Perl Basics with Examples

PERL on Unix/Linux
(Practical Extraction and Reporting
Language)
1
NithinKumar Singani

 Introduction
 Scalar Variables and Lists
 Arrays and Hashes
 Operators and Precedence
 Conditional statements and Loops
 Regular Expressions
 Subroutines
 File and Directory Handling
Contents
2

History
 Developed by Larry Wall in 1987
 Derives from the ubiquitous C programming language
and to a lesser extent from sed, awk, the Unix Shell.
 PERL was originally designed under Unix, but now also
runs under all OS(Including Windows).
3

Introduction
What is PERL?
 Interpreted language that is optimized for string
manipulation, I/O and system tasks.
Why PERL?
 Speed of development – Don't have to compile create object
file and then execute.
 Power of flexibility of a high programming language.
 Easy to use, freely available and portable.
 Makes easy jobs easy, without making hard jobs impossible.
4

Beginning with Perl
 perl –v gives the version of the Perl the user is using.
 perldoc perl gives the list of manual pages as a part of
every Perl installation.
 perldoc –h gives the brief summary of options available.
 To create a Perl program, only a text editor and the perl
interpreter are required.
 Perl file ends with .pl (simple.pl)
5

Beginning with Perl (Contd..)
 Execution Command :
 perl filename.pl, or
 ./filename.pl
 When Unix has to execute Perl Script It first looks for “#!
(Shebang)” , it executes the remainder of the line and passes
the name of the script to it as an argument.
 “#! /usr/bin/perl” is the command used to run the Perl
Interpreter.(Location of the Perl binary, which perl gives the
pointer for perl binary)
 So to start a script we need to add above line as the first line
to make Perl script executable. 6

Beginning with Perl (Contd..)
 The core of Perl is Perl Interpreter the engine that actually
interprets, compiles, and runs Perl scripts.
 All Perl programs go through two phases:
 a compile phase where the syntax is checked and the
source code, including any modules used, is converted
into bytecode.
 a run-time phase where the bytecode is processed into
machine instructions and executed.
7

Man Pages
 Man command used to read the documentation.
Command Description
perl Overview (top level)
perldelta Changes since last version
perlfaq Frequently asked questions
perltoc Table of contents for Perl documentation
perlsyn Perl Syntax
perlop Operators and precedence
perlre Perl Regular Expression
perlfunc Built in functions
perlsub Subroutines
perlvar Predefined Variables
8

Basic Syntax
 “#” is used for commenting the line.
 All statements should end with ”;”.
 “$_” is the special variable called default variable.
 Perl is case sensitive.
 Perl program is compiled and run in a single operation.
9

Example
#! /usr/local/bin/perl
# Directs to perl interpreter on the system.
print “Welcome to perl”;
# Prints a message on the output.
This Program displays:
Welcome to perl
Example:
#! /usr/local/bin/perl –c
Print “welcome to perl”;
Syntax ok
Simple Programs
10

Basic Options
 -c : Check syntax and exit without executing the script.
 -v : Prints the version of perl executable.
 -w : Prints warnings
 -e : Used to enter and execute a line of script on the
command line
11

Standard Files
 STDIN : It is a normal input channel for the script.
 STDOUT : It is an normal output channel.
 STDERR : It is the normal output channel for errors.
12

Example
#! /usr/local/bin/perl –w
print “Enter the Text”;
$input = <STDIN> ; #Reads the input and stores in the variable input
Chomp(); #will remove new line character.
Print “entered text =$input” ; #Prints the input on the command line
Enter the Text
Perl is awesome #Perl will read this “Perl is awesomen”, by
default it will add n character to your
entered text. So use “chomp”
entered text =Perl is awesome
Standard Files (Contd..)
13

Variables
 Variables are used to refer data which is held as value.
 Perl defines three basic data types: scalars, arrays, and
hashes.
Scalars :
 Holds a single value it may be a string ,number or reference.
 Begin with “$”, followed by a letter then by letters, digits or
underscores.
Example : $var =1 # integer
$var = “Hello_world” # string
$var=2.65 # Decimal number
$3var = 123 #Error, Shouldn’t start with
#number
14

 Interpolation takes place only in double quotation marks.
Example
$x = 12 ; #Assign the value to the variable
print ‘ Value of x is $x ’ ; #Prints the output
Value of x is $x #Single quotation will not interpolate
#(no processing is done) the values
Example
$x = 12 ; #Assign the value to the variable
print “Value of x is $x” ; #Prints the output
Value of x is 12 #Double quotation interpolates the values.
#(Variable is replaced by its content )
Variable Interpolation
15

Integers
 Integers are usually expressed as decimal(10) but can be
specified in several different formats.
234  decimal integer
0765  octal integer
0b1101  binary integer
0xcae  hexadecimal integer
 Converting a number from one base to another base can be
done using “sprintf” function.
 Variables of different base can be displayed using “printf ”
function
16

Example
$bin = 0b1010;
$hex = sprint f ‘ %x’, $bin;
$oct = sprint f ‘ %o’ ,45;
print “binary =$bin n hexa =$hex n octal =$oct”;
binary= 1010
hexa = a
octal = 55
Integers (Contd..)
17

Example
$x = 98 ;
print f (“ Value in decimal =%dn”, $x ) ;
print f (“ Value in octal=%on”, $x ) ;
print f (“ Value in binary =%bn”, $x ) ;
print f (“ Value in hexadecimal=%xn”, $x ) ;
Value in decimal =98
Value in octal =142
Value in binary =1100010
Value in hexadecimal =62
Integers (Contd..)
18

Escape
Sequence
Description
b Backspace
e escape
f Form feed
l Forces the next letter into lowercase
L All following letters are lower case
u Forces the next letter into upper case
U All following letters are upper case
r Carriage Return
v Vertical Tab
Escaped Sequences
 Character strings that are enclosed in double quotes accept
escape sequences for special characters.
 The escape sequences consist of a backslash () followed by
one or more characters
19

Built-in functions
Function Description
chomp( ) The chomp() function will remove (usually) any newline
character from the end of a string.
The reason we say usually is that it actually removes any
character that matches the current value of $/ (the input
record separator), and $/ defaults to a newline.
Ex :chomp($text);
Chop( ) The chop() function will remove the last character of a string
(or group of strings) regardless of what that character is.
Ex:chop($text)
Chr () Returns the character represented by that number in the
character set
Ex: chr(65 ) gives A.
Ord() Returns the ASCII numeric value of the character specified by
expression.
Ex:ord(‘A) gives 65.
20

