Perl Workshop

C. David SherrillCenter for Computational Molecular

Science & TechnologyGeorgia Institute of Technology

References

• These notes follow the progression given by the introductory book, “PERL in easy steps,” by Mike McGrath (Computer Step, Warwickshire, UK, 2004)

• Another good book is “Learning PERL,” by Randal L. Schwartz, Tom Phoenix, and Brian D. Foy (O’Reilly, 2005)

• See also www.perl.org and www.perl.com

Perl at a Glance

• High-level language• Popular• Easy to use for processing outputs• Good for web CGI scripts• Interpreted language --- not high-

performance• Remember to make your scripts

executable (e.g., chmod u+x [scriptname])

Part 1: Variables and Printing

Printing in Perl

#!/usr/bin/perl

print “this is a test\n”;# slash will escape quotesprint “I said \”hello!\” \n”;print << “DOC”;Any stuff between here & DOC will be printedDOC

Scalar variables

• Perl doesn’t have strong typing like C/C++ for Fortran

• Perl tries to be smart about how to handle the type of a variable depending on context

• Can have scalar floating point numbers, integers, strings (in C, a string is not a fundamental scalar type)

• Scalars are designated by the $ symbol, e.g., $x

Scalar variable example#!/usr/bin/perl

# initialize a string$greeting = “hello”;

# initialize an integer $number = 5;

# initialize a floating point number$energy = -10.823;

print “Let me say $greeting\n”;print “There are $number problems on the test\n”;print “The energy is $energy\n”;

Formatted output

• It is also possible to print according to a specified format, like the printf() function in C

#!/usr/bin/perl$pi = 3.1415926;printf “%6.3f\n”, $pi;# prints pi in a field 6 characters long with# 3 digits after the decimal, rounding up# 3.142

Array variables

• Unlike C or Fortran, an array in Perl can contain a mixture of any kinds of scalars

• Assigning an array to a scalar makes the scalar equal the length of the array (example of Perl trying to be smart)

• Arrays are designated by the @ symbol, e.g., @a

Array example#!/usr/bin/perl

# set up an array@array = (“hi”, 42, “hello”, 99.9);

# print the whole arrayprint “The array contains: @array\n”;

# access the 2nd element --- counting starts from 0# note also we use scalar syntax ($) for a particular element# because a single element is a scalarprint “The second element is $array[1]\n”;# this prints 42 not “hi”

$length = @array;print “There are $length elements in the array\n”;

Hash variables• These contain key/value pairs and start with the % symbol, e.g., %h

#!/usr/bin/perl %h = (“name”, “David”, “height”, 6.1, “degree”, “Ph.D.”);

# Note that each element of %h when accessed is a scalar, so# use $ syntax to access an element, not %

print << “DOC”;Name: $h{“name”}Height: $h{“height”}Degree: $h{“degree”}DOC

Part 2: Operators

Arithmetic operators

• + : Addition• - : Subtraction• * : Multiplication• ** : Exponential• / : Division• % : Modulus (remainder)• ++: Increment • -- : Decrement

Arithmetic operators example#!/usr/bin/perl

$x = 3;$y = 5;

$z = $x + $y;print "$x + $y = $z\n";# 3 + 5 = 8

$z = ++$x + $y;print "$x + $y = $z\n";# 4 + 5 = 9

$x = 3;

# watch out for this one$z = $x++ + $y;print "$x + $y = $z\n";# 4 + 5 = 8

Assignment operators

Operator Example Same as

= a = b a = b

+= a += b a = a + b

-= a -= b a = a – b

*= a *= b a = a * b

/= a /= b a = a / b

%= a %= b a = a % b

Logical operators

Operator Does&& Logical AND|| Logical OR! Logical NOT

•These logical operators are very similar to those in C•Used with operands that have boolean values TRUE •and FALSE, or which can be converted to these values; typically 1 means TRUE and 0 means FALSE•Unlike in C, FALSE is not always evaluated as 0. In the case of ! for NOT, !1 evaluates as a blank

