mpenning/perl_notes

A quick exploration of perl's magical syntax

Background

perl is famously cryptic and many loathe it because of its large number of implicit behaviors. Contrast this with python, which strives to do nothing implicit by default.

I spent ten years with perl as my weapon of choice. However, in 2007, I migrated 100% to python as my primary langauge, and forgot most of my perl chops. Yet, fifteen years down that road, I've concluded that perl should have a place in my life... at least for some tasks (such as text manipulation and munging).

This repo will keep notes about perl use-cases.

Use-case 1: Find and replace with perl

Introduction

Let's do a common task with perl, find and replace text.

Use-case-01: Find and replace text with perl

To illustrate, lets use perl to open a file and replace all occurances of 'free' with 'TREE'. Consider EXAMPLE_SCRIPT_01_01...

$ # EXAMPLE_SCRIPT_01_01: Find and replace with perl
$ perl -pi.bak -e 's/free/TREE/;' my_file.txt

That's pretty short, which is good for now. Find and replace shouldn't be a complicated task.

Let's dissect EXAMPLE_SCRIPT_01_01...

• The command-line flags are important...
  • The -p flag tells perl to implicitly loop over all lines in my_file.txt; each looped line will be assigned to a magic variable called $_.
  • The -i.bak flag tells perl to implicitly edit the my_file.txt in-place, and back up the original with a .bak extension. The -i flag MUST have an extension or no backup is created.
  • The -e flag tells perl to execute the quoted command-line script

Now, consider EXAMPLE_SCRIPT_01_02. With NO implicit behavior, EXAMPLE_SCRIPT_01_01 becomes...

$ # EXAMPLE_SCRIPT_01_02: Find-and-replace with perl
$ perl -e '`cp my_file.txt my_file.txt.bak`; open $fhin, "<", "my_file.txt"; open $fhout, ">", "my_file.txt.new"; foreach my $line (<$fhin>) { chomp $line; $line =~ s/free/TREE/g; print $fhout "$line\n"; } `mv my_file.txt.new my_file.txt`;'

I'll reformat it for our review...

# EXAMPLE_SCRIPT_01_02: Find-and-replace with perl
# WARNING this script is intentionally verbose to highlight perl's shortcuts
`cp my_file.txt my_file.txt.bak`;      # backup my_file.txt

open $fhin, "<", "my_file.txt";        # read from my_file.txt into $fhin
open $fhout, ">", "my_file.txt.new";   # write to my_file.txt.new with $fhout

foreach my $line (<$fhin>) {           # loop over $fhin
    chomp $line;                       # strip off new-lines in $line
    $line =~ s/free/TREE/g;            # replace 'free' with 'TREE'
    print $fhout "$line\n";            # write the modified line into $fhout
}

`mv my_file.txt.new my_file.txt`;

Now let's look back at EXAMPLE_SCRIPT_01_01...

$ # EXAMPLE_SCRIPT_01_01: Find-and-replace with perl
$ perl -pi.bak -e 's/free/TREE/;' my_file.txt

If I had to fix a file quickly, I'd DEFINITELY use EXAMPLE_SCRIPT_01_01. However, if I had to maintain something for a long time, the verbosity in EXAMPLE_SCRIPT_01_02 is my choice.

Use-case 2: Delete all files matching a regular expression with perl

unlink( grep { /\W\d+\.\d+/ } glob "*");