this?
723 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
727 How do I count the number of lines in a file?
731 How can I use Perl's C<-i> option from within a program?
735 How can I copy a file?
739 How do I make a temporary file name?
743 How can I manipulate fixed-record-length files?
747 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?
751 How can I use a filehandle indirectly?
755 How can I set up a footer format to be used with write()?
759 How can I write() into a string?
763 How can I open a filehandle to a string?
767 How can I output my numbers with commas added?
771 How can I translate tildes (~) in a filename?
775 How come when I open a file read-write it wipes it out?
779 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
783 Is there a leak/bug in glob()?
787 How can I open a file with a leading ">" or trailing blanks?
791 How can I reliably rename a file?
795 How can I lock a file?
799 Why can't I just open(FH, "E<gt>file.lock")?
803 I still don't get locking. I just want to increment the number in the file. How can I do this?
807 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?
811 How do I randomly update a binary file?
815 How do I get a file's timestamp in perl?
819 How do I set a file's timestamp in perl?
823 How do I print to more than one file at once?
827 How can I read in an entire file all at once?
831 How can I read in a file by paragraphs?
835 How can I read a single character from a file? From the keyboard?
839 How can I tell whether there's a character waiting on a filehandle?
843 How do I do a C<tail -f> in perl?
847 How do I dup() a filehandle in Perl?
851 How do I close a file descriptor by number?
855 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
859 Why doesn't glob("*.*") get all the files?
863 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
867 How do I select a random line from a file?
871 Why do I get weird spaces when I print an array of lines?
875 How do I traverse a directory tree?
879 How do I delete a directory tree?
883 How do I copy an entire directory?
888 =head2 L<perlfaq6>: Regular Expressions
890 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).
896 How can I hope to use regular expressions without creating illegible and unmaintainable code?
900 I'm having trouble matching over more than one line. What's wrong?
904 How can I pull out lines between two patterns that are themselves on different lines?
908 How do I match XML, HTML, or other nasty, ugly things with a regex?
912 I put a regular expression into $/ but it didn't work. What's wrong?
916 How do I substitute case insensitively on the LHS while preserving case on the RHS?
920 How can I make C<\w> match national character sets?
924 How can I match a locale-smart version of C</[a-zA-Z]/>?
928 How can I quote a variable to use in a regex?
932 What is C</o> really for?
936 How do I use a regular expression to strip C style comments from a file?
940 Can I use Perl regular expressions to match balanced text?
944 What does it mean that regexes are greedy? How can I get around it?
948 How do I process each word on each line?
952 How can I print out a word-frequency or line-frequency summary?
956 How can I do approximate matching?
960 How do I efficiently match many regular expressions at once?
964 Why don't word-boundary searches with C<\b> work for me?
968 Why does using $&, $`, or $' slow my program down?
972 What good is C<\G> in a regular expression?
976 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
980 What's wrong with using grep in a void context?
984 How can I match strings with multibyte characters?
988 How do I match a regular expression that's in a variable?
993 =head2 L<perlfaq7>: General Perl Language Issues
995 General Perl language issues that don't clearly fit into any of the other sections.
1001 Can I get a BNF/yacc/RE for the Perl language?
1005 What are all these $@%&* punctuation signs, and how do I know when to use them?
1009 Do I always/never have to quote my strings or use semicolons and commas?
1013 How do I skip some return values?
1017 How do I temporarily block warnings?
1021 What's an extension?
1025 Why do Perl operators have different precedence than C operators?
1029 How do I declare/create a structure?
1033 How do I create a module?
1037 How do I adopt or take over a module already on CPAN?
1041 How do I create a class?
1045 How can I tell if a variable is tainted?
1053 What is variable suicide and how can I prevent it?
1057 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1061 How do I create a static variable?
1065 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1069 How can I access a dynamic variable while a similarly named lexical is in scope?