Example of logical operators#!/usr/bin/perl

$x = 1; $y = 0;

# example of AND$z = $x && $y;print "$x && $y = $z\n";# prints 1 && 0 = 0

# example of OR$z = $x || $y;print "$x || $y = $z\n";# prints 1 || 0 = 1

# example of NOT$z = !$y;print "!$y = $z\n";# prints !0 = 1

# example of NOT$z = !$x;print "!$x = $z\n";# prints !1 = 0 ? No, actually it leaves $z as a blank!

Numerical comparison

• < = > returns -1, 0, or 1 if the left side is less than, equal to, or greater than the right side

• Other operators return TRUE if the comparison is true, otherwise it will be blank!

Operator Comparison== Is equal?!= Not equal?< = > Left-to-right comp> Greater?< Less than?>= Greater or equal?<= Less than or

equal?

Numerical comparison example

#!/usr/bin/perl

$z = (2 != 3);print "(2 != 3) = $z\n";# prints (2 != 3) = 1

$z = (2 == 3);print "(2 == 3) = $z\n";# prints (2 == 3) =

String comparison

• Every individual character, like “A”, has a numerical code equivalent given by the ASCII table

Operator Comparison/Actioneq is equal?

ne not equal?

gt greater than?

Lt less than?

cmp -1, 0, or 1, depending

. concatenation

x repeat

uc(string) convert to upper case

lc(string) convert to lower case

chr(num) get char for ASCII num

ord(char) get ASCII num of char

String comparison example#!/usr/bin/perl

$a = "hi";$b = "hello";

$equal = $a eq $b;print "$a eq $b = $equal\n";

$equal = $a eq $a;print "$a eq $a = $equal\n";

$equal = $a ne $b;print "$a ne $b = $equal\n";

$compare = $a cmp $b;print "$a cmp $b = $compare\n";

$compare = $b cmp $a;print "$b cmp $a = $compare\n";

String operators example#!/usr/bin/perl

$a = "hi";$b = "hello";

$c = $a . $b;print "c = $c\n";# prints "c = hihello"

$c = uc($a);print "uc($a) = $c\n";# prints "uc(hi) = HI"

$c = $a x 5;print "$a x 5 = $c\n";# prints "hi x 5 = hihihihihi"

The range operator

• The range operator, .., fills in a range of values in between the endpoints

• @numbers = (1..10) gives @numbers = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

• @letters = (“a”..”z”) gives an array with all letters “a” through “z”

• A “for” statement can also use a range operator to loop through a range, e.g., “for (1..10) { print “hi” };” would print “hi” 10 times

Math functions• PERL has several built-in mathematical functions

Function Operation

abs(x) return absolute value of x

sin(x) return sine of x

cos(x) return cosine of x

hex(string) decimal value of hexadecimal string

oct(string) decimal value of octal string

sqrt(x) return square root of x

Part 3: Loops and Conditions

IF statements• If the test expression is true, then execute the statement(s) following

#!/usr/bin/perl$major = “chemistry”;

if ($major eq “chemistry”) { print “Welcome, chemistry student!\n”;}if ($major ne “chemistry”) { print “You’re not a chemistry student.\n”; print “Why not?\n”;}# note: need the curly braces

IF/ELSE statements• Sometimes more convenient than just “IF” statements

#!/usr/bin/perl

$major = "chemistry";

if ($major eq "chemistry") { print "Welcome, chemistry student!\n";}else { print "You're not a chemistry student.\n"; print "Why not?\n";}# note: need the curly braces

ELSIF statements• “elsif” is read as “else if”. It’s an “else” that has an “if” condition attached to it; useful

in picking one possibility out of a list of several

#!/usr/bin/perl

$grade = "F";

if ($grade eq "A") { print "Excellent!\n";}elsif ($grade eq "B") { print "Good work.\n";}elsif ($grade eq "C") { print "Needs improvement.\n";}else { print "I suggest you start coming to office hours.\n";}

FOR loop• Loop (repeatedly execute a statement block) until a

given condition is met• for (initializer, test, increment/decrement) {statement

block}

for ($i=0; $i<3; $i++) { print "i = $i\n";}# prints the following:# i = 0# i = 1# i = 2

WHILE loops• Execute the statement block while a certain condition holds; watch

out to avoid infinite loops!

