1. Introduction to Perl Part I, II, and III By: Bridget Thomson McInnes 20 January 2004

2. 2 What is Perl? Perl is a Portable Scripting Language Perl is a Portable Scripting Language –No compiling is needed. –Runs on Windows, UNIX and LINUX Fast and easy text processing capability Fast and easy text processing capability Fast and easy file handling capability Fast and easy file handling capability Written by Larry Wall Written by Larry Wall “Perl is the language for getting your job done.” “Perl is the language for getting your job done.”

3. 3 How to Access Perl Off the school network Off the school network –Located on the csdev machines at: /usr/local/bin/perl To install at home To install at home –www.perl.com Has rpm's for Linux www.perl.com –www.activestate.com Has binaries for Windows www.activestate.com Latest Version is 5.8 Latest Version is 5.8 –To check if Perl is working and the version number  % perl -v

4. 4 Resources For Perl Books: Books: –Learning Perl  By Larry Wall  Published by O'Reilly –Programming Perl  By Larry Wall,Tom Christiansen and Jon Orwant  Published by O'Reilly Web Site Web Site –http://safari.oreilly.com http://safari.oreilly.com  Contains both Learning Perl and Programming Perl in ebook form

5. 5 Web Sources for Perl Web Web –www.perl.com www.perl.com –www.perldoc.com www.perldoc.com –www.perl.org www.perl.org –www.perlmonks.org www.perlmonks.org

6. 6 The Basic Hello World Program Program: Program: #!/usr/local/bin/perl -w print “Hello World!\n”; Save this as “hello.pl” Save this as “hello.pl” Give it executable permissions Give it executable permissions –chmod ug+x hello.pl Run it as follows: Run it as follows: –./hello.pl

7. 7 “Hello World” Observations “.pl” extension is optional but is commonly used “.pl” extension is optional but is commonly used The first line “#!/usr/local/bin/perl” tells UNIX where to find Perl The first line “#!/usr/local/bin/perl” tells UNIX where to find Perl “-w” switches on warning : not required but a really good idea “-w” switches on warning : not required but a really good idea Second Line: brackets are not needed around the argument of the print function Second Line: brackets are not needed around the argument of the print function

8. 8 Numerical Literals Numerical Literals Numerical Literals –6Integer –12.6Floating Point –1e10Scientific Notation –6.4E-33Scientific Notation –4_348_348Underscores instead of commas for long numbers

9. 9 String Literals String Literals String Literals –“There is more than on way to do it!” –'Just don't create a file called -rf.' –“Beauty?\nWhat's that?\n” –“” –“Real programmers can write assembly in any language.  Quotes from Larry Wall

10. 10 Types of Variables Types of variables: Types of variables: –Scalar variables : $a, $b, $c –Array variables : @array –Hash variables : %hash –File handles : STDIN, SRC, DEST Variables do not need to be declared Variables do not need to be declared Variable type (int, char,...) is decided at run time Variable type (int, char,...) is decided at run time –$a = 5; # now an integer –$a = “perl”; # now a string

11. 11 Operators on Scalar Variables Numeric and Logic Operators Numeric and Logic Operators –Typical : +, -, *, /, %, ++, --, +=, -=, *=, /=, ||, &&, ! ect … –Not typical: ** for exponentiation String Operators String Operators –Concatenation: “.” - similar to strcat $first_name = “Larry”; $last_name = “Wall”; $full_name = $first_name. “ “. $last_name;

12. 12 Equality Operators for Strings Equality/ Inequality : eq and ne Equality/ Inequality : eq and ne $language = “Perl”; if ($language == “Perl”)...# Wrong! if ($language eq “Perl”)...#Correct –Use eq / ne rather than == / != for strings

13. 13 Relational Operators for Strings Greater than Greater than –Numeric : >String : gt Greater than or equal to Greater than or equal to –Numeric : >=String : ge Less than Less than –Numeric : >String : lt Less than or equal to Less than or equal to –Numeric : >=String : le

14. 14 String Functions Convert to upper case Convert to upper case –$name = uc($name); Convert only the first char to upper case Convert only the first char to upper case –$name = ucfirst($name); Convert to lower case Convert to lower case –$name = lc($name); Convert only the first char to lower case Convert only the first char to lower case –$name = lcfirst($name);

15. 15 A String Example Program #!/usr/local/bin/perl $var1 = “larry”; $var2 = “moe”; $var3 = “shemp”; print ucfirst($var1);# Prints 'Larry' print uc($var2);# Prints 'MOE' print lcfirst(uc($var3));# Prints 'sHEMP'

