perl

PERL on Unix/Linux

Practical Extraction and Reporting Language

§ Introduction

§ Scalar Variables and Lists

§ Arrays and Hashes

§ Operators and Precedence

§ Conditional statements and Loops

§ Regular Expressions

§ Subroutines

§ File and Directory Handling

Contents

History

§ Developed by Larry Wall in 1987

§ Derives from the ubiquitous C programming languageand to a lesser extent from sed, awk, the Unix Shell.

§ PERL was originally designed under Unix, but now alsoruns under all OS(Including Windows).

IntroductionWhat 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.

Beginning with Perl§ perldoc perl gives the list of manual pages as a part of

every Perl installation.

§ perldoc –h gives the brief summary of options available.

§ perl –v gives the version of the Perl the user is using.

§ To create a Perl program, only a text editor and the perlinterpreter are required.

§ Perl file ends with .pl (simple.pl)

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 andpasses the name of the script to it as an argument.

§ “#! /usr/bin/perl” or “#!/usr/local/bin/perl” is the commandused to run the Perl Interpreter.

§ So to start a script we need to add above line as the first lineto make Perl script executable.

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 thesource code, including any modules used, is convertedinto bytecode.

§ a run-time phase where the bytecode is processed intomachine instructions and executed.

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

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.

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 –cPrint “welcome to perl”;

This Program displays:Syntax ok

Simple Programs

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 thecommand line

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.

Example#! /usr/local/bin/perl –wprint “Enter the Text”;$input = <STDIN> ; #Reads the input and stores in the variable inputChomp(); #will remove new line character.Print “entered text =$input” ; #Prints the input on the command line\

This Program displays:Enter the TextPerl is awesome #Perl will read this “Perl is awesome\n”, by

default it will add \n character to your entered text. So use “chomp”

entered text =Perl is awesome

Standard Files (Contd..)

Variables§ Variables are used to refer data which is held as value.

§ Perl defines three basic data types: scalars, arrays, andhashes.

Scalars :§ Holds a single value it may be a string ,number or reference.

§ Begin with “$”, followed by a letter then by letters, digits orunderscores.

Example : $var =1 # integer$var = “Hello_world” # string$var=2.65 # Decimal number$3var = 123 #Error, Shouldn’t start with

#number

§ Interpolation takes place only in double quotation marks.

Example#! /usr/local/bin/perl –w$x = 12 ; #Assign the value to the variableprint ‘ Value of x is $x ’ ; #Prints the output

This Program displays:Value of x is $x #Single quotation will not interpolate

#(no processing is done) the values

Example#! /usr/local/bin/perl –w$x = 12 ; #Assign the value to the variableprint “Value of x is $x” ; #Prints the output

This Program displays:Value of x is 12 #Double quotation interpolates the values.

#(Variable is replaced by its content )

Variable Interpolation

Integers§ Integers are usually expressed as decimal(10) but can be

specified in several different formats.

234 è decimal integer0765 è octal integer0b1101 è binary integer0xcae è hexadecimal integer

§ Converting a number from one base to another base can bedone using “sprintf” function.

§ Variables of different base can be displayed using “printf ”function

Example#! /usr/local/bin/perl –w$bin = 0b1010;$hex = sprint f ‘ %x’, $bin;$oct = sprint f ‘ %o’ ,45;print “binary =$bin \n hexa =$hex \n octal =$oct”;

This Program displays:binary= 1010 hexa = aoctal = 55

Integers (Contd..)

Example#! /usr/local/bin/perl –w$x = 98 ;print f (“ Value in decimal =%d\n”, $x ) ; print f (“ Value in octal=%o\n”, $x ) ;print f (“ Value in binary =%b\n”, $x ) ;print f (“ Value in hexadecimal=%x\n”, $x ) ;

This Program displays:Value in decimal =98Value in octal =142Value in binary =1100010Value in hexadecimal =62

Integers (Contd..)

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

Built-in functionsFunction 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 setEx: chr(65 ) gives A.

Ord() Returns the ASCII numeric value of the character specified by expression.Ex:ord(‘A) gives 65.

§ List is a group of scalar used to initialize array or hash.

§ The elements of a list can be numbers, strings or any othertypes of scalar data.

§ Each element of the Perl lists can be accessed by a numericalindex.

§ The elements of a list are enclosed in a pair of roundparenthesis and are generally separated by commas.

Lists

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..)

