| perlfaq - frequently asked questions about Perl |
perlfaq - frequently asked questions about Perl ($Date: 2003/01/31 17:37:17 $)
The perlfaq is divided into several documents based on topics. A table of contents is at the end of this document.
Extracts of the perlfaq are posted regularly to comp.lang.perl.misc. It is available on many web sites: http://www.perldoc.com/ and http://faq.perl.org/
You may mail corrections, additions, and suggestions to perlfaq-workers@perl.org . 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. You can view the source tree at http://cvs.perl.org/cvsweb/perlfaq/ (which is outside of the main Perl source tree). The CVS repository notes all changes to the FAQ.
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-workers@perl.org 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.
The original perlfaq was written by Tom Christiansen, then expanded by collaboration between Tom and Nathan Torkington. The current document is maintained by the perlfaq-workers (perlfaq-workers@perl.org) Several people have contributed answers, corrections, and comments.
Copyright (c) 1997-2003 Tom Christiansen, Nathan Torkington, and other contributors noted in the answers.
All rights reserved.
This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.
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.
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.
Very general, high-level questions about Perl.
Where to find source and documentation for Perl, support, and related matters.
Programmer tools and programming support.
#!perl to work on [MS-DOS,NT,...]?
Can I write useful Perl programs on the command line?
Why don't Perl one-liners work on my DOS/Mac/VMS system?
Where can I learn about CGI or Web programming in Perl?
Where can I learn about object-oriented Perl programming?
Where can I learn about linking C with Perl? [h2xs, xsubpp]
I've read perlembed, perlguts, etc., but I can't embed perl in
my C program; what am I doing wrong?
When I tried to run my script, I got this message. What does it mean?
What's MakeMaker?
Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
round() function? What about ceil() and floor()? Trig functions?
How do I convert between numeric representations?
Why doesn't & work the way I want it to?
How do I multiply matrices?
How do I perform an operation on a series of integers?
How can I output Roman numerals?
Why aren't my random numbers random?
How do I get a random number between X and Y?
How do I find the day or week of the year?
How do I find the current century or millennium?
How can I compare two dates and find the difference?
How can I take a string and turn it into epoch seconds?
How can I find the Julian Day?
How do I find yesterday's date?
Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
How do I validate input?
How do I unescape a string?
How do I remove consecutive pairs of characters?
How do I expand function calls in a string?
How do I find matching/nesting anything?
How do I reverse a string?
How do I expand tabs in a string?
How do I reformat a paragraph?
How can I access or change N characters of a string?
How do I change the Nth occurrence of something?
How can I count the number of occurrences of a substring within a string?
How do I capitalize all the words on one line?
How can I split a [character] delimited string except when inside [character]?
How do I strip blank space from the beginning/end of a string?
How do I pad a string with blanks or pad a number with zeroes?
How do I extract selected columns from a string?
How do I find the soundex value of a string?
How can I expand variables in text strings?
What's wrong with always quoting ``$vars''?
Why don't my <<HERE documents work?
What is the difference between a list and an array?
What is the difference between $array[1] and @array[1]?
How can I remove duplicate elements from a list or array?
How can I tell whether a certain element is contained in a list or array?
How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
How do I test whether two arrays or hashes are equal?
How do I find the first array element for which a condition is true?
How do I handle linked lists?
How do I handle circular lists?
How do I shuffle an array randomly?
How do I process/modify each element of an array?
How do I select a random element from an array?
How do I permute N elements of a list?
How do I sort an array by (anything)?
How do I manipulate arrays of bits?
Why does defined() return true on empty arrays and hashes?
How do I process an entire hash?
What happens if I add or remove keys from a hash while iterating over it?
How do I look up a hash element by value?
How can I know how many entries are in a hash?
How do I sort a hash (optionally by value instead of key)?
How can I always keep my hash sorted?
What's the difference between ``delete'' and ``undef'' with hashes?
Why don't my tied hashes make the defined/exists distinction?
How do I reset an each() operation part-way through?
How can I get the unique keys from two hashes?
How can I store a multidimensional array in a DBM file?
How can I make my hash remember the order I put elements into it?
Why does passing a subroutine an undefined element in a hash create it?
How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
How can I use a reference as a hash key?
How do I handle binary data correctly?
How do I determine whether a scalar is a number/whole/integer/float?
How do I keep persistent data across program calls?
How do I print out or copy a recursive data structure?
How do I define methods for every class/object?
How do I verify a credit card checksum?
How do I pack arrays of doubles or floats for XS code?
I/O and the ``f'' issues: filehandles, flushing, formats, and footers.
-i option from within a program?
How do I make a temporary file name?
How can I manipulate fixed-record-length files?
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?
How can I use a filehandle indirectly?
How can I set up a footer format to be used with write()?
