PERL

1
PERL

• Practical Extraction and Report Language

2
Introduction

• PERL is designed to assist the programmer with
  common tasks that are probably too heavy or too
  portability-sensitive for the shell, and yet too
  weird or short-lived or complicated to code in C
  or some other UNIX glue language.
• Common on UNIX systems.
• Good for administrators
• Create your own tools
• C-like language, but no main!
• Examples come from OReilly text Learning Perl

3
Compiled vs Interpreted Languages

• Compiled reduced from source code to native
  (binary) code.
• Interpreted computer executes the program line
  by line.
• No compile errors (obviously)
• Runtime errors
• PERL completely reads and parses the program
  before execution.

4
Hello World

• !/usr/bin/perl w
• About to print Hello!
• print ("Hello, world!\n")
• Perl must be the first line in your program (the
  w is to produce extra warning info)

5
Declaring Simple Variables

• Not necessary to declare the type!
• print "What is your name? "
• read from the command line
• name ltSTDINgt
• remove any whitespace
• chomp (name)

6
Conditionals

• !/usr/bin/perl
• Get user name
• print "What is your name? "
• name ltSTDINgt
• chomp (name)
• Note eq compares two strings (dont use )
• if (name eq "Randal")
• print "Hello, Randal! How good of you to be
  here!\n"
• else
• print "Hello, name!\n" ordinary greeting

7
Guessing the Secret Word

• !/usr/bin/perl -w
• secretword "llama"
• the secret word
• print "What is your name? "
• name ltSTDINgt
• chomp name
• if (name eq "Randal")
• print "Hello, Randal! How good of you to be
  here!\n"
• else
• print "Hello, name!\n" ordinary greeting
• print "What is the secret word? "
• guess ltSTDINgt
• chomp (guess)
• while (guess ne secretword)
• print "Wrong, try again. What is the secret
  word? "
• guess ltSTDINgt
• chomp (guess)

8
Arrays/Lists

• Creating an array
• _at_words ("camel","llama","alpaca")
• _at_words qw(camel llama alpaca)
• Accessing an array
• if (words0 eq Chihuahua)
• Note the , not _at_

9
Hashes

• Hash holds a key and value
• Creating a hash
• words qw(
• fred camel
• barney llama
• betty alpaca
• wilma alpaca
• )

10

• !/usr/bin/perl
• words qw(
• fred camel
• barney llama
• betty alpaca
• wilma alpaca
• )
• print "What is your name? "
• name ltSTDINgt
• chomp (name)
• if (name eq "Randal")
• print "Hello, Randal! How good of you to be
  here!\n"
• else
• print "Hello, name!\n" ordinary
  greeting
• secretword wordsname get the secret
  word
• print "What is the secret word? "
• guess ltSTDINgt
• chomp (guess)
• while (guess ne secretword)

11
Regular Expressions

• The strength of PERL
• Regular expressions are denoted using / /
• means all

12
The Match Expression

• is the match operator (begin with)
• Example
• if (name /Randal/) ...
• Says match if begins with Randal
• Unfortunately,doesnt match randal, and matches
  Randall
• Better example
• if (name /Randal\b/i) ...
• \b says it cant be followed by a number or
  character
• i means ignore case

13
Another Match Example

• print "any last request? "
• does the input begin with a y?
• note could be (ltSTDINgt /y/i)
• if (ltSTDINgt /yY/)
• print Just what might that request be?"
• discard a line of standard input
• ltSTDINgt
• print "Sorry, I'm unable to do that.\n"

14
Substitution

• Substitution finds a regular expression and
  replaces it with a string
• s/ltwhat youre looking forgt/ltreplacementgt/
• Example
• s/abc/def/
• Replaces all occurrences of a, any number of bs,
  ending in c with def
• Another Example
• which "this is a test"
• which s/test/quiz/ which is now
  "this is a quiz"
• Advanced
• \W is a non-word character and is all
• name s/\W.//
• Replaces everything after the first word with
  nothing

15
Translation

• Translation converts characters
• name tr/A-Z/a-z/
• This converts upper to lower case
• The A-Z denotes all the characters in between

16
Example

• Find if the sentence contains bird
• what "bird"
• sentence "Every good bird does fly."
• if (sentence /\bwhat\b/)
• print "The sentence contains the word
  what!\n"

17
Splitting and Joining

• Splitting breaks strings into fields, and puts
  the into an array
• Joining glues fields back together
• Split example
• line "merlyn11810Randal/home/merlyn/usr/b
  in/perl"
• split line, using as delimiter
• _at_fields split(//,line)
• now _at_fields is ("merlyn","","118","10","Randal",
  
• "/home/merlyn","/usr/bin/perl")
• Join example
• bigstring join(,_at_fields)
• glue is the delimiter and list is the array

18
Functions

• Use the sub keyword
• Example
• sub myFunction
• code
• Call the function by its name
• Can create local variables using my keyword
• my (local_var)

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Parameters

• Parameters are passed through something called _at__
• Example
• sub addTwoNums
• my (num1, num2) _at__
• return (num1num2)
• addTwoNums(1, 7)

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Files

• Create a filehandle
• Note that you get 3 of them for free ltSTDINgt
• Example
• sub init_words
• open (WORDSLIST, "wordslist.txt")
• reads an entire line
• while (name ltWORDSLISTgt)
• chomp (name)
• word ltWORDSLISTgt
• chomp (word)
• wordsname word
• close (WORDSLIST)

wordslist file fred camel barney llama betty alpac
a wilma alpaca
21
Writing Modes

• A file can be opened for
• Reading open INFILE, lt, filename
• Writing open OUTFILE, gt, filename
• Appending open OUTFILE, gtgt, filename

22
Conclusions

• PERL is great for
• Learn the basics of programming
• Parsing files
• Writing simple tools
• UNIX environments
• System administrators
• PERL isnt popular because
• Lack of graphics (though there is Tcl/Tk)
• LOTS of options for parsing
• Better for UNIX than Windows?
• Looks cryptic at times