 List is a group of scalar used to initialize array or hash.
 The elements of a list can be numbers, strings or any other
types of scalar data.
 Each element of the Perl lists can be accessed by a numerical
index.
 The elements of a list are enclosed in a pair of round
parenthesis and are generally separated by commas.
Lists
21

Example :
$var = “welcome” # normal variable
$var2 = (12,24,”kacper”, $var,36.48) #first list
#first list contains 5 elements and two of are strings(‘kacper’, ’welcome’)
$var3 = (12,24,’Kacper’ ,’$var’ ,36.48) #second list
#second list contains 5 elements and two of are strings(‘kacper’, ’$var’)
Lists (Contd..)
22

 Flexible way of defining list is “qw (quote word)“ operator
which helps avoiding to too many quotation marks, but be
cautious if white spaces are there.
Example
print (“sachin” ,”dravid”, “ganguly”, “kumble” , “n”);
print qw(sachin dravid ganguly kumble”);
print “n”;
print (“sachin”, “dravid” , “ganguly”, ” anil kumble” ,”n”);
print( ‘k’, ’ a’, qw(c p e r ),’t’ , ‘e’, ‘c ‘ , ‘h’ );
print “n”;
Sachindravidgangulykumble
Sachindravidgangulykumble
Sachindravidgangulyanil kumble
Kacpertech
Lists (Contd..)
23

Lists (Contd..)
 Difference between list and array(or hash) is, array is a
variable that can be initialized with a list.
Range Operator
 Defined by symbol “..”
 Used to create a list from a range of letters or numbers.
Example :
Print (2 .. 4 “**” a .. d )
This program displays:
234**abcd
24

List functions
 A list is joined into a string using “join “ function.
Example:
Print join(“ “,(“perl” ,”is” ,”a”, ”scripting” ,”language”));
Perl is a scripting language
 A string is splited into a list using “split” function.
Example:
$var = "where-there-is a-will,there-is a-way";
print split( "-", ("$var"));
wherethereis awill,thereis away
Lists (Contd..)
25

List functions (Cont..)
 “map” evaluates expression or block for each element of
list.
Example:
Print join(“, ”,(map lc, A, B, C));
This program displays
a, b, c
 “grep” returns a sublist of a list for which a specific
criterion is true.
Example:
Print grep(!/x/ , a, b, x, d);
abd
Lists (Contd..)
26

Arrays
 An one dimensional ordered list of scalar variables.
 Array provides dynamic storage for a list ,and so can be
shrunk ,grown ,and manipulated by altering values.
 Represented using “@(at)” symbol.
 Array without a name is called a list.
 Elements of an array are accessed using the index number
(first element has index zero, next has one, and so on)
 Each element in an array is a scalar.
 “$#”array holds the last index value in the array.
27

Arrays (Contd..)
 ( ) represents the empty list.
Example:
@arr= (“perl” ,2 , 5.143 );
print “ @ arr”;
perl 2 5.143 #displays all the elements
Example:
@num = (1,2,3,4,5) ;
print “@num has “($#num + 1)” elements”;
This program displays
@num has 5 elements #Displays (last index number i.e, 4 + 1) which is
#the length of array
28

Example
@a =(a .. z) ;
@len1 =@a; #Assign the array to the variable
@len2 =scalar (@arr); #using scalar method
Print “ length of a =$len1 n” ;
Print “length of a =$len2”;
length of a = 26
length of a = 26
Example
@arr = ( “one “ ,2 ,”three” ,4.4);
$arr[2] = “kacper”; #second element(three) is replaced by
print “@arr”; new element(kacper)
one 2 kacper 4.4
Arrays (Contd..)
29

Array Methods
push
 Push function adds a value or values to the end of an array.
Example:
@num = (1,2,3,4,5) ;
Push (@num , 6) ; #pushes 6 at the end of array
Print @num ;
123456 #Displays all the elements of array.
pop
 Pop function gets a value or values from an array.
Example:
@num = (1,2,3,4,5) ;
Pop (@num) ; #Removes the last element of an array
Print @num ;
30

Array Methods (Contd..)
unshift
 unshift function adds a value or values at the start of an
array.
Example:
@num = (1,2,3,4,5) ;
unshift (@num , 6) ; #Adds 6 at the beginning of array
Print @num ;
31

shift
 shift function shifts off the first value of the array.
Example:
@num = (1,2,3,4,5) ;
$x = shift(@num) ; #Shifts the first element of an array
Print “$x” ;
2,3,4,5 #Displays the value stored in x.
32

map
 Array processing method converts one array to another.
 Syntax : map Expression(or Block) , list
 Runs an expression on each element of an array(like loop)
 Locally assigns $_ as an alias to the current array item.
Example:
@small = qw( “one” ,”two”, “three”) ;
@caps = map (uc ,@small); #uc returns an upper case version
print ( “@val”) ;
ONE TWO THREE #Displays in upper case
33

map (Contd..)
Example:
@num = (65, 66,67 ,68) ;
@num2 = map(2*$_ , @num); # multiplies each element by 2
@char = map(chr $_ ,@num); #chr returns the character represented by
that number
print “@num2 n @charn”;
130 132 134 136
A B C D
34

Array Slice
 Array slice is a section of an array.
Example:
@num = (1 ,2 ,3 ,4 ,5) ;
@val = @num[0 ,1]; #Array slice of first two element of @num
print join (“, ”, @val) ;
1 ,2 #Displays first two elements of @num.
35

