Perl 101 - The Basics of Perl Programming

Perl -101

Objectives
To introduce Perl Scripting Language
To introduce the concept of regular expressions and pattern matching
To introduce various file operations in Perl
Linux User Group at San Jose State University

Agenda
Part 1 (1:30PM to 2:45PM)
Introduction to Perl
– Types of Variables
– I/O Functions
– Operators
– String and formatting strings
– Selectional, iterational and miscellaneous control statements
Data Structures
– Arrays and various operations on arrays
– Hashes and various operations on hashes
Functions:
– Subroutines
– Command line arguments
Linux User Group at San Jose State University Part 2 (2:50PM to 3:45PM) Regular Expressions: – Regular Expressions – Meta characters – Character classes – Pattern matching File IO: – Reading, Writing text files – Various file I/O modes Misc: – Installing modules Questions and Answers

What is PERL and where PERL is being used?
PERL is scripting language
PERL is mostly used for extracting information from text files and generating various reports based on that
PERL is also used for developing web applications using CGI standards
It is also used for developing scripts of automated testing, system administration etc.
It is one of the most popular scripting language

Perl Introduction
PERL – Practical Extraction and Report Language
Developed by Larry Wall in 1987
Originally designed for reading text files and preparing reports based on that information
Combines features of C ,sed, awk and sh
Gives unlimited data size
Recursion of unlimited depth
Sophisticated pattern matching techniques
Used for web programming
Widely used for system administration
Wide range of third party modules

The PERL Interpreter
Perl Interpreter
Converts the scripts in to a parse tree and executes immediately
Internally Perl maintains a byte code for execution
Known as interpreter/ compiler

PERL - Variables
Scalar Variables
Simple variables containing an element, either a number or a string
Defined by $ symbol
Array
List of scalar variables
Defined by @ symbol
Hashes
Similar to arrays
Each item is identified by a key value
Defined by % symbol

PERL – Scalar Variables
Defined by $ Symbol
Stores either a number or a String
No declaration required, creation is done when a variable is referred first in the program,known as auto vivification
Strings and Numbers are interchangeable
Variable names are case sensitive
Variable name can have alphabets, numbers and underscore (_)
Should not start with a number
Variables starting with _ is special

PERL - Lists
Collection of scalar data, Arrays and Hashes
Represents a logical group of items
Scalar data inside ( and ) separated using , represents a list
A set of operators works on list – called as list operator
E.g. print
print (1,2,3) displays 123

PERL – Display Functions
print
Unformatted output statement which can print a string, Number or a List
Default outputs to STDOUT, can be redirected to STDERR or a File
Returns ‘true’ if successful else ‘false’ [1 represents ‘true’]
print List
print FILEHANDLE List
printf
Formatted output statement
By defaults outputs to STDOUT. Can be redirected to STDERR or a File
printf Format, List
printf FILEHANDLE, Format, List
warn
Outputs a message on the STDERR stream (unbuffered)
Used to trace error when STDOUT is redirected
Program is neither terminated nor an exception is thrown
warn “Message”

PERL - Operators
Assignment Operator
Auto increment and decrement operators
Additive operators
Multiplicative operators
Exponentiation Operator
Relational Operators
Logical Operators
Range Operator
Binding Operator
Arrow operator
File I/O Operator

PERL – Operators contd
Assignment Operator (=)
Auto increment and decrement operator (++, --)
Additive Operator (+, -, .)
Addition & Subtraction operator
Concatenation Operator (.)
Multiplicative Operators
Multiplication & Division
Repetition Operator (x)
Exponential Operator

PERL – operators contd
Relational Operators
Operating on Numeric data
<, <=, >, >=, ==, !=, 
Operating on String data
lt – less than E.g. ($Myname lt $YourName)
gt – greater than
le – less than or equal to
ge – greater than or equal to
eq – equal to
ne – not equal to
cmp – compare (returns -1, 0 , 1)
Logical Operator
AND - && or ‘ and ’
OR - || or ‘ or ’
NOT - not

PERL – Operators contd
Range Operator (..)
Returns a list of sequence of values
print (1..50);
print (a..z);
Bind Operator (=~, !~)
Used for binding scalar expressions to a pattern match
Arrow Operator (->)
Used to retrieve elements from a reference
File Operator (<File_Handle>)
Used for File I/O Operations
<> represents <STDIN>
Note: Bind, Arrow and File operators will be explained later

