MirBSD manpage: perlfaq4(1)

PERLFAQ4(1)     Perl Programmers Reference Guide      PERLFAQ4(1)


     perlfaq4 - Data Manipulation


     This section of the FAQ answers questions related to manipu-
     lating numbers, dates, strings, arrays, hashes, and miscel-
     laneous data issues.

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

     Internally, your computer represents floating-point numbers
     in binary. Digital (as in powers of two) computers cannot
     store all numbers exactly.  Some real numbers lose precision
     in the process.  This is a problem with how computers store
     numbers and affects all computer languages, not just Perl.

     perlnumber show the gory details of number representations
     and conversions.

     To limit the number of decimal places in your numbers, you
     can use the printf or sprintf function.  See the "Floating
     Point Arithmetic" for more details.

             printf "%.2f", 10/3;

             my $number = sprintf "%.2f", 10/3;

     Why is int() broken?

     Your int() is most probably working just fine.  It's the
     numbers that aren't quite what you think.

     First, see the above item "Why am I getting long decimals
     (eg, 19.9499999999999) instead of the numbers I should be
     getting (eg, 19.95)?".

     For example, this

         print int(0.6/0.2-2), "\n";

     will in most computers print 0, not 1, because even such
     simple numbers as 0.6 and 0.2 cannot be presented exactly by
     floating-point numbers.  What you think in the above as
     'three' is really more like 2.9999999999999995559.

     Why isn't my octal data interpreted correctly?

     Perl only understands octal and hex numbers as such when
     they occur as literals in your program.  Octal literals in
     perl must start with a leading "0" and hexadecimal literals

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     must start with a leading "0x". If they are read in from
     somewhere and assigned, no automatic conversion takes place.
     You must explicitly use oct() or hex() if you want the
     values converted to decimal.  oct() interprets hex
     ("0x350"), octal ("0350" or even without the leading "0",
     like "377") and binary ("0b1010") numbers, while hex() only
     converts hexadecimal ones, with or without a leading "0x",
     like "0x255", "3A", "ff", or "deadbeef". The inverse mapping
     from decimal to octal can be done with either the "%o" or
     "%O" sprintf() formats.

     This problem shows up most often when people try using
     chmod(), mkdir(), umask(), or sysopen(), which by widespread
     tradition typically take permissions in octal.

         chmod(644,  $file); # WRONG
         chmod(0644, $file); # right

     Note the mistake in the first line was specifying the
     decimal literal 644, rather than the intended octal literal
     0644.  The problem can be seen with:

         printf("%#o",644); # prints 01204

     Surely you had not intended "chmod(01204, $file);" - did
     you?  If you want to use numeric literals as arguments to
     chmod() et al. then please try to express them as octal con-
     stants, that is with a leading zero and with the following
     digits restricted to the set 0..7.

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

     Remember that int() merely truncates toward 0.  For rounding
     to a certain number of digits, sprintf() or printf() is usu-
     ally the easiest route.

         printf("%.3f", 3.1415926535);       # prints 3.142

     The POSIX module (part of the standard Perl distribution)
     implements ceil(), floor(), and a number of other mathemati-
     cal and trigonometric functions.

         use POSIX;
         $ceil   = ceil(3.5);                        # 4
         $floor  = floor(3.5);                       # 3

     In 5.000 to 5.003 perls, trigonometry was done in the
     Math::Complex module.  With 5.004, the Math::Trig module
     (part of the standard Perl distribution) implements the tri-
     gonometric functions. Internally it uses the Math::Complex
     module and some functions can break out from the real axis

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     into the complex plane, for example the inverse sine of 2.

     Rounding in financial applications can have serious implica-
     tions, and the rounding method used should be specified pre-
     cisely.  In these cases, it probably pays not to trust
     whichever system rounding is being used by Perl, but to
     instead implement the rounding function you need yourself.

     To see why, notice how you'll still have an issue on half-
     way-point alternation:

         for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

         0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
         0.8 0.8 0.9 0.9 1.0 1.0

     Don't blame Perl.  It's the same as in C.  IEEE says we have
     to do this. Perl numbers whose absolute values are integers
     under 2**31 (on 32 bit machines) will work pretty much like
     mathematical integers.  Other numbers are not guaranteed.

     How do I convert between numeric

     As always with Perl there is more than one way to do it.
     Below are a few examples of approaches to making common
     conversions between number representations.  This is
     intended to be representational rather than exhaustive.

     Some of the examples below use the Bit::Vector module from
     CPAN. The reason you might choose Bit::Vector over the perl
     built in functions is that it works with numbers of ANY
     size, that it is optimized for speed on some operations, and
     for at least some programmers the notation might be fami-

     How do I convert hexadecimal into decimal
         Using perl's built in conversion of 0x notation:

             $dec = 0xDEADBEEF;

         Using the hex function:

             $dec = hex("DEADBEEF");

         Using pack:

             $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));

         Using the CPAN module Bit::Vector:

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             use Bit::Vector;
             $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
             $dec = $vec->to_Dec();

     How do I convert from decimal to hexadecimal
         Using sprintf:

             $hex = sprintf("%X", 3735928559); # upper case A-F
             $hex = sprintf("%x", 3735928559); # lower case a-f

         Using unpack:

             $hex = unpack("H*", pack("N", 3735928559));

         Using Bit::Vector:

             use Bit::Vector;
             $vec = Bit::Vector->new_Dec(32, -559038737);
             $hex = $vec->to_Hex();

         And Bit::Vector supports odd bit counts:

             use Bit::Vector;
             $vec = Bit::Vector->new_Dec(33, 3735928559);
             $vec->Resize(32); # suppress leading 0 if unwanted
             $hex = $vec->to_Hex();

     How do I convert from octal to decimal
         Using Perl's built in conversion of numbers with leading

             $dec = 033653337357; # note the leading 0!

         Using the oct function:

             $dec = oct("33653337357");

         Using Bit::Vector:

             use Bit::Vector;
             $vec = Bit::Vector->new(32);
             $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
             $dec = $vec->to_Dec();

     How do I convert from decimal to octal
         Using sprintf:

             $oct = sprintf("%o", 3735928559);

         Using Bit::Vector:

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             use Bit::Vector;
             $vec = Bit::Vector->new_Dec(32, -559038737);
             $oct = reverse join('', $vec->Chunk_List_Read(3));

     How do I convert from binary to decimal
         Perl 5.6 lets you write binary numbers directly with the
         0b notation:

             $number = 0b10110110;

         Using oct:

             my $input = "10110110";
             $decimal = oct( "0b$input" );

         Using pack and ord:

             $decimal = ord(pack('B8', '10110110'));

         Using pack and unpack for larger strings:

             $int = unpack("N", pack("B32",
                 substr("0" x 32 . "11110101011011011111011101111", -32)));
             $dec = sprintf("%d", $int);

             # substr() is used to left pad a 32 character string with zeros.

         Using Bit::Vector:

             $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
             $dec = $vec->to_Dec();

     How do I convert from decimal to binary
         Using sprintf (perl 5.6+):

             $bin = sprintf("%b", 3735928559);

         Using unpack:

             $bin = unpack("B*", pack("N", 3735928559));

         Using Bit::Vector:

             use Bit::Vector;
             $vec = Bit::Vector->new_Dec(32, -559038737);
             $bin = $vec->to_Bin();

         The remaining transformations (e.g. hex -> oct, bin ->
         hex, etc.) are left as an exercise to the inclined

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     Why doesn't & work the way I want it to?

     The behavior of binary arithmetic operators depends on
     whether they're used on numbers or strings.  The operators
     treat a string as a series of bits and work with that (the
     string "3" is the bit pattern 00110011).  The operators work
     with the binary form of a number (the number 3 is treated as
     the bit pattern 00000011).

     So, saying "11 & 3" performs the "and" operation on numbers
     (yielding 3).  Saying "11" & "3" performs the "and" opera-
     tion on strings (yielding "1").

