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 quotes

print “I said \”hello!\” \n”;

print << “DOC”;

Any stuff between here & DOC will be printed

DOC

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/Action

eq 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 lines

close(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 writing

print TXT "hello, testing!\n"; # write a line

print TXT "end of test.\n"; # write another line

close(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 writing

print TXT "Add a line!\n"; # write an additional line

close(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() example

if (-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, …