3 perlfaq - frequently asked questions about Perl ($Date: 2002/11/13 06:23:50 $)
7 The perlfaq is structured into the following documents:
10 =head2 perlfaq: Structural overview of the FAQ.
14 =head2 L<perlfaq1>: General Questions About Perl
16 Very general, high-level questions about Perl.
26 Who supports Perl? Who develops it? Why is it free?
30 Which version of Perl should I use?
34 What are perl4 and perl5?
46 Is Perl difficult to learn?
50 How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
54 Can I do [task] in Perl?
58 When shouldn't I program in Perl?
62 What's the difference between "perl" and "Perl"?
66 Is it a Perl program or a Perl script?
74 Where can I get a list of Larry Wall witticisms?
78 How can I convince my sysadmin/supervisor/employees to use version 5/5.6.1/Perl instead of some other language?
83 =head2 L<perlfaq2>: Obtaining and Learning about Perl
85 Where to find source and documentation for Perl, support,
92 What machines support Perl? Where do I get it?
96 How can I get a binary version of Perl?
100 I don't have a C compiler on my system. How can I compile perl?
104 I copied the Perl binary from one machine to another, but scripts don't work.
108 I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
112 What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
116 Is there an ISO or ANSI certified version of Perl?
120 Where can I get information on Perl?
124 What are the Perl newsgroups on Usenet? Where do I post questions?
128 Where should I post source code?
140 Perl on the Net: FTP and WWW Access
144 What mailing lists are there for Perl?
148 Archives of comp.lang.perl.misc
152 Where can I buy a commercial version of Perl?
156 Where do I send bug reports?
160 What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org?
165 =head2 L<perlfaq3>: Programming Tools
167 Programmer tools and programming support.
173 How do I do (anything)?
177 How can I use Perl interactively?
181 Is there a Perl shell?
185 How do I find which modules are installed on my system?
189 How do I debug my Perl programs?
193 How do I profile my Perl programs?
197 How do I cross-reference my Perl programs?
201 Is there a pretty-printer (formatter) for Perl?
205 Is there a ctags for Perl?
209 Is there an IDE or Windows Perl Editor?
213 Where can I get Perl macros for vi?
217 Where can I get perl-mode for emacs?
221 How can I use curses with Perl?
225 How can I use X or Tk with Perl?
229 How can I generate simple menus without using CGI or Tk?
233 How can I make my Perl program run faster?
237 How can I make my Perl program take less memory?
241 Is it safe to return a reference to local or lexical data?
245 How can I free an array or hash so my program shrinks?
249 How can I make my CGI script more efficient?
253 How can I hide the source for my Perl program?
257 How can I compile my Perl program into byte code or C?
261 How can I compile Perl into Java?
265 How can I get C<#!perl> to work on [MS-DOS,NT,...]?
269 Can I write useful Perl programs on the command line?
273 Why don't Perl one-liners work on my DOS/Mac/VMS system?
277 Where can I learn about CGI or Web programming in Perl?
281 Where can I learn about object-oriented Perl programming?
285 Where can I learn about linking C with Perl? [h2xs, xsubpp]
289 I've read perlembed, perlguts, etc., but I can't embed perl in
290 my C program; what am I doing wrong?
294 When I tried to run my script, I got this message. What does it mean?
303 =head2 L<perlfaq4>: Data Manipulation
305 Manipulating numbers, dates, strings, arrays, hashes, and
306 miscellaneous data issues.