     Most problems with "&" and "|" arise because the programmer
     thinks they have a number but really it's a string.  The
     rest arise because the programmer says:

         if ("\020\020" & "\101\101") {
             # ...

     but a string consisting of two null bytes (the result of
     ""\020\020" & "\101\101"") is not a false value in Perl.
     You need:

         if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
             # ...

     How do I multiply matrices?

     Use the Math::Matrix or Math::MatrixReal modules (available
     from CPAN) or the PDL extension (also available from CPAN).

     How do I perform an operation on a series of integers?

     To call a function on each element in an array, and collect
     the results, use:

         @results = map { my_func($_) } @array;

     For example:

         @triple = map { 3 * $_ } @single;

     To call a function on each element of an array, but ignore
     the results:

         foreach $iterator (@array) {

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     To call a function on each integer in a (small) range, you
     can use:

         @results = map { some_func($_) } (5 .. 25);

     but you should be aware that the ".." operator creates an
     array of all integers in the range.  This can take a lot of
     memory for large ranges.  Instead use:

         @results = ();
         for ($i=5; $i < 500_005; $i++) {
             push(@results, some_func($i));

     This situation has been fixed in Perl5.005. Use of ".." in a
     "for" loop will iterate over the range, without creating the
     entire range.

         for my $i (5 .. 500_005) {
             push(@results, some_func($i));

     will not create a list of 500,000 integers.

     How can I output Roman numerals?

     Get the http://www.cpan.org/modules/by-module/Roman module.

     Why aren't my random numbers random?

     If you're using a version of Perl before 5.004, you must
     call "srand" once at the start of your program to seed the
     random number generator.

              BEGIN { srand() if $] < 5.004 }

     5.004 and later automatically call "srand" at the beginning.
     Don't call "srand" more than once---you make your numbers
     less random, rather than more.

     Computers are good at being predictable and bad at being
     random (despite appearances caused by bugs in your programs
     :-).  see the random article in the "Far More Than You Ever
     Wanted To Know" collection in
     http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
     Tom Phoenix, talks more about this.  John von Neumann said,
     "Anyone who attempts to generate random numbers by deter-
     ministic means is, of course, living in a state of sin."

     If you want numbers that are more random than "rand" with
     "srand" provides, you should also check out the
     Math::TrulyRandom module from CPAN.  It uses the

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     imperfections in your system's timer to generate random
     numbers, but this takes quite a while.  If you want a better
     pseudorandom generator than comes with your operating sys-
     tem, look at "Numerical Recipes in C" at http://www.nr.com/

     How do I get a random number between X and Y?

     "rand($x)" returns a number such that "0 <= rand($x) < $x".
     Thus what you want to have perl figure out is a random
     number in the range from 0 to the difference between your X
     and Y.

     That is, to get a number between 10 and 15, inclusive, you
     want a random number between 0 and 5 that you can then add
     to 10.

         my $number = 10 + int rand( 15-10+1 );

     Hence you derive the following simple function to abstract
     that. It selects a random integer between the two given
     integers (inclusive), For example: "random_int_in(50,120)".

        sub random_int_in ($$) {
          my($min, $max) = @_;
           # Assumes that the two arguments are integers themselves!
          return $min if $min == $max;
          ($min, $max) = ($max, $min)  if  $min > $max;
          return $min + int rand(1 + $max - $min);

Data: Dates
     How do I find the day or week of the year?

     The localtime function returns the day of the year.  Without
     an argument localtime uses the current time.

             $day_of_year = (localtime)[7];

     The POSIX module can also format a date as the day of the
     year or week of the year.

             use POSIX qw/strftime/;
             my $day_of_year  = strftime "%j", localtime;
             my $week_of_year = strftime "%W", localtime;

     To get the day of year for any date, use the Time::Local
     module to get a time in epoch seconds for the argument to

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             use POSIX qw/strftime/;
             use Time::Local;
             my $week_of_year = strftime "%W",
                     localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );

     The Date::Calc module provides two functions to calculate

             use Date::Calc;
             my $day_of_year  = Day_of_Year(  1987, 12, 18 );
             my $week_of_year = Week_of_Year( 1987, 12, 18 );

     How do I find the current century or millennium?

     Use the following simple functions:

         sub get_century    {
             return int((((localtime(shift || time))[5] + 1999))/100);

         sub get_millennium {
             return 1+int((((localtime(shift || time))[5] + 1899))/1000);

     On some systems, the POSIX module's strftime() function has
     been extended in a non-standard way to use a %C format,
     which they sometimes claim is the "century".  It isn't,
     because on most such systems, this is only the first two
     digits of the four-digit year, and thus cannot be used to
     reliably determine the current century or millennium.

     How can I compare two dates and find the difference?

     (contributed by brian d foy)

     You could just store all your dates as a number and then
     subtract. Life isn't always that simple though. If you want
     to work with formatted dates, the Date::Manip, Date::Calc,
     or DateTime modules can help you.

     How can I take a string and turn it into epoch seconds?

     If it's a regular enough string that it always has the same
     format, you can split it up and pass the parts to "timelo-
     cal" in the standard Time::Local module.  Otherwise, you
     should look into the Date::Calc and Date::Manip modules from

     How can I find the Julian Day?

     (contributed by brian d foy and Dave Cross)

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     You can use the Time::JulianDay module available on CPAN.
     Ensure that you really want to find a Julian day, though, as
     many people have different ideas about Julian days.  See
     http://www.hermetic.ch/cal_stud/jdn.htm for instance.

     You can also try the DateTime module, which can convert a
     date/time to a Julian Day.

       $ perl -MDateTime -le'print DateTime->today->jd'

     Or the modified Julian Day

       $ perl -MDateTime -le'print DateTime->today->mjd'

     Or even the day of the year (which is what some people think
     of as a Julian day)

       $ perl -MDateTime -le'print DateTime->today->doy'

     How do I find yesterday's date?

     (contributed by brian d foy)

     Use one of the Date modules. The "DateTime" module makes it
     simple, and give you the same time of day, only the day

             use DateTime;

             my $yesterday = DateTime->now->subtract( days => 1 );

             print "Yesterday was $yesterday\n";

     You can also use the "Date::Calc" module using its
     Today_and_Now function.

             use Date::Calc qw( Today_and_Now Add_Delta_DHMS );

             my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );

             print "@date\n";

     Most people try to use the time rather than the calendar to
     figure out dates, but that assumes that days are twenty-four
     hours each.  For most people, there are two days a year when
     they aren't: the switch to and from summer time throws this
     off. Let the modules do the work.

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     Does Perl have a Year 2000 problem?  Is Perl Y2K compliant?

     Short answer: No, Perl does not have a Year 2000 problem.
     Yes, Perl is Y2K compliant (whatever that means).  The pro-
     grammers you've hired to use it, however, probably are not.

     Long answer: The question belies a true understanding of the
     issue. Perl is just as Y2K compliant as your pencil--no
     more, and no less. Can you use your pencil to write a
     non-Y2K-compliant memo?  Of course you can.  Is that the
     pencil's fault?  Of course it isn't.

     The date and time functions supplied with Perl (gmtime and
     localtime) supply adequate information to determine the year
     well beyond 2000 (2038 is when trouble strikes for 32-bit
     machines).  The year returned by these functions when used
     in a list context is the year minus 1900. For years between
     1910 and 1999 this happens to be a 2-digit decimal number.
     To avoid the year 2000 problem simply do not treat the year
     as a 2-digit number.  It isn't.

     When gmtime() and localtime() are used in scalar context
     they return a timestamp string that contains a fully-
     expanded year.  For example, "$timestamp =
     gmtime(1005613200)" sets $timestamp to "Tue Nov 13 01:00:00
     2001".  There's no year 2000 problem here.