Array Splice
 Array splicing means adding elements from a list to the
array.
Example:
@num = (1 ,2 ,3 ,4 ,5) ;
@val =(6 , 7);
splice(@num,4 ,0 , @val) ; #Adds the element of @val to @num
Print join(“ ,”,@num);
1 ,2 ,3 ,4 , 5 , 6 , 7 #Displays all the elements of @num after
#splicing.
36

sort
 Sorts the elements in the ASCII order.
 Defines the global variables $a and $b by default ,using
these we can specify our own sort.
Example:
@str = qw(sachin dravid ganguly kumble) ;
@val =( 56,13,45,11);
@str_sort 1 =sort ( @str);
@val_sort1 =sort (@val);
print “@str_sort 1 n”;
Print “@val_sort1 ”;
dravid ganguly kumble sachin
11 13 45 56
37

sort (Contd..)
Example:
#!usr /local/bin/perl
@str = qw(sachin dravid ganguly kumble) ;
@val =( 56,13,45,11);
@str_sort2 = sort{$a cmp $b}@str; #sorted in alphabetical order
@str_rev = sort{$b cmp $a}@str; # sorted in reverse order
@val_sort2 = sort{$a <=> $b}@val; # sorted in ascending order
@val_rev = sort{$b , < = > $a}@val; #sorted in descending order
print “@str_sort2 t @str_revn”;
print “@val_sort2 t val_rev n”;
dravid ganguly kumble sachin sachin kumble ganguly dravid
11 13 45 56 56 45 13 11
38

join
 Perl join function is used to concatenate the elements of an
array or a list into a string, using a separator given by a
scalar variable value.
 Syntax : $string = join (EXPR, LIST);
Example:
#!usr/local/bin/perl
@arr = ("mukesh“ ,"anil“ ,"prem“ ,"ratan");
$arr = join " t", @arr;
print "business Tycoons: $arrn";
print join "-CEOt", @arr, "n“;
business Tycoons: mukesh anil prem ratan
mukesh-CEO anil-CEO prem-CEO ratan-CEO
39

Array Reversal
 Reverse function is used to reverse the elements of an array.
Example:
@num = (1 ,2 ,3 ,4 ,5) ;
@rev =reverse @num ; #Reversing the elements of @num
Print join(“ ,”, @rev) ;
5 ,4 ,3 ,2 ,1 #Displays all the elements of @rev.
spilt
 Split() function is the opposite of join function.
 Syntax :LIST = split(/PATTERN/, EXPR, LIMIT)
40

 LIST – represents a list, array or hash that is returned by
the split function
 PATTERN – usually is a regular expression but could be a
single character or a string
 EXPR – is the string expression that will be split into an
array or a list.
 LIMIT is the maximum number of fields the EXPR will be
split into
Example:
!usr/local/bin/perl
$string = “gandhi-ind-nehru-ind-sastri-ind-kalam-ind”;
@colors = split('ind', $string); print @colors,"n";
gandhi--nehru--sastri--kalam-
41

Hashes
 An associative array ideal for handling attribute/value pair.
 Lists and arrays are ordered and accessed by index ,hashes
are ordered and accessed by specified key.
 Represented using “%” symbol.
 First element in each row is called a Key and the second
element is a Value associated with that key.
 Example : %coins = (“quarter”,25, “dime”,5); or
%coins = ( quarter => 25 , dime => 5);
Key Value
42

Hashes (Contd..)
 Hah values can be any scalar ,just like an array ,but hash
keys can only be strings.
Example: Printing the hash.
%hash1 = ( one => 1 ,two => 2 ,three =>3 ,four =>4);
print %hash1; #we cant use print “%hash1”;
print “@{[hash1]} n”;
@temp = %hash1;
Print “@temp”;
three3one1two2four4
three 3 one 1 two 2 four 4
three 3 one 1 two 2 four 4
 The print order determined by how the Perl chooses to store
internally.
43

Hashes (Contd..)
 Hash can have only scalars as values.
 “{ }” are used to access individual elements of the hash.
Example:
$ele = $hash1(‘three’); #single key, use scalar
@mul_ele = @hash1(‘four’ ,’one’); #multiple key ,use array
print “single element =$ele “;
print “multiple elements =@mul_ele”;
single element =3
multiple elements = 4 1
44

Hashes (Contd..)
 “keys” function can be used to find the no. of keys and list of
entries in a hash.
 “values ” function can be used to find the no. of values list of
values in a hash.
Example:
$ele = $hash1(‘three’); #single key, use scalar
@mul_ele = @hash1(‘four’ ,’one’); #multiple key ,use array
print “single element =$ele “;
print “multiple elements =@mul_ele”;
single element =3
multiple elements = 4 1
45

Manipulating Hashes
 To add or change the value key we can do like this
$hash1{ ‘three’ } = ‘PERL’ .
 It will overwrite the previous value if already existing.
Otherwise it is added as a new key.
 “undef “ function is used to remove the value of the key, but
key will still exists.
Example: undef $hash1{‘ two’} ;
 “delete” function is used to remove the value as well as key
from the hash.
Example :delete $hash1 {‘four ‘};
46

Hash Sorting
 Hashes are not ordered and we must not rely on the order in
which we added the hash items – Perl uses internally its own
way to store the items.
 We can sort hashes either by key or value ,using sort
function.
Example: Sort by key
%data = ( sachin => 10,
dravid => 19,
dhoni => 7,
rohit => 45 );
foreach $key(sort (keys(%data))) {
print “t$key t $data{$key}”;}
dhoni 7 dravid 19 rohit 45 sachin 10
47

Hash Sorting (contd..)
 Sort function returns least (or greatest) element among all
elements in the first iteration.
Example: Sort by value
%data = ( sachin => 10,
dravid => 19,
dhoni => 7,
rohit => 45 );
foreach $key (sort{$data {$a} <= > $data{$b}} keys %data) {
print “t $key tt $data{$key} n“; }
dhoni 7 sachin 10 dravid 19 rohit 45
 In the above example first values are compared(using
sort{$data {$a} <= > $data{$b}} ), found least value and that
is assigned to key (using keys% data) in every iteration
48

Operators
 Operators can be broadly divided into 4 types.
 Unary operator which takes one operand.
Example: not operator i.e. !
 Binary operator which take two operands
Example: addition operator i.e. +
 Ternary operator which take three operands.
Example: conditional operator i.e. ?:
 List operator which take list operands
Example: print operator
49

Arithmetic Operators
Operator Description
+ Adds two numbers
- Subtracts two numbers
* Multiplies two numbers
/ Divides two numbers
++ Increments by one.(same like C)
-- Decrements by one
% Gives the remainder (10%2 gives five)
** Gives the power of the number.
Print 2**5 ; #prints 32.
50

Shift Operators
 shift operators manipulate integer values as binary numbers,
shifting their bits one to the left and one to the right
respectively.
<< Left Shift
Print 2 >>3 ; left shift by three positions, prints 8
>> Right Shift
Print 42 >>2; #right shift by two positions, prints 10
x Repetition Operator.
Ex: print “ hi ” x 3;
Output : hihihi
Ex2: @array = (1, 2, 3) x 3; #array
contains(1,2,3,1,2,3,1,2,3)
Ex3 :@arr =(2)x80 #80 element array of value 2
51

