


PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


NAME
       perlfaq - frequently asked questions about Perl ($Date:
       2001/05/24 18:35:53 $)

DESCRIPTION
       The perlfaq is structured into the following documents:

       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?

       o   How does Perl compare with other languages like Java,
           Python, REXX, Scheme, or Tcl?

       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?

       o   How can I convince my sysadmin/supervisor/employees to
           use version 5/5.005/Perl instead of some other lan-
           guage?

       the perlfaq2 manpage: Obtaining and Learning about Perl

       Where to find source and documentation to Perl, support,
       and related matters.





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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       o   What machines support Perl?  Where do I get it?

       o   How can I get a binary version of Perl?

       o   I don't have a C compiler on my system.  How can I
           compile perl?

       o   I copied the Perl binary from one machine to another,
           but scripts don't work.

       o   I grabbed the sources and tried to compile but
           gdbm/dynamic loading/malloc/linking/... failed.  How
           do I make it work?

       o   What modules and extensions are available for Perl?
           What is CPAN?  What does CPAN/src/... mean?

       o   Is there an ISO or ANSI certified version of Perl?

       o   Where can I get information on Perl?

       o   What are the Perl newsgroups on Usenet?  Where do I
           post questions?

       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? Perl Mongers? pm.org? perl.org?

       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?




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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       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 sys-
           tem?

       o   Where can I learn about CGI or Web programming in
           Perl?

       o   Where can I learn about object-oriented Perl program-
           ming?

       o   Where can I learn about linking C with Perl? [h2xs,
           xsubpp]



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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       o   I've read perlembed, perlguts, etc., but I can't embed
           perl in my C program; what am I doing wrong?

       o   When I tried to run my script, I got this message.
           What does it mean?

       o   What's MakeMaker?

       the perlfaq4 manpage: Data Manipulation

       Manipulating numbers, dates, strings, arrays, hashes, and
       miscellaneous data issues.

       o   Why am I getting long decimals (eg, 19.9499999999999)
           instead of the numbers I should be getting (eg,
           19.95)?

       o   Why isn't my octal data interpreted correctly?

       o   Does Perl have a round() function?  What about ceil()
           and floor()?  Trig functions?

       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 sec-
           onds?

       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 com-
           pliant?

       o   How do I validate input?

       o   How do I unescape a string?




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       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?

       o   How can I count the number of occurrences of a sub-
           string within a string?

       o   How do I capitalize all the words on one line?

       o   How can I split a [character] delimited string except
           when inside [character]? (Comma-separated files)

       o   How do I strip blank space from the beginning/end of a
           string?

       o   How do I pad a string with blanks or pad a number with
           zeroes?

       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?

       o   How can I tell whether a list or array contains a cer-
           tain element?

       o   How do I compute the difference of two arrays?  How do
           I compute the intersection of two arrays?



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       o   How do I test whether two arrays or hashes are equal?

       o   How do I find the first array element for which a con-
           dition is true?

       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?

       o   What happens if I add or remove keys from a hash while
           iterating over it?

       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?

       o   What's the difference between "delete" and "undef"
           with hashes?

       o   Why don't my tied hashes make the defined/exists dis-
           tinction?

       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?

       o   How can I make my hash remember the order I put ele-
           ments into it?




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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       o   Why does passing a subroutine an undefined element in
           a hash create it?

       o   How can I make the Perl equivalent of a C
           structure/C++ class/hash or array of hashes or arrays?

       o   How can I use a reference as a hash key?

       o   How do I handle binary data correctly?

       o   How do I determine whether a scalar is a num-
           ber/whole/integer/float?

       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.

       o   How do I flush/unbuffer an output filehandle?  Why
           must I do this?

       o   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?

       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?

       o   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?

       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?




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       o   How can I translate tildes (~) in a filename?

       o   How come when I open a file read-write it wipes it
           out?

       o   Why do I sometimes get an "Argument list too long"
           when I use <*>?

       o   Is there a leak/bug in glob()?

       o   How can I open a file with a leading ">" or trailing
           blanks?

       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")?

       o   I still don't get locking.  I just want to increment
           the number in the file.  How can I do this?

       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?

       o   How can I read a single character from a file?  From
           the keyboard?

       o   How can I tell whether there's a character waiting on
           a filehandle?

       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?

       o   Why can't I use "C:\temp\foo" in DOS paths?  What
           doesn't `C:\temp\foo.exe` work?

       o   Why doesn't glob("*.*") get all the files?

       o   Why does Perl let me delete read-only files?  Why does
           "-i" clobber protected files?  Isn't this a bug in
           Perl?



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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       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.

       o   How can I hope to use regular expressions without cre-
           ating illegible and unmaintainable code?

       o   I'm having trouble matching over more than one line.
           What's wrong?

       o   How can I pull out lines between two patterns that are
           themselves on different lines?

       o   I put a regular expression into $/ but it didn't work.
           What's wrong?

       o   How do I substitute case insensitively on the LHS
           while preserving case on the RHS?