     That doesn't mean that Perl can't be used to create non-Y2K
     compliant programs.  It can.  But so can your pencil.  It's
     the fault of the user, not the language.  At the risk of
     inflaming the NRA: "Perl doesn't break Y2K, people do."  See
     http://www.perl.org/about/y2k.html for a longer exposition.

Data: Strings
     How do I validate input?

     (contributed by brian d foy)

     There are many ways to ensure that values are what you
     expect or want to accept. Besides the specific examples that
     we cover in the perlfaq, you can also look at the modules
     with "Assert" and "Validate" in their names, along with
     other modules such as "Regexp::Common".

     Some modules have validation for particular types of input,
     such as "Business::ISBN", "Business::CreditCard",
     "Email::Valid", and "Data::Validate::IP".

     How do I unescape a string?

     It depends just what you mean by "escape".  URL escapes are
     dealt with in perlfaq9.  Shell escapes with the backslash

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     ("\") character are removed with


     This won't expand "\n" or "\t" or any other special escapes.

     How do I remove consecutive pairs of characters?

     (contributed by brian d foy)

     You can use the substitution operator to find pairs of char-
     acters (or runs of characters) and replace them with a sin-
     gle instance. In this substitution, we find a character in
     "(.)". The memory parentheses store the matched character in
     the back-reference "\1" and we use that to require that the
     same thing immediately follow it. We replace that part of
     the string with the character in $1.


     We can also use the transliteration operator, "tr///". In
     this example, the search list side of our "tr///" contains
     nothing, but the "c" option complements that so it contains
     everything. The replacement list also contains nothing, so
     the transliteration is almost a no-op since it won't do any
     replacements (or more exactly, replace the character with
     itself). However, the "s" option squashes duplicated and
     consecutive characters in the string so a character does not
     show up next to itself

             my $str = 'Haarlem';   # in the Netherlands
         $str =~ tr///cs;       # Now Harlem, like in New York

     How do I expand function calls in a string?

     (contributed by brian d foy)

     This is documented in perlref, and although it's not the
     easiest thing to read, it does work. In each of these exam-
     ples, we call the function inside the braces used to
     dereference a reference. If we have a more than one return
     value, we can construct and dereference an anonymous array.
     In this case, we call the function in list context.

             print "The time values are @{ [localtime] }.\n";

     If we want to call the function in scalar context, we have
     to do a bit more work. We can really have any code we like
     inside the braces, so we simply have to end with the scalar
     reference, although how you do that is up to you, and you
     can use code inside the braces.

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             print "The time is ${\(scalar localtime)}.\n"

             print "The time is ${ my $x = localtime; \$x }.\n";

     If your function already returns a reference, you don't need
     to create the reference yourself.

             sub timestamp { my $t = localtime; \$t }

             print "The time is ${ timestamp() }.\n";

     The "Interpolation" module can also do a lot of magic for
     you. You can specify a variable name, in this case "E", to
     set up a tied hash that does the interpolation for you. It
     has several other methods to do this as well.

             use Interpolation E => 'eval';
             print "The time values are $E{localtime()}.\n";

     In most cases, it is probably easier to simply use string
     concatenation, which also forces scalar context.

             print "The time is " . localtime . ".\n";

     How do I find matching/nesting anything?

     This isn't something that can be done in one regular expres-
     sion, no matter how complicated.  To find something between
     two single characters, a pattern like "/x([^x]*)x/" will get
     the intervening bits in $1. For multiple ones, then some-
     thing more like "/alpha(.*?)omega/" would be needed.  But
     none of these deals with nested patterns.  For balanced
     expressions using "(", "{", "[" or "<" as delimiters, use
     the CPAN module Regexp::Common, or see "(??{ code })" in
     perlre.  For other cases, you'll have to write a parser.

     If you are serious about writing a parser, there are a
     number of modules or oddities that will make your life a lot
     easier.  There are the CPAN modules Parse::RecDescent,
     Parse::Yapp, and Text::Balanced; and the byacc program.
     Starting from perl 5.8 the Text::Balanced is part of the
     standard distribution.

     One simple destructive, inside-out approach that you might
     try is to pull out the smallest nesting parts one at a time:

         while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
             # do something with $1

     A more complicated and sneaky approach is to make Perl's
     regular expression engine do it for you.  This is courtesy

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     Dean Inada, and rather has the nature of an Obfuscated Perl
     Contest entry, but it really does work:

         # $_ contains the string to parse
         # BEGIN and END are the opening and closing markers for the
         # nested text.

         @( = ('(','');
         @) = (')','');
         @$ = (eval{/$re/},$@!~/unmatched/i);
         print join("\n",@$[0..$#$]) if( $$[-1] );

     How do I reverse a string?

     Use reverse() in scalar context, as documented in "reverse"
     in perlfunc.

         $reversed = reverse $string;

     How do I expand tabs in a string?

     You can do it yourself:

         1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;

     Or you can just use the Text::Tabs module (part of the stan-
     dard Perl distribution).

         use Text::Tabs;
         @expanded_lines = expand(@lines_with_tabs);

     How do I reformat a paragraph?

     Use Text::Wrap (part of the standard Perl distribution):

         use Text::Wrap;
         print wrap("\t", '  ', @paragraphs);

     The paragraphs you give to Text::Wrap should not contain
     embedded newlines.  Text::Wrap doesn't justify the lines

     Or use the CPAN module Text::Autoformat.  Formatting files
     can be easily done by making a shell alias, like so:

         alias fmt="perl -i -MText::Autoformat -n0777 \
             -e 'print autoformat $_, {all=>1}' $*"

     See the documentation for Text::Autoformat to appreciate its
     many capabilities.

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     How can I access or change N characters of a string?

     You can access the first characters of a string with
     substr(). To get the first character, for example, start at
     position 0 and grab the string of length 1.

             $string = "Just another Perl Hacker";
         $first_char = substr( $string, 0, 1 );  #  'J'

     To change part of a string, you can use the optional fourth
     argument which is the replacement string.

         substr( $string, 13, 4, "Perl 5.8.0" );

     You can also use substr() as an lvalue.

         substr( $string, 13, 4 ) =  "Perl 5.8.0";

     How do I change the Nth occurrence of something?

     You have to keep track of N yourself.  For example, let's
     say you want to change the fifth occurrence of "whoever" or
     "whomever" into "whosoever" or "whomsoever", case insensi-
     tively.  These all assume that $_ contains the string to be

         $count = 0;
             ++$count == 5           # is it the 5th?
                 ? "${2}soever"      # yes, swap
                 : $1                # renege and leave it there

     In the more general case, you can use the "/g" modifier in a
     "while" loop, keeping count of matches.

         $WANT = 3;
         $count = 0;
         $_ = "One fish two fish red fish blue fish";
         while (/(\w+)\s+fish\b/gi) {
             if (++$count == $WANT) {
                 print "The third fish is a $1 one.\n";

     That prints out: "The third fish is a red one."  You can
     also use a repetition count and repeated pattern like this:


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     How can I count the number of occurrences of a substring
     within a string?

     There are a number of ways, with varying efficiency.  If you
     want a count of a certain single character (X) within a
     string, you can use the "tr///" function like so:

         $string = "ThisXlineXhasXsomeXx'sXinXit";
         $count = ($string =~ tr/X//);
         print "There are $count X characters in the string";

     This is fine if you are just looking for a single character.
     However, if you are trying to count multiple character sub-
     strings within a larger string, "tr///" won't work.  What
     you can do is wrap a while() loop around a global pattern
     match.  For example, let's count negative integers:

         $string = "-9 55 48 -2 23 -76 4 14 -44";
         while ($string =~ /-\d+/g) { $count++ }
         print "There are $count negative numbers in the string";

     Another version uses a global match in list context, then
     assigns the result to a scalar, producing a count of the
     number of matches.

             $count = () = $string =~ /-\d+/g;

     How do I capitalize all the words on one line?

