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perlfaq - frequently asked questions about Perl ($Date: 1999/05/23 20:38:02 $)DESCRIPTION
This document is structured into the following sections: perlfaq: Structural overview of the FAQ. This document. the perlfaq1 manpage: General Questions About Perl Very general, high-level information about Perl. o What is Perl? o Who supports Perl? Who develops it? Why is it free? o Which version of Perl should I use? o What are perl4 and perl5? o What is perl6? o How stable is Perl? o Is Perl difficult to learn? Python, REXX, Scheme, or Tcl? o How does Perl compare with other languages like Java, o Can I do [task] in Perl? o When shouldn't I program in Perl? o What's the difference between "perl" and "Perl"? o Is it a Perl program or a Perl script? o What is a JAPH? o Where can I get a list of Larry Wall witticisms? version (5/5.005/Perl instead of some other language)? o How can I convince my sysadmin/supervisor/employees to use the perlfaq2 manpage: Obtaining and Learning about Perl Where to find source and documentation to Perl, support, and related matters. o What machines support Perl? Where do I get it? o How can I get a binary version of Perl? perl? o I don't have a C compiler on my system. How can I compile scripts don't work. o I copied the Perl binary from one machine to another, but loading/malloc/linking/... failed. How do I make it work? o I grabbed the sources and tried to compile but gdbm/dynamic is CPAN? What does CPAN/src/... mean? o What modules and extensions are available for Perl? What o Is there an ISO or ANSI certified version of Perl? o Where can I get information on Perl? questions? o What are the Perl newsgroups on USENET? Where do I post o Where should I post source code? o Perl Books o Perl in Magazines o Perl on the Net: FTP and WWW Access o What mailing lists are there for perl? o Archives of comp.lang.perl.misc o Where can I buy a commercial version of Perl? o Where do I send bug reports? o What is perl.com? the perlfaq3 manpage: Programming Tools Programmer tools and programming support. o How do I do (anything)? o How can I use Perl interactively? o Is there a Perl shell? o How do I debug my Perl programs? o How do I profile my Perl programs? o How do I cross-reference my Perl programs? o Is there a pretty-printer (formatter) for Perl? o Is there a ctags for Perl? o Is there an IDE or Windows Perl Editor? o Where can I get Perl macros for vi? o Where can I get perl-mode for emacs? o How can I use curses with Perl? o How can I use X or Tk with Perl? o How can I generate simple menus without using CGI or Tk? o What is undump? o How can I make my Perl program run faster? o How can I make my Perl program take less memory? o Is it unsafe to return a pointer to local data? o How can I free an array or hash so my program shrinks? o How can I make my CGI script more efficient? o How can I hide the source for my Perl program? o How can I compile my Perl program into byte code or C? o How can I compile Perl into Java? o How can I get `#!perl' to work on [MS-DOS,NT,...]? o Can I write useful perl programs on the command line? o Why don't perl one- liners work on my DOS/Mac/VMS system? o Where can I learn about CGI or Web programming in Perl? o Where can I learn about object- oriented Perl programming? o Where can I learn about linking C with Perl? [h2xs, xsubpp] in my C program, what am I doing wrong? o I've read perlembed, perlguts, etc., but I can't embed perl does it mean? o When I tried to run my script, I got this message. What o What's MakeMaker? the perlfaq4 manpage: Data Manipulation Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues. instead of the numbers I should be getting (eg, 19.95)? o Why am I getting long decimals (eg, 19.9499999999999) o Why isn't my octal data interpreted correctly? floor()? Trig functions? o Does Perl have a round() function? What about ceil() and o How do I convert bits into ints? o Why doesn't & work the way I want it to? o How do I multiply matrices? o How do I perform an operation on a series of integers? o How can I output Roman numerals? o Why aren't my random numbers random? o How do I find the week-of-the-year/day-of-the-year? o How do I find the current century or millennium? o How can I compare two dates and find the difference? o How can I take a string and turn it into epoch seconds? o How can I find the Julian Day? o How do I find yesterday's date? o Does Perl have a year 2000 problem? Is Perl Y2K compliant? o How do I validate input? o How do I unescape a string? o How do I remove consecutive pairs of characters? o How do I expand function calls in a string? o How do I find matching/nesting anything? o How do I reverse a string? o How do I expand tabs in a string? o How do I reformat a paragraph? o How can I access/change the first N letters of a string? o How do I change the Nth occurrence of something? within a string? o How can I count the number of occurrences of a substring o How do I capitalize all the words on one line? inside [character]? (Comma-separated files) o How can I split a [character] delimited string except when string? o How do I strip