312 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
316 Why isn't my octal data interpreted correctly?
320 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
324 How do I convert between numeric representations?
328 Why doesn't & work the way I want it to?
332 How do I multiply matrices?
336 How do I perform an operation on a series of integers?
340 How can I output Roman numerals?
344 Why aren't my random numbers random?
348 How do I get a random number between X and Y?
352 How do I find the week-of-the-year/day-of-the-year?
356 How do I find the current century or millennium?
360 How can I compare two dates and find the difference?
364 How can I take a string and turn it into epoch seconds?
368 How can I find the Julian Day?
372 How do I find yesterday's date?
376 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
380 How do I validate input?
384 How do I unescape a string?
388 How do I remove consecutive pairs of characters?
392 How do I expand function calls in a string?
396 How do I find matching/nesting anything?
400 How do I reverse a string?
404 How do I expand tabs in a string?
408 How do I reformat a paragraph?
412 How can I access or change N characters of a string?
416 How do I change the Nth occurrence of something?
420 How can I count the number of occurrences of a substring within a string?
424 How do I capitalize all the words on one line?
428 How can I split a [character] delimited string except when inside [character]?
432 How do I strip blank space from the beginning/end of a string?
436 How do I pad a string with blanks or pad a number with zeroes?
440 How do I extract selected columns from a string?
444 How do I find the soundex value of a string?
448 How can I expand variables in text strings?
452 What's wrong with always quoting "$vars"?
456 Why don't my <<HERE documents work?
460 What is the difference between a list and an array?
464 What is the difference between $array[1] and @array[1]?
468 How can I remove duplicate elements from a list or array?
472 How can I tell whether a certain element is contained in a list or array?
476 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
480 How do I test whether two arrays or hashes are equal?
484 How do I find the first array element for which a condition is true?
488 How do I handle linked lists?
492 How do I handle circular lists?
496 How do I shuffle an array randomly?
500 How do I process/modify each element of an array?
504 How do I select a random element from an array?
508 How do I permute N elements of a list?
512 How do I sort an array by (anything)?
516 How do I manipulate arrays of bits?
520 Why does defined() return true on empty arrays and hashes?
524 How do I process an entire hash?
528 What happens if I add or remove keys from a hash while iterating over it?
532 How do I look up a hash element by value?
536 How can I know how many entries are in a hash?
540 How do I sort a hash (optionally by value instead of key)?
544 How can I always keep my hash sorted?
548 What's the difference between "delete" and "undef" with hashes?
552 Why don't my tied hashes make the defined/exists distinction?
556 How do I reset an each() operation part-way through?
560 How can I get the unique keys from two hashes?
564 How can I store a multidimensional array in a DBM file?
568 How can I make my hash remember the order I put elements into it?
572 Why does passing a subroutine an undefined element in a hash create it?
576 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
580 How can I use a reference as a hash key?
584 How do I handle binary data correctly?
588 How do I determine whether a scalar is a number/whole/integer/float?
592 How do I keep persistent data across program calls?
596 How do I print out or copy a recursive data structure?
600 How do I define methods for every class/object?
604 How do I verify a credit card checksum?
608 How do I pack arrays of doubles or floats for XS code?
613 =head2 L<perlfaq5>: Files and Formats
615 I/O and the "f" issues: filehandles, flushing, formats, and footers.
621 How do I flush/unbuffer an output filehandle? Why must I do this?
625 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?
629 How do I count the number of lines in a file?
633 How can I use Perl's C<-i> option from within a program?
637 How do I make a temporary file name?
641 How can I manipulate fixed-record-length files?
645 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?
649 How can I use a filehandle indirectly?
653 How can I set up a footer format to be used with write()?
657 How can I write() into a string?
661 How can I output my numbers with commas added?
665 How can I translate tildes (~) in a filename?
669 How come when I open a file read-write it wipes it out?
673 Why do I sometimes get an "Argument list too long" when I use <*>?
677 Is there a leak/bug in glob()?
681 How can I open a file with a leading ">" or trailing blanks?
685 How can I reliably rename a file?
689 How can I lock a file?
693 Why can't I just open(FH, ">file.lock")?
697 I still don't get locking. I just want to increment the number in the file. How can I do this?
701 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?
