Introduction to Perl Part II

1
Introduction to PerlPart II

• By Bridget Thomson McInnes
• 22 January 2004

2
File Handlers

• Very simple compared to C/ C !!!
• Are not prefixed with a symbol (, _at_, , ect)
• Opening a File
• open (SRC, my_file.txt)
• Reading from a File
• line ltSRCgt reads upto a newline character
• Closing a File
• close (SRC)

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File Handlers cont...

• Opening a file for output
• open (DST, gtmy_file.txt)
• Opening a file for appending
• open (DST, gtgtmy_file.txt)
• Writing to a file
• print DST Printing my first line.\n
• Safeguarding against opening a non existent file
• open (SRC, file.txt) die Could not open
  file.\n

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File Test Operators

• Check to see if a file exists
• if ( -e file.txt)
• The file exists!
• Other file test operators
• -r readable
• -x executable
• -d is a directory
• -T is a text file

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Quick Program with File Handles

• Program to copy a file to a destination file
• !/usr/local/bin/perl -w
• open(SRC, file.txt) die Could not open
  source file.\n
• open(DSTlt gtnewfile.txt)
• while ( line ltSRCgt )
• print DST line
• close SRC
• close DST

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Some Default File Handles

• STDIN Standard Input
• line ltSTDINgt takes input from stdin
• STDOUT Standard output
• print STDOUT File handling in Perl is sweet!\n
• STDERR Standard Error
• print STDERR Error!!\n

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The ltgt File Handle

• The empty file handle takes the command line
  file(s) or STDIN
• line ltgt
• If program is run ./prog.pl file.txt, this will
  automatically open file.txt and read the first
  line.
• If program is run ./prog.pl file1.txt file2.txt,
  this will first read in file1.txt and then
  file2.txt ... you will not know when one ends and
  the other begins.

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The ltgt File Handle cont...

• If program is run ./prog.pl, the program will
  wait for you to enter text at the prompt, and
  will continue until you enter the EOF character
• CTRL-D in UNIX

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Example Program with STDIN

• Suppose you want to determine if you are one of
  the three stooges
• !/usr/local/bin/perl
• stooges (larry gt 1, moe gt 1, curly gt 1 )
• print Enter your name ?
• name ltSTDINgt chomp name
• if(stoogeslc(name))
• print You are one of the Three
  Stooges!!\n
• else
• print Sorry, you are not a Stooge!!\n

10
Chomp and Chop

• Chomp function that deletes a trailing newline
  from the end of a string.
• line this is the first line of text\n
• chomp line removes the new line character
• print line prints this is the first
  line of text without returning
• Chop function that chops off the last character
  of a string.
• line this is the first line of text
• chop line
• print line prints this is the first line
  of tex

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Regular Expressions

• What are Regular Expressions .. a few
  definitions.
• Specifies a class of strings that belong to the
  formal / regular languages defined by regular
  expressions
• In other words, a formula for matching strings
  that follow a specified pattern.
• Some things you can do with regular expressions
• Parse the text
• Add and/or replace subsections of text
• Remove pieces of the text

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Regular Expressions cont..

• A regular expression characterizes a regular
  language
• Examples in UNIX
• ls .c
• Lists all the files in the current directory that
  are postfixed '.c'
• ls .txt
• Lists all the files in the current directory that
  are postfixed '.txt'

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Simple Example for ... ? Clarity

• In the simplest form, a regular expression is a
  string of characters that you are looking for
• We want to find all the words that contain the
  string 'ing' in our text.
• The regular expression we would use
• /ing/

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Simple Example cont...

• What would are program then look like
• !/usr/local/bin/perl
• while(ltgt)
• chomp
• _at_words split/ /
• foreach word(_at_words)
• if(wordm/ing/) print word\n
  

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Regular Expressions Types

• Regular expressions are composed of two types of
  characters
• Literals
• Normal text characters
• Like what we saw in the previous program (
  /ing/ )
• Metacharacters
• special characters
• Add a great deal of flexibility to your search

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Metacharacters

• Match more than just characters
• Match line position
• start of a line ( carat )
• end of a line ( dollar sign )
• Match any characters in a list ...
• Example
• /Bbridget/ matches Bridget or bridget
• /McIinnes/ matches McInnes or Mcinnes

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Our Simple Example Revisited

• Now suppose we only want to match words that end
  in 'ing' rather than just contain 'ing'.
• How would we change are regular expressions to
  accomplish this
• Previous Regular Expression
• word m/ ing /
• New Regular Expression
• wordm/ ing /
• 
  

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Ranges of Regular Expressions

• Ranges can be specified in Regular Expressions
• Valid Ranges
• A-Z Upper Case Roman Alphabet
• a-z Lower Case Roman Alphabet
• A-Za-z Upper or Lower Case Roman Alphabet
• A-F Upper Case A through F Roman
  Characters
• A-z Valid but be careful
• Invalid Ranges
• a-Z Not Valid
• F-A Not Valid

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Ranges cont ...

