=head1 NAME
-perlfaq - frequently asked questions about Perl ($Date: 1997/03/17 22:17:56 $)
+perlfaq - frequently asked questions about Perl ($Date: 1999/05/23 20:38:02 $)
=head1 DESCRIPTION
Very general, high-level information about Perl.
+=over 4
+
+=item * What is Perl?
+
+=item * Who supports Perl? Who develops it? Why is it free?
+
+=item * Which version of Perl should I use?
+
+=item * What are perl4 and perl5?
+
+=item * What is perl6?
+
+=item * How stable is Perl?
+
+=item * Is Perl difficult to learn?
+
+=item * How does Perl compare with other languages like Java, Python, REXX, Scheme, or Tcl?
+
+=item * Can I do [task] in Perl?
+
+=item * When shouldn't I program in Perl?
+
+=item * What's the difference between "perl" and "Perl"?
+
+=item * Is it a Perl program or a Perl script?
+
+=item * What is a JAPH?
+
+=item * Where can I get a list of Larry Wall witticisms?
+
+=item * How can I convince my sysadmin/supervisor/employees to use version (5/5.005/Perl instead of some other language)?
+
+=back
+
+
=item L<perlfaq2>: Obtaining and Learning about Perl
-Where to find source and documentation to Perl, support and training,
+Where to find source and documentation to Perl, support,
and related matters.
+=over 4
+
+=item * What machines support Perl? Where do I get it?
+
+=item * How can I get a binary version of Perl?
+
+=item * I don't have a C compiler on my system. How can I compile perl?
+
+=item * I copied the Perl binary from one machine to another, but scripts don't work.
+
+=item * I grabbed the sources and tried to compile but gdbm/dynamic loading/malloc/linking/... failed. How do I make it work?
+
+=item * What modules and extensions are available for Perl? What is CPAN? What does CPAN/src/... mean?
+
+=item * Is there an ISO or ANSI certified version of Perl?
+
+=item * Where can I get information on Perl?
+
+=item * What are the Perl newsgroups on USENET? Where do I post questions?
+
+=item * Where should I post source code?
+
+=item * Perl Books
+
+=item * Perl in Magazines
+
+=item * Perl on the Net: FTP and WWW Access
+
+=item * What mailing lists are there for perl?
+
+=item * Archives of comp.lang.perl.misc
+
+=item * Where can I buy a commercial version of Perl?
+
+=item * Where do I send bug reports?
+
+=item * What is perl.com?
+
+=back
+
+
=item L<perlfaq3>: Programming Tools
Programmer tools and programming support.
+=over 4
+
+=item * How do I do (anything)?
+
+=item * How can I use Perl interactively?
+
+=item * Is there a Perl shell?
+
+=item * How do I debug my Perl programs?
+
+=item * How do I profile my Perl programs?
+
+=item * How do I cross-reference my Perl programs?
+
+=item * Is there a pretty-printer (formatter) for Perl?
+
+=item * Is there a ctags for Perl?
+
+=item * Is there an IDE or Windows Perl Editor?
+
+=item * Where can I get Perl macros for vi?
+
+=item * Where can I get perl-mode for emacs?
+
+=item * How can I use curses with Perl?
+
+=item * How can I use X or Tk with Perl?
+
+=item * How can I generate simple menus without using CGI or Tk?
+
+=item * What is undump?
+
+=item * How can I make my Perl program run faster?
+
+=item * How can I make my Perl program take less memory?
+
+=item * Is it unsafe to return a pointer to local data?
+
+=item * How can I free an array or hash so my program shrinks?
+
+=item * How can I make my CGI script more efficient?
+
+=item * How can I hide the source for my Perl program?
+
+=item * How can I compile my Perl program into byte code or C?
+
+=item * How can I compile Perl into Java?
+
+=item * How can I get C<#!perl> to work on [MS-DOS,NT,...]?
+
+=item * Can I write useful perl programs on the command line?
+
+=item * Why don't perl one-liners work on my DOS/Mac/VMS system?
+
+=item * Where can I learn about CGI or Web programming in Perl?
+
+=item * Where can I learn about object-oriented Perl programming?
+
+=item * Where can I learn about linking C with Perl? [h2xs, xsubpp]
+
+=item * I've read perlembed, perlguts, etc., but I can't embed perl in
+my C program, what am I doing wrong?
+
+=item * When I tried to run my script, I got this message. What does it
+mean?
+
+=item * What's MakeMaker?