§ Flexible way of defining list is “qw (quote word)“ operatorwhich helps avoiding to too many quotation marks, but becautious if white spaces are there.

Example#! /usr/local/bin/perl –wprint (“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”;print (“sachin” ,”dravid”, “ganguly”, “kumble” )[1];

This Program displays:SachindravidgangulykumbleSachindravidgangulykumbleSachindravidgangulyanil kumblekacpertechdravid

Lists (Contd..)

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

List functions§ A list is joined into a string using “join “ function.

Example:Print join(“ “,(“perl” ,”is” ,”a”, ”scripting” ,”language”));This program displays:Perl is a scripting language

§ A string is splited into a list using “split” function.

Example:Print (split “|“, ”perl”) ;This program displays:P|e|r|l

Lists (Contd..)

List functions (Cont..)§ “map” evaluates expression or block for each element of list.

Example:Print join(“, ”,(map lc, A, B, C));This program displaysa, b, c

§ “grep” returns a sublist of a list for which a specific criterion is true.

Example:Print grep(!/x/ , a, b, x, d);This program displays:abd

Lists (Contd..)

Arrays§ An one dimensional ordered list of scalar variables.

§ Array provides dynamic storage for a list ,and so can beshrunk ,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.

Arrays (Contd..)§ ( ) represents the empty list.

Example:@arr= (“perl” ,2 , 5.143 );print “ @ arr”;This program displays:

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

Example#! /usr/local/bin/perl –w@a =(a .. z) ;@len1 =@a; #Assign the array to the variable@len2 =scalar (@arr); #using scalar methodPrint “ length of a =$len1 \n” ; Print “length of a =$len2”;

This Program displays:length of a = 26length of a = 26

Example#! /usr/local/bin/perl –w@arr = ( “one “ ,2 ,”three” ,4.4); $arr[2] = “kacper”; #second element(three) is replaced byprint “@arr”; new element(kacper)

This Program displays:one 2 kacper 4.4

Arrays (Contd..)

Array Methodspush§ 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 arrayPrint @num ;

This program displays: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 arrayPrint @num ;


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 arrayPrint @num ;


Array Methods (Contd..)shift§ shift function shifts off the first value of the array.

Example:@num = (1,2,3,4,5) ;$x = shift(@num) ; #Shifts the first element of an arrayPrint “$x” ;

This program displays:2,3,4,5 #Displays the value stored in x.

Array Methods (Contd..)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”) ;

This program displays:ONE TWO THREE #Displays in upper case

Array Methods (Contd..)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 numberprint “@num2 \n @char\n”;

This program displays:130 132 134 136A B C D

Array Methods (Contd..)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 @numprint join (“, ”, @val) ;

This program displays:1 ,2 #Displays first two elements of @num.

Array Methods (Contd..)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 @numPrint join(“ ,”,@num);

This program displays:1 ,2 ,3 ,4 , 5 , 6 , 7 #Displays all the elements of @num after

#splicing.

Array Methods (Contd..)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 ”;

This program displays:dravid ganguly kumble sachin11 13 45 56

Array Methods (Contd..)sort (Contd..)

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

This program displays:dravid ganguly kumble sachin sachin kumble ganguly dravid11 13 45 56 56 45 13 11

Array Methods (Contd..)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: $arr\n";print join "-CEO\t", @arr, "\n“;

This program displays:business Tycoons: mukesh anil prem ratanmukesh-CEO anil-CEO prem-CEO ratan-CEO

Array Methods (Contd..)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 @numPrint join(“ ,”, @rev) ;

This program displays: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)

Array Methods (Contd..)§ LIST – represents a list, array or hash that is returned by

the split function

§ PATTERN – usually is a regular expression but could be asingle character or a string

§ EXPR – is the string expression that will be split into anarray or a list.

§ LIMIT is the maximum number of fields the EXPR will besplit into

Example:!usr/local/bin/perl$string = “gandhi-ind-nehru-ind-sastri-ind-kalam-ind”;@colors = split('ind', $string); print @colors,"\n";

This program displays:gandhi--nehru--sastri--kalam-

Hashes§ An associative array ideal for handling attribute/value pair.

§ Lists and arrays are ordered and accessed by index ,hashesare ordered and accessed by specified key.

§ Represented using “%” symbol.

§ First element in each row is called a Key and the secondelement is a Value associated with that key.