PERL – Standard Input
Input operator
<STDIN> or <>
Read input from the user including ‘ ’ from the user
chomp operator can be used for removing the ‘ ’ character
By default input is stored in a default variable $_
chomp
Usually used to discard the ‘ ’ character at the end of input string

PERL - Strings
Sequence of characters, each character is an 8 bit value
Single quoted string
Escape sequence except & ’ are not interpreted so
Double quoted string
All escape sequences are interpreted so
Concatenation Operator
‘ .’ can be used for concatenating string values
Repetition Operator
‘ x’ makes as many number of concatenated copies of string

PERL – String Functions
Converting to lower case using – lc STRING
Converting to upper case using – uc STRING
Converting initial case to lower using – lcfirst STRING
Converting initial case to upper using – ucfirst STRING
Getting string length using – length (STRING)
Returns the length of the string
If STRING is omitted, it returns the length of the string stored in $_

PERL – String Functions contd
Searching string using – index (STRING, SUBSTRING, [POSITION])
returns the index of the first occurrence of the SUBSTRING in STRING on or after POSITION
If POSITION is ignored, it starts searching from the first location
returns -1 if not found
rindex can be used to find the index of the last occurrence of the SUBSTRING in STRING on or before POSITION

PERL – String Functions contd
Extracting/Replacing substring using –
substr STRING,OFFSET,[LENGTH], REPLACEMENT
Extracts and returns the substring from OFFSET to LENGTH+OFFSET
If LENGTH is omitted, extraction starts from OFFSET to end of STRING
If LENGTH is negative, extraction omits that much number of characters from the end
If OFFSET is negative extraction starts from end of the string and moves back wards
If REPLACEMENT is specified, extracted substring will be replaced by REPLACEMENT

PERL – Control Structures
Branching Statements
if
if – else
if – elsif
Looping Statements
while / until
do – while/ until
for
foreach
Miscellaneous Control Statements
last, next, redo
Expression modifiers
exit, die

PERL – Control Structures contd
if statement
if ( condition ) {
#True Block
}
If - else statement
if( condition ) {
#True Block
}else {
#False Block
}
if – elsif statement
If( condition1) {
#True block for Condition1
}elsif(condition2) {
#True block for Condition2
}else {
#False block
}

while loop
while (condition) {
#while block statements
}
Control comes out of the loop when the condition is FALSE
until loop
until (condition) {
#until block statements
}
Control comes out of the loop when the condition is TRUE

do while loop
do{
#while block statements
}while (condition)
Control comes out of the loop when the condition is FALSE
do until loop
do {
#until block statements
}until (condition)
Control comes out of the loop when the condition is TRUE

for
for(init expr ; condition ; increment/decrement ) {
#for block
}
foreach
mostly used when iteration has to be done for different values in a list
foreach Variable (LIST) {
#Block of Statements
}

continue
flow control statement follows while or foreach loop
continue block will be executed after every successful completion of an iteration
next
used to continue with the next iteration of the loop by stopping current iteration
cannot be used in side do-while/do-until loop
if continue block is present it will also be executed
redo
restarts the loop without evaluating the condition again
continue block is not executed, if present
last
exits the loop
continue block is not executed, if present

exit
used to terminate the execution of a script
exit 0 denotes exit on success and exit 1 denotes exit on error
cannot display error messages while exiting
$Choice=<STDIN>
if ($Choice==4) { exit 0};
die
displays the error message to STDERR stream
and terminates the execution of a script
copy($TargetFile,$SourceFile) or die “File cannot be copied”

PERL - Arrays
Set of scalar values
Dynamically grows/shrinks when ever required
Can store dissimilar data types
Prefixed with ‘@’ symbol
Un-initialized locations and locations beyond the end of array will be undef
Can be initialized using
List
qw operator
Repetition operator (X)
Range operator (..)
Individual locations

PERL – Arrays contd
Creation using Range operator
@EmpNumber = (1001..1200) ;
- Can be used if array has to be initialized by a range of elements
Creation using Repetition operator
@Number = (0) X 10;
- Can be used if array has to be initialized by same value in all locations
Creation of array with dissimilar elements
@Details=(1001,”John”,3400.50);

