Perl

Looks like the main use of perl these days is to maintain old code which used perl. Perhaps, Python has taken over the role that perl used to play earlier. Still, perl run faster than python (especially for regex operations).

• Possible expansion of PERL: Practical Extraction and Reporting Language
• Perl 5 is current perl
• Perl 6 turned out to be Raku

Resource:

• Perl Hacks: Tips & Tools for Programming, Debugging, and Surviving - Shane Warden, Damian Conway, Curtis 'Ovid' Poe

General

• Comments: Starts with #

Strings

• qw(): quoted word
• chomp(): remove newline from end of string ??
• lc(): convert string to lowercase
• length(): Lents of a string
• String concatenation: $str1 . $str2
• String formatting: sprintf
  • Also see string interpolation: https://perldoc.perl.org/perlop#Gory-details-of-parsing-quoted-constructs

Quoted strings

• q: single quoted string
• qq: double quoted string
  • Delimiter is usually /, but can be other symbols too.

> my $s = q/"String" not yet over/;
'"String" not yet over'

> print $s;
"String" not yet over

Variables

https://perldoc.perl.org/variables

• Scalar variables start with: $
  • The $ looks like 's'
• Array variables start with: @
  • The @ looks like 'a'
• By default, variables are global.
  • Include use strict; to make perl warn you of implicit global variables.
• Use my to declare local variables.
  • Declaration of multiple my vars: my ($a, $b);

Multiple variable with same name

If there are two variables in current scope which has the same name (say, $x), use ::$x to unshadow outer variable.

// Global var set
$pi = 3.14;

print "Pi is $pi\n";
{
  print "Inside block\n";

  // Global var reset
  $pi = 3.1415;
  print "Inner Pi is $pi\n";
  print "Outer pi is $::pi\n";

  // Local var set
  my $pi = 3.141592;
  print "Inner Pi is $pi\n";
  print "Outer pi is $::pi\n";

  print "Leaving block\n";
}

// The local var is no longer in scope.
// Latest value of global var is printed.
print "Pi is $pi\n";

# Pi is 3.14
# Inside block
# Inner Pi is 3.1415
# Outer pi is 3.1415
# Inner Pi is 3.141592
# Outer pi is 3.1415
# Leaving block
# Pi is 3.1415

Arrays

https://www.perltutorial.org/perl-array/

• Number of elements: @arr

• Highest index: $#arr

• Stack operations:

  • push()
  • pop()

• Queue operations:

  • shift(): remove first element of array
  • unshift(): add elements to front of array
  • pop(): remove last element of array

• Sort array: sort @arr

Indexing

• Indexing starts from 0: $arr[i]
• Negative indexing and index slices possible.
  • Eg: $arr[-2..-1]

Hashes

Hash variable names start with a %.

%hash={"one"=>1,"two"=>2,"three"=>3}
print $hash{"one"}

Inline perl

Use -e option.

perl -e 'print("Hey!\n")'
Hey!

Loop

# $elem is scalar, @elems is an array
foreach $elem in @elems {
  # ...
}

cpan

WARNING: Probably never tried any of the following…

• Install a module: cpan Module::Name

  • Same as: cpan -i Module::Name (∵ the -i switch is default)
  • May take some time to finish.

• Installation of new packages: cpan <package-name>

  • Examples:
    • cpan Unicode::GCString
    • cpan YAML::Tiny
    • cpan File::HomeDir

• Run o conf init inside cpan to re-run configuration

• cpanm (cpanminus): something to supplement cpan ??

  • Considered simpler to use than cpan.
  • Installable with cpan App::cpanminus or using distro package manager.

Misc

• perltidy: automatic code formatter
  • Package: Perl::Tidy
• A perl repl: Many options, each with their own pros and cons.
  • rlwrap perl -de1
    • Doesn't support my variables ??
  • reply: https://metacpan.org/pod/Reply
    • Install with cpanm Reply

More

• Perl regex
• String formatting/templating
• Time management and formatting
• Example perl script (LaTeXTidy.pl): http://web.archive.org/web/20160822025224/http://bfc.sfsu.edu/LaTeXTidy-0.31.pl