§ Example : %coins = (“quarter”,25, “dime”,5); or%coins = ( quarter => 25 , dime => 5);

Key Value

Hashes (Contd..)§ Hah values can be any scalar ,just like an array ,but hash

keys can only be strings.

Example: Printing the hash.#!usr/local/bin/perl%hash1 = ( one => 1 ,two => 2 ,three =>3 ,four =>4);print %hash1; #we cant use print “%hash1”;print “@{[hash1]} \n”;@temp = %hash1; Print “@temp”;

This program displays:three3one1two2four4three 3 one 1 two 2 four 4three 3 one 1 two 2 four 4

§ The print order determined by how the Perl chooses to storeinternally.

Hashes (Contd..)§ Hash can have only scalars as values.

§ “{ }” are used to access individual elements of the hash.

Example:#!usr/local/bin/perl%hash1 = ( one => 1 ,two => 2 ,three =>3 ,four =>4);$ele = $hash1(‘three’); #single key, use scalar @mul_ele = @hash1(‘four’ ,’one’); #multiple key ,use arrayprint “single element =$ele “;print “multiple elements =@mul_ele”;

This program displays:single element =3multiple elements = 4 1

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 ofvalues in a hash.

Example:#!usr/local/bin/perl%hash1 = ( one => 1 ,two => 2 ,three =>3 ,four =>4);$ele = $hash1(‘three’); #single key, use scalar @mul_ele = @hash1(‘four’ ,’one’); #multiple key ,use arrayprint “single element =$ele “;print “multiple elements =@mul_ele”;

This program displays:single element =3multiple elements = 4 1

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 isadded as a new key.

§ “undef “ function is used to remove the value of the key, but key willstill exists.

Example: undef $hash1{‘ two’} ;

§ “delete” function is used to remove the value as well as key from thehash.

Example :delete $hash1 {‘four ‘};

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 ownway to store the items.

§ We can sort hashes either by key or value ,using sortfunction.

Example: Sort by key%data = ( sachin => 10,

dravid => 19,dhoni => 7,rohit => 45 );

foreach $key(sort (keys(%data))) {print “\t$key \t $data{$key}”;}

This program displays:dhoni 7 dravid 19 rohit 45 sachin 10

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 \t\t $data{$key} \n“; }

This program displays:dhoni 7 sachin 10 dravid 19 rohit 45

§ In the above example first values are compared(usingsort{$data {$a} <= > $data{$b}} ), found least value and thatis assigned to key (using keys% data) in every iteration

Operators§ Operators can be broadly divided into 4 types.

§ Unaryoperator which takes one operand.Example: not operator i.e. !

§ Binaryoperator which take two operandsExample: addition operator i.e. +

§ Ternaryoperator which take three operands.Example: conditionaloperator i.e. ?:

§ List operator which take list operandsExample: print operator

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.

Shift Operators§ shift operators manipulate integer values as binary numbers,

shifting their bits one to the left and one to the rightrespectively.


<< Left ShiftPrint 2 >>3 ; left shift by three positions, prints 8

>> Right ShiftPrint 42 >>2; #right shift by two positions, prints 10

x Repetition Operator.Ex: print “ hi ” x 3;

Output : hihihiEx2: @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

Logical Operators§ Logical operators represented by either symbols or names.

These two sets are identical in operation, but have differentprecedence.

§ The “!” operator has a much higher precedence than even“&& “and “|| “.

§ The “not”, “and”, “or”, and “xor” operators have the lowestprecedence of all Perl's operators, with “not” being thehighest 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

Bitwise Operators§ Bitwise operators treat their operands as binary values and

perform a logical operation between the corresponding bits ofeach value.


& Bitwise AND

| Bitwise OR

^ Bitwise XOR

~ Bitwise NOT

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 right0 if left is equal to right1 if left is greater than right

!= Inequality operator

Comparison Operators on stringsString 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 right0 if left is equal to right1 if left is greater than right

ne Return True if operands are not equal

.(dot) Concatenation operator. It takes two strings and joins themEx: print “System” .”Verilog”It prints SystemVerilog.

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 variableother than “$_ .”

§ The value returned from =~ is the return value of the regularexpression function, returns undef if match failed.

§ The !~ operator performs a logical negation of the returnedvalue for conditional expressions, that is 1 for failure and'' “ for success in both scalar and list contexts.

