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 The perlfaq documents are in the perl source tree (see
27 L<perlrepository>). The git repository notes all changes to the FAQ and
28 holds the latest version of the working documents and may vary
29 significantly from the version distributed with the latest version of
30 Perl. Check the repository before sending your corrections.
32 =head2 How to contribute to the perlfaq
34 You can mail corrections, additions, and suggestions to
35 C<< <perlfaq-workers AT perl DOT org> >>. The perlfaq volunteers use this
36 address to coordinate their efforts and track the perlfaq development.
37 They appreciate your contributions to the FAQ but do not have time to
38 provide individual help, so don't use this address to ask FAQs.
40 The perlfaq server posts extracts of the perlfaq to that newsgroup
41 every 6 hours (or so), and the community of volunteers reviews and
42 updates the answers. If you'd like to help review and update the
43 answers, check out comp.lang.perl.misc.
45 You can also fork the perl repository, make your changes, and send them
46 to Perl 5 Porters. See L<perlrepository>.
48 =head2 What will happen if you mail your Perl programming problems to the authors?
50 The perlfaq-workers like to keep all traffic on the perlfaq-workers list
51 so that everyone can see the work being done (and the work that needs to
52 be done). The mailing list serves as an official record. If you email the
53 authors or maintainers directly, you'll probably get a reply asking you
54 to post to the mailing list. If you don't get a reply, it probably means
55 that the person never saw the message or didn't have time to deal with
56 it. Posting to the list allows the volunteers with time to deal with it
59 If you have a question that isn't in the FAQ and you would like help with
60 it, try the resources in L<perlfaq2>.
64 Tom Christiansen wrote the original perlfaq then expanded it with the
65 help of Nat Torkington. The perlfaq-workers maintain current document
66 and the denizens of comp.lang.perl.misc regularly review and update the
67 FAQ. Several people have contributed answers, corrections, and comments,
68 and the perlfaq notes those contributions wherever appropriate.
70 =head1 AUTHOR AND COPYRIGHT
72 Tom Christiansen wrote the original version of this document.
73 brian d foy C<< <bdfoy@cpan.org> >> wrote this version. See the
74 individual perlfaq documents for additional copyright information.
76 This document is available under the same terms as Perl itself. Code
77 examples in all the perlfaq documents are in the public domain. Use
78 them as you see fit (and at your own risk with no warranty from anyone).
