Learn perl

1. Practical Approach to PERL
(Day2)
Rakesh Mukundan

2. String Comparison

We want to check if a string contains the pattern
“blahblah”

Consider the strings

“I am so bored blahblah”

“blahblahblah”

“And so blahblah am I!”

“Blahblah is so blahblah!! “

3. Regex: Perl's Way

To check if a pattern exists in a string variable

$MyString =~ /blahblah/;

The expression will return 1 if a match is
found else 0

Use it inside an if condition

if($MyString =~ /blahblah/){ }

To check if a pattern is not present in a string

$MyString !~ /blahblah/

4. Contd..

Check if the line starts with a string : ^

/^blahblah/

Matches only to “blahblah is me”

Don't match to “am so blahblah”

Check if the line ends with a string : $

/blahblah$/
• Don't match “blahblah is me”
• Only match “am so blahblah”

5. Contd..

To match any charecter use dot(.)operator

/b.t/

will match to bit,bat,b1t,b0t etc

To match one or more character: plus(+)

/sho+t/

will match shot,shoot,shooot,shooooooot

But will not match to sht

6. Contd..

To match zero or more characters: star(*)

/sho*t/

Will match to sht,shot,shoot,shooooot

To match any charecter any number of times

/b.*t/

Will match bt,bot,bit,boot,boooot,baaaaaaat

How to match an operator say plus(+) in string?

Use escape char()

/B+/ will match “B+”

7. Few more

Matching a digit: d

Matching a non digit : D

Matching white space : s

Matching any of the specified char square bracker[]:

Eg:/[abc]cat/ will match to acat,bcat,ccat

/[123]456/ will match 1456,2456,3456

Fancy way: /[0-4]/ is same as /[01234]/

/[a-d]/ is same as /[abcd]/

/[0-2a-c]/ is same as /[012abc]/

8. Example Regex
An IP adress:192.168.1.1
d
d+
.
d+
.
d+
.
d+
d+.d+.d+.d+

9. Date
03/03/2012
d+
/
d+
/
d+
d+/d+/d+

10. Extracting Matches

Consider /alpha.+gamma/

It matches string “xxxalphazzzzgamma”

Suppose we want to extract the match

Place the match in single bracket() matched
value will be available in the variable $1

if($MyString =~/alpha(.+)gamma/){

Print $1

}

11. Extracting Date

Extract date/month/year from the string
“20/10/2012”

if($MyString =~/(d+)/(d+)/(d+)/){

$date= $1;

$month=$2;

$year=$3;

}

12. FILE Opening

open(FILEHANDLE, MODE, EXPR)

open($FH,"<","trace.txt") or die $!;

Modes:

> Write/Create

< Read

>> Appened/Create

+< Read/Write

+> Read/Write/Create

+>> Read/Append/Create

13. Reading a File

To read a single line $MyLine = <$FH>

To read the whole file $MyFile = <@FH>

Not recommended as it will try to load the
entire file into memory

Instead use a loop

Safer way to process a large file

while($MyLine= <$FH>){

#process a line

}

14. File Closing

Use close function along with file handler

close($FH);

15. File Closing

Use close function along with file handler

close($FH);

16. Log Parser

Open the log file and count the number of lines

Count the number of packets

Identify Unique IPs and number of occurances of
each IP

Identify the IPs exchanging ICMP traffic

Identify missed pings if any

17. Functions(Sub Routines)

Used for code re use and maintainability

No need to declare subroutines, define and use

Defining

Sub MyFunction {

#code to be executed

}

Calling a function

MyFunction();

18. Passing values to sub

Values passed to a sub routine will be available in a
special array named @_

sub MyFunction{

@argArray = @_;

print Dumper @argArray;

}

MyFunction(‘arg1’,789);

19. Returning Values From sub

Use return $variable;

sub MyFunction{

$num1 = shift(@_);

$num2 = shift(@_);

$sum = $num1 + $num2;

return $sum;

}

20. Perl Modules

Modules are similar to libraries

For code re usability

Standard modules are available with perl installation
 Eg: Data::Dumper

Non standard modules can be downloaded and
installed

CPAN : Comprehensive Perl Archive Network

Typically will be in a .pm file

21. Creating a Module

Similar to normal Perl code

Start module package MyModule;

The file should be named MyModule.pm

End Module with 1;

To use a modulein code use MyModule;

To call a sub routine in module MyModule-
>MyFuntion();

22. Strict Usage

By default perl doesn't need any variable to be
declared before use

Simple spelling mistakes in variable names can lead
to hours of code debugging!

By using the strict method,perl will strictly ask you
declare variable

my $MyFirstVar;

my @MyFirstArray;

my %MyFirstHash;