blank space from the beginning/end of a zeroes? o How do I pad a string with blanks or pad a number with o How do I extract selected columns from a string? o How do I find the soundex value of a string? o How can I expand variables in text strings? o What's wrong with always quoting "$vars"? o Why don't my <<HERE documents work? o What is the difference between a list and an array? o What is the difference between $array[1] and @array[1]? o How can I remove duplicate elements from a list or array? element? o How can I tell whether a list or array contains a certain compute the intersection of two arrays? o How do I compute the difference of two arrays? How do I o How do I test whether two arrays or hashes are equal? is true? o How do I find the first array element for which a condition o How do I handle linked lists? o How do I handle circular lists? o How do I shuffle an array randomly? o How do I process/modify each element of an array? o How do I select a random element from an array? o How do I permute N elements of a list? o How do I sort an array by (anything)? o How do I manipulate arrays of bits? o Why does defined() return true on empty arrays and hashes? o How do I process an entire hash? iterating over it? o What happens if I add or remove keys from a hash while o How do I look up a hash element by value? o How can I know how many entries are in a hash? o How do I sort a hash (optionally by value instead of key)? o How can I always keep my hash sorted? hashes? o What's the difference between "delete" and "undef" with distinction? o Why don't my tied hashes make the defined/exists o How do I reset an each() operation part-way through? o How can I get the unique keys from two hashes? o How can I store a multidimensional array in a DBM file? into it? o How can I make my hash remember the order I put elements hash create it? o Why does passing a subroutine an undefined element in a class/hash or array of hashes or arrays? o How can I make the Perl equivalent of a C structure/C++ o How can I use a reference as a hash key? o How do I handle binary data correctly? number/whole/integer/float? o How do I determine whether a scalar is a o How do I keep persistent data across program calls? o How do I print out or copy a recursive data structure? o How do I define methods for every class/object? o How do I verify a credit card checksum? o How do I pack arrays of doubles or floats for XS code? the perlfaq5 manpage: Files and Formats I/O and the "f" issues: filehandles, flushing, formats and footers. do this? o How do I flush/unbuffer an output filehandle? Why must I file/insert a line in the middle of a file/append to the beginning of a file? o How do I change one line in a file/delete a line in a o How do I count the number of lines in a file? o How do I make a temporary file name? o How can I manipulate fixed-record-length files? I pass filehandles between subroutines? How do I make an array of filehandles? o How can I make a filehandle local to a subroutine? How do o How can I use a filehandle indirectly? o How can I set up a footer format to be used with write()? o How can I write() into a string? o How can I output my numbers with commas added? o How can I translate tildes (~) in a filename? o How come when I open a file read- write it wipes it out? use <*>? o Why do I sometimes get an "Argument list too long" when I o Is there a leak/bug in glob()? blanks? o How can I open a file with a leading ">" or trailing o How can I reliably rename a file? o How can I lock a file? o Why can't I just open(FH, ">file.lock")? number in the file. How can I do this? o I still don't get locking. I just want to increment the o How do I randomly update a binary file? o How do I get a file's timestamp in perl? o How do I set a file's timestamp in perl? o How do I print to more than one file at once? o How can I read in an entire file all at once? o How can I read in a file by paragraphs? keyboard? o How can I read a single character from a file? From the filehandle? o How can I tell whether there's a character waiting on a o How do I do a `tail -f' in perl? o How do I dup() a filehandle in Perl? o How do I close a file descriptor by number? `C:\temp\foo.exe` work? o Why can't I use "C:\temp\foo" in DOS paths? What doesn't o Why doesn't glob("*.*") get all the files? clobber protected files? Isn't this a bug in Perl? o Why does Perl let me delete read- only files? Why does `-i' o How do I select a random line from a file? o Why do I get weird spaces when I print an array of lines? the perlfaq6 manpage: Regexps Pattern matching and regular expressions. illegible and unmaintainable code? o How can I hope to use regular expressions without creating What's wrong? o I'm having trouble matching over more than one line. themselves on different lines? o How can I pull out lines between two patterns that are What's wrong? o I put a regular expression into $/ but it didn't work. preserving case on the RHS? o How do I substitute case insensitively on the LHS, but o How can I make `\w' match national character sets? o How can I match a locale- smart version of `/[a-zA-Z]/'? o How can I quote a