705 How do I randomly update a binary file?
709 How do I get a file's timestamp in perl?
713 How do I set a file's timestamp in perl?
717 How do I print to more than one file at once?
721 How can I read in an entire file all at once?
725 How can I read in a file by paragraphs?
729 How can I read a single character from a file? From the keyboard?
733 How can I tell whether there's a character waiting on a filehandle?
737 How do I do a C<tail -f> in perl?
741 How do I dup() a filehandle in Perl?
745 How do I close a file descriptor by number?
749 Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
753 Why doesn't glob("*.*") get all the files?
757 Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
761 How do I select a random line from a file?
765 Why do I get weird spaces when I print an array of lines?
770 =head2 L<perlfaq6>: Regular Expressions
772 Pattern matching and regular expressions.
778 How can I hope to use regular expressions without creating illegible and unmaintainable code?
782 I'm having trouble matching over more than one line. What's wrong?
786 How can I pull out lines between two patterns that are themselves on different lines?
790 I put a regular expression into $/ but it didn't work. What's wrong?
794 How do I substitute case insensitively on the LHS while preserving case on the RHS?
798 How can I make C<\w> match national character sets?
802 How can I match a locale-smart version of C</[a-zA-Z]/>?
806 How can I quote a variable to use in a regex?
810 What is C</o> really for?
814 How do I use a regular expression to strip C style comments from a file?
818 Can I use Perl regular expressions to match balanced text?
822 What does it mean that regexes are greedy? How can I get around it?
826 How do I process each word on each line?
830 How can I print out a word-frequency or line-frequency summary?
834 How can I do approximate matching?
838 How do I efficiently match many regular expressions at once?
842 Why don't word-boundary searches with C<\b> work for me?
846 Why does using $&, $`, or $' slow my program down?
850 What good is C<\G> in a regular expression?
854 Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
858 What's wrong with using grep or map in a void context?
862 How can I match strings with multibyte characters?
866 How do I match a pattern that is supplied by the user?
871 =head2 L<perlfaq7>: General Perl Language Issues
873 General Perl language issues that don't clearly fit into any of the
880 Can I get a BNF/yacc/RE for the Perl language?
884 What are all these $@%&* punctuation signs, and how do I know when to use them?
888 Do I always/never have to quote my strings or use semicolons and commas?
892 How do I skip some return values?
896 How do I temporarily block warnings?
904 Why do Perl operators have different precedence than C operators?
908 How do I declare/create a structure?
912 How do I create a module?
916 How do I create a class?
920 How can I tell if a variable is tainted?
928 What is variable suicide and how can I prevent it?
932 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
936 How do I create a static variable?
940 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
944 How can I access a dynamic variable while a similarly named lexical is in scope?
948 What's the difference between deep and shallow binding?
952 Why doesn't "my($foo) = <FILE>;" work right?
956 How do I redefine a builtin function, operator, or method?
960 What's the difference between calling a function as &foo and foo()?
964 How do I create a switch or case statement?
968 How can I catch accesses to undefined variables, functions, or methods?
972 Why can't a method included in this same file be found?
976 How can I find out my current package?
980 How can I comment out a large block of perl code?
984 How do I clear a package?
988 How can I use a variable as a variable name?
993 =head2 L<perlfaq8>: System Interaction
995 Interprocess communication (IPC), control over the user-interface
996 (keyboard, screen and pointing devices).
1002 How do I find out which operating system I'm running under?
1006 How come exec() doesn't return?
1010 How do I do fancy stuff with the keyboard/screen/mouse?
1014 How do I print something out in color?
1018 How do I read just one key without waiting for a return key?
1022 How do I check whether input is ready on the keyboard?
1026 How do I clear the screen?
1030 How do I get the screen size?
1034 How do I ask the user for a password?
1038 How do I read and write the serial port?
1042 How do I decode encrypted password files?
1046 How do I start a process in the background?
1050 How do I trap control characters/signals?
1054 How do I modify the shadow password file on a Unix system?
1058 How do I set the time and date?
1062 How can I sleep() or alarm() for under a second?
1066 How can I measure time under a second?
1070 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
1074 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
1078 How can I call my system's unique C functions from Perl?
1082 Where do I get the include files to do ioctl() or syscall()?
1086 Why do setuid perl scripts complain about kernel problems?
1090 How can I open a pipe both to and from a command?
1094 Why can't I get the output of a command with system()?
1098 How can I capture STDERR from an external command?
1102 Why doesn't open() return an error when a pipe open fails?
1106 What's wrong with using backticks in a void context?
1110 How can I call backticks without shell processing?
1114 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
1118 How can I convert my shell script to perl?
1122 Can I use perl to run a telnet or ftp session?
1126 How can I write expect in Perl?
1130 Is there a way to hide perl's command line from programs such as "ps"?
1134 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?