# important to initialize variable before loop!$i=0;

while ($i<3) { print "i = $i\n"; $i++; # need this line to avoid infinite loop!}# prints the following:# i = 0# i = 1# i = 2

DO/WHILE loops• Like “WHILE” but always executes at least once; test is made at end not

beginning of statement block• There is a related “DO/UNTIL” loop

# important to initialize variable before loop!$i=0;

do { print "i = $i\n"; $i++; # need this line to avoid infinite loop!}while ($i < 3);# prints the following:# i = 0# i = 1# i = 2

NEXT statement• Skip to next iteration of a loop• Equivalent to C’s “continue” statement

for ($i=0; $i<3; $i++){ if ($i == 1) { next } print "i = $i\n";}# prints the following:# i = 0# i = 2

LAST statement• Skip out of loop and exit it completely• Equivalent to C’s “break” statement

for ($i=0; $i<3; $i++){ if ($i == 1) { last } print "i = $i\n";}# prints the following:# i = 0

Part 4: Arrays

Working with arrays

• Elements are accessed by number, starting from 0; can use -1 to access the last element in the array

• A particular element of an array is accessed using $ syntax not @ (because each element is a scalar, not an array)

• To make an array of strings, the function qw() is a shortcut to put a list of items in quotes

Array example#!/usr/bin/perl

@names1 = ("David", "Daniel", "Justin");@names2 = qw(Mutasem Micah Arteum); # avoid annoying quotes

print "@names1\n";# prints David Daniel Justin

print "@names2\n";# prints Mutasem Micah Arteum

print "$names1[1]\n";# prints Daniel, *not* David!

print “$names1[-1]\n”;# prints last element, Justin

Converting scalars to arrays

• Can take a scalar (like a text string) and split it into components (like individual words) and place them in an array

• Most frequently split using spaces or commas

• Use the split() function

Scalars to arrays example#!/usr/bin/perl

$string = "We are learning PERL";@words = split(/ /,$string);

print "@words\n";# prints "We are learning PERL"

print "$words[1]\n";# prints "are"

$prime_list = "1,3,5,7,11";@primes = split(/,/,$prime_list);

print "@primes\n";# prints 1 3 5 7 11

Going through all elements• “foreach” statement creates a loop that goes through all the elements in an

array

#!/usr/bin/perl

@tasks = qw(plan simulation analysis);

$i=0;foreach $task(@tasks) { print "Task $i: $task\n"; $i++;}

# prints the following:# Task 0: plan# Task 1: simulation# Task 2: analysis

Copying parts of arrays#!/usr/bin/perl

@tasks = qw(plan simulation analysis);@priorities = @tasks[0,1];

print "Tasks are: @tasks\n";print "Priorities are: @priorities\n";

# prints the following:# Tasks are: plan simulation analysis# Priorities are: plan simulation

$tasks[1] = "computation"; #changes @tasks not @prioritiesprint "Tasks are: @tasks\n";print "Priorities are: @priorities\n";

# prints the following:# Tasks are: plan computation analysis# Priorities are: plan simulation

shift/unshift and push/pop functions

• shift() deletes the first element of the array and returns that value

• unshift() adds a new element or elements to the beginning array

• pop() deletes the last element of the array and returns that value

• push() adds an element or elements to the end of the array

Example of shift/unshift#!/usr/bin/perl

@grades = (100, 90, 89);print "Grades are: @grades\n";# Grades are: 100, 90, 89

unshift(@grades,54);print "Grades are: @grades\n";# Grades are: 54, 100, 90, 89

$deleted = shift(@grades);print "Deleted the grade $deleted\n";print "Grades are now: @grades\n";# Deleted the grade 54# Grades are now: 100, 90, 89