80 =head1 Table of Contents
84 =item perlfaq - this document
86 =item perlfaq1 - General Questions About Perl
88 =item perlfaq2 - Obtaining and Learning about Perl
90 =item perlfaq3 - Programming Tools
92 =item perlfaq4 - Data Manipulation
94 =item perlfaq5 - Files and Formats
96 =item perlfaq6 - Regular Expressions
98 =item perlfaq7 - General Perl Language Issues
100 =item perlfaq8 - System Interaction
102 =item perlfaq9 - Networking
108 =head2 L<perlfaq1>: General Questions About Perl
110 Very general, high-level questions about Perl.
120 Who supports Perl? Who develops it? Why is it free?
124 Which version of Perl should I use?
128 What are Perl 4, Perl 5, or Perl 6?
144 Is Perl difficult to learn?
148 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
152 Can I do [task] in Perl?
156 When shouldn't I program in Perl?
160 What's the difference between "perl" and "Perl"?
164 Is it a Perl program or a Perl script?
172 Where can I get a list of Larry Wall witticisms?
176 How can I convince others to use Perl?
181 =head2 L<perlfaq2>: Obtaining and Learning about Perl
183 Where to find source and documentation for Perl, support, and related matters.
189 What machines support perl? Where do I get it?
193 How can I get a binary version of perl?
197 I don't have a C compiler. How can I build my own Perl interpreter?
201 I copied the perl binary from one machine to another, but scripts don't work.
205 I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
209 What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
213 Is there an ISO or ANSI certified version of Perl?
217 Where can I get information on Perl?
221 What are the Perl newsgroups on Usenet? Where do I post questions?
225 Where should I post source code?
233 Which magazines have Perl content?
237 What mailing lists are there for Perl?
241 Where are the archives for comp.lang.perl.misc?
245 Where can I buy a commercial version of perl?
249 Where do I send bug reports?
253 What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org?
258 =head2 L<perlfaq3>: Programming Tools
260 Programmer tools and programming support.
266 How do I do (anything)?
270 How can I use Perl interactively?
274 Is there a Perl shell?
278 How do I find which modules are installed on my system?
282 How do I debug my Perl programs?
286 How do I profile my Perl programs?
290 How do I cross-reference my Perl programs?
294 Is there a pretty-printer (formatter) for Perl?
298 Is there a ctags for Perl?
302 Is there an IDE or Windows Perl Editor?
306 Where can I get Perl macros for vi?
310 Where can I get perl-mode or cperl-mode for emacs?
314 How can I use curses with Perl?
318 How can I write a GUI (X, Tk, Gtk, etc.) in Perl?
322 How can I make my Perl program run faster?
326 How can I make my Perl program take less memory?
330 Is it safe to return a reference to local or lexical data?
334 How can I free an array or hash so my program shrinks?
338 How can I make my CGI script more efficient?
342 How can I hide the source for my Perl program?
346 How can I compile my Perl program into byte code or C?
350 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
354 Can I write useful Perl programs on the command line?
358 Why don't Perl one-liners work on my DOS/Mac/VMS system?
362 Where can I learn about CGI or Web programming in Perl?
366 Where can I learn about object-oriented Perl programming?
370 Where can I learn about linking C with Perl?
374 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?
378 When I tried to run my script, I got this message. What does it mean?
387 =head2 L<perlfaq4>: Data Manipulation
389 Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
395 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
403 Why isn't my octal data interpreted correctly?
407 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
411 How do I convert between numeric representations/bases/radixes?
415 Why doesn't & work the way I want it to?
419 How do I multiply matrices?
423 How do I perform an operation on a series of integers?
427 How can I output Roman numerals?
431 Why aren't my random numbers random?
435 How do I get a random number between X and Y?
439 How do I find the day or week of the year?
443 How do I find the current century or millennium?
447 How can I compare two dates and find the difference?
451 How can I take a string and turn it into epoch seconds?
455 How can I find the Julian Day?
459 How do I find yesterday's date?
463 Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?
