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<perlgit>). 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<perlgit>.
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?
140 Is Perl difficult to learn?
144 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
148 Can I do [task] in Perl?
152 When shouldn't I program in Perl?
156 What's the difference between "perl" and "Perl"?
160 Is it a Perl program or a Perl script?
168 Where can I get a list of Larry Wall witticisms?
172 How can I convince others to use Perl?
177 =head2 L<perlfaq2>: Obtaining and Learning about Perl
179 Where to find source and documentation for Perl, support, and related matters.
185 What machines support perl? Where do I get it?
189 How can I get a binary version of perl?
193 I don't have a C compiler. How can I build my own Perl interpreter?
197 I copied the perl binary from one machine to another, but scripts don't work.
201 I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
205 What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
209 Is there an ISO or ANSI certified version of Perl?
213 Where can I get information on Perl?
217 What are the Perl newsgroups on Usenet? Where do I post questions?
221 Where should I post source code?
229 Which magazines have Perl content?
233 What mailing lists are there for Perl?
237 Where are the archives for comp.lang.perl.misc?
241 Where can I buy a commercial version of perl?
245 Where do I send bug reports?
249 What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org?
254 =head2 L<perlfaq3>: Programming Tools
256 Programmer tools and programming support.
262 How do I do (anything)?
266 How can I use Perl interactively?
270 Is there a Perl shell?
274 How do I find which modules are installed on my system?
278 How do I debug my Perl programs?
282 How do I profile my Perl programs?
286 How do I cross-reference my Perl programs?
290 Is there a pretty-printer (formatter) for Perl?
294 Is there a ctags for Perl?
298 Is there an IDE or Windows Perl Editor?
302 Where can I get Perl macros for vi?
306 Where can I get perl-mode or cperl-mode for emacs?
310 How can I use curses with Perl?
314 How can I write a GUI (X, Tk, Gtk, etc.) in Perl?
318 How can I make my Perl program run faster?
322 How can I make my Perl program take less memory?
326 Is it safe to return a reference to local or lexical data?
330 How can I free an array or hash so my program shrinks?
334 How can I make my CGI script more efficient?
338 How can I hide the source for my Perl program?
342 How can I compile my Perl program into byte code or C?
346 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
350 Can I write useful Perl programs on the command line?
354 Why don't Perl one-liners work on my DOS/Mac/VMS system?
358 Where can I learn about CGI or Web programming in Perl?
362 Where can I learn about object-oriented Perl programming?
366 Where can I learn about linking C with Perl?
370 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?
374 When I tried to run my script, I got this message. What does it mean?
383 =head2 L<perlfaq4>: Data Manipulation
385 Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
391 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
399 Why isn't my octal data interpreted correctly?
403 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
407 How do I convert between numeric representations/bases/radixes?
411 Why doesn't & work the way I want it to?
415 How do I multiply matrices?
419 How do I perform an operation on a series of integers?
423 How can I output Roman numerals?
427 Why aren't my random numbers random?
431 How do I get a random number between X and Y?
435 How do I find the day or week of the year?
439 How do I find the current century or millennium?
443 How can I compare two dates and find the difference?
447 How can I take a string and turn it into epoch seconds?
451 How can I find the Julian Day?
455 How do I find yesterday's date?
459 Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?