Other array tricks• Combine two arrays like

@new = (@arr1, @arr2);• Replace an individual element like

$arr[0] = 42;• Get the length of an array like

$len = @array;• Take a “slice” (subset) of an array

@subset = @arr[0,5];• Get the reverse of an array

@rev = reverse(@arr);

Sorting• Can sort the elements of an array alphabetically; will not change the original

array but can assign result to a new array. $a and $b are temp strings.

@students = qw(Robert Amanda Chris Jan);print "students are: @students\n";# students are: Robert Amanda Chris Jan

@students1 = sort{$a cmp $b}@students;@students2 = sort{$b cmp $a}@students;

print "students1 : @students1\n";# students1 : Amanda Chris Jan Robertprint "students2 : @students2\n";# students2 : Robert Jan Chris Amanda

• Could do similar thing with numbers but using {$a $b} for comparison

Part 5: Hashes

Hashes• Key-value pairs; hash variables start with % symbol• Very useful for keeping data from HTML forms• Access a value by giving its associated key in curly

brackets; the accessed value is a scalar, not a hash, so use $ in front

%hash = qw(first David last Sherrill);# need slash below to distinguish the inner quotes# in the hash lookup# from the outer quotes of the print statementprint "first name: $hash{\"first\"}\n";# first name: David

Slice of a hash• Can take a slice (subset) of hash values, similar to taking

a slice of an array. The result is an array of hash values.• Specify the key names of the desired elements, in

quotes, separated by commas. Taking an array, use array syntax.

%hash = qw(first David last Sherrill job Professor);

@names = @hash{"first","last"};print "names: @names\n";# names: David Sherrill

Getting all keys or all values• Can get a list of all keys or all values in a hash using the keys() and

values() functions, which take the name of the hash as the argument

• Warning: the order of the keys/values is not necessarily the same as the original ordering

%hash = qw(first David last Sherrill job Professor);

@karr = keys(%hash);print "keys: @karr\n";# keys: first last job

@varr = values(%hash);print "values: @varr\n";# values: David Sherrill Professor

Looping through hash elements• Can loop through the elements of a hash using the “foreach”

statement; like a “for” loop but goes through an array of elements• Similar to “foreach” in shells like tcsh• %hash = qw(first David last Sherrill job Professor);

foreach $i (keys(%hash)){ # note: below we do $hash not %hash print "The key is $i and the value is $hash{$i}\n";}

# The key is first and the value is David# The key is last and the value is Sherrill# The key is job and the value is Professor

Deleting key/value pairs• Can delete a pair using the “delete” statement followed by the value (a

scalar) to delete

%hash = qw(first David last Sherrill job Professor);

delete $hash{"job"};

foreach $i (keys(%hash)){ # note: below we do $hash not %hash print "The key is $i and the value is $hash{$i}\n";}

# The key is first and the value is David# The key is last and the value is Sherrill

Does a key exist?• Can check if a key exists in a hash using the “exist” keyword; returns 1 if exists,

“blank” if not (can be converted to 0 when necessary)

%hash = qw(first David last Sherrill);

$check_first = exists $hash{"first"};$check_age = exists $hash{"age"};

# "false" doesn't show up as a 0 unless "forced"$num = ( $check_age == 0 ) ? 0 : 1;

print "Does first exist? $check_first\n";# Does first exist? 1

print "Does age exist? $check_age\n";# Does age exist?