Logical Operators
 Logical operators represented by either symbols or names.
These two sets are identical in operation, but have different
precedence.
 The “!” operator has a much higher precedence than even
“&& “and “|| “.
 The “not”, “and”, “or”, and “xor” operators have the lowest
precedence of all Perl's operators, with “not” being the
highest of the four
&& or AND Return True if operands are both True
|| or OR Return True if either operand is True
XOR Return True if only one operand is True
! or NOT (Unary) Return True of operand is False
52

Bitwise Operators
 Bitwise operators treat their operands as binary values and
perform a logical operation between the corresponding bits of
each value.
& Bitwise AND
| Bitwise OR
^ Bitwise XOR
~ Bitwise NOT
53

Comparison Operators
 The comparison operators are binary, returning a value
based on a comparison of the expression
< Lessthan
> Greaterthan
== Equality
<= Lessthan or equal
>= Greaterthan or equal
<= > It does not return a Boolean value. It returns
-1 if left is less than right
0 if left is equal to right
1 if left is greater than right
!= Inequality operator
54

Comparison Operators on strings
String Description
eq Return True if operands are equal
le Return True if left operand is less than right
ge Return True if left operand is greater or equal to right
gt Return True if left operand is less than or equal to right
gt Return True if left operand is greater than right
cmp It does not return a Boolean value. It returns
-1 if left is less than right
0 if left is equal to right
1 if left is greater than right
ne Return True if operands are not equal
.(dot) Concatenation operator. It takes two strings and joins
them
Ex: print “System” .”Verilog”
It prints SystemVerilog.
55

Binding operator
 The binding operator ,=~ ,binds a scalar expression into a
pattern match.
 String operations like s/// ,m//,tr/// work with $_ by default.
 By using these operators you can work on scalar variable
other than “$_ .”
 The value returned from =~ is the return value of the regular
expression function, returns undef if match failed.
 The !~ operator performs a logical negation of the returned
value for conditional expressions, that is 1 for failure and
'' “ for success in both scalar and list contexts.
56

Conditional Statements
if Statement
if keyword to execute a statement block based on the
evaluation of an expression or to choose between executing one
of two statement blocks based on the evaluation of an
expression
Example :-
$firstVar = 2;
if ($var == 1) { print “we are in first if n“; }
elsif( $var ==2) { print “we are in second if n“; }
else { print “we are in third ifn”; }
we are in second if
57

Conditional Statements (Contd..)
until Loops
Until loops are used to repeat a block of statements while some
condition is false.
Example :- do … until loop
$firstVar = 10;
do {
print("inside: firstVar = $firstVarn");
$firstVar++;
} until ($firstVar < 2);
print("outside: firstVar = $firstVarn");
58

do-until Loops
Example :- until loop
$firstVar = 10;
until ($firstVar < 20) {
$firstVar++;
};
print("outside: firstVar = $firstVarn");
outside: firstVar = 10
59

for Loops
Example : - for loops
for ($firstVar = 0; $firstVar < 100; $firstVar++)
{
}
This program will display:
inside: firstVar = 0
...
60

foreach Loops
The foreach statement is used solely to iterate over the
elements of an array. It is very handy for finding the largest
element, printing the elements, or simply seeing if a given value
is a member of an array.
Example :- foreach loop
@array = (1..5, 5..10);
print("@arrayn");
foreach (@array) { $_ = “ ** " if ($_ == 5); }
print("@arrayn");
1 2 3 4 5 5 6 7 8 9 10
1 2 3 4 ** ** 6 7 8 9 10
61

Jump Keywords
The last Keyword
The last keyword is used to exit from a statement block.
Example :- last
@array = ("A".."Z");
for ($index = 0; $index < @array; $index++) {
if ($array[$index] eq "T") {
{ last } }
print("$indexn");
19
62

Jump Keywords (Cont..)
The next Keyword
The next keyword use to skip the rest of the statement block
and start the next iteration.
Example : - next keyword
@array = (0..9);
print("@arrayn");
for ($index = 0; $index < @array; $index++) {
if ($index == 3 || $index == 5) {
next; } $array[$index] = "*";
} print("@arrayn");
0 1 2 3 4 5 6 7 8 9
* * * 3 * 5 * * * *
63

Jump Keywords (Cont..)
The redo Keyword
The redo keyword causes Perl to restart the current statement
block.
Example :- redo
print("What is your name? ");
$name = <STDIN>;
chop($name);
if (! length($name)) {
print("Msg: Zero length input. Please try againn");
redo;
} print("Thank you, " . uc($name) . "n"); }
64

Regular Expressions
 Regular expression(regexps) is simply a string that describes
the pattern (example for pattern ,to find files in a directory
which ends with “.sv” i.e. ls *.sv )
 Used for finding and extracting patterns within the text.
 The role of regexp engine is to take a search pattern and
apply it to the supplied text.
 The following operators use regular expressions.
 Matching Operator (m//)
 Substitution Operator(s///)
 Transliteration(Translation) Operator(tr///)
65

The Matching Operator (m//)
 The matching operator (m//) is used to find patterns in
strings.
Example :
$_ = “success is a progressive journey”;
$var = “success is not a destination”;
If( /success/) { # the initial m is optional
print “String success Found”; }
If ( $var =~ /destination/) { print “String destination Found”; }
String success Found
String destination Found
66

 When regular expression is enclosed in slashes(/success/), $_
is tested against the regular expression ,returning TRUE
if there is a match , false otherwise
Finding multiple matches
Example :
$txt =“winn-ers see ga-in, lose-rs see pa-in”
while ($txt =~ m/-/g) {
print “ Found another -n” ;}
found another -
found another -
found another -
The Matching Operator (Contd..)
67

The Substitution Operator (s///)
 The substitution operator (s///) is used to change strings.
 Syntax :LVALUE =~ s/PATTERN/REPLACEMENT/
 The return value of an s/// operation (in scalar and list
contexts alike) is the number of times it succeeded (which
can be more than once if used with the /g modifier).
 On failure, since it substituted zero times, it returns false
(""), which is numerically equivalent to 0.
 If PATTERN is a null string, the last successfully executed
regular expression is used instead
68