16. 16 Variable Interpolation Perl looks for variables inside strings and replaces them with their value Perl looks for variables inside strings and replaces them with their value $stooge = “Larry” print “$stooge is one of the three stooges.\n”; Produces the output: Larry is one of the three stooges. This does not happen when you use single quotes This does not happen when you use single quotes print '$stooge is one of the three stooges.\n”; Produces the output: $stooge is one of the three stooges.\n

17. 17 Character Interpolation List of character escapes that are recognized when using double quoted strings List of character escapes that are recognized when using double quoted strings –\nnewline –\ttab –\rcarriage return Common Example : Common Example : –print “Hello\n”; # prints Hello and then a return

18. 18 Numbers and Strings are Interchangeable If a scalar variable looks like a number and Perl needs a number, it will use it as a number If a scalar variable looks like a number and Perl needs a number, it will use it as a number $a = 4;# a number print $a + 18;# prints 60 $b = “50”;# looks like a string, but... print $b – 10; # will print 40!

19. 19 If... else... statements Similar to C/C++ - except the scope braces are REQUIRED!! Similar to C/C++ - except the scope braces are REQUIRED!! if ( $os eq “Linux” ) { print “Sweet!\n”; print “Sweet!\n”;} elsif ( $os eq “Windows” ) { print “Time to move to Linux, buddy!\n”; print “Time to move to Linux, buddy!\n”;} else { print “Hmm...!\n”; print “Hmm...!\n”;}

20. 20 Unless... else Statements Unless Statements are the opposite of if... else statements. Unless Statements are the opposite of if... else statements. Unless ($os eq “Linux”) { print “Time to move to Linux, buddy!\n”; print “Time to move to Linux, buddy!\n”;} else { print “Sweet!\n”; print “Sweet!\n”;} And again remember the braces are required! And again remember the braces are required!

21. 21 While Loop While loop: Similar to C/C++ but again the braces are required!! While loop: Similar to C/C++ but again the braces are required!! Example : Example : $i = 0; while ( $i <= 1000 ) { print “$i\n”; print “$i\n”; $i++; $i++;}

22. 22 Until Loop The until function evaluates an expression repeatedly until a specific condition is met. The until function evaluates an expression repeatedly until a specific condition is met. Example: Example: $i = 0; $i = 0; until ($i == 1000) { until ($i == 1000) { print “$i\n”; print “$i\n”; $i++; $i++;}

23. 23 For Loops Like C/C++ Like C/C++ –Example :  for ( $i = 0; $i <= 1000; $i++ ) { print “$i\n”; print “$i\n”; } Another way to create a for loop Another way to create a for loop –Example  for $i(0..1000) { print “$i\n”; print “$i\n”; }

24. 24 Moving around in a Loop Where you would use continue in C, use next. Where you would use continue in C, use next. Where you would use break in C, use last. Where you would use break in C, use last. What is the output for the following code snippet: What is the output for the following code snippet: for ( $i = 0; $i < 10; $i++) { if ($i == 1 || $i == 3) { next; } if ($i == 1 || $i == 3) { next; } if($i == 5) { last; } if($i == 5) { last; } print “$i\n”; print “$i\n”;}

25. Answer 024

26. 26 Arrays Array variable is denoted by the @ symbol Array variable is denoted by the @ symbol –@array = ( “Larry”, Curly”, “Moe” ); To access the whole array, use the whole array To access the whole array, use the whole array –print @array; # prints : Larry Curly Moe  Notice that you do not need to loop through the whole array to print it – Perl does this for you

27. 27 Arrays cont… To access one element of the array : use $ To access one element of the array : use $ –Why? Because every element in the array is scalar –print “$array[0]\n”; # prints : Larry Question: Question: –What happens if we access $array[3] ?  Answer : Nothing

28. 28 Arrays cont... To find the index of the last element in the array To find the index of the last element in the array print $#array; # prints 2 in the previous # example Note another way to find the number of elements in the array: Note another way to find the number of elements in the array: $array_size = @array; –$array_size now has 3 in the above example because there are 3 elements in the array

29. 29 Sorting Arrays Perl has a built in sort function Perl has a built in sort function Two ways to sort: Two ways to sort: –Default : sorts in a standard string comparisons order  sort LIST –Usersub: create your own subroutine that returns an integer less than, equal to or greater than 0  Sort USERSUB LIST  The and cmp operators make creating sorting subroutines very easy