print "variable num = $num\n";# variable num = 0

Part 6: Text Files

Reading a text file• Use “open” and “close” functions• Need a “file handle” to represent the file• Use equality operator to read a line or an array of (all)

lines

# Note: file random.txt must be in same directory, or else# must specify an absolute path

open(TXT, "<random.txt"); # open the file for reading$line = <TXT>; # get the first line (note scalar)close(TXT); # close file again

print "The first line of the file is: $line\n";

Reading the whole file

• To get all the lines, simply assign <filehandle> to an array variable

open(TXT, "<random.txt"); # open the file for reading@lines = <TXT>; # get all the linesclose(TXT); # close file again

print "The file contains:\n";print @lines;

Writing to a text file

• Use the > symbol in front of the filename to write, instead of < to read

open(TXT, ">written.txt"); # open the file for writingprint TXT "hello, testing!\n"; # write a lineprint TXT "end of test.\n"; # write another lineclose(TXT); # close file again

Appending to a text file

• To append (add to the end of an existing file), use the >> symbol before the filename instead of >

open(TXT, ">>written.txt"); # open the file for writingprint TXT "Add a line!\n"; # write an additional lineclose(TXT); # close file again

Exclusive access

• Errors or unexpected behavior might result if two programs tried to write to the same file at the same time

• Can prevent this by putting a “lock” on the file, preventing other programs from accessing the file until the first program has completed the essential operation

#!/usr/bin/perl

# Note: file testfile.txt must be in same directory, or else# must specify an absolute path

open(FP, ">testfile.txt"); # open the file for writing# note - not all platforms support flock()flock(FP, 2); # lock the fileprint FP "Hello!\n"; # write a lineflock(FP, 8); # release the fileclose(FP); # close file again

File locking example

Detecting read/write errors• If a file operation has an error, it typically returns an error message

to the $! variable• This example previews subroutines

open(FP, "<junk.txt") || &pr_error($!);@lines = <FP>;close(FP);

foreach $line(@lines){ print "$line";}

sub pr_error{ print "Received error on opening file.\n"; print "Error message: $_[0]\n"; exit;}

Renaming and deleting files

• To rename a filerename(“old_filename”, “new_filename”);

• To delete a file (don’t use unless you’re sure!) unlink(“file_to_delete”);

File status checks

Operator Operation

-e Does file exist?

-d Is the “file” a directory?

-r Is the file readable?

-w Is the file writable?

-x Is the file executable?

Status check example$file = "crazy_file.txt";

# Another example of TRUE=1, FALSE=blank# Will print blank if file doesn't exist$e = (-e $file);print "Variable \$e = $e\n";

# The following ? : logic still works thoughprint "The file $file ";print $e ? "exists\n" : "does not exist\n";

Files in a directory• Can get all the files in a given directory using the

opendir() function

opendir(CDIR, "."); # . gives current directory@filenames = readdir(CDIR); # get all the filenames@filenames = sort(@filenames); # sort them!closedir(CDIR);

foreach $filename(@filenames){ print "$filename\n";}

Selecting certain filenames• Can use the grep() function, in conjunction with

a “regular expression” (see later), to select only certain filenames

opendir(CDIR, "."); # . gives current directory# get only filenames ending in .txt; escape the . character@filenames = grep( /\.txt/, readdir(CDIR));@filenames = sort(@filenames); # sort them!closedir(CDIR);

foreach $filename(@filenames){ print "$filename\n";}

Setting permissions

• Can set the file permissions on a file or directory using the chmod() function which works like the UNIX command

Permissions Owner Group Others

0777 rwx rwx rwx

0755 rwx r-x r-x

0644 rw- r-- r--

chmod() exampleif (-e "chmodtest"){ chmod(0755, "chmodtest") || &pr_error($!);}else{ print "Can't find file chmodtest\n";}

sub pr_error{ print "Error: $_[0]\n"; exit;}

Making and deleting directories

• Make a directory (needs UNIX permissions code)

mkdir(“subdir”, 0755);• Delete a directory rmdir(“subdir”);• Best to check for errors, e.g., rmdir(“subdir”) || &pr_error($!);