       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?

       o   How do I use a regular expression to strip C style
           comments from a file?

       o   Can I use Perl regular expressions to match balanced
           text?

       o   What does it mean that regexes are greedy?  How can I
           get around it?

       o   How do I process each word on each line?

       o   How can I print out a word-frequency or line-frequency
           summary?

       o   How can I do approximate matching?

       o   How do I efficiently match many regular expressions at
           once?

       o   Why don't word-boundary searches with "\b" work for
           me?



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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       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 compli-
           ant?

       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?

       o   What are all these $@%&* punctuation signs, and how do
           I know when to use them?

       o   Do I always/never have to quote my strings or use
           semicolons and commas?

       o   How do I skip some return values?

       o   How do I temporarily block warnings?

       o   What's an extension?

       o   Why do Perl operators have different precedence than C
           operators?

       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?

       o   How can I pass/return a {Function, FileHandle, Array,
           Hash, Method, Regex}?

       o   How do I create a static variable?

       o   What's the difference between dynamic and lexical
           (static) scoping?  Between local() and my()?



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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       o   How can I access a dynamic variable while a similarly
           named lexical is in scope?

       o   What's the difference between deep and shallow bind-
           ing?

       o   Why doesn't "my($foo) = <FILE>;" work right?

       o   How do I redefine a builtin function, operator, or
           method?

       o   What's the difference between calling a function as
           &foo and foo()?

       o   How do I create a switch or case statement?

       o   How can I catch accesses to undefined variables/func-
           tions/methods?

       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 key-
           board/screen/mouse?

       o   How do I print something out in color?

       o   How do I read just one key without waiting for a
           return key?

       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?




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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       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?

       o   How can I do an atexit() or setjmp()/longjmp()?
           (Exception handling)

       o   Why doesn't my sockets program work under System V
           (Solaris)?  What does the error message "Protocol not
           supported" mean?

       o   How can I call my system's unique C functions from
           Perl?

       o   Where do I get the include files to do ioctl() or
           syscall()?

       o   Why do setuid perl scripts complain about kernel prob-
           lems?

       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?

       o   Why can't my script read from STDIN after I gave it
           EOF (^D on Unix, ^Z on MS-DOS)?

       o   How can I convert my shell script to perl?





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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       o   Can I use perl to run a telnet or ftp session?

       o   How can I write expect in Perl?

       o   Is there a way to hide perl's command line from pro-
           grams such as "ps"?

       o   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 vis-
           ible?

       o   How do I close a process's filehandle without waiting
           for it to complete?

       o   How do I fork a daemon process?

       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?

       o   How do I add the directory my program lives in to the
           module/library search path?

       o   How do I add a directory to my include path at run-
           time?

       o   What is socket.ph and where do I get it?

       the perlfaq9 manpage: Networking

       Networking, the Internet, and a few on the web.

       o   My CGI script runs from the command line but not the
           browser.  (500 Server Error)

       o   How can I get better error messages from a CGI pro-
           gram?



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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       o   How do I remove HTML from a string?

       o   How do I extract URLs?

       o   How do I download a file from the user's machine?  How
           do I open a file on another machine?

       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?

       o   How do I make sure users can't enter values into a
           form that cause my CGI script to do bad things?

       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 news-
           groups?

       o   How do I fetch/put an FTP file?

       o   How can I do RPC in Perl?

About the perlfaq documents







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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       Where to get the perlfaq

       This document is posted regularly to
       comp.lang.perl.announce and several other related news-
       groups.  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 the perlfaq

       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 prob-
       lems 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-sugges-
       tions@perl.com instead.

       You should have read section 2 of this faq.  There you
       would have learned that comp.lang.perl.misc is the appro-
       priate 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 Tork-
       ington.  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
       requires 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



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PERLFAQ(1)       Perl Programmers Reference Guide      PERLFAQ(1)


       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 cor-
       rect, up to date, or suitable for any particular purpose
       whatsoever.  The authors accept no liability in respect of
       this information or its use.

Changes
       1/November/2000
           A few grammatical fixes and updates implemented by
           John Borwick.

       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 arith-
           metic, cross-references, caveat for Text::Wrap, alter-
           native answer for initial capitalizing, fixed incor-
           rect regexp, added example of Tie::IxHash to the perl-
           faq4 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 ques-
           tion, 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



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           in the perlfaq9 manpage.

       25/March/97
           Added more info to the binary distribution section of
           the perlfaq2 manpage.  Added Net::Telnet to the perl-
           faq6 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 fork-
           ing open answer in the perlfaq8 manpage.

           Fixed description of a regular expression in the perl-
           faq4 manpage.

       17/March/97 Version
           Various typos fixed throughout.

           Added new question on Perl BNF on the perlfaq7 man-
           page.

       Initial Release: 11/March/97
           This is the initial release of version 3 of the FAQ;
           consequently there have been no changes since its ini-
           tial release.




























2001-05-24                 perl v5.6.1                         17