30. 30 Numerical Sorting Example #!/usr/local/bin/perl -w @unsortedArray = (3, 10, 76, 23, 1, 54); @sortedArray = sort numeric @unsortedArray; print “@unsortedArray\n”; # prints 3 10 76 23 1 54 print “@sortedArray\n”; # prints 1 3 10 23 54 76 sub numeric { $a $b $a $b}

31. 31 #!/usr/local/bin/perl -w @unsortedArray = (Larry”, “Curly”, “moe”); @sortedArray = sort { lc($a) cmp lc($b)} @unsortedArray; print “@unsortedArray\n”; # prints Larry Curly moe print “@sortedArray\n”; # prints Curly Larry moe String Sorting Example

32. 32 Foreach Foreach allows you to iterate over an array Foreach allows you to iterate over an array Example: Example: foreach $element (@array) { print “$element\n”; print “$element\n”;} This is similar to : This is similar to : for ($i = 0; $i <= $#array; $i++) { print “$array[$i]\n”; print “$array[$i]\n”;}

33. 33 Sorting with Foreach The sort function sorts the array and returns the list in sorted order. The sort function sorts the array and returns the list in sorted order. Example : Example : @array( “Larry”, “Curly”, “Moe”); @array( “Larry”, “Curly”, “Moe”); foreach $element (sort @array) { foreach $element (sort @array) { print “$element ”; print “$element ”; } Prints the elements in sorted order: Prints the elements in sorted order: Curly Larry Moe Curly Larry Moe

34. 34 Strings to Arrays : split Split a string into words and put into an array Split a string into words and put into an array @array = split( / /, “Larry Curly Moe” ); # creates the same array as we saw previously # creates the same array as we saw previously Split into characters Split into characters @stooge = split( //, “curly” ); # array @stooge has 5 elements: c, u, r, l, y

35. 35 Split cont.. Split on any character Split on any character @array = split( /:/, “10:20:30:40”); # array has 4 elements : 10, 20, 30, 40 Split on Multiple White Space Split on Multiple White Space @array = split(/\s+/, “this is a test”; # array has 4 elements : this, is, a, test  More on ‘\s+’ later

36. 36 Arrays to Strings Array to space separated string Array to space separated string @array = (“Larry”, “Curly”, “Moe”); $string = join( “ “, @array); # string = “Larry Curly Moe” Array of characters to string Array of characters to string @stooge = (“c”, “u”, “r”, “l”, “y”); $string = join( “”, @stooge ); # string = “curly”

37. 37 Joining Arrays cont… Join with any character you want Join with any character you want @array = ( “10”, “20”, “30”, “40” ); $string = join( “:”, @array); # string = “10:20:30:40” Join with multiple characters Join with multiple characters @array = “10”, “20”, “30”, “40”); $string = join(“->”, @array); # string = “10->20->30->40”

38. 38 Arrays as Stacks and Lists To append to the end of an array : To append to the end of an array : @array = ( “Larry”, “Curly”, “Moe” ); push (@array, “Shemp” ); print $array[3]; # prints “Shemp” To remove the last element of the array (LIFO) To remove the last element of the array (LIFO) $elment = pop @array; print $element; # prints “Shemp” –@array now has the original elements (“Larry”, “Curly”, “Moe”) (“Larry”, “Curly”, “Moe”)

39. 39 Arrays as Stacks and Lists To prepend to the beginning of an array To prepend to the beginning of an array @array = ( “Larry”, “Curly”, “Moe” ); unshift @array, “Shemp”; print $array[3]; # prints “Moe” print “$array[0];# prints “Shemp” To remove the first element of the array To remove the first element of the array $element = shift @array; print $element; # prints “Shemp” –The array now contains only :  “Larry”, “Curly”, “Moe”

40. 40 Hashes Hashes are like array, they store collections of scalars Hashes are like array, they store collections of scalars... but unlike arrays, indexing is by name... but unlike arrays, indexing is by name Two components to each hash entry: Two components to each hash entry: –Keyexample : name –Value example : phone number Hashes denoted with % Hashes denoted with % –Example : %phoneDirectory Elements are accessed using {} (like [] in arrays) Elements are accessed using {} (like [] in arrays)