Example :
$text = “winners see gain, losers see pain”;
$test =~s/winners/WINNERS/;
print $text;
WINNERS see gain, losers see pain
Example :
@arr = qw(sachin dravid ganguly sachin);
foreach(@arr){ # “for(@arr)” and “s/sachin/10/g for @arr”
s/sachin/10/; } do same thing
print "n@arr“;
10 dravid ganguly 10
The Substitution Operator
(Contd..)
69

Example :
@old = qw(sachin-bharat dravid-bharat ganguly-bharat kumble-ind);
for (@new = @old) { s/bharat/india/ }
print "@oldsn“;
print "@newn";
sachin-bharat dravid-bharat ganguly-bharat kumble-ind
sachin-india dravid-india ganguly-india kumble-ind
The Substitution Operator
(Contd..)
70

Using Modifiers with m// and s///
Modifier Description
g(m//g or
s///g)
Works globally to perform all possible
operations.
i Ignores alphabetic case
x Ignores white space in pattern and allows
comments.
gc Doesn’t reset the search position after a failed
match
s Lets the . Character match newlines.
m Lets ^ and $ match embedded n characters
e Evaluate right hand side as an expression
o Compiles the pattern only once
71

The Translation Operator (tr///)
 Syntax :
LVALUE =~ tr/SEARCHLIST/REPLACEMENTLIST/
 It scans a string, character by character, and replaces each
occurrence of a character found in SEARCHLIST (which is
not a regular expression)with the corresponding character
from REPLACEMENTLIST
 It returns the number of characters replaced or deleted.
 If no string is specified via the =~ or !~ operator, the $_
string is altered.
72

Example :
$count = ($test =~tr/e/E/); print $text;
Print “n no..of replacements =$count”;
winnErs sEE gain,losErs sEE pain
no..of replacements =6
The Translation Operator
(Contd..)
Modifier Description
c (tr///c) Complements the search list.
d Deletes unreplaced characters
s Deletes duplicate replaced characters
73

Example :
$count = ($test =~tr/e/E/c);
print $text; #except “e” all other characters
Print “n no..of replacements =$count”; # are replaced.
EEEEeEEEEeeEEEEEEEEEeEEEEeeEEEEE
no..of replacements =26
The Translation Operator
(Contd..)
74

Different Pattern Delimiters
 If the pattern contains lots of slash characters(/) ,we can also
use different pattern delimiter with the pattern.
Example :
$var = "winners / see / gain,losers / see pain";
If( $var =~ m|see|) { # match with pipes
print “String see Foundn”; }
If ( $var =~ m ?gain?) { #match with question marks.
print “String gain Found”; }
String see Found
String gain Found
 Perl also allows paired characters like braces and
brackets.viz { } ,( ), < >,[ ].
Ex: $var =~s{gain}{GAIN};
75

The Parts of regular Expressions
 In general regular expression can be made up of following
parts.
 Characters
 Character Classes
 Alternative Match Patterns
 Quantifiers
 Assertions
76

Characters
 In regular expression any single character matches itself,
unless it is a metacharacters with special meaning.
 Beside normal characters, Perl defines special characters
that you can use in regular expression.
 These character must start with backslash.(Otherwise Perl
treats it as a normal character).
77

Characters (Contd..)
character Description
d It is equivalent to [0 - 9]
Matches any digit.
Ex1 : $var =~ /d/ # Will match any digit.
D It is equivalent to [^0 - 9]
Matches any non-digit.
Ex1 : $var =~ /D/ # Will match any non-digit.
78

w It is equivalent to [0-9a-zA-Z_]
Matches a word character allowable in Perl
variable name.
i.e. Match any 'word' or alphanumeric character,
which is the set of all upper and lower case
letters, the numbers 0..9 and the underscore
character _
Ex :if ( $var =~ /w/)
W It is equivalent to [^0-9a-zA-Z_]
Matches any non-word characters. Inverse of w
Ex :if ($var =~ /W)
79

s It is equivalent to [ tnr]
Matches any white space character.
i.e. a space ,a tab ,a newline ,a return
Ex :if ($var =~ /s/)
S It is equivalent to [^ tnr]
Matches any non-white space character.
Ex :if ($var =~ /S/)
80

Q Quote(disable) pattern metacharacters until E found.
Ex:#usr/local/bin/perl
$var = “success is not a *”;
If($var =~/*/){ print “found in 1st if” ; }
If($var ~=/Q*E) { print “found in 2nd if “ ; }
It will display :
found in 2nd if
E End case modification.
End case modification
81

U Change the following characters to upper case until
a E sequence is encountered.
Ex:$var = “SUCCESS is not a *” ;
If($var =~/success/){ print “found in 1st if” ; }
If($var ~=/UsuccessE) { print “found in 2nd if “ ; }
It will display:
found in 2nd if
L Change the following characters to lower case until
a E sequence is encountered. Same like U like U
82

u Change the next character to uppercase.
Ex:#usr/local/bin/perl
$var = “SUCCESS is not a *”;
If($var =~/us/){ print “found only s ” ; }
If($var ~=/usu/) { print “found su“ ; }
It will display :
found only s
l Change the next character to lower case
83

Character Classes
 A character class allows a set of possible characters, rather
than just a single character, to match at a particular point in
a regular expression.
 Character classes are denoted by brackets [...], with the set of
characters to be possibly matched inside.
 Matches one occurrence of any character inside the bracket
Ex 1: $var =~ /w[aoi]nder/ # will match “wander”, ”wonder”,
”winder”
84

Character Classes (Contd..)
 If you use ^ as the first character in a(if you use ^ outside
the character class[ ] it works as anchor) character class,
then that character class matches any character not in the
class.
Ex1:$var = ~/w[^aoi]nder
# will look for w followed by something that is
# none of “a “or “o” or “ i”.
85

Alternative Match Patterns
 Alternative Match Pattern means that you can specify a
series of alternatives for a pattern using | to separate them.
 |(called alternation) is equivalent to an “or” in regular
expression. It is used to give a chance.
Ex: $var2 =~ /hope|trust/ # will match either “hope” or “trust
 Alternatives are checked from left to right, so the first
alternative that matches is the one that’s used.
86

Grouping Alternatives
 Grouping[ “( ) “] allows parts of a regular expression to be
treated as a single unit.
 Parts of a regular expression are grouped by enclosing them
in parentheses.
 Used to group similar terms by their common characters and
only specified the differences.
Example : $var2 =~ /(while |for)loop/
# will match either “while loop” or “for loop”
87