     To make the first letter of each word upper case:

             $line =~ s/\b(\w)/\U$1/g;

     This has the strange effect of turning ""don't do it"" into
     ""Don'T Do It"".  Sometimes you might want this.  Other
     times you might need a more thorough solution (Suggested by
     brian d foy):

         $string =~ s/ (
                      (^\w)    #at the beginning of the line
                        |      # or
                      (\s\w)   #preceded by whitespace
         $string =~ /([\w']+)/\u\L$1/g;

     To make the whole line upper case:

             $line = uc($line);

     To force each word to be lower case, with the first letter
     upper case:

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             $line =~ s/(\w+)/\u\L$1/g;

     You can (and probably should) enable locale awareness of
     those characters by placing a "use locale" pragma in your
     program. See perllocale for endless details on locales.

     This is sometimes referred to as putting something into
     "title case", but that's not quite accurate.  Consider the
     proper capitalization of the movie Dr. Strangelove or: How I
     Learned to Stop Worrying and Love the Bomb, for example.

     Damian Conway's Text::Autoformat module provides some smart
     case transformations:

         use Text::Autoformat;
         my $x = "Dr. Strangelove or: How I Learned to Stop ".
           "Worrying and Love the Bomb";

         print $x, "\n";
         for my $style (qw( sentence title highlight ))
             print autoformat($x, { case => $style }), "\n";

     How can I split a [character] delimited string except when
     inside [character]?

     Several modules can handle this sort of
     pasing---Text::Balanced, Text::CSV, Text::CSV_XS, and
     Text::ParseWords, among others.

     Take the example case of trying to split a string that is
     comma-separated into its different fields. You can't use
     "split(/,/)" because you shouldn't split if the comma is
     inside quotes.  For example, take a data line like this:

         SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

     Due to the restriction of the quotes, this is a fairly com-
     plex problem.  Thankfully, we have Jeffrey Friedl, author of
     Mastering Regular Expressions, to handle these for us.  He
     suggests (assuming your string is contained in $text):

          @new = ();
          push(@new, $+) while $text =~ m{
              "([^\"\\]*(?:\\.[^\"\\]*)*)",?  # groups the phrase inside the quotes
            | ([^,]+),?
            | ,
          push(@new, undef) if substr($text,-1,1) eq ',';

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     If you want to represent quotation marks inside a quotation-
     mark-delimited field, escape them with backslashes (eg,
     "like \"this\"".

     Alternatively, the Text::ParseWords module (part of the
     standard Perl distribution) lets you say:

         use Text::ParseWords;
         @new = quotewords(",", 0, $text);

     There's also a Text::CSV (Comma-Separated Values) module on

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

     (contributed by brian d foy)

     A substitution can do this for you. For a single line, you
     want to replace all the leading or trailing whitespace with
     nothing. You can do that with a pair of substitutions.


     You can also write that as a single substitution, although
     it turns out the combined statement is slower than the
     separate ones. That might not matter to you, though.


     In this regular expression, the alternation matches either
     at the beginning or the end of the string since the anchors
     have a lower precedence than the alternation. With the "/g"
     flag, the substitution makes all possible matches, so it
     gets both. Remember, the trailing newline matches the "\s+",
     and  the "$" anchor can match to the physical end of the
     string, so the newline disappears too. Just add the newline
     to the output, which has the added benefit of preserving
     "blank" (consisting entirely of whitespace) lines which the
     "^\s+" would remove all by itself.

             while( <> )
                     print "$_\n";

     For a multi-line string, you can apply the regular expres-
     sion to each logical line in the string by adding the "/m"
     flag (for "multi-line"). With the "/m" flag, the "$" matches
     before an embedded newline, so it doesn't remove it. It

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     still removes the newline at the end of the string.

         $string =~ s/^\s+|\s+$//gm;

     Remember that lines consisting entirely of whitespace will
     disappear, since the first part of the alternation can match
     the entire string and replace it with nothing. If need to
     keep embedded blank lines, you have to do a little more
     work. Instead of matching any whitespace (since that
     includes a newline), just match the other whitespace.

             $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;

     How do I pad a string with blanks or pad a number with

     In the following examples, $pad_len is the length to which
     you wish to pad the string, $text or $num contains the
     string to be padded, and $pad_char contains the padding
     character. You can use a single character string constant
     instead of the $pad_char variable if you know what it is in
     advance. And in the same way you can use an integer in place
     of $pad_len if you know the pad length in advance.

     The simplest method uses the "sprintf" function. It can pad
     on the left or right with blanks and on the left with zeroes
     and it will not truncate the result. The "pack" function can
     only pad strings on the right with blanks and it will trun-
     cate the result to a maximum length of $pad_len.

         # Left padding a string with blanks (no truncation):
             $padded = sprintf("%${pad_len}s", $text);
             $padded = sprintf("%*s", $pad_len, $text);  # same thing

         # Right padding a string with blanks (no truncation):
             $padded = sprintf("%-${pad_len}s", $text);
             $padded = sprintf("%-*s", $pad_len, $text); # same thing

         # Left padding a number with 0 (no truncation):
             $padded = sprintf("%0${pad_len}d", $num);
             $padded = sprintf("%0*d", $pad_len, $num); # same thing

         # Right padding a string with blanks using pack (will truncate):
         $padded = pack("A$pad_len",$text);

     If you need to pad with a character other than blank or zero
     you can use one of the following methods.  They all generate
     a pad string with the "x" operator and combine that with
     $text. These methods do not truncate $text.

     Left and right padding with any character, creating a new

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         $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
         $padded = $text . $pad_char x ( $pad_len - length( $text ) );

     Left and right padding with any character, modifying $text

         substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
         $text .= $pad_char x ( $pad_len - length( $text ) );

     How do I extract selected columns from a string?

     Use substr() or unpack(), both documented in perlfunc. If
     you prefer thinking in terms of columns instead of widths,
     you can use this kind of thing:

         # determine the unpack format needed to split Linux ps output
         # arguments are cut columns
         my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);

         sub cut2fmt {
             my(@positions) = @_;
             my $template  = '';
             my $lastpos   = 1;
             for my $place (@positions) {
                 $template .= "A" . ($place - $lastpos) . " ";
                 $lastpos   = $place;
             $template .= "A*";
             return $template;

     How do I find the soundex value of a string?

     (contributed by brian d foy)

     You can use the Text::Soundex module. If you want to do
     fuzzy or close matching, you might also try the
     String::Approx, and Text::Metaphone, and
     Text::DoubleMetaphone modules.

     How can I expand variables in text strings?

     Let's assume that you have a string that contains place-
     holder variables.

         $text = 'this has a $foo in it and a $bar';

     You can use a substitution with a double evaluation.  The
     first /e turns $1 into $foo, and the second /e turns $foo
     into its value.  You may want to wrap this in an "eval": if
     you try to get the value of an undeclared variable while
     running under "use strict", you get a fatal error.

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         eval { $text =~ s/(\$\w+)/$1/eeg };
         die if $@;

     It's probably better in the general case to treat those
     variables as entries in some special hash.  For example:

         %user_defs = (
             foo  => 23,
             bar  => 19,
         $text =~ s/\$(\w+)/$user_defs{$1}/g;

     What's wrong with always quoting "$vars"?

     The problem is that those double-quotes force
     stringification-- coercing numbers and references into
     strings--even when you don't want them to be strings.  Think
     of it this way: double-quote expansion is used to produce
     new strings.  If you already have a string, why do you need

     If you get used to writing odd things like these:

         print "$var";       # BAD
         $new = "$old";      # BAD
         somefunc("$var");   # BAD

     You'll be in trouble.  Those should (in 99.8% of the cases)
     be the simpler and more direct:

         print $var;
         $new = $old;

     Otherwise, besides slowing you down, you're going to break
     code when the thing in the scalar is actually neither a
     string nor a number, but a reference:

         sub func {
             my $aref = shift;
             my $oref = "$aref";  # WRONG

     You can also get into subtle problems on those few opera-
     tions in Perl that actually do care about the difference
     between a string and a number, such as the magical "++"
     autoincrement operator or the syscall() function.

