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Just enough Perl for Rex
Perl is a scripting language designed to keep easy things easy, and make hard things possible. In this tutorial you will learn just enough Perl to write your own Rex tasks.
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Variables
Scalar variables
Scalars can contain single items, like strings, numbers, objects or references.
my $name = 'John'; # this is a string
my $age = 28; # this is a number (integer)
my $float = 28.5; # also a number, but a float
my $car = Car->new(); # this is an object from the class Car
Array variables
Arrays are lists of scalars. Like a grocery list.
my @names = ( 'John', 'Fred', 'Charley' );
my @to_buy = qw( Cheese Butter Salt ); # qw() quotes the words
To access an array element you have to use its index, which starts at zero:
say 'First name is: ' . $names[0];
say 'Last name is: ' . $name[2];
say 'Also the last name: ' . $name[-1];
Split a string into an array:
my $string = 'John,Fred,Charley';
my @names = split( /,/, $string );
Join the items of an array into a string:
my @names = qw( John Fred Charley );
my $string = join( ',', @names ); # -> John,Fred,Charley
If you want to iterate over an array, do it like this:
for my $name (@names) {
say "Current name: $name"; # double quotes make variables interpolated
}
Hash variables
Hashes are like arrays, but with named indexes, called keys.
my %person = (
name => 'John',
age => 28, # good practice to keep a trailing comma
);
To access a hash element you have to use its key:
say 'Name: ' . $person{'name'};
say 'Age: ' . $person{age}; # perl can autoquote simple key names for you
If you want to iterate over a hash, do it like this:
for my $key ( keys %person ) {
say "key: $key -> value: " . $person{$key};
}
But remember an important note: hashes are always unsorted.
Conditional statements
if ( $name eq 'John' ) {
say 'Hello, my name is John!';
}
else {
say 'Well, my name is not John...';
}
if ( $name ne 'John' ) {
say 'Well, my name is not John...';
}
else {
say 'Hello, my name is John!';
}
if ( $age < 30 ) {
say 'I am younger than 30.';
}
elsif ( $age >= 30 && $age <= 50 ) {
say 'Well, I am between 30 and 50.';
}
else {
say 'I am older than 50.';
}
Loops
for my $num ( 1 .. 5 ) {
say "> $num";
}
# looping over an array
for my $item (@array) {
say "> $item";
}
Regular expressions
my $name = 'John';
if ( $name =~ m/john/ ) { # will _not_ match, because the 'J' in $name is uppercase
}
if ( $name =~ m/john/i ) { # _will_ match, because we use the 'i' modifier for case-insensitive matching
}
$name =~ s/john/Fred/i; # this will replace the first match of 'john' (regardless of its case) with 'Fred'
$name =~ s/john/Fred/ig; # this will replace all matches of 'john' (regardless of its case) with 'Fred'
Subroutines
sub my_function { # define the subroutine called 'my_function'
}
sub my_function2 { # @_ contains the parameters passed to the subroutine
my ( $param1, $param2 ) = @_;
}
my_function(); # call the function 'my_function'
my_function; # also calls 'my_function', but harder to read due to missing parentheses
&my_function; # also calls 'my_function', but with a deprecated old notation
my_function2( 'john', 28 ); # call 'my_function2' with 2 parameters
my_function2 'john', 28; # does the same, but harder to read due to missing parentheses
Useful helpers
Dump the content of a scalar, array or hash:
use Data::Dumper;
say Dumper($scalar);
say Dumper(@array);
say Dumper(%hash);
More documentation
If you want to learn more Perl you can find a great online tutorial on Perl Maven.