Download - An Introduction to Perl

An Introduction to

PerlMarch 2001

Hesham WahbyMentor Graphics Egypt

Presentation Outline

➢ Introduction

➢ Hello world

➢ Basic Perl language

➢ Regular expressions

➢ Some common Perl functions

➢ A glimpse of some advanced features

➢ Conclusion

Introduction:

What is Perl?➢ Practical Extraction and Report Language.

➢ Created in 1987 by Larry Wall, now maintained by hundreds of people.

➢ A high-level programming/scripting language, combining features from C, Sed, Awk, Unix shells and many others.

➢ Originally designed for text (and binary) string processing.

➢ "Perl is designed to make the easy jobs easy and the hard jobs possible."

➢ Perl motto: "There's more than one way to do it."

➢ "Pathologically Eclectic Rubbish Lister"?

Introduction:

Features of Perl➢ Language features:

➢ Simple to start using. Very rich set of tools.

➢ Basically function oriented, with OO extensions.

➢ C-style program structure. Free-style command syntax.

➢ Weakly-typed variables. Implicit declaration. Sophisticated data structures.

➢ Built-in regular expressions.

➢ Built-in database access.

➢ POSIX compliant.

➢ Extensible using modules.

Introduction:

Features of Perl

➢ Compiler features:➢ Compiled at load-time.

➢ Can be translated to optimized C code.

➢ Can also be directly integrated with other C/C++ code by either calling it or getting called by it.

➢ Built-in debugger.

Introduction:

Getting Perl

➢ Perl is GPL: freely distributable, open-source.

➢ Where to get Perl:➢ The Perl Homepage:

http://www.perl.com/➢ CPAN (Comprehensive Perl Archive Network):

http://www.cpan.org/➢ ActivePerl Homepage (for Win32):

http://www.activestate.com/ActivePerl/

Hello world!➢ Example Perl program: "hello.pl":

#!/usr/mgc/bin/perlprint "Hello world!"

➢ Command line:> perl hello.plHello world!>

➢ To run as executable:> chmod +x hello.pl> ./hello.pl

Hello world:

Command-line options> perl [options] program.pl program_arguments

> perl -e '$x=7; print 23*$x; print "\n"'

> preprocess test.pl | perl

> perl -h

> perl -v

> perl -w program.pl

> perl -d program.pl

> perl -c program.pl

> perl -Ipath_for_modules program.pl

> perl -Mmodule_name program.pl

#!/usr/mgc/bin/perl

%types = (int => 'Integers', float => 'Reals');

open INFILE, $ARGV[0];@lines = <INFILE>;close INFILE;

# Search for variablesforeach (@lines) {

if (/^\s*(int|float)\s*(\w*)/) {push @list[$1], $2;}

}

$" = ', '; # Print them outforeach $type (keys %list) {

print "$types[$type]: @list[$type].\n"}

Hello world:A more extensive example

Basic Perl language:

Data types

➢ Scalars

$var➢ Lists (Arrays)

@var $var[n]➢ Hashes (Associative arrays)

%var $var[???]➢ Complex data structures


Special variables➢ Default argument: $_➢ Input record separator: $/➢ Output field separator: $,➢ List separator: $"➢ Process Id: $$➢ Program name: $0➢ Command-line arguments: @ARGV➢ Subroutine arguments: @_➢ Environment variables: %ENV


Context$a = 'Take'; $x = 2; $y = '007';@list = ('red','green','blue');

print $a . ' ' . $x;$b = "Take $x\n";$z = $x + $y; $c = "$x + $y";

print "@list", @list, $#list;$i = @list; print $i;print $list[2];print $list;$listLen = ("purple", @list, 'yellow', @list+2);

$, = "\n"; print %ENV;print (keys %ENV); print (values %ENV);

Basic Perl Language:

Control constructs

if (expression) {block}

if (expression) {block} else {block}

if (expression) {block}elsif (expression) {block}elsif (expression) {block}...else {block}

unless (expression) {block}

unless (expression) {block} else {block}

Basic Perl Language:

Control constructs

while (expression) {block}

while (expression) {block} continue {block}

do {block} while (expression);

until (expression) {block}

for (statement; expression; statement) {block}

foreach variable (list) {block}

label:goto label;


File-handling➢ Opening a file:

open FILE, $file #inputopen FILE, "<$file"; #inputopen (FILE, ">$file"); #outputopen FILE, ">>$file"; #append

➢ Closing a file:close FILE;

➢ Reading from a file:$line = <FILE>; @lines = <FILE>;read FILE, $data, 78, 10;