463 How do I validate input?
467 How do I unescape a string?
471 How do I remove consecutive pairs of characters?
475 How do I expand function calls in a string?
479 How do I find matching/nesting anything?
483 How do I reverse a string?
487 How do I expand tabs in a string?
491 How do I reformat a paragraph?
495 How can I access or change N characters of a string?
499 How do I change the Nth occurrence of something?
503 How can I count the number of occurrences of a substring within a string?
507 How do I capitalize all the words on one line?
511 How can I split a [character]-delimited string except when inside [character]?
515 How do I strip blank space from the beginning/end of a string?
519 How do I pad a string with blanks or pad a number with zeroes?
523 How do I extract selected columns from a string?
527 How do I find the soundex value of a string?
531 How can I expand variables in text strings?
535 What's wrong with always quoting "$vars"?
539 Why don't my E<lt>E<lt>HERE documents work?
543 What is the difference between a list and an array?
547 What is the difference between $array[1] and @array[1]?
551 How can I remove duplicate elements from a list or array?
555 How can I tell whether a certain element is contained in a list or array?
559 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
563 How do I test whether two arrays or hashes are equal?
567 How do I find the first array element for which a condition is true?
571 How do I handle linked lists?
575 How do I handle circular lists?
579 How do I shuffle an array randomly?
583 How do I process/modify each element of an array?
587 How do I select a random element from an array?
591 How do I permute N elements of a list?
595 How do I sort an array by (anything)?
599 How do I manipulate arrays of bits?
603 Why does defined() return true on empty arrays and hashes?
607 How do I process an entire hash?
611 How do I merge two hashes?
615 What happens if I add or remove keys from a hash while iterating over it?
619 How do I look up a hash element by value?
623 How can I know how many entries are in a hash?
627 How do I sort a hash (optionally by value instead of key)?
631 How can I always keep my hash sorted?
635 What's the difference between "delete" and "undef" with hashes?
639 Why don't my tied hashes make the defined/exists distinction?
643 How do I reset an each() operation part-way through?
647 How can I get the unique keys from two hashes?
651 How can I store a multidimensional array in a DBM file?
655 How can I make my hash remember the order I put elements into it?
659 Why does passing a subroutine an undefined element in a hash create it?
663 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
667 How can I use a reference as a hash key?
671 How can I check if a key exists in a multilevel hash?
675 How do I handle binary data correctly?
679 How do I determine whether a scalar is a number/whole/integer/float?
683 How do I keep persistent data across program calls?
687 How do I print out or copy a recursive data structure?
691 How do I define methods for every class/object?
695 How do I verify a credit card checksum?
699 How do I pack arrays of doubles or floats for XS code?
704 =head2 L<perlfaq5>: Files and Formats
706 I/O and the "f" issues: filehandles, flushing, formats, and footers.
712 How do I flush/unbuffer an output filehandle? Why must I do this?
716 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
720 How do I count the number of lines in a file?
724 How do I delete the last N lines from a file?
728 How can I use Perl's C<-i> option from within a program?
732 How can I copy a file?
736 How do I make a temporary file name?
740 How can I manipulate fixed-record-length files?
744 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?
748 How can I use a filehandle indirectly?
752 How can I set up a footer format to be used with write()?
756 How can I write() into a string?
760 How can I open a filehandle to a string?
764 How can I output my numbers with commas added?
768 How can I translate tildes (~) in a filename?
772 How come when I open a file read-write it wipes it out?
776 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
780 Is there a leak/bug in glob()?
784 How can I open a file with a leading ">" or trailing blanks?
788 How can I reliably rename a file?
792 How can I lock a file?
796 Why can't I just open(FH, "E<gt>file.lock")?
800 I still don't get locking. I just want to increment the number in the file. How can I do this?
804 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?
808 How do I randomly update a binary file?
812 How do I get a file's timestamp in perl?
816 How do I set a file's timestamp in perl?
820 How do I print to more than one file at once?
824 How can I read in an entire file all at once?
828 How can I read in a file by paragraphs?
832 How can I read a single character from a file? From the keyboard?
836 How can I tell whether there's a character waiting on a filehandle?
840 How do I do a C<tail -f> in perl?
844 How do I dup() a filehandle in Perl?
848 How do I close a file descriptor by number?
852 Why can't I use "C:\temp\foo" in DOS paths? Why doesn't `C:\temp\foo.exe` work?
856 Why doesn't glob("*.*") get all the files?
860 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
864 How do I select a random line from a file?
868 Why do I get weird spaces when I print an array of lines?
872 How do I traverse a directory tree?
876 How do I delete a directory tree?
880 How do I copy an entire directory?
885 =head2 L<perlfaq6>: Regular Expressions
887 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).
893 How can I hope to use regular expressions without creating illegible and unmaintainable code?
897 I'm having trouble matching over more than one line. What's wrong?
901 How can I pull out lines between two patterns that are themselves on different lines?
905 How do I match XML, HTML, or other nasty, ugly things with a regex?
909 I put a regular expression into $/ but it didn't work. What's wrong?
913 How do I substitute case-insensitively on the LHS while preserving case on the RHS?
917 How can I make C<\w> match national character sets?
921 How can I match a locale-smart version of C</[a-zA-Z]/>?
925 How can I quote a variable to use in a regex?
929 What is C</o> really for?
933 How do I use a regular expression to strip C-style comments from a file?
937 Can I use Perl regular expressions to match balanced text?
941 What does it mean that regexes are greedy? How can I get around it?
945 How do I process each word on each line?
949 How can I print out a word-frequency or line-frequency summary?
953 How can I do approximate matching?
957 How do I efficiently match many regular expressions at once?
961 Why don't word-boundary searches with C<\b> work for me?
965 Why does using $&, $`, or $' slow my program down?
969 What good is C<\G> in a regular expression?
973 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
977 What's wrong with using grep in a void context?
981 How can I match strings with multibyte characters?
985 How do I match a regular expression that's in a variable?
990 =head2 L<perlfaq7>: General Perl Language Issues
992 General Perl language issues that don't clearly fit into any of the other sections.
998 Can I get a BNF/yacc/RE for the Perl language?
1002 What are all these $@%&* punctuation signs, and how do I know when to use them?
1006 Do I always/never have to quote my strings or use semicolons and commas?
1010 How do I skip some return values?
1014 How do I temporarily block warnings?
1018 What's an extension?
1022 Why do Perl operators have different precedence than C operators?
1026 How do I declare/create a structure?
1030 How do I create a module?
1034 How do I adopt or take over a module already on CPAN?
1038 How do I create a class?
1042 How can I tell if a variable is tainted?
1050 What is variable suicide and how can I prevent it?
1054 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1058 How do I create a static variable?
