3 perlfaq - frequently asked questions about Perl ($Date: 2003/01/31 17:37:17 $)
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
101 =head2 L<perlfaq1>: General Questions About Perl
103 Very general, high-level questions about Perl.
113 Who supports Perl? Who develops it? Why is it free?
117 Which version of Perl should I use?
121 What are perl4 and perl5?
133 Is Perl difficult to learn?
137 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
141 Can I do [task] in Perl?
145 When shouldn't I program in Perl?
149 What's the difference between "perl" and "Perl"?
153 Is it a Perl program or a Perl script?
161 Where can I get a list of Larry Wall witticisms?
165 How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language?
170 =head2 L<perlfaq2>: Obtaining and Learning about Perl
172 Where to find source and documentation for Perl, support,
179 What machines support Perl? Where do I get it?
183 How can I get a binary version of Perl?
187 I don't have a C compiler on my system. How can I compile perl?
191 I copied the Perl binary from one machine to another, but scripts don't work.
195 I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
199 What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
203 Is there an ISO or ANSI certified version of Perl?
207 Where can I get information on Perl?
211 What are the Perl newsgroups on Usenet? Where do I post questions?
215 Where should I post source code?
227 Perl on the Net: FTP and WWW Access
231 What mailing lists are there for Perl?
235 Archives of comp.lang.perl.misc
239 Where can I buy a commercial version of Perl?
243 Where do I send bug reports?
247 What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org?
252 =head2 L<perlfaq3>: Programming Tools
254 Programmer tools and programming support.
260 How do I do (anything)?
264 How can I use Perl interactively?
268 Is there a Perl shell?
272 How do I find which modules are installed on my system?
276 How do I debug my Perl programs?
280 How do I profile my Perl programs?
284 How do I cross-reference my Perl programs?
288 Is there a pretty-printer (formatter) for Perl?
292 Is there a ctags for Perl?
296 Is there an IDE or Windows Perl Editor?
300 Where can I get Perl macros for vi?
304 Where can I get perl-mode for emacs?
308 How can I use curses with Perl?
312 How can I use X or Tk with Perl?
316 How can I generate simple menus without using CGI or Tk?
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 compile Perl into Java?
352 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
356 Can I write useful Perl programs on the command line?
360 Why don't Perl one-liners work on my DOS/Mac/VMS system?
364 Where can I learn about CGI or Web programming in Perl?
368 Where can I learn about object-oriented Perl programming?
372 Where can I learn about linking C with Perl? [h2xs, xsubpp]
376 I've read perlembed, perlguts, etc., but I can't embed perl in
377 my C program; what am I doing wrong?
381 When I tried to run my script, I got this message. What does it mean?
390 =head2 L<perlfaq4>: Data Manipulation
392 Manipulating numbers, dates, strings, arrays, hashes, and
393 miscellaneous data issues.
399 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?
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 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 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 do I handle binary data correctly?
675 How do I determine whether a scalar is a number/whole/integer/float?
679 How do I keep persistent data across program calls?
683 How do I print out or copy a recursive data structure?
687 How do I define methods for every class/object?
691 How do I verify a credit card checksum?
695 How do I pack arrays of doubles or floats for XS code?
700 =head2 L<perlfaq5>: Files and Formats
702 I/O and the "f" issues: filehandles, flushing, formats, and footers.
708 How do I flush/unbuffer an output filehandle? Why must I do this?
712 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?
716 How do I count the number of lines in a file?
720 How can I use Perl's C<-i> option from within a program?
724 How do I make a temporary file name?
728 How can I manipulate fixed-record-length files?
732 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?
736 How can I use a filehandle indirectly?
740 How can I set up a footer format to be used with write()?
744 How can I write() into a string?
748 How can I output my numbers with commas added?
752 How can I translate tildes (~) in a filename?
756 How come when I open a file read-write it wipes it out?
760 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
764 Is there a leak/bug in glob()?
768 How can I open a file with a leading ">" or trailing blanks?
772 How can I reliably rename a file?
776 How can I lock a file?
780 Why can't I just open(FH, "E<gt>file.lock")?
784 I still don't get locking. I just want to increment the number in the file. How can I do this?
788 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?