Changing working directory

• The script usually assumes it is working in the same directory it resides in

• This means files in other locations need to be addressed with full or relative paths

• Instead, can tell PERL to use a different “working” directory and then use “local” filenames

• chdir(“../docs”); # go back up to the “docs” directory and do all subsequent work in there

Part 7: Subroutines

Subroutines

• When a complex operation is required several times, it is best to move it into a subroutine, which is a block of code that can be “called” from anywhere in the main body of the script

• Subroutines are generally defined at the bottom of the file; definitions use the “sub” statement

• Subroutines are called using the name of the subroutine preceded by an ampersand

Subroutine exampleprint "The start of the main body.\n";&great_perl;print "The end of the main body.\n";

sub great_perl{ print "From the subroutine; PERL is great!\n";}

# This prints:# The start of the main body.# From the subroutine; PERL is great!# The end of the main body.

Passing values

• Values are passed to a subroutine using a special “underscore array” @_

• Use $_[0], $_[1], etc, to access each element

• PERL doesn’t necessarily enforce the number and/or type of arguments to a subroutine

Example of arguments&print_args("One", "Two", "Three");

sub print_args{ $i = 1; foreach $arg(@_) { print "Argument $i is $arg.\n"; $i++; }}

# This prints:# Argument 1 is One# Argument 2 is Two# Argument 3 is Three

Library files

• Subroutines may be collected into a library file with a filename ending in “.lib”.

• Library files do not need to start with a #! line

• Library files do need to end with a “1” on the last line

Library exampleContents of file “test.7.3.lib”:

sub howdy{ print "hello!\n";}

sub goodbye{ print "goodbye!\n";}

1

Calling library subroutines#!/usr/bin/perl

require "test.7.3.lib";

&howdy;&goodbye;

# Prints:# hello!# goodbye!

Return values• Subroutines can return values, like a function; in this

instance, we do not use the & prefix

$x = 5;$y = square($x);print "$x squared is $y\n";

sub square{ return $_[0] * $_[0];}

# Prints:# 5 squared is 25

Returning a series of values@z = square(4,5);print "The squares are: @z\n";

sub square{ # return a comma-separated series of values return (($_[0] * $_[0]),($_[1] * $_[1]));}

# Prints:# The squares are: 16 25

Returning an explicit array@z = square(1,2,3,4);print "The squares are: @z\n";

sub square{ $i = 0; foreach $element(@_) { # note that we never declared @z or said how big $z[$i] = $element * $element; $i++; } return(@z);}

# Prints:# The squares are: 1 4 9 16

Part 8: Miscellaneous Functions

Miscellaneous Functions

• rand(): generate a random number between 0 and the argument

• int(): convert the argument to an integer• chop(): remove the final character of a text

string• chomp(): remove any unseen characters

(e.g., newlines) from the end of a string

Random example#!/usr/bin/perl

$x = rand(100);print "x = $x\n";# prints a floating point number between 0 and100

$x_int = int($x);print "x_int = $x_int\n";# prints the integer part of $x

Chop example#!/usr/bin/perl

$a = "I like PERL";

print "String is : $a\n";# prints "String is : I like PERL

$c = chop($a);

print "Chopped character : $c\n";# prints "Chopped character : L

print "Chopped string : $a\n";# prints "Chopped string : I like PER

Chomp example#!/usr/bin/perl

print "What is your name? ";$name = <STDIN>;

print "Hello $name, welcome to PERL.\n";# prints# Hello [$name]# , welcome to PERL.chomp $name;print "Hello $name, welcome to PERL.\n";# prints# Hello [$name], welcome to PERL.