variable to use in a regex? o What is `/o' really for? from a file? o How do I use a regular expression to strip C style comments o Can I use Perl regular expressions to match balanced text? around it? o What does it mean that regexes are greedy? How can I get o How do I process each word on each line? summary? o How can I print out a word-frequency or line-frequency o How can I do approximate matching? once? o How do I efficiently match many regular expressions at o Why don't word- boundary searches with `\b' work for me? o Why does using $&, $`, or $' slow my program down? o What good is `\G' in a regular expression? o Are Perl regexes DFAs or NFAs? Are they POSIX compliant? o What's wrong with using grep or map in a void context? o How can I match strings with multibyte characters? o How do I match a pattern that is supplied by the user? the perlfaq7 manpage: General Perl Language Issues General Perl language issues that don't clearly fit into any of the other sections. o Can I get a BNF/yacc/RE for the Perl language? know when to use them? o What are all these $@%&* punctuation signs, and how do I semicolons and commas? o Do I always/never have to quote my strings or use o How do I skip some return values? o How do I temporarily block warnings? o What's an extension? operators? o Why do Perl operators have different precedence than C o How do I declare/create a structure? o How do I create a module? o How do I create a class? o How can I tell if a variable is tainted? o What's a closure? o What is variable suicide and how can I prevent it? Method, Regex}? o How can I pass/return a {Function, FileHandle, Array, Hash, o How do I create a static variable? scoping? Between local() and my()? o What's the difference between dynamic and lexical (static) lexical is in scope? o How can I access a dynamic variable while a similarly named o What's the difference between deep and shallow binding? o Why doesn't "my($foo) = <FILE>;" work right? o How do I redefine a builtin function, operator, or method? and foo()? o What's the difference between calling a function as &foo o How do I create a switch or case statement? variables/functions/methods? o How can I catch accesses to undefined o Why can't a method included in this same file be found? o How can I find out my current package? o How can I comment out a large block of perl code? o How do I clear a package? o How can I use a variable as a variable name? the perlfaq8 manpage: System Interaction Interprocess communication (IPC), control over the user-interface (keyboard, screen and pointing devices). o How do I find out which operating system I'm running under? o How come exec() doesn't return? o How do I do fancy stuff with the keyboard/screen/mouse? o How do I print something out in color? key? o How do I read just one key without waiting for a return o How do I check whether input is ready on the keyboard? o How do I clear the screen? o How do I get the screen size? o How do I ask the user for a password? o How do I read and write the serial port? o How do I decode encrypted password files? o How do I start a process in the background? o How do I trap control characters/signals? o How do I modify the shadow password file on a Unix system? o How do I set the time and date? o How can I sleep() or alarm() for under a second? o How can I measure time under a second? handling) o How can I do an atexit() or setjmp()/longjmp()? (Exception (Solaris)? What does the error message "Protocol not supported" mean? o Why doesn't my sockets program work under System V o How can I call my system's unique C functions from Perl? syscall()? o Where do I get the include files to do ioctl() or o Why do setuid perl scripts complain about kernel problems? o How can I open a pipe both to and from a command? o Why can't I get the output of a command with system()? o How can I capture STDERR from an external command? o Why doesn't open() return an error when a pipe open fails? o What's wrong with using backticks in a void context? o How can I call backticks without shell processing? on Unix, ^Z on MS-DOS)? o Why can't my script read from STDIN after I gave it EOF (^D o How can I convert my shell script to perl? o Can I use perl to run a telnet or ftp session? o How can I write expect in Perl? such as "ps"? o Is there a way to hide perl's command line from programs script. How come the change disappeared when I exited the script? How do I get my changes to be visible? o I {changed directory, modified my environment} in a perl it to complete? o How do I close a process's filehandle without waiting for o How do I fork a daemon process? o How do I make my program run with sh and csh? o How do I find out if I'm running interactively or not? o How do I timeout a slow event? o How do I set CPU limits? o How do I avoid zombies on a Unix system? o How do I use an SQL database? o How do I make a system() exit on control-C? o How do I open a file without blocking? o How do I install a module from CPAN? o What's the difference between require and use? o How do I keep my own module/library directory? module/library search path? o How do I add the directory my program