     Stringification also destroys arrays.

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         @lines = `command`;
         print "@lines";             # WRONG - extra blanks
         print @lines;               # right

     Why don't my <<HERE documents work?

     Check for these three things:

     There must be no space after the << part.
     There (probably) should be a semicolon at the end.
     You can't (easily) have any space in front of the tag.

     If you want to indent the text in the here document, you can
     do this:

         # all in one
         ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
             your text
             goes here

     But the HERE_TARGET must still be flush against the margin.
     If you want that indented also, you'll have to quote in the

         ($quote = <<'    FINIS') =~ s/^\s+//gm;
                 ...we will have peace, when you and all your works have
                 perished--and the works of your dark master to whom you
                 would deliver us. You are a liar, Saruman, and a corrupter
                 of men's hearts.  --Theoden in /usr/src/perl/taint.c
         $quote =~ s/\s+--/\n--/;

     A nice general-purpose fixer-upper function for indented
     here documents follows.  It expects to be called with a here
     document as its argument. It looks to see whether each line
     begins with a common substring, and if so, strips that sub-
     string off.  Otherwise, it takes the amount of leading whi-
     tespace found on the first line and removes that much off
     each subsequent line.

         sub fix {
             local $_ = shift;
             my ($white, $leader);  # common whitespace and common leading string
             if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
                 ($white, $leader) = ($2, quotemeta($1));
             } else {
                 ($white, $leader) = (/^(\s+)/, '');
             return $_;

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     This works with leading special strings, dynamically deter-

         $remember_the_main = fix<<'    MAIN_INTERPRETER_LOOP';
             @@@ int
             @@@ runops() {
             @@@     SAVEI32(runlevel);
             @@@     runlevel++;
             @@@     while ( op = (*op->op_ppaddr)() );
             @@@     TAINT_NOT;
             @@@     return 0;
             @@@ }

     Or with a fixed amount of leading whitespace, with remaining
     indentation correctly preserved:

         $poem = fix<<EVER_ON_AND_ON;
            Now far ahead the Road has gone,
               And I must follow, if I can,
            Pursuing it with eager feet,
               Until it joins some larger way
            Where many paths and errands meet.
               And whither then? I cannot say.
                     --Bilbo in /usr/src/perl/pp_ctl.c

Data: Arrays
     What is the difference between a list and an array?

     An array has a changeable length.  A list does not.  An
     array is something you can push or pop, while a list is a
     set of values.  Some people make the distinction that a list
     is a value while an array is a variable. Subroutines are
     passed and return lists, you put things into list context,
     you initialize arrays with lists, and you foreach() across a
     list.  "@" variables are arrays, anonymous arrays are
     arrays, arrays in scalar context behave like the number of
     elements in them, subroutines access their arguments through
     the array @_, and push/pop/shift only work on arrays.

     As a side note, there's no such thing as a list in scalar
     context. When you say

         $scalar = (2, 5, 7, 9);

     you're using the comma operator in scalar context, so it
     uses the scalar comma operator.  There never was a list
     there at all!  This causes the last value to be returned: 9.

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     What is the difference between $array[1] and @array[1]?

     The former is a scalar value; the latter an array slice,
     making it a list with one (scalar) value.  You should use $
     when you want a scalar value (most of the time) and @ when
     you want a list with one scalar value in it (very, very
     rarely; nearly never, in fact).

     Sometimes it doesn't make a difference, but sometimes it
     does. For example, compare:

         $good[0] = `some program that outputs several lines`;


         @bad[0]  = `same program that outputs several lines`;

     The "use warnings" pragma and the -w flag will warn you
     about these matters.

     How can I remove duplicate elements from a list or array?

     (contributed by brian d foy)

     Use a hash. When you think the words "unique" or "dupli-
     cated", think "hash keys".

     If you don't care about the order of the elements, you could
     just create the hash then extract the keys. It's not impor-
     tant how you create that hash: just that you use "keys" to
     get the unique elements.

        my %hash   = map { $_, 1 } @array;
        # or a hash slice: @hash{ @array } = ();
        # or a foreach: $hash{$_} = 1 foreach ( @array );

        my @unique = keys %hash;

     You can also go through each element and skip the ones
     you've seen before. Use a hash to keep track. The first time
     the loop sees an element, that element has no key in %Seen.
     The "next" statement creates the key and immediately uses
     its value, which is "undef", so the loop continues to the
     "push" and increments the value for that key. The next time
     the loop sees that same element, its key exists in the hash
     and the value for that key is true (since it's not 0 or
     undef), so the next skips that iteration and the loop goes
     to the next element.

             my @unique = ();
             my %seen   = ();

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             foreach my $elem ( @array )
                     next if $seen{ $elem }++;
                     push @unique, $elem;

     You can write this more briefly using a grep, which does the
     same thing.

        my %seen = ();
        my @unique = grep { ! $seen{ $_ }++ } @array;

     How can I tell whether a certain element is contained in a
     list or array?

     (portions of this answer contributed by Anno Siegel)

     Hearing the word "in" is an indication that you probably
     should have used a hash, not a list or array, to store your
     data.  Hashes are designed to answer this question quickly
     and efficiently.  Arrays aren't.

     That being said, there are several ways to approach this.
     If you are going to make this query many times over arbi-
     trary string values, the fastest way is probably to invert
     the original array and maintain a hash whose keys are the
     first array's values.

         @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
         %is_blue = ();
         for (@blues) { $is_blue{$_} = 1 }

     Now you can check whether $is_blue{$some_color}.  It might
     have been a good idea to keep the blues all in a hash in the
     first place.

     If the values are all small integers, you could use a simple
     indexed array.  This kind of an array will take up less

         @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
         @is_tiny_prime = ();
         for (@primes) { $is_tiny_prime[$_] = 1 }
         # or simply  @istiny_prime[@primes] = (1) x @primes;

     Now you check whether $is_tiny_prime[$some_number].

     If the values in question are integers instead of strings,
     you can save quite a lot of space by using bit strings

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         @articles = ( 1..10, 150..2000, 2017 );
         undef $read;
         for (@articles) { vec($read,$_,1) = 1 }

     Now check whether "vec($read,$n,1)" is true for some $n.

     These methods guarantee fast individual tests but require a
     re-organization of the original list or array.  They only
     pay off if you have to test multiple values against the same

     If you are testing only once, the standard module List::Util
     exports the function "first" for this purpose.  It works by
     stopping once it finds the element. It's written in C for
     speed, and its Perl equivalant looks like this subroutine:

             sub first (&@) {
                     my $code = shift;
                     foreach (@_) {
                             return $_ if &{$code}();

     If speed is of little concern, the common idiom uses grep in
     scalar context (which returns the number of items that
     passed its condition) to traverse the entire list. This does
     have the benefit of telling you how many matches it found,

             my $is_there = grep $_ eq $whatever, @array;

     If you want to actually extract the matching elements, sim-
     ply use grep in list context.

             my @matches = grep $_ eq $whatever, @array;

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

     Use a hash.  Here's code to do both and more.  It assumes
     that each element is unique in a given array:

         @union = @intersection = @difference = ();
         %count = ();
         foreach $element (@array1, @array2) { $count{$element}++ }
         foreach $element (keys %count) {
             push @union, $element;
             push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;

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     Note that this is the symmetric difference, that is, all
     elements in either A or in B but not in both.  Think of it
     as an xor operation.