467 How do I validate input?
471 How do I unescape a string?
475 How do I remove consecutive pairs of characters?
479 How do I expand function calls in a string?
483 How do I find matching/nesting anything?
487 How do I reverse a string?
491 How do I expand tabs in a string?
495 How do I reformat a paragraph?
499 How can I access or change N characters of a string?
503 How do I change the Nth occurrence of something?
507 How can I count the number of occurrences of a substring within a string?
511 How do I capitalize all the words on one line?
515 How can I split a [character] delimited string except when inside [character]?
519 How do I strip blank space from the beginning/end of a string?
523 How do I pad a string with blanks or pad a number with zeroes?
527 How do I extract selected columns from a string?
531 How do I find the soundex value of a string?
535 How can I expand variables in text strings?
539 What's wrong with always quoting "$vars"?
543 Why don't my E<lt>E<lt>HERE documents work?
547 What is the difference between a list and an array?
551 What is the difference between $array[1] and @array[1]?
555 How can I remove duplicate elements from a list or array?
559 How can I tell whether a certain element is contained in a list or array?
563 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
567 How do I test whether two arrays or hashes are equal?
571 How do I find the first array element for which a condition is true?
575 How do I handle linked lists?
579 How do I handle circular lists?
583 How do I shuffle an array randomly?
587 How do I process/modify each element of an array?
591 How do I select a random element from an array?
595 How do I permute N elements of a list?
599 How do I sort an array by (anything)?
603 How do I manipulate arrays of bits?
607 Why does defined() return true on empty arrays and hashes?
611 How do I process an entire hash?
615 How do I merge two hashes?
619 What happens if I add or remove keys from a hash while iterating over it?
623 How do I look up a hash element by value?
627 How can I know how many entries are in a hash?
631 How do I sort a hash (optionally by value instead of key)?
635 How can I always keep my hash sorted?
639 What's the difference between "delete" and "undef" with hashes?
643 Why don't my tied hashes make the defined/exists distinction?
647 How do I reset an each() operation part-way through?
651 How can I get the unique keys from two hashes?
655 How can I store a multidimensional array in a DBM file?
659 How can I make my hash remember the order I put elements into it?
663 Why does passing a subroutine an undefined element in a hash create it?
667 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
671 How can I use a reference as a hash key?
675 How can I check if a key exists in a multilevel hash?
679 How do I handle binary data correctly?
683 How do I determine whether a scalar is a number/whole/integer/float?
687 How do I keep persistent data across program calls?
691 How do I print out or copy a recursive data structure?
695 How do I define methods for every class/object?
699 How do I verify a credit card checksum?
703 How do I pack arrays of doubles or floats for XS code?
708 =head2 L<perlfaq5>: Files and Formats
710 I/O and the "f" issues: filehandles, flushing, formats, and footers.
716 How do I flush/unbuffer an output filehandle? Why must I do this?
720 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
724 How do I count the number of lines in a file?
728 How do I delete the last N lines from a file?
732 How can I use Perl's C<-i> option from within a program?
736 How can I copy a file?
740 How do I make a temporary file name?
744 How can I manipulate fixed-record-length files?
748 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?
752 How can I use a filehandle indirectly?
756 How can I set up a footer format to be used with write()?
760 How can I write() into a string?
764 How can I open a filehandle to a string?
768 How can I output my numbers with commas added?
772 How can I translate tildes (~) in a filename?
776 How come when I open a file read-write it wipes it out?
780 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
784 Is there a leak/bug in glob()?
788 How can I open a file with a leading ">" or trailing blanks?
792 How can I reliably rename a file?
796 How can I lock a file?
800 Why can't I just open(FH, "E<gt>file.lock")?
804 I still don't get locking. I just want to increment the number in the file. How can I do this?
808 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?
812 How do I randomly update a binary file?
816 How do I get a file's timestamp in perl?
820 How do I set a file's timestamp in perl?
824 How do I print to more than one file at once?
828 How can I read in an entire file all at once?
832 How can I read in a file by paragraphs?
836 How can I read a single character from a file? From the keyboard?
840 How can I tell whether there's a character waiting on a filehandle?
844 How do I do a C<tail -f> in perl?
848 How do I dup() a filehandle in Perl?
852 How do I close a file descriptor by number?
856 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
860 Why doesn't glob("*.*") get all the files?
864 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
868 How do I select a random line from a file?
872 Why do I get weird spaces when I print an array of lines?
876 How do I traverse a directory tree?
880 How do I delete a directory tree?
884 How do I copy an entire directory?
889 =head2 L<perlfaq6>: Regular Expressions
891 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).
897 How can I hope to use regular expressions without creating illegible and unmaintainable code?
901 I'm having trouble matching over more than one line. What's wrong?
905 How can I pull out lines between two patterns that are themselves on different lines?
909 How do I match XML, HTML, or other nasty, ugly things with a regex?
913 I put a regular expression into $/ but it didn't work. What's wrong?
917 How do I substitute case insensitively on the LHS while preserving case on the RHS?
921 How can I make C<\w> match national character sets?
925 How can I match a locale-smart version of C</[a-zA-Z]/>?
929 How can I quote a variable to use in a regex?
933 What is C</o> really for?
937 How do I use a regular expression to strip C style comments from a file?
941 Can I use Perl regular expressions to match balanced text?
945 What does it mean that regexes are greedy? How can I get around it?
949 How do I process each word on each line?
953 How can I print out a word-frequency or line-frequency summary?
957 How can I do approximate matching?
961 How do I efficiently match many regular expressions at once?
965 Why don't word-boundary searches with C<\b> work for me?
969 Why does using $&, $`, or $' slow my program down?
973 What good is C<\G> in a regular expression?
977 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
981 What's wrong with using grep in a void context?
985 How can I match strings with multibyte characters?
989 How do I match a regular expression that's in a variable?
994 =head2 L<perlfaq7>: General Perl Language Issues
996 General Perl language issues that don't clearly fit into any of the other sections.
1002 Can I get a BNF/yacc/RE for the Perl language?
1006 What are all these $@%&* punctuation signs, and how do I know when to use them?
1010 Do I always/never have to quote my strings or use semicolons and commas?
1014 How do I skip some return values?
1018 How do I temporarily block warnings?
1022 What's an extension?
1026 Why do Perl operators have different precedence than C operators?
1030 How do I declare/create a structure?
1034 How do I create a module?
1038 How do I adopt or take over a module already on CPAN?
1042 How do I create a class?
1046 How can I tell if a variable is tainted?
1054 What is variable suicide and how can I prevent it?
1058 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1062 How do I create a static variable?