lives in to the o How do I add a directory to my include path at runtime? o What is socket.ph and where do I get it? the perlfaq9 manpage: Networking Networking, the Internet, and a few on the web. browser. (500 Server Error) o My CGI script runs from the command line but not the o How can I get better error messages from a CGI program? o How do I remove HTML from a string? o How do I extract URLs? open a file on another machine? o How do I download a file from the user's machine? How do I o How do I make a pop-up menu in HTML? o How do I fetch an HTML file? o How do I automate an HTML form submission? o How do I decode or create those %-encodings on the web? o How do I redirect to another page? o How do I put a password on my web pages? o How do I edit my .htpasswd and .htgroup files with Perl? that cause my CGI script to do bad things? o How do I make sure users can't enter values into a form o How do I parse a mail header? o How do I decode a CGI form? o How do I check a valid mail address? o How do I decode a MIME/BASE64 string? o How do I return the user's mail address? o How do I send mail? o How do I read mail? o How do I find out my hostname/domainname/IP address? o How do I fetch a news article or the active newsgroups? o How do I fetch/put an FTP file? o How can I do RPC in Perl? Where to get this document This document is posted regularly to comp.lang.perl.announce and several other related newsgroups. It is available in a variety of formats from CPAN in the /CPAN/doc/FAQs/FAQ/ directory, or on the web at http://www.perl.com/perl/faq/ . How to contribute to this document You may mail corrections, additions, and suggestions to perlfaq-suggestions@perl.com . This alias should not be used to ask FAQs. It's for fixing the current FAQ. Send questions to the comp.lang.perl.misc newsgroup. What will happen if you mail your Perl programming problems to the authors Your questions will probably go unread, unless they're suggestions of new questions to add to the FAQ, in which case they should have gone to the perlfaq- suggestions@perl.com instead. You should have read section 2 of this faq. There you would have learned that comp.lang.perl.misc is the appropriate place to go for free advice. If your question is really important and you require a prompt and correct answer, you should hire a consultant.Credits
When I first began the Perl FAQ in the late 80s, I never realized it would have grown to over a hundred pages, nor that Perl would ever become so popular and widespread. This document could not have been written without the tremendous help provided by Larry Wall and the rest of the Perl Porters.Author and Copyright Information
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. All rights reserved. Bundled Distributions 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 outside of that package require that special arrangements be made with copyright holder. 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. Disclaimer This information is offered in good faith and in the hope that it may be of use, but is not guaranteed to be correct, up to date, or suitable for any particular purpose whatsoever. The authors accept no liability in respect of this information or its use.Changes
23/May/99 Extensive updates from the net in preparation for 5.6 release. 13/April/99 More minor touch-ups. Added new question at the end of perlfaq7 on variable names within variables. 7/January/99 Small touchups here and there. Added all questions in this document as a sort of table of contents. 22/June/98 Significant changes throughout in preparation for the 5.005 release. 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. 23/April/97 Added http://www.oasis.leo.org/perl/ to the perlfaq2 manpage. Style fix to the perlfaq3 manpage. 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 the perlfaq4 manpage. Added example of passing and storing filehandles, added commify to the perlfaq5 manpage. Restored variable suicide, and added mass commenting to the perlfaq7 manpage. Added Net::Telnet, fixed backticks, added reader/writer pair to telnet question, added FindBin, grouped module questions together in the perlfaq8 manpage. Expanded caveats for the simple URL extractor, gave LWP example, added CGI security question, expanded on the mail address answer in the perlfaq9 manpage. 25/March/97 Added more info to the binary distribution section of the perlfaq2 manpage. Added Net::Telnet to the perlfaq6 manpage. Fixed typos in the perlfaq8 manpage. Added mail sending example to the perlfaq9 manpage. Added Merlyn's columns to the perlfaq2 manpage. 18/March/97 Added the DATE to the NAME section, indicating which sections have changed. Mentioned SIGPIPE and the perlipc manpage in the forking open answer in the perlfaq8 manpage. Fixed description of a regular expression in the perlfaq4 manpage. 17/March/97 Version Various typos fixed throughout. Added new question on Perl BNF on the perlfaq7 manpage. Initial Release: 11/March/97 This is the initial release of version 3 of the FAQ; consequently there have been no changes since its initial release.
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