Length of the array
$#<array name> stores the last index of the array
$#<array name> +1 will give the number of elements
Emptying an array
$#Emp = -1;
OR
@Emp = ( );
OR
@Emp = undef;
Looping through an Array using foreach statement
converting scalar into an array using split function
Sorting an Array using – sort @<array_name>
Reversing an Array using – reverse @<array_name>

Pushing and Popping elements
push (<Array>, <Value>/<List>)
Will add the element to the end of array
Array expands automatically
pop (<Array>)
Will remove the element from the end of the array
Array shrinks automatically
Shifting and Unshifting
unshift (<Array>, <Value>/<List>)
Will add the element/list to the front of the array & returns number of elements
Array expands automatically
shift (<Array>)
Will remove the element from front of the array
Array shrinks automatically

PERL – Hashes
Hashes
Unordered collection of values
Each element is linked to a key, which uniquely identifies that element
Any element from the hash can be retrieved, added, deleted using the key
A hash variable is prefixed with %
Also called as associative arrays
Accessing elements
Elements can be retrieved using key
$RetrievedElement = $Hash{Key};
Adding new elements
$Hash{ new key}= new element
Changing the existing elements
$Hash{key} = new element

PERL – Hashes contd
Deleting key / value pairs
delete ($Hash{Key})
Getting all keys or all values
keys (%hash)
values (%hash)
Does a key exist??
Using exists keyword
Looping through hash elements
using foreach and keys

Perl – more data structures
Combination of arrays and hashes
Arrays of Arrays
Hashes of Arrays
Arrays of Hashes
Hashes of Hashes
and so on…
http://perldoc.perl.org/perldsc.html

PERL – Regular Expressions
A regular expression , often called a pattern in Perl, is a template that either matches or doesn't match a given string.
Using simple patterns:
To compare a pattern (regular expression) to the contents of $_, simply put the pattern between a pair of forward slashes (/)
$_ = “perl script";
if (/perl/) {
print "It matched! ";
}
Note: $_ is the default variable

PERL – Regular Expression contd
Metacharacters
Dot (.) is a wildcard character - it matches any single character except a newline (which is represented by " ")
/b.t/ will match bat, bet, bit, but etc. It will not match bt or boat etc.
A backslash in front of any metacharacter makes it non-special.
/3.14159/ doesn't have a wildcard character.
Star (*) is a wildcard character – it matches the preceding character zero or more times
/perl *script/ matches any number of tab characters between perl and script. i.e. it matches “perl script" with one tab, or "perl script" with two tabs, or “perl script" with three tabs, or even “perlscript" with nothing in between at all.

Metacharacters
Plus (+) is to match the preceding character one or more times
/perl +script/ matches if perl and script are separated only by spaces. This pattern won't match “perlscript”, since the plus matches only if there are one or more spaces between the two words.
Question mark ("?") means that the preceding character is optional
/perl ? script/ matches it perl and script are separated by one space or no space. This will match “perl script” as well as “perlscript”.
Parentheses ("( )") may be used for grouping
/(perl)+/ matches strings like perlperlperl
The vertical bar (|), often pronounced "or" means that either the left side may match, or the right side
/unix|perl/ will match any string that mentions unix or perl

Character class
a list of possible characters inside square brackets ([]), matches any single character from within the class.
[abcwxyz] may match any one of the seven characters.
[a-zA-Z] may match any of the alphabets (both uppercase and lowercase).
A caret (^) at the start of the character class negates it.
[^def] will match any single character except “d”, “e” or “f”.
[d] or [0-9] will match any digit
[w] is a shortcut for any "word" character:
[w] or [A-Za-z0-9_] will match any “word”
s is a shortcut for white space character

PERL – Regular Expression Contd
Matches with m//
m// (pattern match) operator allows you to choose any pair of delimiters to quote the contents
m(perl), m<perl>, m{perl}, or m[perl]
Case Insensitive Matching with /i
To make a case-insensitive pattern match, so that you can match PERL as easily as perl or Perl, use the /i modifier:
Binding Operator =~
Matching against $_ is merely the default; the binding operator (=~) tells Perl to match the pattern on the right against the string on the left, instead of matching against $_
Substitution with s///
s/// is the search and replace feature. This simply replaces whatever part of a variable matches a pattern with a replacement string.
If the match fails, nothing happens, and the variable is untouched