+
+=back
+
+
=item L<perlfaq4>: Data Manipulation
Manipulating numbers, dates, strings, arrays, hashes, and
miscellaneous data issues.
+=over 4
+
+=item * Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
+
+=item * Why isn't my octal data interpreted correctly?
+
+=item * Does Perl have a round() function? What about ceil() and floor()? Trig functions?
+
+=item * How do I convert bits into ints?
+
+=item * Why doesn't & work the way I want it to?
+
+=item * How do I multiply matrices?
+
+=item * How do I perform an operation on a series of integers?
+
+=item * How can I output Roman numerals?
+
+=item * Why aren't my random numbers random?
+
+=item * How do I find the week-of-the-year/day-of-the-year?
+
+=item * How do I find the current century or millennium?
+
+=item * How can I compare two dates and find the difference?
+
+=item * How can I take a string and turn it into epoch seconds?
+
+=item * How can I find the Julian Day?
+
+=item * How do I find yesterday's date?
+
+=item * Does Perl have a year 2000 problem? Is Perl Y2K compliant?
+
+=item * How do I validate input?
+
+=item * How do I unescape a string?
+
+=item * How do I remove consecutive pairs of characters?
+
+=item * How do I expand function calls in a string?
+
+=item * How do I find matching/nesting anything?
+
+=item * How do I reverse a string?
+
+=item * How do I expand tabs in a string?
+
+=item * How do I reformat a paragraph?
+
+=item * How can I access/change the first N letters of a string?
+
+=item * How do I change the Nth occurrence of something?
+
+=item * How can I count the number of occurrences of a substring within a string?
+
+=item * How do I capitalize all the words on one line?
+
+=item * How can I split a [character] delimited string except when inside
+[character]? (Comma-separated files)
+
+=item * How do I strip blank space from the beginning/end of a string?
+
+=item * How do I pad a string with blanks or pad a number with zeroes?
+
+=item * How do I extract selected columns from a string?
+
+=item * How do I find the soundex value of a string?
+
+=item * How can I expand variables in text strings?
+
+=item * What's wrong with always quoting "$vars"?
+
+=item * Why don't my E<lt>E<lt>HERE documents work?
+
+=item * What is the difference between a list and an array?
+
+=item * What is the difference between $array[1] and @array[1]?
+
+=item * How can I remove duplicate elements from a list or array?
+
+=item * How can I tell whether a list or array contains a certain element?
+
+=item * How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
+
+=item * How do I test whether two arrays or hashes are equal?
+
+=item * How do I find the first array element for which a condition is true?
+
+=item * How do I handle linked lists?
+
+=item * How do I handle circular lists?
+
+=item * How do I shuffle an array randomly?
+
+=item * How do I process/modify each element of an array?
+
+=item * How do I select a random element from an array?
+
+=item * How do I permute N elements of a list?
+
+=item * How do I sort an array by (anything)?
+
+=item * How do I manipulate arrays of bits?
+
+=item * Why does defined() return true on empty arrays and hashes?
+
+=item * How do I process an entire hash?
+
+=item * What happens if I add or remove keys from a hash while iterating over it?
+
+=item * How do I look up a hash element by value?
+
+=item * How can I know how many entries are in a hash?
+
+=item * How do I sort a hash (optionally by value instead of key)?
+
+=item * How can I always keep my hash sorted?
+
+=item * What's the difference between "delete" and "undef" with hashes?
+
+=item * Why don't my tied hashes make the defined/exists distinction?
+
+=item * How do I reset an each() operation part-way through?
+
+=item * How can I get the unique keys from two hashes?
+
+=item * How can I store a multidimensional array in a DBM file?
+
+=item * How can I make my hash remember the order I put elements into it?
+
+=item * Why does passing a subroutine an undefined element in a hash create it?
+
+=item * How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
+
+=item * How can I use a reference as a hash key?
+
+=item * How do I handle binary data correctly?
+
+=item * How do I determine whether a scalar is a number/whole/integer/float?
+
+=item * How do I keep persistent data across program calls?
+
+=item * How do I print out or copy a recursive data structure?
+
+=item * How do I define methods for every class/object?
+
+=item * How do I verify a credit card checksum?
+
+=item * How do I pack arrays of doubles or floats for XS code?
+
+=back
+
+
=item L<perlfaq5>: Files and Formats
I/O and the "f" issues: filehandles, flushing, formats and footers.
+=over 4
+
+=item * How do I flush/unbuffer an output filehandle? Why must I do this?