41. 41 Hashes continued... Adding a new key-value pair Adding a new key-value pair $phoneDirectory{“Shirly”} = 7267975 $phoneDirectory{“Shirly”} = 7267975 –Note the $ to specify “scalar” context! Each key can have only one value Each key can have only one value $phoneDirectory{“Shirly”} = 7265797 $phoneDirectory{“Shirly”} = 7265797 # overwrites previous assignment # overwrites previous assignment Multiple keys can have the same value Multiple keys can have the same value Accessing the value of a key Accessing the value of a key $phoneNumber =$phoneDirectory{“Shirly”}; $phoneNumber =$phoneDirectory{“Shirly”};

42. 42 Hashes and Foreach Foreach works in hashes as well! Foreach works in hashes as well! foreach $person (keys %phoneDirectory) { print “$person: $phoneDirectory{$person}”; print “$person: $phoneDirectory{$person}”;} Never depend on the order you put key/values in the hash! Perl has its own magic to make hashes amazingly fast!! Never depend on the order you put key/values in the hash! Perl has its own magic to make hashes amazingly fast!!

43. 43 Hashes and Sorting The sort function works with hashes as well The sort function works with hashes as well Sorting on the keys Sorting on the keys foreach $person (sort keys %phoneDirectory) { print “$person : $directory{$person}\n”; print “$person : $directory{$person}\n”;} –This will print the phoneDirectory hash table in alphabetical order based on the name of the person, i.e. the key.

44. 44 Hash and Sorting cont... Sorting by value Sorting by value foreach $person (sort {$phoneDirectory{$a} $phoneDirectory{$b}} keys %phoneDirectory) { print “$person : $phoneDirectory{$person}\n”; print “$person : $phoneDirectory{$person}\n”;} –Prints the person and their phone number in the order of their respective phone numbers, i.e. the value.

45. 45 A Quick Program using Hashes Count the number of Republicans in an array Count the number of Republicans in an array %seen = (); # initialize hash to empty @politArray = ( “R”, “R”, “D”, “I”, “D”, “R”, “G” ); foreach $politician (@politArray) { $seen{$politician}++; $seen{$politician}++;} print “Number of Republicans = $seen{'R'}\n”;

46. 46 Slightly more advanced program Count the number of parties represented, and by how much! Count the number of parties represented, and by how much! %seen = (); # initialize hash to empty @politArray = ( “R”, “R”, “D”, “I”, “D”, “R”, “G” ); foreach $politician (@politArray) { $seen{$politician}++; $seen{$politician}++;} foreach $party (keys %seen) { print “Party : $party. Num reps: $seen{$party}\n”; print “Party : $party. Num reps: $seen{$party}\n”;}

47. 47 Command Line Arguments Command line arguments in Perl are extremely easy. @ARGV is the array that holds all arguments passed in from the command line. – –Example:   %./prog.pl arg1 arg2 arg3 – –@ARGV would contain ('arg1', arg2', 'arg3) $#ARGV returns the number of command line arguments that have been passed. – –Remember $#array is the size of the array!

48. 48 Quick Program with @ARGV Simple program called log.pl that takes in a number and prints the log base 2 of that number; #!/usr/local/bin/perl -w $log = log($ARGV[0]) / log(2); print “The log base 2 of $ARGV[0] is $log.\n”; Run the program as follows: – –% log.pl 8 This will return the following: – –The log base 2 of 8 is 3.

49. 49 Another Example Program You want to print the binary form of an integer You want to print the binary form of an integer #!/usr/local/bin/perl -w foreach $integer (@ARGV) { # converts the integer to a 32 bit binary number # converts the integer to a 32 bit binary number @binary=split//,unpack(“B32”,pack(“N”,$integer)); @binary=split//,unpack(“B32”,pack(“N”,$integer)); # Store the last 4 elements of @binary into @bits # Store the last 4 elements of @binary into @bits @bits = @binary[28..$#binary]; @bits = @binary[28..$#binary]; # Print the integer and its binary form # Print the integer and its binary form print “$integer : @bits\n”; print “$integer : @bits\n”;}

50. 50 File Handlers Very simple compared to C/ C++ !!! Very simple compared to C/ C++ !!! Are not prefixed with a symbol ($, @, %, ect) Are not prefixed with a symbol ($, @, %, ect) Opening a File: Opening a File: open (SRC, “my_file.txt”); Reading from a File Reading from a File $line = ; # reads upto a newline character Closing a File Closing a File close (SRC);

51. 51 File Handlers cont... Opening a file for output: Opening a file for output: open (DST, “>my_file.txt”); Opening a file for appending Opening a file for appending open (DST, “>>my_file.txt”); Writing to a file: Writing to a file: print DST “Printing my first line.\n”; Safeguarding against opening a non existent file Safeguarding against opening a non existent file open (SRC, “file.txt”) || die “Could not open file.\n”;