Conditional Statementsif Statement

§ if keyword to execute a statement block based on the evaluation of anexpression or to choose between executing one of two statementblocks 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 if\n”; }

This program displays: we are in second if

Conditional Statements (Contd..)until Loops

§ Until loops are used to repeat a block of statements while somecondition is false.

Example :- do … until loop

$firstVar = 10;

do {

print("inside: firstVar = $firstVar\n");

$firstVar++;

} until ($firstVar < 2);

print("outside: firstVar = $firstVar\n");

Conditional Statements (Contd..)do-until Loops

Example :- until loop

$firstVar = 10;

until ($firstVar < 20) {


$firstVar++;

};

print("outside: firstVar = $firstVar\n");

This program displays:outside: firstVar = 10

Conditional Statements (Contd..)for Loops

Example : - for loops

for ($firstVar = 0; $firstVar < 100; $firstVar++)

{


}

This program will display:inside: firstVar = 0inside: firstVar = 1...inside: firstVar = 98inside: firstVar = 99

Conditional Statements (Contd..)foreach Loops

§ The foreach statement is used solely to iterate over the elements of anarray. It is very handy for finding the largest element, printing theelements, or simply seeing if a given value is a member of an array.

Example :- foreach loop

@array = (1..5, 5..10);

print("@array\n");

foreach (@array) { $_ = “ ** " if ($_ == 5); }

print("@array\n");


1 2 3 4 5 5 6 7 8 9 10

1 2 3 4 ** ** 6 7 8 9 10

Jump KeywordsThe 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("$index\n");


19

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("@array\n");

for ($index = 0; $index < @array; $index++) {

if ($index == 3 || $index == 5) {

next; } $array[$index] = "*";

} print("@array\n");


0 1 2 3 4 5 6 7 8 9

* * * 3 * 5 * * * *

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 again\n");

redo;

} print("Thank you, " . uc($name) . "\n"); }

Regular Expressions§ Regular expression(regexps) is simply a string that describes

the pattern (example for pattern ,to find files in a directorywhich 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 andapply it to the supplied text.

§ The following operators use regular expressions.§ Matching Operator (m//)§ Substitution Operator(s///)§ Transliteration(Translation) Operator(tr///)

The Matching Operator (m//)§ The matching operator (m//) is used to find patterns in

strings.

Example :#!usr/local/bin/perl$_ = “success is a progressive journey”;$var = “success is not a destination”;If( /success/) { # the initial m is optional print “String success Found”; }

If ( $var =~ /destination/) { print “String destination Found”; }

This program displays: String success FoundString destination Found

§ When regular expression is enclosed in slashes(/success/), $_is tested against the regular expression ,returning TRUE ifthere is a match , false otherwise

Finding multiple matchesExample :#!usr/local/bin/perl$txt =“winn-ers see ga-in, lose-rs see pa-in”while ($txt =~ m/-/g) {print “ Found another -\n” ;}

This program displays: found another -found another -found another -

The Matching Operator (Contd..)

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 listcontexts alike) is the number of times it succeeded (which canbe 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 executedregular expression is used instead

Example :#!usr/local/bin/perl$text = “winners see gain, losers see pain”;$test =~s/winners/WINNERS/;print $text;

This program displays: WINNERS see gain, losers see pain

Example :#!usr/local/bin/perl@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“;

This program displays:10 dravid ganguly 10

The Substitution Operator (Contd..)

Example :#!usr/local/bin/perl@old = qw(sachin-bharat dravid-bharat ganguly-bharat kumble-ind);for (@new = @old) { s/bharat/india/ }print "@olds\n“;print "@new\n";

This program displays:sachin-bharat dravid-bharat ganguly-bharat kumble-indsachin-india dravid-india ganguly-india kumble-ind

The Substitution Operator (Contd..)

Using Modifiers with m// and s///

Modifier Description

g(m//g or s///g) Works globally to perform all possible operations.i Ignores alphabetic casex Ignores white space in pattern and allows

comments.gc Doesn’t reset the search position after a failed

matchs Lets the . Character match newlines.m Lets ^ and $ match embedded \n characterse Evaluate right hand side as an expressiono Compiles the pattern only once

The Translation Operator (tr///)§ Syntax :

LVALUE =~ tr/SEARCHLIST/REPLACEMENTLIST/

§ It scans a string, character by character, and replaces eachoccurrence of a character found in SEARCHLIST (which isnot a regular expression)with the corresponding characterfrom REPLACEMENTLIST

§ It returns the number of characters replaced or deleted.