+
+=item * 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?
+
+=item * How do I count the number of lines in a file?
+
+=item * How do I make a temporary file name?
+
+=item * How can I manipulate fixed-record-length files?
+
+=item * 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?
+
+=item * How can I use a filehandle indirectly?
+
+=item * How can I set up a footer format to be used with write()?
+
+=item * How can I write() into a string?
+
+=item * How can I output my numbers with commas added?
+
+=item * How can I translate tildes (~) in a filename?
+
+=item * How come when I open a file read-write it wipes it out?
+
+=item * Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
+
+=item * Is there a leak/bug in glob()?
+
+=item * How can I open a file with a leading "E<gt>" or trailing blanks?
+
+=item * How can I reliably rename a file?
+
+=item * How can I lock a file?
+
+=item * Why can't I just open(FH, ">file.lock")?
+
+=item * I still don't get locking. I just want to increment the number in the file. How can I do this?
+
+=item * How do I randomly update a binary file?
+
+=item * How do I get a file's timestamp in perl?
+
+=item * How do I set a file's timestamp in perl?
+
+=item * How do I print to more than one file at once?
+
+=item * How can I read in an entire file all at once?
+
+=item * How can I read in a file by paragraphs?
+
+=item * How can I read a single character from a file? From the keyboard?
+
+=item * How can I tell whether there's a character waiting on a filehandle?
+
+=item * How do I do a C<tail -f> in perl?
+
+=item * How do I dup() a filehandle in Perl?
+
+=item * How do I close a file descriptor by number?
+
+=item * Why can't I use "C:\temp\foo" in DOS paths? What doesn't `C:\temp\foo.exe` work?
+
+=item * Why doesn't glob("*.*") get all the files?
+
+=item * Why does Perl let me delete read-only files? Why does C<-i> clobber protected files? Isn't this a bug in Perl?
+
+=item * How do I select a random line from a file?
+
+=item * Why do I get weird spaces when I print an array of lines?
+
+=back
+
+
=item L<perlfaq6>: Regexps
Pattern matching and regular expressions.
+=over 4
+
+=item * How can I hope to use regular expressions without creating illegible and unmaintainable code?
+
+=item * I'm having trouble matching over more than one line. What's wrong?
+
+=item * How can I pull out lines between two patterns that are themselves on different lines?
+
+=item * I put a regular expression into $/ but it didn't work. What's wrong?
+
+=item * How do I substitute case insensitively on the LHS, but preserving case on the RHS?
+
+=item * How can I make C<\w> match national character sets?
+
+=item * How can I match a locale-smart version of C</[a-zA-Z]/>?
+
+=item * How can I quote a variable to use in a regex?
+
+=item * What is C</o> really for?
+
+=item * How do I use a regular expression to strip C style comments from a file?
+
+=item * Can I use Perl regular expressions to match balanced text?
+
+=item * What does it mean that regexes are greedy? How can I get around it?
+
+=item * How do I process each word on each line?
+
+=item * How can I print out a word-frequency or line-frequency summary?
+
+=item * How can I do approximate matching?
+
+=item * How do I efficiently match many regular expressions at once?
+
+=item * Why don't word-boundary searches with C<\b> work for me?
+
+=item * Why does using $&, $`, or $' slow my program down?
+
+=item * What good is C<\G> in a regular expression?
+
+=item * Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
+
+=item * What's wrong with using grep or map in a void context?
+
+=item * How can I match strings with multibyte characters?
+
+=item * How do I match a pattern that is supplied by the user?
+
+=back
+
+
=item L<perlfaq7>: General Perl Language Issues
General Perl language issues that don't clearly fit into any of the
other sections.
+=over 4
+
+=item * Can I get a BNF/yacc/RE for the Perl language?
+
+=item * What are all these $@%&* punctuation signs, and how do I know when to use them?
+
+=item * Do I always/never have to quote my strings or use semicolons and commas?
+
+=item * How do I skip some return values?
+
+=item * How do I temporarily block warnings?
+
+=item * What's an extension?
+
+=item * Why do Perl operators have different precedence than C operators?
+
+=item * How do I declare/create a structure?
+
+=item * How do I create a module?
+
+=item * How do I create a class?
+
+=item * How can I tell if a variable is tainted?
+
+=item * What's a closure?
+
+=item * What is variable suicide and how can I prevent it?
+
+=item * How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
+
+=item * How do I create a static variable?
