Introduction to Perl

Overview

Basics of the language:

Scalars, arrays, hashes
Function definition
Regexes
Control structures (if, else, for, while)
IO (screen, keyboard, files)
References
Introduce multi dimensional data structures
Packages
OOP
Anonymous functions, dispatch tables
Introduce CPAN (MetaCPAN)
Go over a number of modules for text processing

First script

#!/usr/bin/env perl
use v5.10;
use strict;
use warnings;

say 'Hello World';

#!/usr/bin/env perl - Path to perl interpreter
use v5.10; - Minimal version number of Perl, turn on new features. Same as use 5.010;
use strict; - Require variable declarations, avoid stupid mistakes.
use warnigs; - Ask for warnings. Avoid further stupid mistakes.
say - print with newline at the end

Always use strict and use warnings!

Avoid global variables

Not really Perl specific, but declare variables as lates as possible!
Don't use global variables!
You will thank yourself a year later.

Keep it simple

A script has to be simple, readable and reusable.
Sophisticated optimization and algorithms are less interesting.

Editors

vim
emacs
Notepad++
Atom
...

IDEs

Eclipse + EPIC or Perlipse.
Jetbrains IntelliJ IDEA + perl plugin.
Komodo

Debugging

The IDEs have their debuggers
print statements
use Data::Dumper qw(Dumper)
perl -d mycode.pl

First script with main function

#!/usr/bin/env perl
use v5.10;
use strict;
use warnings;


sub main {
    say 'Hello World';
}

main();

Putting all your code in small(!) functions will make your code better.

Comments

Anywhere (except the first row) a # starts a comment till the end of the row.

# Comment
say "somthing"; # comment

IO: Output on screen - print vs. say

say "hello"; # print a newline at the end. (available since perl 5.10)
print "hello"; # print as it is
print "hello\n"; # print a newline at the end.

IO: Input from keyboard

my $input = <STDIN>;
chomp $input;

Read one line till newline.
Remove newline from the end.

Scalar values

undef
a number
a string
a reference to any other data structure or function

Scalar values - undef

undef corresponds to NULL in SQL, None in Python, and null in PHP or JavaScript.

Scalar values - strings

A string can be either double quoted or single quoted. Differences explained later.

"A string"

'Another string'

Scalar values - numbers

For numbers there is no need for quotes.

integer

26
1_234_567                # like 1,234,567 in human writing

integer (hex/oct/binary)

0x1a            # hex     also written as    hex("1a");
032             # oct     also written as    oct("32");
0b11010         # binary  also written as    oct("0b11010");
                # all 3 equal to 26 decimal

real or floating-point

3.5e+3          # 3500

Lists

A list is a series of scalar values separated by comma and enclosed in parentheses. The scalar values themselves can be references to other data structures. (An array, explained later is a variable holding the content of a list.)

('string', 42, 2.3, undef, ['one', 'two'], { 'name' => 'Foo Bar' })

Hash

A hash is a set of key-value pairs. The keys are strings that are unique in the given hash. The values are scalars, including references to other data structures. In PHP they are calle "associative arrays". In Python "dictionaries".

(
    'name'  => 'Foo Bar',
    'email' => 'foo@bar.com',
)

(
    'banana' => 7,
    'apple' => 3,
    'lemon' => 1,
)

Boolean values

In Perl there is no special True and False values. Any value can be checked for True-ness. The following values are considered to be False:

undef
0        # the number 0.
''       # empty string.
'0'      # a string with a single 0 in it.

Everything else is considered to be True.

single-quoted and double-quotes strings

$name    = 'Foo';
$message = "Hello $name, how are you?"; # Hello Foo, how are you?

$other   = 'Hello $name, how are you?'; # Hello $name, how are you?

print "\n";    # is a real newline
print '\n';    # will print \n

also use q or qq:

$message = qq{Hello $name, how are you?}; # Hello Foo, how are you?

$other   = q{Hello $name, how are you?}; # Hello $name, how are you?

String Operators

"abc" . "def"     # concatenation  "abcdef"
"abc" x 3         # repetition operator "abcabcabc"

Numerical Operators

2 + 3        # 5
2 * 3        # 6
2 ** 3       # 2*2*2 = 8
7 / 2        # 3.5
...

Boolean Operators

and            &&
or             ||
not            !
xor

Precedence!

