---
name: perl
category: language
language: perl
filename: learnperl-tw.pl
contributors:
    - ["Korjavin Ivan", "http://github.com/korjavin"]
    - ["Dan Book", "http://github.com/Grinnz"]
translators:
    - ["Kang-min Liu", "https://gugod.org"]
lang: zh-tw
---

Perl 5 為超過 25 年以來持續地發展，是具高度能力、豐富機能之程式語言。

自大型主機至攜帶裝置，Perl 5 能於上百種平台上執行，既適於快速打造產品原型、亦合於大規模專案之發展。


```perl
# 註解列皆以井字號為開頭

#### 嚴謹度

use strict;
use warnings;

# 所有的 perl 程式檔案都應當包含此兩列程式碼。在如變數名稱有拼寫錯誤之時，
# strict 能使編譯過程失敗。而對於像是將未定義值接到字串中等等易犯之錯誤，
# warnings 則能提供適當的警告訊息。

#### Perl 變數與其型別

# 變數的開頭皆為一印記（sigil），是為一符號，用以標示其型別。
# 變數名稱唯有以字母或底線開頭，後接字母、數字、底線若干，方為有效。

### 在 Perl 語言中，主要的變數型別有三種：$純量、@陣列、%雜湊。

## 純量
#  一個純量變數，只能裝一個值：
my $animal = "camel";
my $answer = 42;
my $display = "You have $answer ${animal}s.\n";

# 純量值可為字串、整數、浮點數。Perl 會自動地在需要之時進行轉換。

# 以單引號括住的字串內容與其字面之值完全相同。而以雙引號括住的字串，其中則能內插變數
# 與像是表示換列的 "\n" 這種控制碼。

## 陣列
# 一個陣列，可以裝下很多值：
my @animals = ("camel", "llama", "owl");
my @numbers = (23, 42, 69);
my @mixed   = ("camel", 42, 1.23);

# 陣列元素的存取，需要角括號。前方的印記為 $ 符號，表示只取一個值。
my $second = $animals[1];

# The size of an array is retrieved by accessing the array in a scalar
# context, such as assigning it to a scalar variable or using the
# "scalar" operator.

# 欲知陣列之大小，在純量語境之下使用陣列便可。例如，將陣列裝到一個純量變數中。
# 又或者是使用 scalar 算符。

my $num_animals = @animals;
print "Number of numbers: ", scalar(@numbers), "\n";

# 陣列也能夠被安插在雙引號字串之內。各內容元素間隔，預設是一個空白字符。

print "We have these numbers: @numbers\n";

# 雙引號字串中，若有像電子郵件地址的部分，會被視為是在內插某個陣列的內容物。
# 請稍加留意。

my @example = ('secret', 'array');
my $oops_email = "foo@example.com"; # 'foosecret array.com'
my $ok_email = 'foo@example.com';

## Hashes
#   A hash represents a set of key/value pairs:

my %fruit_color = ("apple", "red", "banana", "yellow");

#  You can use whitespace and the "=>" operator to lay them out more
#  nicely:

my %fruit_color = (
  apple  => "red",
  banana => "yellow",
);

# Hash elements are accessed using curly braces, again with the $ sigil.
my $color = $fruit_color{apple};

# All of the keys or values that exist in a hash can be accessed using
# the "keys" and "values" functions.
my @fruits = keys %fruit_color;
my @colors = values %fruit_color;

# Scalars, arrays and hashes are documented more fully in perldata.
# (perldoc perldata).

#### References

# More complex data types can be constructed using references, which
# allow you to build arrays and hashes within arrays and hashes.

my $array_ref = \@array;
my $hash_ref = \%hash;
my @array_of_arrays = (\@array1, \@array2, \@array3);

# You can also create anonymous arrays or hashes, returning a reference:

my $fruits = ["apple", "banana"];
my $colors = {apple => "red", banana => "yellow"};

# References can be dereferenced by prefixing the appropriate sigil.

my @fruits_array = @$fruits;
my %colors_hash = %$colors;

# As a shortcut, the arrow operator can be used to dereference and
# access a single value.

my $first = $array_ref->[0];
my $value = $hash_ref->{banana};

# See perlreftut and perlref for more in-depth documentation on
# references.

#### Conditional and looping constructs

# Perl has most of the usual conditional and looping constructs.

if ($var) {
  ...
} elsif ($var eq 'bar') {
  ...
} else {
  ...
}

unless (condition) {
  ...
}
# This is provided as a more readable version of "if (!condition)"

# the Perlish post-condition way
print "Yow!" if $zippy;
print "We have no bananas" unless $bananas;

#  while
while (condition) {
  ...
}

my $max = 5;
# for loops and iteration
for my $i (0 .. $max) {
  print "index is $i";
}

for my $element (@elements) {
  print $element;
}

map {print} @elements;

# implicitly

for (@elements) {
  print;
}

# iterating through a hash (for and foreach are equivalent)

foreach my $key (keys %hash) {
  print $key, ': ', $hash{$key}, "\n";
}

# the Perlish post-condition way again
print for @elements;

# iterating through the keys and values of a referenced hash
print $hash_ref->{$_} for keys %$hash_ref;