• Ranges of Digits can also be specified
• 0-9 Valid
• 9-0 Invalid
• Negating Ranges
• / 0-9 /
• Match anything except a digit
• / a /
• Match anything except an a
• / A-Z /
• Match anything that starts with something
  other than a single upper case
  letter
• First start of line
• Second negation

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Our Simple Example Again

• Now suppose we want to create a list of all the
  words in our text that do not end in 'ing'
• How would we change are regular expressions to
  accomplish this
• Previous Regular Expression
• word m/ ing /
• New Regular Expression
• wordm/ ing /

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Literal Metacharacters

• Suppose that you actually want to look for all
  strings that equal '' in your text
• Use the \ symbol
• / \ / Regular expression to search for
• What does the following Regular Expressions
  Match?
• / A - Z /
• Matches any line that contains ( A-Z or )
  followed by

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Patterns provided in Perl

• Some Patterns
• \d 0 9
• \w a z A z 0 9 _
• \s \r \t \n \f (white space pattern)
• \D 0 - 9
• \W a z A Z 0 9
• \S \r \t \n \f
• Example 19\d\d
• Looks for any year in the 1900's

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Using Patterns in our Example

• Commonly words are not separated by just a single
  space but by tabs, returns, ect...
• Let's modify our split function to incorporate
  multiple white space
• !/usr/local/bin/perl
• while(ltgt)
• chomp
• _at_words split/\s/, _
• foreach word(_at_words)
• if(wordm/ing/) print word\n

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Word Boundary Metacharacter

• Regular Expression to match the start or the end
  of a 'word' \b
• Examples
• / Jeff\b / Match Jeff but not Jefferson
• / Carol\b / Match Chris but not Caroline
• / Rollin\b / Match Rollin but not Rolling
• /\bform / Match form or formation but not
  Information
• /\bform\b/ Match form but neither information
  nor formation

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DOT Metacharacter

• The DOT Metacharacter, '.' symbolizes any
  character except a new line
• / b . bble/
• Would possibly return bobble, babble, bubble
• / . oat/
• Would possibly return boat, coat, goat
• Note remember '.' usually means a bunch of
  anything, this can be handy but also can have
  hidden ramifications.

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PIPE Metacharacter

• The PIPE Metacharacter is used for alternation
• / Bridget (Thomson McInnes) /
• Match Bridget Thomson or Bridget McInnes but
  NOT Bridget Thomson McInnes
• / B bridget /
• Match B or bridget
• / ( B b ) ridget /
• Match Bridget or bridget at the beginning of a
  line

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Our Simple Example

• Now with our example, suppose that we want to not
  only get all words that end in 'ing' but also
  'ed'.
• How would we change are regular expressions to
  accomplish this
• Previous Regular Expression
• word m/ ing /
• New Regular Expression
• wordm/ (inged) /

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The ? Metacharacter

• The metacharacter, ?, indicates that the
  character immediately preceding it occurs zero or
  one time
• Examples
• / worl?ds /
• Match either 'worlds' or 'words'
• / m?ethane /
• Match either 'methane' or 'ethane'

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The Metacharacter

• The metacharacter, , indicates that the
  characterer immediately preceding it occurs zero
  or more times
• Example
• / abc/ Match 'ac', 'abc', 'abbc', 'abbbc'
  ect...
• Matches any string that starts with an a, if
  possibly followed by a sequence of b's and ends
  with a c.
• Sometimes called Kleene's star

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Our Simple Example again

• Now suppose we want to create a list of all the
  words in our text that end in 'ing' or 'ings'
• How would we change are regular expressions to
  accomplish this
• Previous Regular Expression
• word m/ ing /
• New Regular Expression
• wordm/ ings? /

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Modifying Text

• Match
• Up to this point, we have seen attempt to match a
  given regular expression
• Example variable m/ regex /
• Substitution
• Takes match one step further if there is a
  match, then replace it with the given string
• Example variable s/ regex / replacement
• var / Thomson / McInnes /
• var / Bridgette / Bridget /

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Substitution Example

• Suppose when we find all our words that end in
  'ing' we want to replace the 'ing' with 'ed'.
• !/usr/local/bin/perl -w
• while(ltgt)
• chomp _
• _at_words split/ \s/, _
• foreach word(_at_words)
• if(words/ing/ed/) print
  word\n

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Special Variable Modified by a Match

• Copy of text matched by the regex
• '
• A copy of the target text in from of the match
• A copy of the target text after the match
• 1, 2, 3, ect
• The text matched by 1st, 2nd, ect., set of
  parentheses. Note 0 is not included here
• A copy of the highest numbered 1, 2, 3, ect..

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Our Simple Example once again

• Now lets revise are program to find all the words
  that end in 'ing' without splitting our line of
  text into an array of words
• !/usr/local/bin/perl -w
• while(ltgt)
• chomp _
• if(_/(A-Za-zing\b)/) print "\n"
  

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Example

• !/usr/local/bin
• exp ltSTDINgt chomp exp
• if(exp/(A-Za-z\s)\bcrave\b(\sA-Za-z)/)
  
• print 1\n
• print 2\n
• Run Program with string I crave to rule the
  world!
• Results
• I
• to rule the world!

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Example

• !/usr/local/bin
• exp ltSTDINgt chomp exp
• if(exp/\bcrave\b/)
• print \n print \n print \n
• Run Program with string I crave to rule the
  world!
• Results
• I
• crave
• to rule the world!

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Thank you ?