Perl is an Operator and Context driven language

"2" . "3"     # "23"
"2" . 3       # "23"
2   . 3       # "23"

"2" + "3"     # 5
"2" + 3       # 5

"abc" . "def"    # "abcdef"
"abc" + "def"    # 0  + 2 warnings about Argument isn't numeric in addition (+)

String Functions

length STRING - number of characters
lc STRING - lower case
uc STRING - upper case
index STRING - the location of a substring given its content
substr STRING, OFFSET, LENGTH - the content of a substring given its location

say length 'abc';     # 3
say length 23;        # 2

Variables

Sigil + variable name:

$name_of_a_scalar
@name_of_an_array
%name_of_a_hash

Variables (and other names) are case sensitive

$h
$H

Sigils

$ - scalars
@ - arrays
% - hashes
& - functions

Variable declaration

my $name;                 # Without assignment the value is undef
$name = 'Foo Bar';

my $email = 'foo@bar.com';

Arrays

my @people;                           # Declare empty array.
my @names = (                         # Declare array and assign 3 elements.
    'Foo',
    'Bar',
    'Moo',                            # Trailing comma is ok.
);
say $names[0];                        # Access the 0th element 'Foo'

scalar @names;      # number of elements in the array
$#names;            # the largest index (one less than number of elements)

push @names, 'Minimacko';
pop @names;
shift @names;
unshift @names, 'Malacka';

Hashes

my %phone = (
    'Foo' => '01-23456',
    'Bar' => '98-76543',
);
say $phone{'Foo'};
$phone{'Zorg'} = '999'

for my $name (keys %phone) {
    say "$name $phone{$name}";
}

IO (files)

open my $fh, '<', $filename or die "Could not open '$filename' for reading $!";

open my $fh, '>', $filename or die "Could not open '$filename' for writing $!";

open my $fh, '>>', $filename or die "Could not open '$filename' to append $!";

my $line = <$fh>;   # Read a line from a text file

Control structures (if, else, elsif)

For Loops (for, foreach)

C-style for loop

Possible, but not recommended in Perl.

While Loops

my $counter = 0;
while ( $counter < 10 ) {
    say $counter;
    $counter++;
}

Loop controls (next, last, redo)

next - evaluate the loop condition and if it is true go to the next iteration
last - exit the loop
redo - start the iteration again without evaluating the loop condition

Infinite while loop

while (1) {
    if (cond) {
        last;
    }
}

References

\$x    # reference to scalar
\@y    # reference to array
\%z    # reference to hash
\&f    # reference to function

References to Array

my @names = ('foo', 'bar', 'baz')
my $ref_to_names = \@names;

@names             @$ref_to_names        # de-reference the array

$name[0]           $$ref_to_names[0]
                   $ref_to_names->[0]    # same but looks better

sacalar @names     scalar @$ref_to_names
$#names            $#$ref_to_name        # I never liked this

Functions

sub greeting {
    say 'Hello World';
}

greeting();

Function parameters

sub greeting {
    my ($name) = @_;
    say "Hello $name!";
}

greeting('Foo');

References to functions

sub greeting {
    my ($name) = @_;
    say "Hello $name!";
}

Anonymous functions (state machine as an example), dispatch tables

sub {
    # do something
}

Dispatch table

Linux environment variables (%ENV)

#!/usr/bin/env perl
use v5.10;
use strict;
use warnings;

say $ENV{PATH};

for my $name (sort keys %ENV) {
    say $name;
}

$ENV{LANG} = 'Klingon';

List all defined variables

#!/usr/bin/env perl
use v5.10;
use strict;
use warnings;

my $x;
my @y;

our $xx;

foreach my $variable (sort keys %main::) {
    say $variable;
}

Regexes

Perl 5 Regex cheat sheet

Packages

OOP

Perl core
Moo
Moose
...

OOP Perl core

package MyClass;
use v5.10;
use strict;
use warnings;

sub new {
    my ($class, %params) = @_;
    my $self = bless {}, $class;
    for my $field (keys %params) {
        $self->{$field} = $params{$field};
    }
}

sub some_action {
    my ($slef, @params) = @_;

    # $self->{field} 

    my $result = 42;

    return $result;
} 

1;

Perl internal variables

$! - error of the last system call e.g. open file
$/ - input record separator, slurp mode
$_ - the default variable for many operations
$0 - Name of the current program
$1, $2, .. Regex match variables
@ARGV - command line parameters
@_ - parameters of the current function

Full list at perldoc perlvar

CPAN

CPAN
sco old, but well indexed by Google
Meta::CPAN new, maintaned, open source
mcpan - search.mcpan.org

Install CPAN Modules

Use the packaging system of your OS
cpan
cpanm from cpanmin.us
Ask your sysadmin

Selected Standard Perl Modules

Data::Dumper
File::Basename
Cwd

Selected Perl modules

Datetime (date manipulation)
Datetime::Tiny
DBI for database access
DBD:: database drivers
XML
Config file (INI)
CSV
YAML
JSON
Logging

Keyboard shortcuts

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