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://svn.perl.org/modules/perlfaq/trunk/ (which is outside of the
28 main Perl source tree). The SVN 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 =head2 What will happen if you mail your Perl programming problems to the authors?
48 The perlfaq-workers like to keep all traffic on the perlfaq-workers list
49 so that everyone can see the work being done (and the work that needs to
50 be done). The mailing list serves as an official record. If you email the
51 authors or maintainers directly, you'll probably get a reply asking you
52 to post to the mailing list. If you don't get a reply, it probably means
53 that the person never saw the message or didn't have time to deal with
54 it. Posting to the list allows the volunteers with time to deal with it
57 If you have a question that isn't in the FAQ and you would like help with
58 it, try the resources in L<perlfaq2>.
62 Tom Christiansen wrote the original perlfaq then expanded it with the
63 help of Nat Torkington. The perlfaq-workers maintain current document
64 and the denizens of comp.lang.perl.misc regularly review and update the
65 FAQ. Several people have contributed answers, corrections, and comments,
66 and the perlfaq notes those contributions wherever appropriate.
68 =head1 AUTHOR AND COPYRIGHT
70 Tom Christiansen wrote the original version of this document.
71 brian d foy C<< <bdfoy@cpan.org> >> wrote this version. See the
72 individual perlfaq documents for additional copyright information.
74 This document is available under the same terms as Perl itself. Code
75 examples in all the perlfaq documents are in the public domain. Use
76 them as you see fit (and at your own risk with no warranty from anyone).
78 =head1 Table of Contents
82 =item perlfaq - this document
84 =item perlfaq1 - General Questions About Perl
86 =item perlfaq2 - Obtaining and Learning about Perl
88 =item perlfaq3 - Programming Tools
90 =item perlfaq4 - Data Manipulation
92 =item perlfaq5 - Files and Formats
94 =item perlfaq6 - Regular Expressions
96 =item perlfaq7 - General Perl Language Issues
98 =item perlfaq8 - System Interaction
100 =item perlfaq9 - Networking
106 =head2 L<perlfaq1>: General Questions About Perl
108 Very general, high-level questions about Perl.
118 Who supports Perl? Who develops it? Why is it free?
122 Which version of Perl should I use?
126 What are Perl 4, Perl 5, or Perl 6?
142 Is Perl difficult to learn?
146 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
150 Can I do [task] in Perl?
154 When shouldn't I program in Perl?
158 What's the difference between "perl" and "Perl"?
162 Is it a Perl program or a Perl script?
170 Where can I get a list of Larry Wall witticisms?
174 How can I convince others to use Perl?
179 =head2 L<perlfaq2>: Obtaining and Learning about Perl
181 Where to find source and documentation for Perl, support, and related matters.
187 What machines support perl? Where do I get it?
191 How can I get a binary version of perl?
195 I don't have a C compiler. How can I build my own Perl interpreter?
199 I copied the perl binary from one machine to another, but scripts don't work.
203 I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
207 What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
211 Is there an ISO or ANSI certified version of Perl?
215 Where can I get information on Perl?
219 What are the Perl newsgroups on Usenet? Where do I post questions?
223 Where should I post source code?
231 Which magazines have Perl content?
235 What mailing lists are there for Perl?
239 Where are the archives for 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 write a GUI (X, Tk, Gtk, etc.) in 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 get C<#!perl> to work on [MS-DOS,NT,...]?