#### Regular expressions

# Perl's regular expression support is both broad and deep, and is the
# subject of lengthy documentation in perlrequick, perlretut, and
# elsewhere. However, in short:

# Simple matching
if (/foo/)       { ... }  # true if $_ contains "foo"
if ($x =~ /foo/) { ... }  # true if $x contains "foo"

# Simple substitution

$x =~ s/foo/bar/;         # replaces foo with bar in $x
$x =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar in $x


#### Files and I/O

# You can open a file for input or output using the "open()" function.

# For reading:
open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
# For writing (clears file if it exists):
open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
# For writing (appends to end of file):
open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";

# You can read from an open filehandle using the "<>" operator.  In
# scalar context it reads a single line from the filehandle, and in list
# context it reads the whole file in, assigning each line to an element
# of the list:

my $line  = <$in>;
my @lines = <$in>;

# You can write to an open filehandle using the standard "print"
# function.

print $out @lines;
print $log $msg, "\n";

#### Writing subroutines

# Writing subroutines is easy:

sub logger {
  my $logmessage = shift;

  open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";

  print $logfile $logmessage;
}

# Now we can use the subroutine just as any other built-in function:

logger("We have a logger subroutine!");

#### Modules

# A module is a set of Perl code, usually subroutines, which can be used
# in other Perl code. It is usually stored in a file with the extension
# .pm so that Perl can find it.

package MyModule;
use strict;
use warnings;

sub trim {
  my $string = shift;
  $string =~ s/^\s+//;
  $string =~ s/\s+$//;
  return $string;
}

1;

# From elsewhere:

use MyModule;
MyModule::trim($string);

# The Exporter module can help with making subroutines exportable, so
# they can be used like this:

use MyModule 'trim';
trim($string);

# Many Perl modules can be downloaded from CPAN (http://www.cpan.org/)
# and provide a range of features to help you avoid reinventing the
# wheel.  A number of popular modules like Exporter are included with
# the Perl distribution itself. See perlmod for more details on modules
# in Perl.

#### Objects

# Objects in Perl are just references that know which class (package)
# they belong to, so that methods (subroutines) called on it can be
# found there. The bless function is used in constructors (usually new)
# to set this up. However, you never need to call it yourself if you use
# a module like Moose or Moo (see below).

package MyCounter;
use strict;
use warnings;

sub new {
  my $class = shift;
  my $self = {count => 0};
  return bless $self, $class;
}

sub count {
  my $self = shift;
  return $self->{count};
}

sub increment {
  my $self = shift;
  $self->{count}++;
}

1;

# Methods can be called on a class or object instance with the arrow
# operator.

use MyCounter;
my $counter = MyCounter->new;
print $counter->count, "\n"; # 0
$counter->increment;
print $counter->count, "\n"; # 1

# The modules Moose and Moo from CPAN can help you set up your object
# classes. They provide a constructor and simple syntax for declaring
# attributes. This class can be used equivalently to the one above.

package MyCounter;
use Moo; # imports strict and warnings

has 'count' => (is => 'rwp', default => 0, init_arg => undef);

sub increment {
  my $self = shift;
  $self->_set_count($self->count + 1);
}

1;

# Object-oriented programming is covered more thoroughly in perlootut,
# and its low-level implementation in Perl is covered in perlobj.
```

#### FAQ

perlfaq contains questions and answers related to many common tasks, and often provides suggestions for good CPAN modules to use.

#### Further Reading

 - [perl-tutorial](http://perl-tutorial.org/)
 - [Learn at www.perl.com](http://www.perl.org/learn.html)
 - [perldoc](http://perldoc.perl.org/)
 - and perl built-in : `perldoc perlintro`