Grouping Alternatives (Contd..)
 The pairs of parentheses are numbered from left to right by
the positions of the left parentheses.
 Perl places the text that is matched by the regular expression
in the first pair of parentheses into the variable $1, and the
text matched by the regular expression in the second pair of
parentheses into $2,and so on.
Example :
my $text= "Testing";
if ($text =~ /((T|N)est(ing|er))/) {
print " $1 = $1 t $2 = $2 t $3 = $3 n ";
$1 = Testing $2 = T $3 = ing
88

Grouping Alternatives (Contd..)
 There are three pairs of parentheses in the above example.
 The first one is that which surrounds the whole regular
expression, hence $1 evaluates to the whole matched text,
which is “Testing”.
 The match caused by the second pair of parentheses (T|N),
which is “T”, is assigned to $2.
 The third pair of parentheses (ing|er) causes $3 to be
assigned the value “ing”.
89

Quantifiers
 Quantifiers says how many times something may match,
instead of the default of matching just once.
 You can use quantifier to specify that a pattern must match
a specific number of times.
 Quantifiers in a regular expression are like loops in a
program.
90

Quantifiers (Contd..)
* It indicates that the string Immediately to the left
should be matched zero or more times in order to be
evaluated as a true.
Ex1 : $var =~ /st*/ # Will match for the strings like
“st”, ”sttr”, “sts ”, “star”, “son “….
The regexp “a*” will search for a followed by either
“a” or any other character. It matches all strings
which contain the character “a”.
+ It indicates that the string Immediately to the left
should be matched one or more times in order to be
Ex:$var =~ /st*/ # Will match for the strings like “st”,
”sttr”, “sts” ,”star “, but not “son”.
91

{ } It indicates that how many times the string
immediately to the left should be matched.
{n}  should match exactly n times.
{n,} should match at least n times
{n, m} Should match at least n times but not more
than m times.
Ex : $var =~ /mn{2,4}p/ # will match “mnnp”,
“mnnnp”, ”mnnnnp” .
? It indicates that the string Immediately to the left
should be matched zero or one times in order to be
Ex : $var =~ /st?r/ # will match either “star” or
“sttr”.
$var = ~/comm?a/ # will match either “coma” or
“comma”
92

.(period) Used to match any single character except newline
character
Ex :$var1 = ~ /r.n/ # will match “run” , “ran”, “ ron “…
93

 Quantifiers are greedy by default, which means they
will try to match as much as they can.
Ex :$str =“they are players, aren’t they ? “
$str1 = “they are players, weren’t they ?
$str =~s/.*are/were/;
$str1 =~ s/are.*/
print “$strn”; print “$str1”
Str will print : weren’t they ?
Str1 will print : they
 Perl will use the .* Preceding “are” to match all the
characters upto the last “are” in the str.
94

Making Quantifiers Less Greedy
 To make Quantifiers less greedy –that is ,to match the
minimum number of times possible –you follow the
quantifier with a ?
 *?  Matches zero or more times.
 +?  Matches one or more times.
 ??  Matches zero or one times.
 {n}?  Matches n times.
 {n,}?  Matches at least n times
 {m,n}  Matches at least n times but more than m
times.
95

Example :
$text = “They are players ,aren’t they?”;
$text =~s/.*?are/were/;
print $text;
Were players ,aren’t they?
Example :
$txt = “no, these are the documents, over there.”;
$txt = ~ s/the(.*?)e/those/;
print $txt;
no, those are the documents, over there
Making Quantifiers Less Greedy
(Contd.)
96

Metacharacters
 A metacharacter is a character or sequence of characters that
has special meaning.
 Perl recognize the same 12 traditional metacharacters
| ( ) [ { ^ $ * + ? .
 If you want to literally match a metacharacter, you must
escape it with a backslash.
 The regex /Mr./ matches anything that contains ``Mr''
followed by another character. If you only want to match a
string that actually contains ``Mr.,'' you must use /Mr./.
97

Example :
$text = “#25 $finish”;
$text =~ s/#25///#25/;
print $text;
//#25
Example :
$text = “//#25 $finish”;
$text =~ s///#25/#25/;
print $text;
#25
Metacharacters (Contd..)
98

Assertions
 Assertions (also called anchors) used to match conditions
within a string, not actual data.
 Assertions are zero width because they do not consume any
characters when they match.
Anchor Description
^(caret) Appears at the beginning of the pattern and finds for a
match at beginning of the line
Ex : $var =~ /^su/ # Will match the strings those are
starting with “su “ i.e. . “sun”, “success”, “super “..
99

Assertions (Contd..)
$ Appears at the end of the pattern and finds for a match
at end of the line
Ex : $var =~ /at$/ # Will match the strings those ends
with “at” i.e. . “cat”, “rat “, “beat”…
Z Matches only at the end of a string, or before a new
line at the end.
It matches at the end of the match text, before the
newline if any is present. Otherwise it is the same as z.
z Matches only at the end of string.
100

 The difference between “^” and “A” is that when you use the
m –multiline-modifier, “^ “ matches the beginning of every
line, but A retains its original meaning and matches only at
the very beginning of the whole string.
Character Description
A Matches only at the beginning of a string.(Similar to ^)
G It applies when we use a regular expression to
produce multiple matches using the g pattern
modifier.
It re-anchors the regular expression at the end of the
previous match, so that previously matched text takes
no part in further matches.
Works only with “/g” .
101

Word Boundaries
 b matches on a word boundary.
 This occurs whenever a word character is adjacent to a non-
word character .
 It is equivalent to “wW|Ww” .
 Within character classes b represents backspace rather
than a word boundary, just as it normally does in any double-
quoted string.
102

Word Boundaries
 B matches on a non-word boundary .
 This occurs whenever two word characters or two non-word
characters fall adjacent to each other.
 It is equivalent to “ww|WW”.
103

Word Boundaries
Example :
$text = "one, ****, three, four"; # “*” is not a word
$text1 = "one,****,three, four"; # character
foreach ($text =~ /bw+b/g){
print $_, "t"; }
print "n using Bn";
foreach ($text1 =~ /Bw+B/g){
print $_, "t"; }
one three four
using B
n hre ou
104

