Perl Workshop

C. David Sherrill

Center for Computational Molecular Science & Technology Georgia 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

#!/usr/bin/perl

Printing in 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 array print “The array contains: @array\n”; # access the 2 nd element --- counting starts from 0 # note also we use scalar syntax ($) for a particular element # because a single element is a scalar print “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

a = b a += b a -= b a *= b a /= b a %= b

Same as

a = b a = a + b a = a – b a = a * b a = 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

Operator

== != < = > > < >= <=

Comparison

Is equal?

Not equal?

Left-to-right comp Greater?

Less than?

Greater or equal?

Less than or equal?

• < = > 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!

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

Operator

eq ne gt Lt cmp .

x uc(string) lc(string) chr(num) ord(char)

Comparison/Action

is equal?

not equal?

greater than?

less than?

-1, 0, or 1, depending concatenation repeat convert to upper case convert to lower case get char for ASCII num get ASCII num of char • Every individual character, like “A”, has a numerical code equivalent given by the ASCII table

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

abs(x) sin(x) cos(x) hex(string) oct(string) sqrt(x)

Operation

return absolute value of x return sine of x return cosine of x decimal value of hexadecimal string decimal value of octal string 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 @priorities print "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

and returns that value element of the array • 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 $a}@students; print "students1 : @students1\n"; # students1 : Amanda Chris Jan Robert print "students2 : @students2\n"; # students2 : Robert Jan Chris Amanda • Could do similar thing with numbers but using {$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 statement print "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

scalar) to delete (a %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, "; # 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 to an array variable open(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

File locking example

#!/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 file print FP "Hello!\n"; # write a line flock(FP, 8); # release the file close(FP); # close file again

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, "; 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 file rename(“old_filename”, “new_filename”); • To delete a file

(don’t use unless you’re sure!)

unlink(“file_to_delete”);

File status checks

Operator

-e -d

Operation

Does file exist?

Is the “file” a directory?

-r -w -x Is the file readable?

Is the file writable?

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 though print "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 0777 rwx 0755 rwx 0644 rw Group Others rwx rwx r-x r-x 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 example

print "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 example

Contents 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 = ; 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_time Date: $mon-$mday-$yr Time: $hr:$min:$sec Daylight saving: $ds Day of week: $wday Day 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 $a # the trailing i makes it case insensitive if ($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

\d \D \s Class \w \W \S

Character classes

Equivalent [a-zA-z0-9_] [^a-zA-Z0-9_] [0-9] [^0-9] [\n\t\r\f] [^\n\t\r\f] Match Letters, digits, underscores All but letters, digits, underscores All digits All but digits All spaces, newlines, tabs, carriage returns, form feeds All but spaces, newlines, tabs, …