§ If no string is specified via the =~ or !~ operator, the $_ stringis altered.

Example :#!usr/local/bin/perl$text = “winners see gain, losers see pain”;$count = ($test =~tr/e/E/); print $text;Print “\n no..of replacements =$count”;

This program displays: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

Example :#!usr/local/bin/perl$text = “winners see gain, losers see pain”;$count = ($test =~tr/e/E/c); print $text; #except “e” all other charactersPrint “\n no..of replacements =$count”; # are replaced.

This program displays:EEEEeEEEEeeEEEEEEEEEeEEEEeeEEEEEno..of replacements =26

The Translation Operator (Contd..)

Different Pattern Delimiters§ If the pattern contains lots of slash characters(/) ,we can also

use different pattern delimiter with the pattern.

Example :#!usr/local/bin/perl$var = "winners / see / gain,losers / see pain"; If( $var =~ m|see|) { # match with pipes print “String see Found\n”; }If ( $var =~ m ?gain?) { #match with question marks.print “String gain Found”; }

This program displays: String see FoundString gain Found

§ Perl also allows paired characters like braces andbrackets.viz { } ,( ), < >,[ ].

Ex: $var =~s{gain}{GAIN};

The Parts of regular Expressions§ In general regular expression can be made up of following

parts.

§ Characters§ Character Classes§ Alternative Match Patterns§ Quantifiers§ Assertions

Characters§ In regular expression any single character matches itself,

unless it is a metacharacters with special meaning.

§ Beside normal characters, Perl defines special charactersthat you can use in regular expression.

§ These character must start with backslash.(Otherwise Perltreats it as a normal character).

Character Description

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

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.


\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 \wEx :if ($var =~ /\W)


\s It is equivalent to [ \t\n\r]Matches any white space character.i.e. a space ,a tab ,a newline ,a returnEx :if ($var =~ /\s/)

\S It is equivalent to [^ \t\n\r]Matches any non-white space character.Ex :if ($var =~ /\S/)


\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.


\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 ~=/\Usuccess\E) { 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


\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.

Character Classes§ A character class allows a set of possible characters, rather

than just a single character, to match at a particular point ina regular expression.

§ Character classes are denoted by brackets [...], with the set ofcharacters 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”

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 theclass.

Ex1:$var = ~/w[^aoi]nder# will look for w followed by something that is# none of “a “or “o” or “ i”.

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 regularexpression. 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 firstalternative that matches is the one that’s used.

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 themin parentheses.

§ Used to group similar terms by their common characters andonly specified the differences.

Example : $var2 =~ /(while |for)loop/# will match either “while loop” or “for loop”

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 expressionin the first pair of parentheses into the variable $1, and thetext matched by the regular expression in the second pair ofparentheses into $2,and so on.

Example :

#!usr/local/bin/perl

my $text= "Testing";

if ($text =~ /((T|N)est(ing|er))/) {

print " \$1 = $1 \t \$2 = $2 \t \$3 = $3 \n ";

This program displays:$1 = Testing $2 = T $3 = ing

Grouping Alternatives (Contd..)§ There are three pairs of parentheses in the above example.

§ The first one is that which surrounds the whole regularexpression, 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 beassigned the value “ing”.

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 aspecific number of times.

§ Quantifiers in a regular expression are like loops in aprogram.

Quantifiers (Contd..)character Description

* 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 evaluated as a true.Ex:$var =~ /st*/ # Will match for the strings like “st”, ”sttr”, “sts” ,”star “, but not “son”.

Quantifiers (Contd..)character Description

{ } It indicates that how many times the string immediately to the left should be matched.{n} è should match exactly n times.{n,} èshould match at least n times{n, m} èShould 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 evaluated as a true.Ex : $var =~ /st?r/ # will match either “star” or “sttr”.

$var = ~/comm?a/ # will match either “coma” or “comma”

Quantifiers (Contd..)§ Quantifiers are greedy by default, which means they

will try to match as much as they can.

Ex :$str =“they are players, aren’t they ? “$str =~s/.*are/were/;print $str;

It will print :weren’t they ?

§ Perl will use the *. Preceding “are” to match all thecharacters upto the last “are” in the str.

Making Quantifiers Less Greedy § To make Quantifiers less greedy –that is ,to match the

minimum number of times possible –you follow the quantifierwith a ?