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

     The following code works for single-level arrays.  It uses a
     stringwise comparison, and does not distinguish defined
     versus undefined empty strings.  Modify if you have other

         $are_equal = compare_arrays(\@frogs, \@toads);

         sub compare_arrays {
             my ($first, $second) = @_;
             no warnings;  # silence spurious -w undef complaints
             return 0 unless @$first == @$second;
             for (my $i = 0; $i < @$first; $i++) {
                 return 0 if $first->[$i] ne $second->[$i];
             return 1;

     For multilevel structures, you may wish to use an approach
     more like this one.  It uses the CPAN module FreezeThaw:

         use FreezeThaw qw(cmpStr);
         @a = @b = ( "this", "that", [ "more", "stuff" ] );

         printf "a and b contain %s arrays\n",
             cmpStr(\@a, \@b) == 0
                 ? "the same"
                 : "different";

     This approach also works for comparing hashes.  Here we'll
     demonstrate two different answers:

         use FreezeThaw qw(cmpStr cmpStrHard);

         %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
         $a{EXTRA} = \%b;
         $b{EXTRA} = \%a;

         printf "a and b contain %s hashes\n",
             cmpStr(\%a, \%b) == 0 ? "the same" : "different";

         printf "a and b contain %s hashes\n",
             cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";

     The first reports that both those the hashes contain the
     same data, while the second reports that they do not.  Which
     you prefer is left as an exercise to the reader.

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     How do I find the first array element for which a condition
     is true?

     To find the first array element which satisfies a condition,
     you can use the first() function in the List::Util module,
     which comes with Perl 5.8.  This example finds the first
     element that contains "Perl".

             use List::Util qw(first);

             my $element = first { /Perl/ } @array;

     If you cannot use List::Util, you can make your own loop to
     do the same thing.  Once you find the element, you stop the
     loop with last.

             my $found;
             foreach ( @array )
                     if( /Perl/ ) { $found = $_; last }

     If you want the array index, you can iterate through the
     indices and check the array element at each index until you
     find one that satisfies the condition.

             my( $found, $index ) = ( undef, -1 );
             for( $i = 0; $i < @array; $i++ )
                     if( $array[$i] =~ /Perl/ )
                             $found = $array[$i];
                             $index = $i;

     How do I handle linked lists?

     In general, you usually don't need a linked list in Perl,
     since with regular arrays, you can push and pop or shift and
     unshift at either end, or you can use splice to add and/or
     remove arbitrary number of elements at arbitrary points.
     Both pop and shift are both O(1) operations on Perl's
     dynamic arrays.  In the absence of shifts and pops, push in
     general needs to reallocate on the order every log(N) times,
     and unshift will need to copy pointers each time.

     If you really, really wanted, you could use structures as
     described in perldsc or perltoot and do just what the algo-
     rithm book tells you to do.  For example, imagine a list
     node like this:

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         $node = {
             VALUE => 42,
             LINK  => undef,

     You could walk the list this way:

         print "List: ";
         for ($node = $head;  $node; $node = $node->{LINK}) {
             print $node->{VALUE}, " ";
         print "\n";

     You could add to the list this way:

         my ($head, $tail);
         $tail = append($head, 1);       # grow a new head
         for $value ( 2 .. 10 ) {
             $tail = append($tail, $value);

         sub append {
             my($list, $value) = @_;
             my $node = { VALUE => $value };
             if ($list) {
                 $node->{LINK} = $list->{LINK};
                 $list->{LINK} = $node;
             } else {
                 $_[0] = $node;      # replace caller's version
             return $node;

     But again, Perl's built-in are virtually always good enough.

     How do I handle circular lists?

     Circular lists could be handled in the traditional fashion
     with linked lists, or you could just do something like this
     with an array:

         unshift(@array, pop(@array));  # the last shall be first
         push(@array, shift(@array));   # and vice versa

     How do I shuffle an array randomly?

     If you either have Perl 5.8.0 or later installed, or if you
     have Scalar-List-Utils 1.03 or later installed, you can say:

         use List::Util 'shuffle';

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             @shuffled = shuffle(@list);

     If not, you can use a Fisher-Yates shuffle.

         sub fisher_yates_shuffle {
             my $deck = shift;  # $deck is a reference to an array
             my $i = @$deck;
             while (--$i) {
                 my $j = int rand ($i+1);
                 @$deck[$i,$j] = @$deck[$j,$i];

         # shuffle my mpeg collection
         my @mpeg = <audio/*/*.mp3>;
         fisher_yates_shuffle( \@mpeg );    # randomize @mpeg in place
         print @mpeg;

     Note that the above implementation shuffles an array in
     place, unlike the List::Util::shuffle() which takes a list
     and returns a new shuffled list.

     You've probably seen shuffling algorithms that work using
     splice, randomly picking another element to swap the current
     element with

         @new = ();
         @old = 1 .. 10;  # just a demo
         while (@old) {
             push(@new, splice(@old, rand @old, 1));

     This is bad because splice is already O(N), and since you do
     it N times, you just invented a quadratic algorithm; that
     is, O(N**2).  This does not scale, although Perl is so effi-
     cient that you probably won't notice this until you have
     rather largish arrays.

     How do I process/modify each element of an array?

     Use "for"/"foreach":

         for (@lines) {
                     s/foo/bar/;     # change that word
                     tr/XZ/ZX/;      # swap those letters

     Here's another; let's compute spherical volumes:

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         for (@volumes = @radii) {   # @volumes has changed parts
                     $_ **= 3;
                     $_ *= (4/3) * 3.14159;  # this will be constant folded

     which can also be done with map() which is made to transform
     one list into another:

             @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;

     If you want to do the same thing to modify the values of the
     hash, you can use the "values" function.  As of Perl 5.6 the
     values are not copied, so if you modify $orbit (in this
     case), you modify the value.

         for $orbit ( values %orbits ) {
                     ($orbit **= 3) *= (4/3) * 3.14159;

     Prior to perl 5.6 "values" returned copies of the values, so
     older perl code often contains constructions such as
     @orbits{keys %orbits} instead of "values %orbits" where the
     hash is to be modified.

     How do I select a random element from an array?

     Use the rand() function (see "rand" in perlfunc):

         $index   = rand @array;
         $element = $array[$index];

     Or, simply:
         my $element = $array[ rand @array ];

     How do I permute N elements of a list?

     Use the List::Permutor module on CPAN.  If the list is actu-
     ally an array, try the Algorithm::Permute module (also on
     CPAN).  It's written in XS code and is very efficient.

             use Algorithm::Permute;
             my @array = 'a'..'d';
             my $p_iterator = Algorithm::Permute->new ( \@array );
             while (my @perm = $p_iterator->next) {
                print "next permutation: (@perm)\n";

     For even faster execution, you could do:

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        use Algorithm::Permute;
        my @array = 'a'..'d';
        Algorithm::Permute::permute {
           print "next permutation: (@array)\n";
        } @array;

     Here's a little program that generates all permutations of
     all the words on each line of input. The algorithm embodied
     in the permute() function is discussed in Volume 4 (still
     unpublished) of Knuth's The Art of Computer Programming and
     will work on any list:

             #!/usr/bin/perl -n
             # Fischer-Kause ordered permutation generator

             sub permute (&@) {
                     my $code = shift;
                     my @idx = 0..$#_;
                     while ( $code->(@_[@idx]) ) {
                             my $p = $#idx;
                             --$p while $idx[$p-1] > $idx[$p];
                             my $q = $p or return;
                             push @idx, reverse splice @idx, $p;
                             ++$q while $idx[$p-1] > $idx[$q];

             permute {print"@_\n"} split;

     How do I sort an array by (anything)?

     Supply a comparison function to sort() (described in "sort"
     in perlfunc):

         @list = sort { $a <=> $b } @list;

     The default sort function is cmp, string comparison, which
     would sort "(1, 2, 10)" into "(1, 10, 2)".  "<=>", used
     above, is the numerical comparison operator.

     If you have a complicated function needed to pull out the
     part you want to sort on, then don't do it inside the sort
     function.  Pull it out first, because the sort BLOCK can be
     called many times for the same element.  Here's an example
     of how to pull out the first word after the first number on
     each item, and then sort those words case-insensitively.

