PERLFAQ(1) Perl Programmers Reference Guide PERLFAQ(1)
NAME
perlfaq - frequently asked questions about Perl
DESCRIPTION
The perlfaq comprises several documents that answer the most commonly
asked questions about Perl and Perl programming. Its divided by topic
into nine major sections outlined in this document.
Where to get the perlfaq
The perlfaq comes with the standard Perl distribution, so if you have
Perl you should have the perlfaq. You should also have the "perldoc"
tool that lets you read the perlfaq:
$ perldoc perlfaq
Besides your local system, you can find the perlfaq on the web, includ
ing at http://perldoc.perl.org/ .
The perlfaq is an evolving document and you can read the latest version
at http://faq.perl.org/ . The perlfaq-workers periodically post
extracts of the latest perlfaq to comp.lang.perl.misc.
You can view the source tree at http://cvs.perl.org/viewcvs/cvs-pub
lic/perlfaq/ (which is outside of the main Perl source tree). The CVS
repository notes all changes to the FAQ and holds the latest version of
the working documents and may vary significantly from the version dis
tributed with the latest version of Perl. Check the repository before
sending your corrections.
How to contribute to the perlfaq
You can mail corrections, additions, and suggestions to "". The perlfaq volunteers use this address to coor
dinate their efforts and track the perlfaq development. They appreci
ate your contributions to the FAQ but do not have time to provide indi
vidual help, so dont use this address to ask FAQs.
The perlfaq server posts extracts of the perlfaq to that newsgroup
every 6 hours (or so), and the community of volunteers reviews and
updates the answers. If youd like to help review and update the
answers, check out comp.lang.perl.misc.
What will happen if you mail your Perl programming problems to the
authors?
The perlfaq-workers like to keep all traffic on the perlfaq-workers
list so that everyone can see the work being done (and the work that
needs to be done). The mailing list serves as an official record. If
you email the authors or maintainers directly, youll probably get a
reply asking you to post to the mailing list. If you dont get a reply,
it probably means that the person never saw the message or didnt have
time to deal with it. Posting to the list allows the volunteers with
time to deal with it when others are busy.
If you have a question that isnt in the FAQ and you would like help
with it, try the resources in perlfaq2.
CREDITS
Tom Christiansen wrote the original perlfaq then expanded it with the
help of Nat Torkington. The perlfaq-workers maintain current document
and the dezinens of comp.lang.perl.misc regularly review and update the
FAQ. Several people have contributed answers, corrections, and com
ments, and the perlfaq notes those contributions wherever appropriate.
AUTHOR AND COPYRIGHT
Tom Christainsen wrote the original version of this document. brian d
foy "" wrote this version. See the individual perlfaq
documents for additional copyright information.
This document is available under the same terms as Perl itself. Code
examples in all the perlfaq documents are in the public domain. Use
them as you see fit (and at your own risk with no warranty from any
one).
Table of Contents
perlfaq - this document
perlfaq1 - General Questions About Perl
perlfaq2 - Obtaining and Learning about Perl
perlfaq3 - Programming Tools
perlfaq4 - Data Manipulation
perlfaq5 - Files and Formats
perlfaq6 - Regular Expressions
perlfaq7 - General Perl Language Issues
perlfaq8 - System Interaction
perlfaq9 - Networking
The Questions
perlfaq1: General Questions About Perl
Very general, high-level questions about Perl.
What is Perl?
Who supports Perl? Who develops it? Why is it free?
Which version of Perl should I use?
What are perl4, perl5, or perl6?
What is Ponie?
What is perl6?
How stable is Perl?
Is Perl difficult to learn?
How does Perl compare with other languages like Java, Python, REXX,
Scheme, or Tcl?
Can I do [task] in Perl?
When shouldnt I program in Perl?
Whats the difference between "perl" and "Perl"?
Is it a Perl program or a Perl script?
What is a JAPH?
Where can I get a list of Larry Wall witticisms?
How can I convince my sysadmin/supervisor/employees to use version
5/5.6.1/Perl instead of some other language?
perlfaq2: Obtaining and Learning about Perl
Where to find source and documentation for Perl, support, and related
matters.
What machines support perl? Where do I get it?
How can I get a binary version of perl?
I dont have a C compiler. How can I build my own Perl interpreter?
I copied the perl binary from one machine to another, but scripts
dont work.
I grabbed the sources and tried to compile but gdbm/dynamic load
ing/malloc/linking/... failed. How do I make it work?
What modules and extensions are available for Perl? What is CPAN?
What does CPAN/src/... mean?
Is there an ISO or ANSI certified version of Perl?
Where can I get information on Perl?
What are the Perl newsgroups on Usenet? Where do I post questions?
Where should I post source code?
Perl Books
Which magazines have Perl content?
What mailing lists are there for Perl?
Where are the archives for comp.lang.perl.misc?
Where can I buy a commercial version of perl?
Where do I send bug reports?
What is perl.com? Perl Mongers? pm.org? perl.org? cpan.org?
perlfaq3: Programming Tools
Programmer tools and programming support.
How do I do (anything)?
How can I use Perl interactively?
Is there a Perl shell?
How do I find which modules are installed on my system?
How do I debug my Perl programs?
How do I profile my Perl programs?
How do I cross-reference my Perl programs?
Is there a pretty-printer (formatter) for Perl?
Is there a ctags for Perl?
Is there an IDE or Windows Perl Editor?
Where can I get Perl macros for vi?
Where can I get perl-mode for emacs?
How can I use curses with Perl?
How can I use X or Tk with Perl?
How can I make my Perl program run faster?
How can I make my Perl program take less memory?