1138 How do I close a process's filehandle without waiting for it to complete?
1142 How do I fork a daemon process?
1146 How do I find out if I'm running interactively or not?
1150 How do I timeout a slow event?
1154 How do I set CPU limits?
1158 How do I avoid zombies on a Unix system?
1162 How do I use an SQL database?
1166 How do I make a system() exit on control-C?
1170 How do I open a file without blocking?
1174 How do I install a module from CPAN?
1178 What's the difference between require and use?
1182 How do I keep my own module/library directory?
1186 How do I add the directory my program lives in to the module/library search path?
1190 How do I add a directory to my include path at runtime?
1194 What is socket.ph and where do I get it?
1199 =head2 L<perlfaq9>: Networking
1201 Networking, the internet, and a few on the web.
1207 What is the correct form of response from a CGI script?
1211 My CGI script runs from the command line but not the browser. (500 Server Error)
1215 How can I get better error messages from a CGI program?
1219 How do I remove HTML from a string?
1223 How do I extract URLs?
1227 How do I download a file from the user's machine? How do I open a file on another machine?
1231 How do I make a pop-up menu in HTML?
1235 How do I fetch an HTML file?
1239 How do I automate an HTML form submission?
1243 How do I decode or create those %-encodings on the web?
1247 How do I redirect to another page?
1251 How do I put a password on my web pages?
1255 How do I edit my .htpasswd and .htgroup files with Perl?
1259 How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
1263 How do I parse a mail header?
1267 How do I decode a CGI form?
1271 How do I check a valid mail address?
1275 How do I decode a MIME/BASE64 string?
1279 How do I return the user's mail address?
1287 How do I use MIME to make an attachment to a mail message?
1295 How do I find out my hostname/domainname/IP address?
1299 How do I fetch a news article or the active newsgroups?
1303 How do I fetch/put an FTP file?
1307 How can I do RPC in Perl?
1312 =head1 About the perlfaq documents
1314 =head2 Where to get the perlfaq
1316 This document is posted regularly to comp.lang.perl.announce and
1317 several other related newsgroups. It is available on many
1318 web sites: http://www.perldoc.com/ and http://perlfaq.cpan.org/ .
1320 =head2 How to contribute to the perlfaq
1322 You may mail corrections, additions, and suggestions to
1323 perlfaq-workers@perl.org . This alias should not be
1324 used to I<ask> FAQs. It's for fixing the current FAQ.
1325 Send questions to the comp.lang.perl.misc newsgroup.
1327 =head2 What will happen if you mail your Perl programming problems to the authors
1329 Your questions will probably go unread, unless they're suggestions of
1330 new questions to add to the FAQ, in which case they should have gone
1331 to the perlfaq-workers@perl.org instead.
1333 You should have read section 2 of this faq. There you would have
1334 learned that comp.lang.perl.misc is the appropriate place to go for
1335 free advice. If your question is really important and you require a
1336 prompt and correct answer, you should hire a consultant.
1340 When I first began the Perl FAQ in the late 80s, I never realized it
1341 would have grown to over a hundred pages, nor that Perl would ever become
1342 so popular and widespread. This document could not have been written
1343 without the tremendous help provided by Larry Wall and the rest of the
1346 =head1 Author and Copyright Information
1348 Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
1349 All rights reserved.
1351 =head2 Bundled Distributions
1353 This documentation is free; you can redistribute it and/or modify it
1354 under the same terms as Perl itself.
1356 Irrespective of its distribution, all code examples in these files
1357 are hereby placed into the public domain. You are permitted and
1358 encouraged to use this code in your own programs for fun
1359 or for profit as you see fit. A simple comment in the code giving
1360 credit would be courteous but is not required.
1364 This information is offered in good faith and in the hope that it may
1365 be of use, but is not guaranteed to be correct, up to date, or suitable
1366 for any particular purpose whatsoever. The authors accept no liability
1367 in respect of this information or its use.