1062 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1066 How can I access a dynamic variable while a similarly named lexical is in scope?
1070 What's the difference between deep and shallow binding?
1074 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
1078 How do I redefine a builtin function, operator, or method?
1082 What's the difference between calling a function as &foo and foo()?
1086 How do I create a switch or case statement?
1090 How can I catch accesses to undefined variables, functions, or methods?
1094 Why can't a method included in this same file be found?
1098 How can I find out my current or calling package?
1102 How can I comment out a large block of Perl code?
1106 How do I clear a package?
1110 How can I use a variable as a variable name?
1114 What does "bad interpreter" mean?
1119 =head2 L<perlfaq8>: System Interaction
1121 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.
1127 How do I find out which operating system I'm running under?
1131 How come exec() doesn't return?
1135 How do I do fancy stuff with the keyboard/screen/mouse?
1139 How do I print something out in color?
1143 How do I read just one key without waiting for a return key?
1147 How do I check whether input is ready on the keyboard?
1151 How do I clear the screen?
1155 How do I get the screen size?
1159 How do I ask the user for a password?
1163 How do I read and write the serial port?
1167 How do I decode encrypted password files?
1171 How do I start a process in the background?
1175 How do I trap control characters/signals?
1179 How do I modify the shadow password file on a Unix system?
1183 How do I set the time and date?
1187 How can I sleep() or alarm() for under a second?
1191 How can I measure time under a second?
1195 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
1199 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1203 How can I call my system's unique C functions from Perl?
1207 Where do I get the include files to do ioctl() or syscall()?
1211 Why do setuid perl scripts complain about kernel problems?
1215 How can I open a pipe both to and from a command?
1219 Why can't I get the output of a command with system()?
1223 How can I capture STDERR from an external command?
1227 Why doesn't open() return an error when a pipe open fails?
1231 What's wrong with using backticks in a void context?
1235 How can I call backticks without shell processing?
1239 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1243 How can I convert my shell script to perl?
1247 Can I use perl to run a telnet or ftp session?
1251 How can I write expect in Perl?
1255 Is there a way to hide perl's command line from programs such as "ps"?
1259 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?
1263 How do I close a process's filehandle without waiting for it to complete?
1267 How do I fork a daemon process?
1271 How do I find out if I'm running interactively or not?
1275 How do I timeout a slow event?
1279 How do I set CPU limits?
1283 How do I avoid zombies on a Unix system?
1287 How do I use an SQL database?
1291 How do I make a system() exit on control-C?
1295 How do I open a file without blocking?
1299 How do I tell the difference between errors from the shell and perl?
1303 How do I install a module from CPAN?
1307 What's the difference between require and use?
1311 How do I keep my own module/library directory?
1315 How do I add the directory my program lives in to the module/library search path?
1319 How do I add a directory to my include path (@INC) at runtime?
1323 What is socket.ph and where do I get it?
1328 =head2 L<perlfaq9>: Networking
1330 Networking, the internet, and a few on the web.
1336 What is the correct form of response from a CGI script?
1340 My CGI script runs from the command line but not the browser. (500 Server Error)
1344 How can I get better error messages from a CGI program?
1348 How do I remove HTML from a string?
1352 How do I extract URLs?
1356 How do I download a file from the user's machine? How do I open a file on another machine?
1360 How do I make an HTML pop-up menu with Perl?
1364 How do I fetch an HTML file?
1368 How do I automate an HTML form submission?
1372 How do I decode or create those %-encodings on the web?
1376 How do I redirect to another page?
1380 How do I put a password on my web pages?
1384 How do I edit my .htpasswd and .htgroup files with Perl?
1388 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1392 How do I parse a mail header?
1396 How do I decode a CGI form?
1400 How do I check a valid mail address?
1404 How do I decode a MIME/BASE64 string?
1408 How do I return the user's mail address?
1416 How do I use MIME to make an attachment to a mail message?
1424 How do I find out my hostname, domainname, or IP address?
1428 How do I fetch a news article or the active newsgroups?
1432 How do I fetch/put an FTP file?
1436 How can I do RPC in Perl?
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