How can I write() into a string?
How can I output my numbers with commas added?
How can I translate tildes (~) in a filename?
How come when I open a file read-write it wipes it out?
Why do I sometimes get an ``Argument list too long'' when I use <*>?
Is there a leak/bug in glob()?
How can I open a file with a leading ``>'' or trailing blanks?
How can I reliably rename a file?
How can I lock a file?
Why can't I just open(FH, ``>file.lock'')?
I still don't get locking. I just want to increment the number in the file. How can I do this?
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?
How do I randomly update a binary file?
How do I get a file's timestamp in perl?
How do I set a file's timestamp in perl?
How do I print to more than one file at once?
How can I read in an entire file all at once?
How can I read in a file by paragraphs?
How can I read a single character from a file? From the keyboard?
How can I tell whether there's a character waiting on a filehandle?
How do I do a tail -f in perl?
How do I dup() a filehandle in Perl?
How do I close a file descriptor by number?
Why can't I use ``C:\temp\foo'' in DOS paths? What doesn't `C:\temp\foo.exe` work?
Why doesn't glob(``*.*'') get all the files?
Why does Perl let me delete read-only files? Why does -i clobber protected files? Isn't this a bug in Perl?
How do I select a random line from a file?
Why do I get weird spaces when I print an array of lines?
Pattern matching and regular expressions.
\w match national character sets?
How can I match a locale-smart version of /[a-zA-Z]/?
How can I quote a variable to use in a regex?
What is /o really for?
How do I use a regular expression to strip C style comments from a file?
Can I use Perl regular expressions to match balanced text?
What does it mean that regexes are greedy? How can I get around it?
How do I process each word on each line?
How can I print out a word-frequency or line-frequency summary?
How can I do approximate matching?
How do I efficiently match many regular expressions at once?
Why don't word-boundary searches with \b work for me?
Why does using $&, $`, or $' slow my program down?
What good is \G in a regular expression?
Are Perl regexes DFAs or NFAs? Are they POSIX compliant?
What's wrong with using grep or map in a void context?
How can I match strings with multibyte characters?
How do I match a pattern that is supplied by the user?
General Perl language issues that don't clearly fit into any of the other sections.
local() and my()?
How can I access a dynamic variable while a similarly named lexical is in scope?
What's the difference between deep and shallow binding?
Why doesn't ``my($foo) = <FILE>;'' work right?
How do I redefine a builtin function, operator, or method?
What's the difference between calling a function as &foo and foo()?
How do I create a switch or case statement?
How can I catch accesses to undefined variables, functions, or methods?
Why can't a method included in this same file be found?
How can I find out my current package?
How can I comment out a large block of perl code?
How do I clear a package?
How can I use a variable as a variable name?
What does ``bad interpreter'' mean?
Interprocess communication (IPC), control over the user-interface (keyboard, screen and pointing devices).
exec() doesn't return?
How do I do fancy stuff with the keyboard/screen/mouse?
How do I print something out in color?
How do I read just one key without waiting for a return key?
How do I check whether input is ready on the keyboard?
How do I clear the screen?
How do I get the screen size?
How do I ask the user for a password?
How do I read and write the serial port?
How do I decode encrypted password files?
How do I start a process in the background?
How do I trap control characters/signals?
How do I modify the shadow password file on a Unix system?
How do I set the time and date?
How can I sleep() or alarm() for under a second?
How can I measure time under a second?
How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
Why doesn't my sockets program work under System V (Solaris)? What does the error message ``Protocol not supported'' mean?
How can I call my system's unique C functions from Perl?
Where do I get the include files to do ioctl() or syscall()?
Why do setuid perl scripts complain about kernel problems?
How can I open a pipe both to and from a command?
Why can't I get the output of a command with system()?
How can I capture STDERR from an external command?
Why doesn't open() return an error when a pipe open fails?
What's wrong with using backticks in a void context?
How can I call backticks without shell processing?
Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
How can I convert my shell script to perl?
Can I use perl to run a telnet or ftp session?
How can I write expect in Perl?
Is there a way to hide perl's command line from programs such as ``ps''?
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?
How do I close a process's filehandle without waiting for it to complete?
How do I fork a daemon process?
How do I find out if I'm running interactively or not?
How do I timeout a slow event?
How do I set CPU limits?
How do I avoid zombies on a Unix system?
How do I use an SQL database?
How do I make a system() exit on control-C?
How do I open a file without blocking?
How do I install a module from CPAN?
What's the difference between require and use?
How do I keep my own module/library directory?
How do I add the directory my program lives in to the module/library search path?
How do I add a directory to my include path at runtime?
What is socket.ph and where do I get it?
Networking, the internet, and a few on the web.
| perlfaq - frequently asked questions about Perl |