1066 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1070 How can I access a dynamic variable while a similarly named lexical is in scope?
1074 What's the difference between deep and shallow binding?
1078 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
1082 How do I redefine a builtin function, operator, or method?
1086 What's the difference between calling a function as &foo and foo()?
1090 How do I create a switch or case statement?
1094 How can I catch accesses to undefined variables, functions, or methods?
1098 Why can't a method included in this same file be found?
1102 How can I find out my current or calling package?
1106 How can I comment out a large block of Perl code?
1110 How do I clear a package?
1114 How can I use a variable as a variable name?
1118 What does "bad interpreter" mean?
1123 =head2 L<perlfaq8>: System Interaction
1125 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.
1131 How do I find out which operating system I'm running under?
1135 How come exec() doesn't return?
1139 How do I do fancy stuff with the keyboard/screen/mouse?
1143 How do I print something out in color?
1147 How do I read just one key without waiting for a return key?
1151 How do I check whether input is ready on the keyboard?
1155 How do I clear the screen?
1159 How do I get the screen size?
1163 How do I ask the user for a password?
1167 How do I read and write the serial port?
1171 How do I decode encrypted password files?
1175 How do I start a process in the background?
1179 How do I trap control characters/signals?
1183 How do I modify the shadow password file on a Unix system?
1187 How do I set the time and date?
1191 How can I sleep() or alarm() for under a second?
1195 How can I measure time under a second?
1199 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
1203 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1207 How can I call my system's unique C functions from Perl?
1211 Where do I get the include files to do ioctl() or syscall()?
1215 Why do setuid perl scripts complain about kernel problems?
1219 How can I open a pipe both to and from a command?
1223 Why can't I get the output of a command with system()?
1227 How can I capture STDERR from an external command?
1231 Why doesn't open() return an error when a pipe open fails?
1235 What's wrong with using backticks in a void context?
1239 How can I call backticks without shell processing?
1243 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1247 How can I convert my shell script to perl?
1251 Can I use perl to run a telnet or ftp session?
1255 How can I write expect in Perl?
1259 Is there a way to hide perl's command line from programs such as "ps"?
1263 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?
1267 How do I close a process's filehandle without waiting for it to complete?
1271 How do I fork a daemon process?
1275 How do I find out if I'm running interactively or not?
1279 How do I timeout a slow event?
1283 How do I set CPU limits?
1287 How do I avoid zombies on a Unix system?
1291 How do I use an SQL database?
1295 How do I make a system() exit on control-C?
1299 How do I open a file without blocking?
1303 How do I tell the difference between errors from the shell and perl?
1307 How do I install a module from CPAN?
1311 What's the difference between require and use?
1315 How do I keep my own module/library directory?
1319 How do I add the directory my program lives in to the module/library search path?
1323 How do I add a directory to my include path (@INC) at runtime?
1327 What is socket.ph and where do I get it?
1332 =head2 L<perlfaq9>: Networking
1334 Networking, the internet, and a few on the web.
1340 What is the correct form of response from a CGI script?
1344 My CGI script runs from the command line but not the browser. (500 Server Error)
1348 How can I get better error messages from a CGI program?
1352 How do I remove HTML from a string?
1356 How do I extract URLs?
1360 How do I download a file from the user's machine? How do I open a file on another machine?
1364 How do I make an HTML pop-up menu with Perl?
1368 How do I fetch an HTML file?
1372 How do I automate an HTML form submission?
1376 How do I decode or create those %-encodings on the web?
1380 How do I redirect to another page?
1384 How do I put a password on my web pages?
1388 How do I edit my .htpasswd and .htgroup files with Perl?
1392 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1396 How do I parse a mail header?
1400 How do I decode a CGI form?
1404 How do I check a valid mail address?
1408 How do I decode a MIME/BASE64 string?
1412 How do I return the user's mail address?
1420 How do I use MIME to make an attachment to a mail message?
1428 How do I find out my hostname, domainname, or IP address?
1432 How do I fetch a news article or the active newsgroups?
1436 How do I fetch/put an FTP file?
1440 How can I do RPC in Perl?
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