+
+=item * What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
+
+=item * How can I access a dynamic variable while a similarly named lexical is in scope?
+
+=item * What's the difference between deep and shallow binding?
+
+=item * Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
+
+=item * How do I redefine a builtin function, operator, or method?
+
+=item * What's the difference between calling a function as &foo and foo()?
+
+=item * How do I create a switch or case statement?
+
+=item * How can I catch accesses to undefined variables/functions/methods?
+
+=item * Why can't a method included in this same file be found?
+
+=item * How can I find out my current package?
+
+=item * How can I comment out a large block of perl code?
+
+=item * How do I clear a package?
+
+=item * How can I use a variable as a variable name?
+
+=back
+
+
=item L<perlfaq8>: System Interaction
Interprocess communication (IPC), control over the user-interface
(keyboard, screen and pointing devices).
+=over 4
+
+=item * How do I find out which operating system I'm running under?
+
+=item * How come exec() doesn't return?
+
+=item * How do I do fancy stuff with the keyboard/screen/mouse?
+
+=item * How do I print something out in color?
+
+=item * How do I read just one key without waiting for a return key?
+
+=item * How do I check whether input is ready on the keyboard?
+
+=item * How do I clear the screen?
+
+=item * How do I get the screen size?
+
+=item * How do I ask the user for a password?
+
+=item * How do I read and write the serial port?
+
+=item * How do I decode encrypted password files?
+
+=item * How do I start a process in the background?
+
+=item * How do I trap control characters/signals?
+
+=item * How do I modify the shadow password file on a Unix system?
+
+=item * How do I set the time and date?
+
+=item * How can I sleep() or alarm() for under a second?
+
+=item * How can I measure time under a second?
+
+=item * How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
+
+=item * Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
+
+=item * How can I call my system's unique C functions from Perl?
+
+=item * Where do I get the include files to do ioctl() or syscall()?
+
+=item * Why do setuid perl scripts complain about kernel problems?
+
+=item * How can I open a pipe both to and from a command?
+
+=item * Why can't I get the output of a command with system()?
+
+=item * How can I capture STDERR from an external command?
+
+=item * Why doesn't open() return an error when a pipe open fails?
+
+=item * What's wrong with using backticks in a void context?
+
+=item * How can I call backticks without shell processing?
+
+=item * Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
+
+=item * How can I convert my shell script to perl?
+
+=item * Can I use perl to run a telnet or ftp session?
+
+=item * How can I write expect in Perl?
+
+=item * Is there a way to hide perl's command line from programs such as "ps"?
+
+=item * 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?
+
+=item * How do I close a process's filehandle without waiting for it to complete?
+
+=item * How do I fork a daemon process?
+
+=item * How do I make my program run with sh and csh?
+
+=item * How do I find out if I'm running interactively or not?
+
+=item * How do I timeout a slow event?
+
+=item * How do I set CPU limits?
+
+=item * How do I avoid zombies on a Unix system?
+
+=item * How do I use an SQL database?
+
+=item * How do I make a system() exit on control-C?
+
+=item * How do I open a file without blocking?
+
+=item * How do I install a module from CPAN?
+
+=item * What's the difference between require and use?
+
+=item * How do I keep my own module/library directory?
+
+=item * How do I add the directory my program lives in to the module/library search path?
+
+=item * How do I add a directory to my include path at runtime?
+
+=item * What is socket.ph and where do I get it?
+
+=back
+
+
=item L<perlfaq9>: Networking
Networking, the Internet, and a few on the web.
+=over 4
+
+=item * My CGI script runs from the command line but not the browser. (500 Server Error)
+
+=item * How can I get better error messages from a CGI program?
+
+=item * How do I remove HTML from a string?
+
+=item * How do I extract URLs?
+
+=item * How do I download a file from the user's machine? How do I open a file on another machine?
+
+=item * How do I make a pop-up menu in HTML?
+
+=item * How do I fetch an HTML file?
+
+=item * How do I automate an HTML form submission?
+
+=item * How do I decode or create those %-encodings on the web?
+
+=item * How do I redirect to another page?
+
+=item * How do I put a password on my web pages?
+
+=item * How do I edit my .htpasswd and .htgroup files with Perl?
+
+=item * How do I make sure users can't enter values into a form that cause my CGI script to do bad things?
+
+=item * How do I parse a mail header?
+
+=item * How do I decode a CGI form?
+
+=item * How do I check a valid mail address?
+
+=item * How do I decode a MIME/BASE64 string?