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         @idx = ();
         for (@data) {
             ($item) = /\d+\s*(\S+)/;
             push @idx, uc($item);
         @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];

     which could also be written this way, using a trick that's
     come to be known as the Schwartzian Transform:

         @sorted = map  { $_->[0] }
                   sort { $a->[1] cmp $b->[1] }
                   map  { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;

     If you need to sort on several fields, the following para-
     digm is useful.

         @sorted = sort { field1($a) <=> field1($b) ||
                          field2($a) cmp field2($b) ||
                          field3($a) cmp field3($b)
                        }     @data;

     This can be conveniently combined with precalculation of
     keys as given above.

     See the sort article in the "Far More Than You Ever Wanted
     To Know" collection in
     http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for more about
     this approach.

     See also the question below on sorting hashes.

     How do I manipulate arrays of bits?

     Use pack() and unpack(), or else vec() and the bitwise

     For example, this sets $vec to have bit N set if $ints[N]
     was set:

         $vec = '';
         foreach(@ints) { vec($vec,$_,1) = 1 }

     Here's how, given a vector in $vec, you can get those bits
     into your @ints array:

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         sub bitvec_to_list {
             my $vec = shift;
             my @ints;
             # Find null-byte density then select best algorithm
             if ($vec =~ tr/\0// / length $vec > 0.95) {
                 use integer;
                 my $i;
                 # This method is faster with mostly null-bytes
                 while($vec =~ /[^\0]/g ) {
                     $i = -9 + 8 * pos $vec;
                     push @ints, $i if vec($vec, ++$i, 1);
                     push @ints, $i if vec($vec, ++$i, 1);
                     push @ints, $i if vec($vec, ++$i, 1);
                     push @ints, $i if vec($vec, ++$i, 1);
                     push @ints, $i if vec($vec, ++$i, 1);
                     push @ints, $i if vec($vec, ++$i, 1);
                     push @ints, $i if vec($vec, ++$i, 1);
                     push @ints, $i if vec($vec, ++$i, 1);
             } else {
                 # This method is a fast general algorithm
                 use integer;
                 my $bits = unpack "b*", $vec;
                 push @ints, 0 if $bits =~ s/^(\d)// && $1;
                 push @ints, pos $bits while($bits =~ /1/g);
             return \@ints;

     This method gets faster the more sparse the bit vector is.
     (Courtesy of Tim Bunce and Winfried Koenig.)

     You can make the while loop a lot shorter with this sugges-
     tion from Benjamin Goldberg:

             while($vec =~ /[^\0]+/g ) {
                push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;

     Or use the CPAN module Bit::Vector:

         $vector = Bit::Vector->new($num_of_bits);
         @ints = $vector->Index_List_Read();

     Bit::Vector provides efficient methods for bit vector, sets
     of small integers and "big int" math.

     Here's a more extensive illustration using vec():

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         # vec demo
         $vector = "\xff\x0f\xef\xfe";
         print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
             unpack("N", $vector), "\n";
         $is_set = vec($vector, 23, 1);
         print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";




         sub set_vec {
             my ($offset, $width, $value) = @_;
             my $vector = '';
             vec($vector, $offset, $width) = $value;
             print "offset=$offset width=$width value=$value\n";

         sub pvec {
             my $vector = shift;
             my $bits = unpack("b*", $vector);
             my $i = 0;
             my $BASE = 8;

             print "vector length in bytes: ", length($vector), "\n";
             @bytes = unpack("A8" x length($vector), $bits);
             print "bits are: @bytes\n\n";

     Why does defined() return true on empty arrays and hashes?

     The short story is that you should probably only use defined
     on scalars or functions, not on aggregates (arrays and
     hashes).  See "defined" in perlfunc in the 5.004 release or
     later of Perl for more detail.

Data: Hashes (Associative Arrays)

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     How do I process an entire hash?

     Use the each() function (see "each" in perlfunc) if you
     don't care whether it's sorted:

         while ( ($key, $value) = each %hash) {
             print "$key = $value\n";

     If you want it sorted, you'll have to use foreach() on the
     result of sorting the keys as shown in an earlier question.

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

     (contributed by brian d foy)

     The easy answer is "Don't do that!"

     If you iterate through the hash with each(), you can delete
     the key most recently returned without worrying about it.
     If you delete or add other keys, the iterator may skip or
     double up on them since perl may rearrange the hash table.
     See the entry for "each()" in perlfunc.

     How do I look up a hash element by value?

     Create a reverse hash:

         %by_value = reverse %by_key;
         $key = $by_value{$value};

     That's not particularly efficient.  It would be more space-
     efficient to use:

         while (($key, $value) = each %by_key) {
             $by_value{$value} = $key;

     If your hash could have repeated values, the methods above
     will only find one of the associated keys.   This may or may
     not worry you.  If it does worry you, you can always reverse
     the hash into a hash of arrays instead:

          while (($key, $value) = each %by_key) {
              push @{$key_list_by_value{$value}}, $key;

     How can I know how many entries are in a hash?

     If you mean how many keys, then all you have to do is use
     the keys() function in a scalar context:

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         $num_keys = keys %hash;

     The keys() function also resets the iterator, which means
     that you may see strange results if you use this between
     uses of other hash operators such as each().

     How do I sort a hash (optionally by value instead of key)?

     (contributed by brian d foy)

     To sort a hash, start with the keys. In this example, we
     give the list of keys to the sort function which then com-
     pares them ASCIIbetically (which might be affected by your
     locale settings). The output list has the keys in ASCIIbeti-
     cal order. Once we have the keys, we can go through them to
     create a report which lists the keys in ASCIIbetical order.

             my @keys = sort { $a cmp $b } keys %hash;

             foreach my $key ( @keys )
                     printf "%-20s %6d\n", $key, $hash{$value};

     We could get more fancy in the "sort()" block though.
     Instead of comparing the keys, we can compute a value with
     them and use that value as the comparison.

     For instance, to make our report order case-insensitive, we
     use the "\L" sequence in a double-quoted string to make
     everything lowercase. The "sort()" block then compares the
     lowercased values to determine in which order to put the

             my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;

     Note: if the computation is expensive or the hash has many
     elements, you may want to look at the Schwartzian Transform
     to cache the computation results.

     If we want to sort by the hash value instead, we use the
     hash key to look it up. We still get out a list of keys, but
     this time they are ordered by their value.

             my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;

     From there we can get more complex. If the hash values are
     the same, we can provide a secondary sort on the hash key.

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             my @keys = sort {
                     $hash{$a} <=> $hash{$b}
                     "\L$a" cmp "\L$b"
                     } keys %hash;

     How can I always keep my hash sorted?

     You can look into using the DB_File module and tie() using
     the $DB_BTREE hash bindings as documented in "In Memory
     Databases" in DB_File. The Tie::IxHash module from CPAN
     might also be instructive.

     What's the difference between "delete" and "undef" with

     Hashes contain pairs of scalars: the first is the key, the
     second is the value.  The key will be coerced to a string,
     although the value can be any kind of scalar: string,
     number, or reference.  If a key $key is present in %hash,
     "exists($hash{$key})" will return true.  The value for a
     given key can be "undef", in which case $hash{$key} will be
     "undef" while "exists $hash{$key}" will return true.  This
     corresponds to ($key, "undef") being in the hash.

     Pictures help...  here's the %hash table:

               keys  values
             |  a   |  3   |
             |  x   |  7   |
             |  d   |  0   |
             |  e   |  2   |

     And these conditions hold

             $hash{'a'}                       is true
             $hash{'d'}                       is false
             defined $hash{'d'}               is true
             defined $hash{'a'}               is true
             exists $hash{'a'}                is true (Perl5 only)
             grep ($_ eq 'a', keys %hash)     is true

     If you now say

             undef $hash{'a'}

     your table now reads:

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               keys  values
             |  a   | undef|
             |  x   |  7   |
             |  d   |  0   |
             |  e   |  2   |

     and these conditions now hold; changes in caps:

             $hash{'a'}                       is FALSE
             $hash{'d'}                       is false
             defined $hash{'d'}               is true
             defined $hash{'a'}               is FALSE
             exists $hash{'a'}                is true (Perl5 only)
             grep ($_ eq 'a', keys %hash)     is true

     Notice the last two: you have an undef value, but a defined

     Now, consider this:

             delete $hash{'a'}

     your table now reads:

               keys  values
             |  x   |  7   |
             |  d   |  0   |
             |  e   |  2   |

     and these conditions now hold; changes in caps:

             $hash{'a'}                       is false
             $hash{'d'}                       is false
             defined $hash{'d'}               is true
             defined $hash{'a'}               is false
             exists $hash{'a'}                is FALSE (Perl5 only)
             grep ($_ eq 'a', keys %hash)     is FALSE

     See, the whole entry is gone!