52. 52 File Test Operators Check to see if a file exists: Check to see if a file exists: if ( -e “file.txt”) { # The file exists! # The file exists!} Other file test operators: Other file test operators: -rreadable -xexecutable -dis a directory -Tis a text file

53. 53 Quick Program with File Handles Program to copy a file to a destination file 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(DST newfile.txt”); while ( $line = ) { print DST $line; print DST $line;} close SRC; close DST;

54. 54 Some Default File Handles STDIN : Standard Input STDIN : Standard Input $line = ; # takes input from stdin STDOUT : Standard output STDOUT : Standard output print STDOUT “File handling in Perl is sweet!\n”; STDERR : Standard Error STDERR : Standard Error print STDERR “Error!!\n”;

55. 55 The <> File Handle The “empty” file handle takes the command line file(s) or STDIN; The “empty” file handle takes the command line file(s) or STDIN; –$line = <>; 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 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. 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.

56. 56 The <> 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 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

57. 57 Example Program with STDIN Suppose you want to determine if you are one of the three stooges Suppose you want to determine if you are one of the three stooges#!/usr/local/bin/perl %stooges = (larry => 1, moe => 1, curly => 1 ); print “Enter your name: ? “; $name = ; chomp $name; if($stooges{lc($name)}) { print “You are one of the Three Stooges!!\n”; print “You are one of the Three Stooges!!\n”; } else { print “Sorry, you are not a Stooge!!\n”; print “Sorry, you are not a Stooge!!\n”;}

58. 58 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”

59. 59 $_ Perl default scalar value that is used when a variable is not explicitly specified. Can be used in – –For Loops – –File Handling – –Regular Expressions – discussed later

60. 60 $_ and For Loops Example using $_ in a for loop @array = ( “Perl”, “C”, “Java” ); for(@array) { print $_. “is a language I know\n”; } – –Output : Perl is a language I know. C is a language I know. Java is a language I know.

61. 61 $_ and File Handlers Example in using $_ when reading in a file; while( <> ) { chomp $_; # remove the newline char @array = split/ /, $_; # split the line on white space # and stores data in an array } Note: – –The line read in from the file is automatically store in the default scalar variable $_

62. 62 $_ and File Handling cont.. Another example similar to the previous example: while(<>) { chomp; # removes trailing newline chars @array = split/ /; # splits the line on white # space and stores the data # in the array } Notes: – –The functions chomp and split automatically perform their respective operations on $_.

63. 63 Example Program Count the number of words in a text and display the top 10 most frequency words. Count the number of words in a text and display the top 10 most frequency words.#!/usr/local/bin/perl %vocab = (); $counter = 0; while(<>) { chomp; foreach $element (split/ /) { $vocab{$element}++; } } foreach $word (sort {$vocab{$b} $vocab{$a}} %vocab) { print “$word $vocab{$word}\n”; print “$word $vocab{$word}\n”; if($counter == 10) { last; } $counter++; if($counter == 10) { last; } $counter++;}

64. 64 Regular Expressions What are Regular Expressions.. a few definitions. 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 Some things you can do with regular expressions –Parse the text –Add and/or replace subsections of text –Remove pieces of the text

65. 65 Regular Expressions cont.. A regular expression characterizes a regular language A regular expression characterizes a regular language Examples in UNIX: 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'

66. 66 Simple Example for... ? Clarity In the simplest form, a regular expression is a string of characters that you are looking for 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. We want to find all the words that contain the string 'ing' in our text. The regular expression we would use : The regular expression we would use : /ing/ /ing/

67. 67 Simple Example cont... What would are program then look like: What would are program then look like:#!/usr/local/bin/perl while(<>) { chomp; chomp; @words = split/ /, $_; @words = split/ /, $_; foreach $word(@words) { foreach $word(@words) { if($word=~m/ing/) { print “$word\n”; } if($word=~m/ing/) { print “$word\n”; } }}

68. 68 Regular Expressions Types Regular expressions are composed of two types of characters: 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

69. 69 Metacharacters Match more than just characters Match more than just characters Match line position Match line position –^start of a line( carat ) –$end of a line( dollar sign ) Match any characters in a list : [... ] Match any characters in a list : [... ] Example : Example : –/[Bb]ridget/matches Bridget or bridget – /Mc[Ii]nnes/matches McInnes or Mcinnes