➢ Writing to a file:print FILE "String.\n";

➢ Binary files:binmode FILE;

➢ Reading output of a command-line:$output = `ls -l $dir`


Subroutines

sub Add {

local ($x, $sum);$sum = 0;foreach $x (@_) {

$sum += $x;}

$sum;}

$test = &Add (2, $number, @listofnumbers);

Regular expressions:

Pattern matching$str = "The large swirls are eddies in the Gulf.";$str =~ m/die/ ; #true$str =~ /gulf/ ; #false$str !~ /gulf/i ; #false$_ = $str;/^the/ ; #false

➢ Modifiers:g: match globallyi: case-insensitive matchingm: multi-lineo: compile onces: single-linex: extended RE


Basic elements

\ Quote next metacharacter. Any character^ Start of line$ End of line\b \B (Non-)Word boundary

\w \W (Non-)Word character\s \S (Non-)Whitespace\d \D (Non-)Digit

\t Tab\n Newline

$var Match contents of variable${var} To explicitly delimit variables


Basic elements

| Alternation() Grouping[] Character class[^] Negative character class

* Match 0 or more+ Match 1 or more? Match 1 or 0{n} Match exectly n{n,} Match n or more{n,m} Match n or more, but less than m

*? +? ?? {}? Reverse 'greedy' behaviour


Extracting matched patterns

if ($match = ($string =~ /\d+/)) {print $match }

($a, $b) = /(\w)\s(\w)/;

@mygroups = (`groups` =~ /\b(\w+)\b/g);

/setenv\s*(\w+)\s*(\w+)/;$vars{

nmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm /mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm /$1} = $2;


Substitution and translation➢ Substitution:

$str =~ s/green/blue/g;

s/\b(.)(.*)(.)\b/\3\2\1/g;

s/(\d+)/1 + $1/eg;

➢ Translation:

tr/abc/ABC/;

tr/A-Z/QWERTYUIOPASDFGHJKLZXCVBNM/;


Some example REs

/\/\s*0*\.0*/ ;/[a-z]['")]*[.!?]+['")]*\s/ ;

s|/usr/bin|/usr/local/bin| ;$count = s/Mister\b/Mr./g ;s/\d+/$&*2/eg ;s/(\$\w+)/$1/eeg ;

$program =~ s {/\* # Match opening delimiter..*? # Match minimal characters.\*/ # Match closing delimiter.

} []gsx;

Some common Perl functions:

Strings➢ length

$l = length $string;$l = length; #uses $_

➢ split@list = split /[,\s]/, $string, 10;($name, $value) = split /=/;

➢ substr$piece = substr $string, 2, 10;

➢ chop & chomp$c = chop $string;chomp @lines;$/ = ' '; chomp;

➢ pack & unpack


Lists➢ push & pop

push @list, $item;$num = push @list, @items;$item = pop @list;

➢ shift & unshift$item = shift @list;unshift @list, $item;

➢ sort@sorted = sort @list;print sort {$a <=> $b} @list;

➢ splice@sublist = splice @list, 2, 5;splice @list, $off, $len, @newitems;


Miscellaneous

➢ time & localtime($sec,$min,$hour,$mday,$mon,$year, $wday,$yday,$isdst) = localtime time;$now = localtime;# "Thu Oct 13 04:54:34 1994"

➢ rand & srandsrand time;$x = rand 10;

A glimpse of someadvanced features

➢ References and complex data structures

➢ Formats

➢ POD: plain old documentation

➢ Modules

➢ Perl/Tk and Perl/CGI

➢ Databases

➢ OO Perl

Conclusion:

When to use Perl

➢ Advanced "shell scripts".

➢ Process management.

➢ Writing quick routines to batch process files or access databases.

➢ Anything involving a lot of string handling (ASCII or binary).

➢ CGI and other web-programming.

Conclusion:

When not to use Perl

➢ Heavy computations.

➢ Large applications.

Conclusion:

Further resources

➢ Books:

➢ "Programming Perl" 2/e, Larry Wall, Tom Christiansen & Randal Schwartz. (The camel book.)

➢ "Learning Perl" 2/e, Randal Schwartz, Tom Christiansen & Larry Wall. (The llama book.)

➢ "Perl in a Nutshell", Ellen Siever, Stephen Spainhour & Nathan Patwardhan.

➢ "Advanced Perl Programming", Sriram Srinivasan.

Conclusion:

Further resources

➢ Web-sites:

➢ Perl Core Documentation:

http://www.perldoc.com/

➢ Nik Silver's Perl Tutorial:

http://www.comp.leeds.ac.uk/nik/start.html