Practical Extraction

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Practical Extraction Report Language

• Picture taken from http//www.wendy.org/DPW2006/sh
  irt.htm

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Agenda

• Why Perl?
• Getting/Installing Perl
• Using Perl
• Structure of basic program (Hello world)
• Variables Operators
• Regular Expressions
• Other Topics

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Why Perl

• Perl is built around regular expressions
• REs are good for string processing
• Therefore Perl is a good scripting language
• Perl is especially popular for CGI scripts
• Perl makes full use of the power of UNIX
• Short Perl programs can be very short
• Perl is designed to make the easy jobs easy,
  without making the difficult jobs impossible. --
  Larry Wall, Programming Perl

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Getting/Installing Perl

• Windows
• www.activestate.com
• Download ActivePerl
• Run installer
• Linux
• Mostly installed when Linux is installed
• userX_at_machineY which perl
• Get it from
• Linux distribution CDs
• Update your installation and during package
  selection, select perl
• ActiveState.com
• CPAN

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Other Possibilities

• Using Virtual Machines
• VMWare
• Install VMWare workstation on windows
• Install Linux under VMware workstation (select
  perl to be installed)
• Cygwin
• Install Cygwin on windows
• It will provide a Linux interface such that perl
  can be used
• http//www.cygwin.com/mirrors.html

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Using Perl

• Windows
• Write a program and save it with .pl extension
• C\perl\bingtperl program_name.pl
• Linux
• Write a program and save it with .pl extension
• userX_at_machineY perl program_name.pl
• userX_at_machineY ./program_name.pl
• Same under VMware Cygwin

chmod x program_name.pl
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Structure of a basic program
First line is special, Path to perl installation
this path can be different e.g., /bin/perl

• !/usr/bin/perl
• Program to do the obvious
• print 'Hello world.'

denotes comment, any thing after till the end
of line is comment
Statement ends with semicolon
Built in function
Function argument, in this case a string constant
userX_at_machineY perl hello.pl
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Variables

• Scalar variables
• Only one value at a time
• List variables
• List of values (Arrays)

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Scalar Variables

• The scalar variable means that it can store only
  one value.
• They should always be preceded with the symbol.
  e.g., var1
• There is no necessity to declare the variable
  before hand. (but recommended)
• There are no data types such as character or
  numeric. If you treat the variable as character
  then it can store a character. If you treat it as
  string it can store one word . if you treat it as
  a number it can store one number.

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Example scalar

• !/perl/bin
• x "100\n"
• print x
• x x 1
• print x

Output 100 101
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List/Array Variables

• They are like arrays. It can be considered as a
  group of scalar variables.
• They are always preceded by the _at_symbol.
• eg _at_items (Apple",Bell",Chair")
• Like in C the index starts from 0.
• If you want the second name you should use
  items1
• Watch the symbol here because each element is
  a scalar variable.
• Followed by the list variable gives the length
  of the list variable.
• items will provide index of last element _at_items
• len _at_items will assign length of array to
  len

Is the result of two statements same len
_at_items and print _at_items ?
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Example List/Array

• !/perl/bin
• _at_myarray (1721, 2974, string")
• print _at_myarray
• myarray0 string
• myarray1 1234
• myarray2 5646
• print _at_myarray
• print myarray0 . myarray1 .
  myarray0

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Operations on Arrays

• Push
• push adds one or more things to the end of a list
• push (_at_items, table", chair")
• push returns the new length of the list
• Pop
• pop removes and returns the last element
• myitem pop(_at_items)
• shift, unshift, reverse

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Example (Push Pop)

• !/perl/bin
• _at_myarray (1721, 2974, string")
• print _at_myarray\n
• push(_at_myarray,newval1,newval2)
• print _at_myarray\n
• popvalue pop(_at_myarray)
• print myarray\n
• print _at_myarray

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Operators

• Arithmetic
• String
• Single and Double quotes
• Conditional

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Arithmetic in Perl
a 1 2 Add 1 and 2 and store in a a
3 - 4 Subtract 4 from 3 and store in
a a 5 6 Multiply 5 and 6 a 7 /
8 Divide 7 by 8 to give 0.875 a 9
10 Nine to the power of 10, that is, 910 a
5 2 Remainder of 5 divided by 2 a
Increment a and then return
it a Return a and then
increment it --a Decrement a
and then return it a-- Return a
and then decrement it
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String and assignment operators
a b . c Concatenate b and c a b x
c b repeated c times a b
Assign b to a a b Add b to a a
- b Subtract b from a a . b
Append b onto a
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Single and double quotes

• a 'apples'
• b 'bananas'
• print a . ' and ' . b
• prints apples and bananas
• print 'a and b'
• prints a and b
• print "a and b"
• prints apples and bananas

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Conditions

• Strings Numbers
• eq equal to
• ne ! not equal to
• lt lt less than
• gt gt greater than
• le lt less then or equal to
• ge gt greater then or equal to
• Logical
• And
• Or
• ! negation

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Control structures

• Loops
• Foreach
• For
• while
• Condition
• If / else
• Subroutines

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foreach
Visit each item in turn and call it
myitem _at_item (item1,item2,item3) forea
ch myitem (_at_items) print "myitem\n"

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for loops

• for loops are just as in C or Java
• for (i 0 i lt 10 i) print
  "i\n"

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while loops
!/usr/local/bin/perl a 1 while (a !
10) a
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do..while loops
!/usr/local/bin/perl a 1 do a while
(a ! 10)
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if statements
if (a) print "The string is not
empty\n" else print "The string is
empty\n"
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if - elsif statements
if (!a) print "The string is empty\n"
elsif (length(a) 1) print "The string
has one character\n" elsif (length(a)
2) print "The string has two characters\n"
else print "The string has many
characters\n"
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Calling subroutines

• Assume you have a subroutine printargs that just
  prints out its arguments
• Subroutine calls
• printargs(arg1", arg2")
• Prints arg1 arg2"
• returnvalue printargs(arg1", arg2")
• Prints arg1 arg2
• returnvalue will be assigned two

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Defining subroutines

• Here's the definition of printargs
• sub printargs
• print "_at__\n"
• Parameters are put in the array _at__ which can be
  accessed using
• _0, _1 etc

How many parameters are passed to sub routine?
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Returning a result

• Use return statement

sub maximum if (_0 gt _1)
return _0 else
return _1 biggest maximum(37,
24)
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Basic pattern matching

• sentence /the/
• True if sentence contains "the"
• sentence "The dog bites."if (sentence
  /the/) is false
• because Perl is case-sensitive
• ! is "does not contain"

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RE special characters
. Any single character except a
newline The beginning of the line or
string The end of the line or
string Zero or more of the last
character One or more of the last
character ? Zero or one of the last
character
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RE examples
. matches the entire string hi.bye
matches from "hi" to "bye" inclusive x y
matches x, one or more blanks, and y Dear
matches "Dear" only at beginning bags?
matches "bag" or "bags" hiss matches
"hiss", "hisss", "hissss", etc.
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Other Topics

• Split() and join()
• File handling
• Perl 5
• Modules
• http//www.pageresource.com/cgirec/index2.htm