Regular Expressions-Examples(1)
$var =~m/^./ Will match for dot(.) at the beginning of the
statement.
“^” Used to match at the beginning of the line ,dot
is a meta character so it has to preceded by “”
$var =~ /w+/ Will match a word, a nonempty sequence of
alphanumeric characters and underscores such as
“trust “ , “12bar8” and “kac_per”.
$var
=~/starts*end/
The strings “start” and “end” optionally separated
by any amount of white space (spaces, tabs,
newlines).
105

$var =~/o.m/ It will match exactly “o.m”
$var =~
/blue(colo(ur|r))/
Will match either “bluecolor “or “bluecolour”
$var =~ s/s+$// Removes(trims) the Trailing white space.
106

$var =~
s/^s+//
Removes(trims) the leading white space.
Ex:$txt = “ trust in god”;
$txt ==~ s/^s+//
Print $txt;
It will print : trust in god
$var =~ m/
(d+)/
Will match complete first number.
Ex :$txt = "day = 86400s or 1440 mor 24 h";
if($txt =~ m/(d+)/){
print "nnFirst Number is $1"; }
It will print :
First Number is 86400
107

Subroutines
 Subroutine is a separate body of code designed to perform a
particular task. It is same as function in C language.
 The Idea behind subroutines is that old programming dictum
–divide and conquer.
 Subroutines allow you to divide your code into manageable
parts, which makes overall programming easier to handle.
 Perl allows you to create subroutines using the “sub” control
structure
108

Example :
$v1 =36;
large_small(); #subroutine call before definition , parentheses must
sub large_small{
if($v1 >40) { print "value is bigger than 40n"; }
else{ print "value is smaller than 40n"; } }
$v1 =45;
large_small; # subroutine call after definition , parentheses are
# optional
value is smaller than 40
value is bigger than 40
Subroutines (Contd..)
109

Scope of variable
 Perl variables have global package scope by default.
 When we change a variable value in subroutine, we’ll be
changing it in the rest of the program by mistake as well.
 We can create a variables that are entirely local to
subroutine by using keyword “local” or “my”.
 They can have same name as global variable and not affect
those global variables at all.
110

Example :
$v1 =36;
incr();
print "value of v1 =$v1n";
sub incr {
my $var =$v1;
print "value before incrementing =$varn";
$var++;
print "value after incrementing =$varn";
}
value before incrementing =36
value after incrementing =37
value of v1 =36
Scope of variable (Contd..)
111

Parameters and Arguments
 You can pass values to subroutine by placing in parentheses.
(ex: incr($v1);)
 When you pass values to a subroutine ,those values are
stored in a special array named “@_”.
 Beside accepting passed values, subroutines can also return
values using “return” keyword.
112

Example :
$sum =add(10,20);
print "sum =$sumn“;
sub add {
($val1, $val2) = @_;
return $val1+$val2; }
sum =30
 Perl returns the last value in a sub routine so you can omit
the return keyword .
 In the above example $val1+$val2 gives the same.
(Contd..)
113

(Contd..)
 Different ways of reading arguments passed to subroutine
 Sub add
{
$val1 =$_[0];
$val2 =$_[1];
}
 Sub add
{
$val1 =shift@_ ; (or) shift ;
$val2 =shift@_ ; (or) shift ;
}
In a subroutine ,shift uses @_ by default so you can use
shift directly.
114

Recursion
 Recursion happens when a subroutine calls itself, either
directly, or indirectly, via another subroutine
Example :
#!usr/local/bin/perl0
$fact = fact(6);
print "factorial of given number =$factn“;
sub fact {
local $val =shift(@_);
if($var ==0) { return 1; }
elsif($val==1) { return $val ; }
else{ return $val*fact($val-1); } }
factorial of given number =720
115

Passing Lists
Example :
@small= qw(sachin dravid ganguly );
@big =case_convert(@small);
print "@big";
sub case_convert {
@low =@_;
@caps =map(uc ,@low); }
SACHIN DRAVID GANGULY
Subroutines –Examples(1)
116

Nested subroutines
Example :
call();
sub call{
display();
sub display {
print "you are in inner subroutinen" ; }
print "you are in outer subroutinen" ; }
print "you are in mainn" ;
you are in inner subroutine
you are in outer subroutine
you are in main
Subroutines –Examples(2)
117

 In general passing arrays or hashes flattens their elements
into one long list, so it’s a problem if you want to send two or
more distinct arrays or hashes.
 To preserve integrity, you can pass references to arrays or
hashes instead.
 References can be created by using a backslash () operator
on a variable. It is similar to address-of (&)operator in C.
 $var Scalar Reference
 @arr Array Reference
 %hash Hash Reference
Pass by Reference
118

Pass by Reference (Contd..)
 If you pass $a (a reference to the $a scalar variable) to a
subroutine, then in the subroutine the variable that receives
that parameter receives a “reference “ (or a "pointer")
pointing to the $a scalar
 Dereferencing references(Using prefixes $, @ ,% ,->)
 $$ref_var Scalar Dereference
 $$ref_arr Array Dereference #array is copied
into scalar( ref_arr)
 $$ref_hash Hash Dereference
119

Example :
@arr = qw(America England france);
print "before: arr = " . join(', ', @arr) . "n";
change (@arr);
print "after: arr = " . join(', ', @arr) . "n“;
sub change {
my $ref_arr =shift;
$$ref_arr[0]= "China";
@{$ref_arr}[1] ="India"; # ”{ }” creates a block.
$ref_arr ->[2] ="Japan"; # “ -> “ called arrow operator.
}
before: arr = America, England, france
after: arr = China, India, Japan
Pass by Reference (Contd..)
120

Returning by Reference
 If you return two arrays normally, their values are flattened
into one long list. If you return references to arrays, you can
deference those arrays and reach the original arrays.
Example :
sub get_strings{
@str1 = qw(Asia Austrelia Africa);
@str2 =qw(Brown White Black);
return @str1 ,@str2; }
($ref_str1 ,$ref_str2) =get_strings;
print "@$ref_str1 n"; print "@$ref_str2 n";
Asia Austrelia Africa
Brown White Black
 You can get lot of material on references that comes with
Perl( perldoc perl)
121

File Handling
 A filehandle is nothing more than a nickname for the files
you intend to use in your PERL scripts and programs.
 Filehandles are a connection between our program and an
external data source
 Filehandles in Perl are a distinct data type.
 STDIN or standard input represents the default input
filehandle and usually connected to the keyboard.
 STDOUT or Standard output represents the default output
filehandle and usually connected to the console
device(screen)
122