792 How do I randomly update a binary file?
796 How do I get a file's timestamp in perl?
800 How do I set a file's timestamp in perl?
804 How do I print to more than one file at once?
808 How can I read in an entire file all at once?
812 How can I read in a file by paragraphs?
816 How can I read a single character from a file? From the keyboard?
820 How can I tell whether there's a character waiting on a filehandle?
824 How do I do a C<tail -f> in perl?
828 How do I dup() a filehandle in Perl?
832 How do I close a file descriptor by number?
836 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
840 Why doesn't glob("*.*") get all the files?
844 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
848 How do I select a random line from a file?
852 Why do I get weird spaces when I print an array of lines?
857 =head2 L<perlfaq6>: Regular Expressions
859 Pattern matching and regular expressions.
865 How can I hope to use regular expressions without creating illegible and unmaintainable code?
869 I'm having trouble matching over more than one line. What's wrong?
873 How can I pull out lines between two patterns that are themselves on different lines?
877 I put a regular expression into $/ but it didn't work. What's wrong?
881 How do I substitute case insensitively on the LHS while preserving case on the RHS?
885 How can I make C<\w> match national character sets?
889 How can I match a locale-smart version of C</[a-zA-Z]/>?
893 How can I quote a variable to use in a regex?
897 What is C</o> really for?
901 How do I use a regular expression to strip C style comments from a file?
905 Can I use Perl regular expressions to match balanced text?
909 What does it mean that regexes are greedy? How can I get around it?
913 How do I process each word on each line?
917 How can I print out a word-frequency or line-frequency summary?
921 How can I do approximate matching?
925 How do I efficiently match many regular expressions at once?
929 Why don't word-boundary searches with C<\b> work for me?
933 Why does using $&, $`, or $' slow my program down?
937 What good is C<\G> in a regular expression?
941 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
945 What's wrong with using grep or map in a void context?
949 How can I match strings with multibyte characters?
953 How do I match a pattern that is supplied by the user?
958 =head2 L<perlfaq7>: General Perl Language Issues
960 General Perl language issues that don't clearly fit into any of the
967 Can I get a BNF/yacc/RE for the Perl language?
971 What are all these $@%&* punctuation signs, and how do I know when to use them?
975 Do I always/never have to quote my strings or use semicolons and commas?
979 How do I skip some return values?
983 How do I temporarily block warnings?
991 Why do Perl operators have different precedence than C operators?
995 How do I declare/create a structure?
999 How do I create a module?
1003 How do I create a class?
1007 How can I tell if a variable is tainted?
1015 What is variable suicide and how can I prevent it?
1019 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
1023 How do I create a static variable?
1027 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
1031 How can I access a dynamic variable while a similarly named lexical is in scope?
1035 What's the difference between deep and shallow binding?
1039 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
1043 How do I redefine a builtin function, operator, or method?
1047 What's the difference between calling a function as &foo and foo()?
1051 How do I create a switch or case statement?
1055 How can I catch accesses to undefined variables, functions, or methods?
1059 Why can't a method included in this same file be found?
1063 How can I find out my current package?
1067 How can I comment out a large block of perl code?
1071 How do I clear a package?
1075 How can I use a variable as a variable name?
1079 What does "bad interpreter" mean?
1084 =head2 L<perlfaq8>: System Interaction
1086 Interprocess communication (IPC), control over the user-interface
1087 (keyboard, screen and pointing devices).
1093 How do I find out which operating system I'm running under?
1097 How come exec() doesn't return?
1101 How do I do fancy stuff with the keyboard/screen/mouse?
1105 How do I print something out in color?
1109 How do I read just one key without waiting for a return key?
1113 How do I check whether input is ready on the keyboard?
1117 How do I clear the screen?
1121 How do I get the screen size?
1125 How do I ask the user for a password?
1129 How do I read and write the serial port?
1133 How do I decode encrypted password files?
1137 How do I start a process in the background?
1141 How do I trap control characters/signals?
1145 How do I modify the shadow password file on a Unix system?
1149 How do I set the time and date?
1153 How can I sleep() or alarm() for under a second?
1157 How can I measure time under a second?
1161 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
1165 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1169 How can I call my system's unique C functions from Perl?
1173 Where do I get the include files to do ioctl() or syscall()?
1177 Why do setuid perl scripts complain about kernel problems?
1181 How can I open a pipe both to and from a command?
1185 Why can't I get the output of a command with system()?
1189 How can I capture STDERR from an external command?
1193 Why doesn't open() return an error when a pipe open fails?
1197 What's wrong with using backticks in a void context?
1201 How can I call backticks without shell processing?
1205 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1209 How can I convert my shell script to perl?
1213 Can I use perl to run a telnet or ftp session?
1217 How can I write expect in Perl?
1221 Is there a way to hide perl's command line from programs such as "ps"?
1225 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?
1229 How do I close a process's filehandle without waiting for it to complete?
1233 How do I fork a daemon process?
1237 How do I find out if I'm running interactively or not?
1241 How do I timeout a slow event?
1245 How do I set CPU limits?
1249 How do I avoid zombies on a Unix system?
1253 How do I use an SQL database?
1257 How do I make a system() exit on control-C?
1261 How do I open a file without blocking?
1265 How do I install a module from CPAN?
1269 What's the difference between require and use?
1273 How do I keep my own module/library directory?
1277 How do I add the directory my program lives in to the module/library search path?
1281 How do I add a directory to my include path at runtime?
1285 What is socket.ph and where do I get it?
1290 =head2 L<perlfaq9>: Networking
1292 Networking, the internet, and a few on the web.
1298 What is the correct form of response from a CGI script?
1302 My CGI script runs from the command line but not the browser. (500 Server Error)
1306 How can I get better error messages from a CGI program?
1310 How do I remove HTML from a string?
1314 How do I extract URLs?
1318 How do I download a file from the user's machine? How do I open a file on another machine?
1322 How do I make a pop-up menu in HTML?
1326 How do I fetch an HTML file?
1330 How do I automate an HTML form submission?
1334 How do I decode or create those %-encodings on the web?
1338 How do I redirect to another page?
1342 How do I put a password on my web pages?
1346 How do I edit my .htpasswd and .htgroup files with Perl?
1350 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1354 How do I parse a mail header?
1358 How do I decode a CGI form?
1362 How do I check a valid mail address?
1366 How do I decode a MIME/BASE64 string?
1370 How do I return the user's mail address?
1378 How do I use MIME to make an attachment to a mail message?
1386 How do I find out my hostname/domainname/IP address?
1390 How do I fetch a news article or the active newsgroups?
1394 How do I fetch/put an FTP file?
1398 How can I do RPC in Perl?
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