§ *? èMatches zero or more times.§ +? èMatches one or more times.§ ?? èMatches zero or one times.§ {n}? èMatches n times.§ {n,}? èMatches at least n times§ {m,n} èMatches at least n times but more than m

times.

Example :#!usr/local/bin/perl$text = “They are players ,aren’t they?”;$text =~s/.*?are/were/;print $text;

This program displays: Were players ,aren’t they?

Example :#!usr/local/bin/perl$txt = “no, these are the documents, over there.”;$txt = ~ s/the(.*?)e/those/;print $txt;

This program displays:no, those are the documents, over there

Making Quantifiers Less Greedy (Contd.)

Assertions§ Assertions (also called anchors) used to match conditions

within a string, not actual data.

§ Assertions are zero width because they do not consume anycharacters when they match.

Anchor Description

^(caret) Appears at the beginning of the pattern and finds for a match at beginning of the lineEx : $var =~ /^su/ # Will match the strings those are starting with “su “ i.e. . “sun”, “success”, “super “..

Assertions (Contd..)character Description

$ Appears at the end of the pattern and finds for a match at end of the lineEx : $var =~ /at$/ # 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.

Assertions (Contd..)

§ The difference between “^” and “\A” is that when you use them –multiline-modifier, “^ “ matches the beginning of everyline, but \A retains its original meaning and matches only atthe 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” .

Assertions (Contd..)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 “\w\W|\W\w” .

§ Within character classes \b represents backspace ratherthan a word boundary, just as it normally does in any double-quoted string.

Assertions (Contd..)Word Boundaries

§ \B matches on a non-word boundary .

§ This occurs whenever two word characters or two non-wordcharacters fall adjacent to each other.

§ It is equivalent to “\w\w|\W\W”.

Word BoundariesExample :#!usr/local/bin/perl$text = "one, ****, three, four"; # “*” is not a word$text1 = "one,****,three, four"; # characterforeach ($text =~ /\b\w+\b/g){print $_, "\t"; }print "\n using \\B\n";foreach ($text1 =~ /\B\w+\B/g){print $_, "\t"; }

This program displays: one three fourusing \Bn hre ou

Assertions (Contd..)

Regular Expressions-Examples(1)character Description

$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 =~/start\s*end/

The strings “start” and “end” optionally separated by any amount of white space (spaces, tabs, newlines).


$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.


$var =~ s/ \̂s+// Removes(trims) the leading white space.Ex:$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 "\n\nFirst Number is $1"; }It will print :

First Number is 86400

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 manageableparts, which makes overall programming easier to handle.

§ Perl allows you to create subroutines using the “sub” controlstructure

Example :#!usr/local/bin/perl$v1 =36;large_small(); #subroutine call before definition , parentheses mustsub large_small{ if($v1 >40) { print "value is bigger than 40\n"; }else{ print "value is smaller than 40\n"; } }$v1 =45;

large_small; # subroutine call after definition , parentheses are# optional

This program displays: value is smaller than 40value is bigger than 40

Subroutines (Contd..)

Scope of variable§ Perl variables have global package scope by default.

§ When we change a variable value in subroutine, we’ll bechanging it in the rest of the program by mistake as well.

§ We can create a variables that are entirely local to subroutineby using keyword “local” or “my”.

§ They can have same name as global variable and not affectthose global variables at all.

Example :#!usr/local/bin/perl$v1 =36;incr(); print "value of v1 =$v1\n";sub incr {

my $var =$v1; print "value before incrementing =$var\n"; $var++;print "value after incrementing =$var\n";

}

This program displays: value before incrementing =36value after incrementing =37value of v1 =36

Scope of variable (Contd..)

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 arestored in a special array named “@_”.

§ Beside accepting passed values, subroutines can also returnvalues using “return” keyword.

Example :#!usr/local/bin/perl$sum =add(10,20);print "sum =$sum\n“;sub add {

($val1, $val2) = @_;return $val1+$val2; }

This program displays: sum =30

§ Perl returns the last value in a sub routine so you can omitthe return keyword .

§ In the above example $val1+$val2 gives the same.

Parameters and Arguments (Contd..)

Parameters and Arguments (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 useshift directly.