PERL – Regular Expression Contd
Global Replacements with /g
s/// will make just one replacement, even if others are possible. The /g modifier tells s/// to make all possible replacements
A fairly common use of a global replacement is to collapse whitespace; that is, to turn any arbitrary whitespace into a single space

PERL - Subroutines
Section of the script that performs a specific task
Values can be passed to subroutine so that it performs the task on these values
Values passed to the subroutines are called arguments
Return value is send back by the subroutine
@_ is an array where the argument values are stored during subroutine call
Subroutine definitions can be placed anywhere in your program text

PERL – Subroutine contd
Subroutine Declaration
sub MySubroutine()
MySubroutine accepts no arguments
sub MySubroutine($)
MySubroutine accepts one scalar argument
sub MySubroutine($$)
MySubroutine accepts 2 scalar arguments
sub MySubroutine(@)
MySubroutine accepts an array as an argument
sub MySubroutine(%)
MySubroutine accepts a hash reference as an argument
sub MySubroutine($@)
MySubroutine accepts a scalar value and an array as argument
sub MySubroutine($;$)
MySubroutine accepts 2 arguments, where the second argument is optional

Argument passing & Returning Values
Arguments passed from the calling function are stored in an array @_
Default arguments
Default arguments are specified in the prototype, separated by ‘;’ from mandatory parameters
Scope of a variable
Default scope of all variables are global
Scope can be changed by using ‘my’ or ‘local’ keyword
Changing the scope of a variable using ‘my’
Restricts the scope of a variable to the block in which it is defined
It is not visible to the subroutine called from the enclosing block
Changing the scope of a variable using ‘local’
Restricts the scope of a variable to the block in which it is defined
It is visible to the subroutine called from the enclosing block

Reference Variable
Reference variable store the reference (address) of the variables
Variables can be directly accessed using reference variables
Reference to a scalar
Command line arguments
Command line arguments are stored in a built in array @ARGV when a perl script is executed

PERL – File I/O
Data that outlives the program
needs to be stored
in a permanent storage
so that it can be referred later on
Example
word processing application might save the text in a linked list or some other data structure when the application is running, but when the application is terminated the contents of the linked list need to stored in a file

PERL – File I/O contd
Opening a File
open FILEHANDLE, FileName
Returns true (1) if successfully opened, false (0) otherwise
open (MyFileHandle,”MyData.txt”)
File will be opened for input if file name is prefixed by ‘<‘
open (MyFileHandle,”<MyData.txt”)
File will be opened for output if the file name is prefixed by ‘>’
If the file exists contents will be erased
If the file doesn’t exists new file will be created
open (MyFileHandle,”>MyData.txt”)
File will be opened for appending if the the file name is prefixed by ‘>>’
If the file doesn’t exist new file will be created
open (MyFileHandle,”>>MyData.txt”)

Opening a File contd
File will be opened for read & write if the file name is prefixed by ‘+>’ or ‘+<‘
‘ +>’ will erase the existing contents and open the file for read and write
‘ +<‘ must be used if the file has to be opened for updating the existing contents
open (MyFileHandle, “+<MyData.txt”)
Handling Errors in File Open
An alternate statement can be attached to the open statement using ‘or’
The alternate statement will be executed if opening a file fails
open FileHandle,FileName or {Alternate Statement}
Closing a File
Closing a file will transfer the un buffered data to the file
Returns true(1) if successful, false(0) otherwise
close FileHandle

Reading from a File using <> (angle) operator
<FileHandle> returns the next line of input from the file
If <> alone is specified, input is taken from STDIN
Returns null when end of file is reached
Reading from file using readline
readline (FileHandle) returns one line of data from the file
Undefined value will be returned if end of file is reached
Read from file using getc
getc(FileHandle) returns a character from the file
Undefined value will be returned if end of file is reached
Writing unformatted records into a file using print
print FileHandle Message
Returns true if successful, false otherwise
print FIleHandle Message or Alternate statement
Returns false and executes alternate statement if unsuccessful, true otherwise

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Perl 101 - The Basics of Perl Programming

by Utkarsh Sengar

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