File Handling (Contd..)
 STDERR or Standard error is the default output error
filehandle and usually connected to screen.
Opening a file
 To open a file ,use the “open “ function.
 Syntax : open FILEHANDLE ,MODE,LIST
open FILEHANDLE ,EXPR
open FILEHANDLE
 The “open “ function takes a filename and creates the handle
for it.
123

Opening a file (Contd..)
 The open function returns a true(nonzero) value if successful
otherwise it returns undefined value.
 The filehandle will create in either case but if the call to
“open” fails, the filehandle will be unopened and unassigned.
 If the “open” fails the reason is stored in special variable
“$!” ,which produces a message in string context.
 File handling is most error prone ,so use “open and “die”
together.
Ex: open (HANDLE, $filename) or die "Can't open $filename:
$!n";
124

 “open” understands total six modes.
MODE Symbol Description
Read < Open file handle for read access only.
Ex :open FILHND “<$file”;
This is the default mode and so the < prefix is
usually optional
Write > Open the file for write access only.
Ex :open FILHND “>$file”;
If the file doesn’t exist then it is created and
opened.
If the file does exist then it overwrite the existing
contents
125

Append >> Open the file for write access only.
Ex :open FILHND “>>$file”;
opened.
If the does exists then it appends that file.
Read-
update
+< Open the file for read and write access.
Ex :open FILHND “+<$file”;
If the file does not exist then the open fails.
If the file does exist then it overwrite(contents are
preserved for reading) the existing contents.
126

Write-
update
>+ Open the file for read and write access.
Ex :open FILHND “>+$file”;
If the file doesn’t exist then it is created.
If the file does exist then it is truncated and its
existing contents are lost.(usually used for
opening a new file)
Append-
update
>>+ Open the file for read and write access only.
Ex :open FILHND “>>+$file”;
opened.
If the file does exist then both read and write
commence from the end of the file.
127

Reading Lines
Example :
exam.txt :: winners dont do different things.
winners do things differently..
success is not a destination.
Perl Script:
open FILE, "exam.txt" or die $!;
$lineno;
while(<FILE>){ print $lineno++," t"; print "$_"; }
0 winners dont do different things.
1 winners do things differently.
2 success is not a destination.
128

 “< >” is called readline or diamond operator.
 In the above example “while (<FILE>)” is equivalent to
“while(defined($_=<FILE>)”.
 Above statement will reads a line from file and assigns it to
“$_” and checks whether it is defined or not.
 If it is not defined ,probably at the end of the file so it will
comes out of the loop.
129

 Perl provides a special handle called “ARGV”.
 It reads the files from the command line and opens them all
if specified.
 It will read from Standard input(STDIN) if nothing is
specified on the command line.
 If you don't specify anything in the angle brackets,
whatever is in @ARGV is used instead.
130

Command line passing
Example :
exam.txt :: winners dont do different things.
Perl Script:
$match =“do”;
while(<FILE>){
If(/$match/) { print “FOUNDn”; }
else { print”NOT FOUND”; } }
FOUND #In first line It finds match
NOT FOUND #In second line it doesn’t find match
File Handling –Examples(1)
131

Substituting the source file contents
Example :
exam.txt :: winners do not do different things.
Perl Script:
While ($line =<FILE>){
$line =~ s/not/NOT/;
push (@out_line ,$line); }
close(FILE);
open OUTFILE ,“>exam.txt" or die $!; #opening the same for
writing
print (OUTFILE @out_line);
close (OUTFILE);
132

Substituting the source file contents
 Source file contents now changed.
 exam.txt will contain modified (below)data
exam.txt :: winners do NOT do different things.
success is NOT a destination.
File Handling –Examples(2) –
contd..
133

Example :
#!/usr/local/bin/perl
print "Filename: "; my $infile = <>; chomp $infile;
print "New name: ";
my $outfile = <>; chomp $outfile;
open IN, $infile;
open OUT, "> $outfile";
print OUT <IN>;
close IN; # syntax :: close <filehandle>
close OUT;
perl filename.pl exam.txt
Filename: exam.txt
New name:copy_exam.txt #it will create this file.
134

Example :-
if (open (LOGFILE, ">>message .log"))
{
print LOGFILE ("This is message number 3.n");
print LOGFILE ("This is message number 4.n");
close (LOGFILE); #! close function
}
This is message number 1.
This is message number 2
135

print, printf, and write Functions
 “print” function writes to the file specified, or to the current
default file if no file is specified.
Ex: print ("Hello, there!n");
print OUTFILE ("Hello, there!n");
 “write” function uses a print format to send formatted
output to the file that is specified or to the current default
file.
Ex : write (CD_REPORT);
136

 You can use Linux commands in Perl.
 Below example explains the usage.(It will execute the
run.csh by traversing each directory)
Example :
#!/usr/local/bin/perl
$com2= `find . -type d -name "^UPF*"| tee list_new`; # It will find all UPF
directories
print"$com2n"; #It willl execute the above command
open LIST ,"list_new" or die $!;
foreach $i (<LIST>){
print "directry name >>>>>$i";
chomp ($i); # list will contain all UPF directories.
chdir ("$i"); # chdir is a Perl function ,It will return one on successful
exectition
Importing Linux commands in
Perl
137

Example :
$exe=`csh run.csh |tee run.log`; # It will execute the run.csh
print"$exen";
chdir ("..")
}
close (LIST)
Importing Linux commands in
Perl(contd..)
138

Directories Handling
 “print” function writes to the file specified, or to the current
default file if no file is specified.
Ex: print ("Hello, there!n");
print OUTFILE ("Hello, there!n");
 “write” function uses a print format to send formatted
output to the file that is specified or to the current default
file.
Ex : write (CD_REPORT);
139

Directories Handling (Contd..)
 To create a new directory, call the function “mkdir”.
 Syntax :mkdir (dirname, permissions);
Ex: mkdir ("/u/public /newdir ", 0777);
 To set a directory to be the current working directory, use
the function “chdir”.
 Syntax: chdir (dirname);
Ex :chdir ("/u/public/newdir");
140

Directories Handling (Contd..)
 To open the directory (already existing) ,use the function
“opendir”
 Syntax : opendir (dirvar, dirname);
Ex: opendir (DIR, "/u/kacper/mydir");
 To close an opened directory, use the “closedir” function
 Syntax: chdir (dirname);
Ex : closedir (mydir);
141

Perl Basics with Examples

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