352 Can I write useful Perl programs on the command line?
356 Why don't Perl one-liners work on my DOS/Mac/VMS system?
360 Where can I learn about CGI or Web programming in Perl?
364 Where can I learn about object-oriented Perl programming?
368 Where can I learn about linking C with Perl?
372 I've read perlembed, perlguts, etc., but I can't embed perl in my C program; what am I doing wrong?
376 When I tried to run my script, I got this message. What does it mean?
385 =head2 L<perlfaq4>: Data Manipulation
387 Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
393 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
401 Why isn't my octal data interpreted correctly?
405 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
409 How do I convert between numeric representations/bases/radixes?
413 Why doesn't & work the way I want it to?
417 How do I multiply matrices?
421 How do I perform an operation on a series of integers?
425 How can I output Roman numerals?
429 Why aren't my random numbers random?
433 How do I get a random number between X and Y?
437 How do I find the day or week of the year?
441 How do I find the current century or millennium?
445 How can I compare two dates and find the difference?
449 How can I take a string and turn it into epoch seconds?
453 How can I find the Julian Day?
457 How do I find yesterday's date?
461 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
465 How do I validate input?
469 How do I unescape a string?
473 How do I remove consecutive pairs of characters?
477 How do I expand function calls in a string?
481 How do I find matching/nesting anything?
485 How do I reverse a string?
489 How do I expand tabs in a string?
493 How do I reformat a paragraph?
497 How can I access or change N characters of a string?
501 How do I change the Nth occurrence of something?
505 How can I count the number of occurrences of a substring within a string?
509 How do I capitalize all the words on one line?
513 How can I split a [character] delimited string except when inside [character]?
517 How do I strip blank space from the beginning/end of a string?
521 How do I pad a string with blanks or pad a number with zeroes?
525 How do I extract selected columns from a string?
529 How do I find the soundex value of a string?
533 How can I expand variables in text strings?
537 What's wrong with always quoting "$vars"?
541 Why don't my E<lt>E<lt>HERE documents work?
545 What is the difference between a list and an array?
549 What is the difference between $array[1] and @array[1]?
553 How can I remove duplicate elements from a list or array?
557 How can I tell whether a certain element is contained in a list or array?
561 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
565 How do I test whether two arrays or hashes are equal?
569 How do I find the first array element for which a condition is true?
573 How do I handle linked lists?
577 How do I handle circular lists?
581 How do I shuffle an array randomly?
585 How do I process/modify each element of an array?
589 How do I select a random element from an array?
593 How do I permute N elements of a list?
597 How do I sort an array by (anything)?
601 How do I manipulate arrays of bits?
605 Why does defined() return true on empty arrays and hashes?
609 How do I process an entire hash?
613 What happens if I add or remove keys from a hash while iterating over it?
617 How do I look up a hash element by value?
621 How can I know how many entries are in a hash?
625 How do I sort a hash (optionally by value instead of key)?
629 How can I always keep my hash sorted?
633 What's the difference between "delete" and "undef" with hashes?
637 Why don't my tied hashes make the defined/exists distinction?
641 How do I reset an each() operation part-way through?
645 How can I get the unique keys from two hashes?
649 How can I store a multidimensional array in a DBM file?
653 How can I make my hash remember the order I put elements into it?
657 Why does passing a subroutine an undefined element in a hash create it?
661 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
665 How can I use a reference as a hash key?
669 How do I handle binary data correctly?
673 How do I determine whether a scalar is a number/whole/integer/float?
677 How do I keep persistent data across program calls?
681 How do I print out or copy a recursive data structure?
685 How do I define methods for every class/object?
689 How do I verify a credit card checksum?
693 How do I pack arrays of doubles or floats for XS code?
698 =head2 L<perlfaq5>: Files and Formats
700 I/O and the "f" issues: filehandles, flushing, formats, and footers.
706 How do I flush/unbuffer an output filehandle? Why must I do this?
710 How do I change, delete, or insert a line in a file, or append to the beginning of a file?
714 How do I count the number of lines in a file?
718 How can I use Perl's C<-i> option from within a program?
722 How can I copy a file?
726 How do I make a temporary file name?
730 How can I manipulate fixed-record-length files?
734 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?
738 How can I use a filehandle indirectly?
742 How can I set up a footer format to be used with write()?
746 How can I write() into a string?
750 How can I open a filehandle to 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 regular expression that's in a variable?
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 $@%&* punctuation signs, and how do I know when to use them?
980 Do I always/never have to quote my strings or use semicolons and commas?
984 How do I skip some return values?
988 How do I temporarily block warnings?
996 Why do Perl operators have different precedence than C operators?
1000 How do I declare/create a structure?
1004 How do I create a module?
1008 How do I adopt or take over a module already on CPAN?
1012 How do I create a class?
1016 How can I tell if a variable is tainted?
1024 What is variable suicide and how can I prevent it?
1028 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1032 How do I create a static variable?
1036 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1040 How can I access a dynamic variable while a similarly named lexical is in scope?
1044 What's the difference between deep and shallow binding?
1048 Why doesn't "my($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 an HTML pop-up menu with Perl?
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, or 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?
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