+
+=item * How do I return the user's mail address?
+
+=item * How do I send mail?
+
+=item * How do I read mail?
+
+=item * How do I find out my hostname/domainname/IP address?
+
+=item * How do I fetch a news article or the active newsgroups?
+
+=item * How do I fetch/put an FTP file?
+
+=item * How can I do RPC in Perl?
+
+=back
+
+
=back
=head2 Where to get this document
=head2 How to contribute to this document
You may mail corrections, additions, and suggestions to
-perlfaq-suggestions@perl.com. Mail sent to the old perlfaq alias will
-merely cause the FAQ to be sent to you.
+perlfaq-suggestions@perl.com . This alias should not be
+used to I<ask> FAQs. It's for fixing the current FAQ.
+Send questions to the comp.lang.perl.misc newsgroup.
=head2 What will happen if you mail your Perl programming problems to the authors
=head1 Author and Copyright Information
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.
-=head2 Non-commercial Reproduction
-
-Permission is granted to distribute this document, in part or in full,
-via electronic means or printed copy providing that (1) that all credits
-and copyright notices be retained, (2) that no charges beyond reproduction
-be involved, and (3) that a reasonable attempt be made to use the most
-current version available.
+=head2 Bundled Distributions
-Furthermore, you may include this document in any distribution of the
-full Perl source or binaries, in its verbatim documentation, or on a
-complete dump of the CPAN archive, providing that the three stipulations
-given above continue to be met.
+When included as part of the Standard Version of Perl, or as part of
+its complete documentation whether printed or otherwise, this work
+may be distributed only under the terms of Perl's Artistic License.
+Any distribution of this file or derivatives thereof I<outside>
+of that package require that special arrangements be made with
+copyright holder.
-=head2 Commercial Reproduction
-
-Requests for all other distribution rights, including the incorporation
-in part or in full of this text or its code into commercial products
-such as but not limited to books, magazine articles, or CD-ROMs, must
-be made to perlfaq-legal@perl.com. Any commercial use of any portion
-of this document without prior written authorization by its authors
-will be subject to appropriate action.
+Irrespective of its distribution, all code examples in these files
+are hereby placed into the public domain. You are permitted and
+encouraged to use this code in your own programs for fun
+or for profit as you see fit. A simple comment in the code giving
+credit would be courteous but is not required.
=head2 Disclaimer
=over 4
+=item 23/May/99
+
+Extensive updates from the net in preparation for 5.006 release.
+
+=item 13/April/99
+
+More minor touch-ups. Added new question at the end
+of perlfaq7 on variable names within variables.
+
+=item 7/January/99
+
+Small touchups here and there. Added all questions in this
+document as a sort of table of contents.
+
+=item 22/June/98
+
+Significant changes throughout in preparation for the 5.005
+release.
+
+=item 24/April/97
+
+Style and whitespace changes from Chip, new question on reading one
+character at a time from a terminal using POSIX from Tom.
+
+=item 23/April/97
+
+Added http://www.oasis.leo.org/perl/ to L<perlfaq2>. Style fix to
+L<perlfaq3>. Added floating point precision, fixed complex number
+arithmetic, cross-references, caveat for Text::Wrap, alternative
+answer for initial capitalizing, fixed incorrect regexp, added example
+of Tie::IxHash to L<perlfaq4>. Added example of passing and storing
+filehandles, added commify to L<perlfaq5>. Restored variable suicide,
+and added mass commenting to L<perlfaq7>. Added Net::Telnet, fixed
+backticks, added reader/writer pair to telnet question, added FindBin,
+grouped module questions together in L<perlfaq8>. Expanded caveats
+for the simple URL extractor, gave LWP example, added CGI security
+question, expanded on the mail address answer in L<perlfaq9>.
+
+=item 25/March/97
+
+Added more info to the binary distribution section of L<perlfaq2>.
+Added Net::Telnet to L<perlfaq6>. Fixed typos in L<perlfaq8>. Added
+mail sending example to L<perlfaq9>. Added Merlyn's columns to
+L<perlfaq2>.
+
+=item 18/March/97
+
+Added the DATE to the NAME section, indicating which sections have
+changed.
+
+Mentioned SIGPIPE and L<perlipc> in the forking open answer in
+L<perlfaq8>.
+
+Fixed description of a regular expression in L<perlfaq4>.
+
=item 17/March/97 Version
Various typos fixed throughout.
https://perl5.git.perl.org / perl5.git / blobdiff