70. 70 Our Simple Example Revisited Now suppose we only want to match words that end in 'ing' rather than just contain 'ing'. 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: How would we change are regular expressions to accomplish this: –Previous Regular Expression: $word =~m/ ing / $word =~m/ ing / –New Regular Expression: $word=~m/ ing$ / $word=~m/ ing$ /

71. 71 Ranges of Regular Expressions Ranges can be specified in Regular Expressions Ranges can be specified in Regular Expressions Valid Ranges 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 Invalid Ranges Invalid Ranges –[a-Z]Not Valid –[A-z]Not Valid –[F-A]Not Valid

72. 72 Ranges cont... Ranges of Digits can also be specified Ranges of Digits can also be specified –[0-9]Valid –[9-0]Invalid Negating Ranges 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

73. 73 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' 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: How would we change are regular expressions to accomplish this: –Previous Regular Expression: $word =~m/ ing$ / $word =~m/ ing$ / –New Regular Expression: $word=~m/ [^ ing]$ / $word=~m/ [^ ing]$ /

74. 74 Literal Metacharacters Suppose that you actually want to look for all strings that equal '^' in your text 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? What does the following Regular Expressions Match? / [ A - Z ^ ] ^ / / [ A - Z ^ ] ^ / –Matches any line that contains ( A-Z or ^) followed by ^

75. 75 Patterns provided in Perl Some Patterns 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 ] Example :[ 19\d\d ] –Looks for any year in the 1900's

76. 76 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(<>) { chomp; @words = split/\s+/, $_; foreach $word(@words) { if($word=~m/ing/) { print “$word\n”; }

77. 77 Word Boundary Metacharacter Regular Expression to match the start or the end of a 'word' : \b Regular Expression to match the start or the end of a 'word' : \b Examples: 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

78. 78 DOT Metacharacter The DOT Metacharacter, '.' symbolizes any character except a new line The DOT Metacharacter, '.' symbolizes any character except a new line / b. bble/ / b. bble/ –Would possibly return : bobble, babble, bubble /. oat/ /. 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. Note: remember '.*' usually means a bunch of anything, this can be handy but also can have hidden ramifications.

79. 79 PIPE Metacharacter The PIPE Metacharacter is used for alternation The PIPE Metacharacter is used for alternation / Bridget (Thomson | McInnes) / / Bridget (Thomson | McInnes) / –Match Bridget Thomson or Bridget McInnes but NOT Bridget Thomson McInnes / B | bridget / / B | bridget / –Match B or bridget / ^( B | b ) ridget / / ^( B | b ) ridget / –Match Bridget or bridget at the beginning of a line

80. 80 Our Simple Example Now with our example, suppose that we want to not only get all words that end in 'ing' but also 'ed'. 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: How would we change are regular expressions to accomplish this: –Previous Regular Expression: $word =~m/ ing$ / $word =~m/ ing$ / –New Regular Expression: $word=~m/ (ing|ed)$ / $word=~m/ (ing|ed)$ /

81. 81 The ? Metacharacter The metacharacter, ?, indicates that the character immediately preceding it occurs zero or one time The metacharacter, ?, indicates that the character immediately preceding it occurs zero or one time Examples: Examples: –/ worl?ds /  Match either 'worlds' or 'words' –/ m?ethane /  Match either 'methane' or 'ethane'

82. 82 The * Metacharacter The metacharacter, *, indicates that the characterer immediately preceding it occurs zero or more times The metacharacter, *, indicates that the characterer immediately preceding it occurs zero or more times Example : Example : –/ ab*c/ 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 Sometimes called Kleene's star

83. 83 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: $word=~m/ ings?$ /

84. 84 Modifying Text Match Match –Up to this point, we have seen attempt to match a given regular expression –Example : $variable =~m/ regex / Substitution 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 =~ / Thomson / McInnes /; $var =~ / Bridgette / Bridget /; $var =~ / Bridgette / Bridget /;

85. 85 Substitution Example Suppose when we find all our words that end in 'ing' we want to replace the 'ing' with 'ed'. 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(<>) { chomp $_; chomp $_; @words = split/ \s+/, $_; @words = split/ \s+/, $_; foreach $word(@words) { foreach $word(@words) { if($word=~s/ing$/ed/) { print “$word\n”; } if($word=~s/ing$/ed/) { print “$word\n”; } }}

86. 86 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 $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..

87. 87 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 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(<>) { chomp $_; chomp $_; if($_=~/([A-Za-z]*ing\b)/) { print "$&\n"; } if($_=~/([A-Za-z]*ing\b)/) { print "$&\n"; }}

88. Thank you Thank you