Is it safe to return a reference to local or lexical data?
How can I free an array or hash so my program shrinks?
How can I make my CGI script more efficient?
How can I hide the source for my Perl program?
How can I compile my Perl program into byte code or C?
How can I compile Perl into Java?
How can I get "#!perl" to work on [MS-DOS,NT,...]?
Can I write useful Perl programs on the command line?
Why dont 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?
Ive read perlembed, perlguts, etc., but I cant 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?
Whats MakeMaker?
perlfaq4: Data Manipulation
Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous
data issues.
Why am I getting long decimals (eg, 19.9499999999999) instead of
the numbers I should be getting (eg, 19.95)?
Why is int() broken?
Why isnt my octal data interpreted correctly?
Does Perl have a round() function? What about ceil() and floor()?
Trig functions?
How do I convert between numeric representations/bases/radixes?
Why doesnt & 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 arent 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 yesterdays 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?
Whats wrong with always quoting "$vars"?
Why dont my <?
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 cant I just open(FH, ">file.lock")?
I still dont 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 files timestamp in perl?
How do I set a files 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 theres 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 cant I use "C:\temp\foo" in DOS paths? Why doesnt
C:\temp\foo.exe work?
Why doesnt glob("*.*") get all the files?
Why does Perl let me delete read-only files? Why does "-i" clobber
protected files? Isnt 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?
perlfaq6: Regular Expressions
This section is surprisingly small because the rest of the FAQ is lit
tered with answers involving regular expressions. For example, decoding
a URL and checking whether something is a number are handled with regu
lar expressions, but those answers are found elsewhere in this document
(in perlfaq9: "How do I decode or create those %-encodings on the web"
and perlfaq4: "How do I determine whether a scalar is a num
ber/whole/integer/float", to be precise).
How can I hope to use regular expressions without creating illegi
ble and unmaintainable code?
Im having trouble matching over more than one line. Whats wrong?
How can I pull out lines between two patterns that are themselves
on different lines?
I put a regular expression into $/ but it didnt work. Whats
wrong?
How do I substitute case insensitively on the LHS while preserving
case on the RHS?
How can I make "\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 dont 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?
Whats wrong with using grep in a void context?
How can I match strings with multibyte characters?
How do I match a pattern that is supplied by the user?
perlfaq7: General Perl Language Issues
General Perl language issues that dont clearly fit into any of the
other sections.
Can I get a BNF/yacc/RE for the Perl language?
What are all these $@%&* punctuation signs, and how do I know when
to use them?
Do I always/never have to quote my strings or use semicolons and
commas?
How do I skip some return values?
How do I temporarily block warnings?
Whats an extension?
Why do Perl operators have different precedence than C operators?
How do I declare/create a structure?
How do I create a module?
How do I create a class?
How can I tell if a variable is tainted?
Whats a closure?
What is variable suicide and how can I prevent it?
How can I pass/return a {Function, FileHandle, Array, Hash, Method,
Regex}?
How do I create a static variable?
Whats the difference between dynamic and lexical (static) scoping?
Between local() and my()?
How can I access a dynamic variable while a similarly named lexical
is in scope?
Whats the difference between deep and shallow binding?
Why doesnt "my($foo) = ;" work right?
How do I redefine a builtin function, operator, or method?
Whats 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 cant 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?
perlfaq8: System Interaction
This section of the Perl FAQ covers questions involving operating sys
tem interaction. Topics include interprocess communication (IPC), con
trol over the user-interface (keyboard, screen and pointing devices),
and most anything else not related to data manipulation. Read the FAQs
and documentation specific to the port of perl to your operating system
(eg, perlvms, perlplan9, ...). These should contain more detailed
information on the vagaries of your perl.
How do I find out which operating system Im running under?
How come exec() doesnt 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 han
dling)
Why doesnt my sockets program work under System V (Solaris)? What
does the error message "Protocol not supported" mean?
How can I call my systems 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 cant I get the output of a command with system()?
How can I capture STDERR from an external command?
Why doesnt open() return an error when a pipe open fails?
Whats wrong with using backticks in a void context?
How can I call backticks without shell processing?
Why cant 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 perls 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 processs filehandle without waiting for it to
complete?
How do I fork a daemon process?
How do I find out if Im 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 tell the difference between errors from the shell and
perl?
How do I install a module from CPAN?
Whats 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 mod
ule/library search path?
How do I add a directory to my include path (@INC) at runtime?
What is socket.ph and where do I get it?
perlfaq9: Networking
Networking, the internet, and a few on the web.
What is the correct form of response from a CGI script?
My CGI script runs from the command line but not the browser. (500
Server Error)
How can I get better error messages from a CGI program?
How do I remove HTML from a string?
How do I extract URLs?
How do I download a file from the users machine? How do I open a
file on another machine?
How do I make a pop-up menu in HTML?
How do I fetch an HTML file?
How do I automate an HTML form submission?
How do I decode or create those %-encodings on the web?
How do I redirect to another page?
How do I put a password on my web pages?
How do I edit my .htpasswd and .htgroup files with Perl?
How do I make sure users cant enter values into a form that cause
my CGI script to do bad things?
How do I parse a mail header?
How do I decode a CGI form?
How do I check a valid mail address?
How do I decode a MIME/BASE64 string?
How do I return the users mail address?
How do I send mail?
How do I use MIME to make an attachment to a mail message?
How do I read mail?
How do I find out my hostname, domainname, or IP address?
How do I fetch a news article or the active newsgroups?
How do I fetch/put an FTP file?
How can I do RPC in Perl?
perl v5.8.8 2008-04-25 PERLFAQ(1)
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