Time#!/usr/bin/perl# Adapted from time.pl from PERL in easy steps, Ch. 8

$machine_time = localtime(time); # local time (there's also gmtime for UT)($sec, $min, $hr, $mday, $mon, $yr, $wday, $yday, $isdst) = localtime(time);

$mon++; # make it start from 1 not 0: month 1-12$yday++; # make it start from 1 not 0: day of year, 1-365$yr += 1900; # yr internally starts from 1900, fix that$hr = sprintf("%02d",$hr); # convert it to formatted string$min = sprintf("%02d",$min); # ..$sec = sprintf("%02d",$sec); # ..$ds = ($isdst == 1) ? "Yes":"no"; # convert daylight savings flag to string

print << "DOC";Machine time: $machine_timeDate: $mon-$mday-$yrTime: $hr:$min:$secDaylight saving: $dsDay of week: $wdayDay of year: $yday

DOC

Part 9: Pattern Matching

Pattern matching

• To search a string, use the “binding” operator “=~”

• To look for a match, specify “m”, then the search pattern, then optionally “i” to specify ignore uppercase/lowercase: e.g., $string =~ m/hi/i looks for “hi” inside $string

• The part of the string that matches, is before the match, and is after the match can be gotten by $&, $`, and $’

Match example$a = "Hello, this is a test.";

# This looks for a match of "IS" inside string $a# the trailing i makes it case insensitiveif ($a =~ m/IS/i ) { print "Found IS in string \$a\n"; print "Matched string : $&\n"; print "Text before match: $`\n"; print "Text after match : $'\n";}else { print "Did not find IS in string\n";}

# Prints:# Found IS in string $a# Matched string : is# Text before match: Hello, th# Text after match : is a test.

Substitution

• To substitute the first occurance of “string1” with “string2” in $a, use the substitution operation: $a =~ s/string1/string2/

• Can again append “i” to make it case insensitive• Careful about spaces• To substitute more than the first pattern match,

use “g” at the end to substitute “globally”

Substitution example$a = "Hello, this is a test.";

# attempt to substitute "was" for "is", case sensitive$a =~ s/is/was/;print "$a\n";# Hello, thwas is a test.

# try again...$a = "Hello, this is a test.";

# attempt to substitute “ was" for “ is", case sensitive$a =~ s/ is/ was/;print "$a\n";# Hello, this was a test.

# global specifier “g” afterward will do all not just 1st one$a = "hi hi hi";$a =~ s/hi/ho/g;print "$a\n";# ho ho ho

Split example#!/usr/bin/perl

# split() uses pattern matching, too# simple example of a pattern

$breakfast = "ham,eggs,cereal,grits";

@items = split(/,/, $breakfast);

foreach $item(@items) { print "$item\n";}

# ham# eggs# cereal# grits

Translate matches

• The “translate” operation, “tr”, will change any matching pattern to a new pattern

• The pattern can be a “class” of characters: e.g., [1-5] matches all numbers 1-5, [a-z] matches all lower-case letters a-z, [A-Z] matches all upper-case letters A-Z

• This is not equivalent to “substitute” – the translation will be done character-by-character, not string-by-string

Translation example#!/usr/bin/perl

# Example of the "translate" operation, to replace# any example of "a", "b", or "c" with uppercase$breakfast = "ham,eggs,cereal,grits";$breakfast =~ tr/[a-c]/[A-C]/;print "$breakfast\n";# hAm,eggs,CereAl,grits

# Look how translation is done character by character# not string by string$breakfast = "ham,eggs,cereal,grits";$breakfast =~ tr/grits/bacon/;print "$breakfast\n";# ham,ebbn,ceaeal,bacon

Character classes

Class Equivalent Match\w [a-zA-z0-9_] Letters, digits,

underscores\W [^a-zA-Z0-9_] All but letters, digits,

underscores\d [0-9] All digits

\D [^0-9] All but digits

\s [\n\t\r\f] All spaces, newlines, tabs, carriage returns, form feeds

\S [^\n\t\r\f] All but spaces, newlines, tabs, …