     Why don't my tied hashes make the defined/exists distinc-

     This depends on the tied hash's implementation of EXISTS().
     For example, there isn't the concept of undef with hashes
     that are tied to DBM* files. It also means that exists() and
     defined() do the same thing with a DBM* file, and what they
     end up doing is not what they do with ordinary hashes.

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     How do I reset an each() operation part-way through?

     Using "keys %hash" in scalar context returns the number of
     keys in the hash and resets the iterator associated with the
     hash.  You may need to do this if you use "last" to exit a
     loop early so that when you re-enter it, the hash iterator
     has been reset.

     How can I get the unique keys from two hashes?

     First you extract the keys from the hashes into lists, then
     solve the "removing duplicates" problem described above.
     For example:

         %seen = ();
         for $element (keys(%foo), keys(%bar)) {
         @uniq = keys %seen;

     Or more succinctly:

         @uniq = keys %{{%foo,%bar}};

     Or if you really want to save space:

         %seen = ();
         while (defined ($key = each %foo)) {
         while (defined ($key = each %bar)) {
         @uniq = keys %seen;

     How can I store a multidimensional array in a DBM file?

     Either stringify the structure yourself (no fun), or else
     get the MLDBM (which uses Data::Dumper) module from CPAN and
     layer it on top of either DB_File or GDBM_File.

     How can I make my hash remember the order I put elements
     into it?

     Use the Tie::IxHash from CPAN.

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         use Tie::IxHash;
         tie my %myhash, 'Tie::IxHash';
         for (my $i=0; $i<20; $i++) {
             $myhash{$i} = 2*$i;
         my @keys = keys %myhash;
         # @keys = (0,1,2,3,...)

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

     If you say something like:

         somefunc($hash{"nonesuch key here"});

     Then that element "autovivifies"; that is, it springs into
     existence whether you store something there or not.  That's
     because functions get scalars passed in by reference.  If
     somefunc() modifies $_[0], it has to be ready to write it
     back into the caller's version.

     This has been fixed as of Perl5.004.

     Normally, merely accessing a key's value for a nonexistent
     key does not cause that key to be forever there.  This is
     different than awk's behavior.

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

     Usually a hash ref, perhaps like this:

         $record = {
             NAME   => "Jason",
             EMPNO  => 132,
             TITLE  => "deputy peon",
             AGE    => 23,
             SALARY => 37_000,
             PALS   => [ "Norbert", "Rhys", "Phineas"],

     References are documented in perlref and the upcoming
     perlreftut. Examples of complex data structures are given in
     perldsc and perllol.  Examples of structures and object-
     oriented classes are in perltoot.

     How can I use a reference as a hash key?

     (contributed by brian d foy)

     Hash keys are strings, so you can't really use a reference
     as the key. When you try to do that, perl turns the

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     reference into its stringified form (for instance,
     "HASH(0xDEADBEEF)"). From there you can't get back the
     reference from the stringified form, at least without doing
     some extra work on your own. Also remember that hash keys
     must be unique, but two different variables can store the
     same reference (and those variables can change later).

     The Tie::RefHash module, which is distributed with perl,
     might be what you want. It handles that extra work.

Data: Misc
     How do I handle binary data correctly?

     Perl is binary clean, so this shouldn't be a problem.  For
     example, this works fine (assuming the files are found):

         if (`cat /vmunix` =~ /gzip/) {
             print "Your kernel is GNU-zip enabled!\n";

     On less elegant (read: Byzantine) systems, however, you have
     to play tedious games with "text" versus "binary" files.
     See "binmode" in perlfunc or perlopentut.

     If you're concerned about 8-bit ASCII data, then see perllo-

     If you want to deal with multibyte characters, however,
     there are some gotchas.  See the section on Regular Expres-

     How do I determine whether a scalar is a

     Assuming that you don't care about IEEE notations like "NaN"
     or "Infinity", you probably just want to use a regular

        if (/\D/)            { print "has nondigits\n" }
        if (/^\d+$/)         { print "is a whole number\n" }
        if (/^-?\d+$/)       { print "is an integer\n" }
        if (/^[+-]?\d+$/)    { print "is a +/- integer\n" }
        if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
        if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
        if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                             { print "a C float\n" }

     There are also some commonly used modules for the task.
     Scalar::Util (distributed with 5.8) provides access to
     perl's internal function "looks_like_number" for determining
     whether a variable looks like a number.  Data::Types exports
     functions that validate data types using both the above and

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     other regular expressions. Thirdly, there is
     "Regexp::Common" which has regular expressions to match
     various types of numbers. Those three modules are available
     from the CPAN.

     If you're on a POSIX system, Perl supports the
     "POSIX::strtod" function.  Its semantics are somewhat
     cumbersome, so here's a "getnum" wrapper function for more
     convenient access.  This function takes a string and returns
     the number it found, or "undef" for input that isn't a C
     float.  The "is_numeric" function is a front end to "getnum"
     if you just want to say, "Is this a float?"

         sub getnum {
             use POSIX qw(strtod);
             my $str = shift;
             $str =~ s/^\s+//;
             $str =~ s/\s+$//;
             $! = 0;
             my($num, $unparsed) = strtod($str);
             if (($str eq '') || ($unparsed != 0) || $!) {
                 return undef;
             } else {
                 return $num;

         sub is_numeric { defined getnum($_[0]) }

     Or you could check out the String::Scanf module on the CPAN
     instead. The POSIX module (part of the standard Perl distri-
     bution) provides the "strtod" and "strtol" for converting
     strings to double and longs, respectively.

     How do I keep persistent data across program calls?

     For some specific applications, you can use one of the DBM
     modules. See AnyDBM_File.  More generically, you should con-
     sult the FreezeThaw or Storable modules from CPAN.  Starting
     from Perl 5.8 Storable is part of the standard distribution.
     Here's one example using Storable's "store" and "retrieve"

         use Storable;
         store(\%hash, "filename");

         # later on...
         $href = retrieve("filename");        # by ref
         %hash = %{ retrieve("filename") };   # direct to hash

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     How do I print out or copy a recursive data structure?

     The Data::Dumper module on CPAN (or the 5.005 release of
     Perl) is great for printing out data structures.  The Stor-
     able module on CPAN (or the 5.8 release of Perl), provides a
     function called "dclone" that recursively copies its argu-

         use Storable qw(dclone);
         $r2 = dclone($r1);

     Where $r1 can be a reference to any kind of data structure
     you'd like. It will be deeply copied.  Because "dclone"
     takes and returns references, you'd have to add extra punc-
     tuation if you had a hash of arrays that you wanted to copy.

         %newhash = %{ dclone(\%oldhash) };

     How do I define methods for every class/object?

     Use the UNIVERSAL class (see UNIVERSAL).

     How do I verify a credit card checksum?

     Get the Business::CreditCard module from CPAN.

     How do I pack arrays of doubles or floats for XS code?

     The kgbpack.c code in the PGPLOT module on CPAN does just
     this. If you're doing a lot of float or double processing,
     consider using the PDL module from CPAN instead--it makes
     number-crunching easy.
     Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington,
     and other authors as noted. 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 this
     file are hereby placed into the public domain.  You are per-
     mitted 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

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