Why Perl?

I've been told by many people now that Perl is an archaic and incomprehensible programming language and that I should write new projects in more popular languages such as Python, JavaScript or even Rust.

Don't get me wrong - other languages have their place. For anything that I want to distribute to others I'd generally choose another language, such as C++ because it can be distributed without a runtime as one big binary. For anything which requires lots of speed I'd choose C.

I could go on but in short I'll use Perl for anything personal which:

• Needs to be written quickly.
• Can run on UNIX like systems (I'm not running Perl on Windows)
• Heavily based around strings (such as web parsing, text processing, etc.) where compiled languages will fall down.

There are many reasons I love Perl, but the main one is just how expressive it can be.

Take the following code in Python that validates an input:

import re

email = ''

while(not re.match(r'.*@.*\.com', email)):
  email = input("Email: ")

print(email)

Looks fairly straight-forward, just reading an input and validating it by a regex.

But here is the same in Perl:

my $email;
do {
  print "Email: ";
} until(($line = <STDIN>) =~ /.*@.*\.com/);

print "$line\n";

Whilst some may disagree, this to me seems so much more natural to read, and the inbuilt regex handling is so much better than any compiled language or Python.

Regex support

Perl is based around the use of regular expressions. They are a first class citizen in the Perl world.

If statements

If statements can be used to find whether there is a match to a string, for example:

my $str = "my name is Geoff";

if($str =~ /my name is (.*)/g) {
  print "Name: $1\n";
}

$1 refers to the first group, $2 the second and so on.

While loops

While loops can be used to repeat for every match, for example:

my $str = "i 'hope' you have a nice 'day'";
while($str =~ /'([^']*)'/g) {
  print "Quoted: $1\n";
}

Syntactic Sugar

Perl has a lot of syntactic sugar, which when unfamiliar with can cause a bit of trouble, but once one has become accustomed to it it becomes extremely useful.

Here are a few of my favourite keywords:

• unless(x) - equal to if(not x)
• until(x) - equal to while(not x)

$_ - The default variable

$_ is the default variable - in many scenarios this can be used in functions without specifying it.

Take for example the following program to print all the words surrounded by quote marks in a string (this can easily be done in a more concise way but I do it like this to demonstrate the feature):

my $str = "i 'hope' you have a nice 'day'";

for my $word (split / /, $str) {
  chomp $word;
  if($word =~ /'[^']*'/g) {
    print "Quoted: $&\n";
  }
}

But using the default variable it can be rewritten as so:

my $str = "i 'hope' you have a nice 'day'";

for (split / /, $str) {
  chomp;
  if(/'[^']*'/g) {
    print "Quoted: $&\n";
  }
}

So for, chomp and if all assume the default variable is used when another is not specified.

qw

qw is an easy way to specify an array of strings. Whereas one can do the following:

my @a = ('hello', 'there', 'cat');

We can now use qw to make it easier:

my @a = qw/ hello there cat /;

This is just a nice feature which is fairly easy to pick up.

The diamond operator

The diamond operator is complicated in its definition, but I will attempt to explain its behaviour.

Empty diamond:

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

This is the same as saying: 'open the command line arguments as files and read their content line by line, and if there are no arguments then read from STDIN'.

Diamond with a file handle:

open FH, '<', 'test.txt';

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

close FH;

This reads the file handle line by line. This can also be done with the file handle of STDIN to read a line of text from standard input.