1073 What's the difference between deep and shallow binding?
1077 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
1081 How do I redefine a builtin function, operator, or method?
1085 What's the difference between calling a function as &foo and foo()?
1089 How do I create a switch or case statement?
1093 How can I catch accesses to undefined variables, functions, or methods?
1097 Why can't a method included in this same file be found?
1101 How can I find out my current or calling package?
1105 How can I comment out a large block of perl code?
1109 How do I clear a package?
1113 How can I use a variable as a variable name?
1117 What does "bad interpreter" mean?
1122 =head2 L<perlfaq8>: System Interaction
1124 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.
1130 How do I find out which operating system I'm running under?
1134 How come exec() doesn't return?
1138 How do I do fancy stuff with the keyboard/screen/mouse?
1142 How do I print something out in color?
1146 How do I read just one key without waiting for a return key?
1150 How do I check whether input is ready on the keyboard?
1154 How do I clear the screen?
1158 How do I get the screen size?
1162 How do I ask the user for a password?
1166 How do I read and write the serial port?
1170 How do I decode encrypted password files?
1174 How do I start a process in the background?
1178 How do I trap control characters/signals?
1182 How do I modify the shadow password file on a Unix system?
1186 How do I set the time and date?
1190 How can I sleep() or alarm() for under a second?
1194 How can I measure time under a second?
1198 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
1202 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1206 How can I call my system's unique C functions from Perl?
1210 Where do I get the include files to do ioctl() or syscall()?
1214 Why do setuid perl scripts complain about kernel problems?
1218 How can I open a pipe both to and from a command?
1222 Why can't I get the output of a command with system()?
1226 How can I capture STDERR from an external command?
1230 Why doesn't open() return an error when a pipe open fails?
1234 What's wrong with using backticks in a void context?
1238 How can I call backticks without shell processing?
1242 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1246 How can I convert my shell script to perl?
1250 Can I use perl to run a telnet or ftp session?
1254 How can I write expect in Perl?
1258 Is there a way to hide perl's command line from programs such as "ps"?
1262 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?
1266 How do I close a process's filehandle without waiting for it to complete?
1270 How do I fork a daemon process?
1274 How do I find out if I'm running interactively or not?
1278 How do I timeout a slow event?
1282 How do I set CPU limits?
1286 How do I avoid zombies on a Unix system?
1290 How do I use an SQL database?
1294 How do I make a system() exit on control-C?
1298 How do I open a file without blocking?
1302 How do I tell the difference between errors from the shell and perl?
1306 How do I install a module from CPAN?
1310 What's the difference between require and use?
1314 How do I keep my own module/library directory?
1318 How do I add the directory my program lives in to the module/library search path?
1322 How do I add a directory to my include path (@INC) at runtime?
1326 What is socket.ph and where do I get it?
1331 =head2 L<perlfaq9>: Networking
1333 Networking, the internet, and a few on the web.
1339 What is the correct form of response from a CGI script?
1343 My CGI script runs from the command line but not the browser. (500 Server Error)
1347 How can I get better error messages from a CGI program?
1351 How do I remove HTML from a string?
1355 How do I extract URLs?
1359 How do I download a file from the user's machine? How do I open a file on another machine?
1363 How do I make an HTML pop-up menu with Perl?
1367 How do I fetch an HTML file?
1371 How do I automate an HTML form submission?
1375 How do I decode or create those %-encodings on the web?
1379 How do I redirect to another page?
1383 How do I put a password on my web pages?
1387 How do I edit my .htpasswd and .htgroup files with Perl?
1391 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1395 How do I parse a mail header?
1399 How do I decode a CGI form?
1403 How do I check a valid mail address?
1407 How do I decode a MIME/BASE64 string?
1411 How do I return the user's mail address?
1419 How do I use MIME to make an attachment to a mail message?
1427 How do I find out my hostname, domainname, or IP address?
1431 How do I fetch a news article or the active newsgroups?
1435 How do I fetch/put an FTP file?
1439 How can I do RPC in Perl?
https://perl5.git.perl.org / perl5.git / blob