perl is beautiful

2012-10-02T02:18:00

Writing the first post on a blog about programming is fairly easy. Now, the second one needed a little thought to come up with. I decided to show a trick I use almost daily, be it at work or on personal projects. And that is perl. Yes, the ancient language from 1987.

If you're familiar with grep (and if you're not, stop reading this right now and get familiarized), you already know the enormous power of a general purpose stream filter. What I want to show you in this post is how you can augment that power using simple (aren't them all?) features from perl, both the language and the homonymous interpreter.

The interpreter

I'll start with the interpreter just to get you started on how to run simple programs. No perl experience is required, since the constructs used here are really simple. So, let's start:

$ perl -v
This is perl 5, version 14, subversion 2 (v5.14.2) built for x86_64-linux-gnu-thread-multi
(with 53 registered patches, see perl -V for more detail)

Copyright 1987-2011, Larry Wall

Perl may be copied only under the terms of either the Artistic License or the
GNU General Public License, which may be found in the Perl 5 source kit.

Complete documentation for Perl, including FAQ lists, should be found on
this system using "man perl" or "perldoc perl".  If you have access to the
Internet, point your browser at http://www.perl.org/, the Perl Home Page.

If everything is setup right, you get a nice message along with the version number. If you get something like

perl: command not found

you probably need to install the perl interpreter or set your PATH right. Installing it may be as simple as

$ sudo apt-get install perl

depending on your system. But your platform's documentation is probably a much better resource on how to install programs. Once we have the interpreter, we can start fiddling with it:

$ perl
print "Hello world!\n";

Press Ctrl-D and you should get the (in)famous message on your screen. For longer scripts, you can save it to a file and pass the file name as an argument to the interpreter. But my personal favorite is the -e switch. It means that the string following the switch is actually the program to be run.

$ perl -e 'print "Hello world!\n"'

You won't have this problem here, but take care when mixing the quotes used inside the program from the quotes used to pass the command to the shell. You may need to escape some of them.

perl

Let's start building some more interesting examples, exploring one of the strongest side of perl: its support for regular expressions. We create a file to contain our script:

#!/usr/bin/perl

use strict;
use warnings;

my $str = <>;
print $str if $str =~ m/g w/;

I'll explain what each line does. You should know what line 1 does. Line 3 and 4 turn on useful warnings (if you don't set warnings to the highest level possible on your compiler/interpreter, shame on you). On line 6, we create a variable called str (the $ character is used to indicate simple variables, while the keyword my defines it as a local variable), and initialize it using the diamond operator. Long story short, this operator reads from the standard input (as long as you don't pass any parameters when executing the program, more on this later).

Line 7 deserves a special paragraph. Here I'm using the inverted if syntax (which is a great feature that most languages lack), but don't get fooled by it. It could be written as

if($str =~ m/g w/) {
    print $str;
}

but I find the inverted syntax much clearer and easier to read. The =~ operator is used to apply regular expressions. It is a binary operator, where the left-hand side is a string or a variable containing a string, and the right-hand side is a regex operator (in this case, the m// operator). The syntax to match a string against a regular expression is

string =~ m/regex/;

Using some of perl's magic, we could rewrite our example as

$_ = <>;
print if m/g w/;

Now things start to get interesting! I decided to go light and present the omnipotent $_ variable explicitly. $_ is a special (a.k.a. magic) variable in perl, which is used as the default value of many functions. Two examples here are print and the match operator.

One last thing before I delegate the rest of this topic to the next post: the diamond operator can be used with a loop to read all the lines from the input:

while(<>) {
    print;
}

Here, <> causes one line from the input to be read and stored on $_. Then, inside the body of the loop, print is called without arguments, causing it to use our old friend $_. Input, when using the diamond operator, can be one of two things. If the program is called with no arguments, input is stdin. If file names are passed as arguments, the input is the content of these files, read sequentially from first to last. The loop keeps reading until the input ends, which means reading until an EOF character on stdin or reading all the files. Congratulations! You have just implemented cat.

Coming next: actually increasing productivity.