Recursion§ Recursion happens when a subroutine calls itself, either

directly, or indirectly, via another subroutine

Example :#!usr/local/bin/perl0$fact = fact(6);print "factorial of given number =$fact\n“;

sub fact {local $val =shift(@_);if($var ==0) { return 1; }elsif($val==1) { return $val ; }else{ return $val*fact($val-1); } }

This program displays: factorial of given number =720

Passing ListsExample :#!usr/local/bin/perl@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)

Nested subroutinesExample :#!usr/local/bin/perlcall();sub call{

display();sub display {

print "you are in inner subroutine\n" ; }print "you are in outer subroutine\n" ; }

print "you are in main\n" ;

This program displays:you are in inner subroutineyou are in outer subroutineyou are in main

Subroutines –Examples(2)

§ In general passing arrays or hashes flattens their elementsinto one long list, so it’s a problem if you want to send two ormore distinct arrays or hashes.

§ To preserve integrity, you can pass references to arrays orhashes instead.

§ References can be created by using a backslash (\) operatoron a variable. It is similar to address-of (&)operator in C.

§ \$var èScalar Reference§ \@arr èArray Reference § \%hash èHash Reference

Pass by Reference

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 receivesthat parameter receives a “reference “ (or a "pointer")pointing to the $a scalar

§ Dereferencing references(Using prefixes $, @ ,% ,->)§ $$ref_var èScalar Dereference§ $$ref_arr èArray Dereference #array is copied

into scalar( ref_arr)§ $$ref_hash èHash Dereference

Example :#!usr/local/bin/perl@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.}

This program displays:before: arr = America, England, franceafter: arr = China, India, Japan

Pass by Reference (Contd..)

Returning by Reference§ If you return two arrays normally, their values are flattened

into one long list. If you return references to arrays, you candeference those arrays and reach the original arrays.

Example :#!usr/local/bin/perlsub 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";

This program displays: Asia Austrelia AfricaBrown White Black

§ You can get lot of material on references that comes withPerl( perldoc perl)

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 anexternal data source

§ Filehandles in Perl are a distinct data type.

§ STDIN or standard input represents the default inputfilehandle and usually connected to the keyboard.

§ STDOUT or Standard output represents the default outputfilehandle and usually connected to the consoledevice(screen)

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,LISTopen FILEHANDLE ,EXPRopen FILEHANDLE

§ The “open “ function takes a filename and creates the handlefor it.

File Handling (Contd..)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";

File Handling (Contd..)Opening a file (Contd..)§ “open” understands total six modes.

MODE Symbol DescriptionRead < 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

File Handling (Contd..)Opening a file (Contd..)

MODE Symbol Description

Append >> 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 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.

File Handling (Contd..)Opening a file (Contd..)MODE Symbol Description

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”;If the file doesn’t exist then it is created and opened.If the file does exist then both read and write commence from the end of the file.

Reading LinesExample :exam.txt :: winners dont do different things.

winners do things differently..success is not a destination.

Perl Script:#!usr/local/bin/perlopen FILE, "exam.txt" or die $!;$lineno;while(<FILE>){ print $lineno++," \t"; print "$_"; }

This program displays:0 winners dont do different things.1 winners do things differently.2 success is not a destination.

File Handling (Contd..)

File Handling (Contd..)§ “< >” 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 willcomes out of the loop.

File Handling (Contd..)§ Perl provides a special handle called “ARGV”.

§ It reads the files from the command line and opens them all ifspecified.

§ It will read from Standard input(STDIN) if nothing isspecified on the command line.

§ If you don't specify anything in the angle brackets,whatever is in @ARGV is used instead.

Command line passingExample :exam.txt :: winners dont do different things.

success is not a destination.Perl Script:#!usr/local/bin/perl$match =“do”;while(){

If(/$match/) { print “FOUND\n”; }else { print”NOT FOUND”; } }

This program displays:FOUNDNOT FOUND

File Handling –Examples(1)

Command line passingExample :#!/usr/local/bin/perlprint "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;

This program displays:perl filename.pl exam.txtFilename: exam.txtNew name:copy_exam.txt #it will create this file.


Command line passingExample :-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 program displays:This is message number 1. This is message number 2This is message number 3. This is message number 4.


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 outputto the file that is specified or to the current default file.

Ex : write (CD_REPORT);

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 outputto the file that is specified or to the current default file.

Ex : write (CD_REPORT);

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 thefunction “chdir”.

§ Syntax: chdir (dirname);

Ex :chdir ("/u/public/newdir");

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);

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