Apr 18, 2023

Perl

Major parts of this lecture adapted from http://www.scs.leeds.ac.uk/Perl/start.html

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Why Perl?

Perl is built around regular expressions REs are good for string processing Therefore Perl is a good scripting language Perl is especially popular for CGI scripts

Perl makes full use of the power of UNIX Short Perl programs can be very short

“Perl is designed to make the easy jobs easy, without making the difficult jobs impossible.” -- Larry Wall, Programming Perl

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Why not Perl?

Perl is very UNIX-oriented Perl is available on other platforms... ...but isn’t always fully implemented there However, Perl is often the best way to get some UNIX

capabilities on less capable platforms Perl does not scale well to large programs

Weak subroutines, heavy use of global variables Perl’s syntax is not particularly appealing

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What is a scripting language?

Operating systems can do many things copy, move, create, delete, compare files execute programs, including compilers schedule activities, monitor processes, etc.

A command-line interface gives you access to these functions, but only one at a time

A scripting language is a “wrapper” language that integrates OS functions

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Major scripting languages

UNIX has sh, Perl Macintosh has AppleScript, Frontier Windows has no major scripting languages

probably due to the weaknesses of DOS Generic scripting languages include:

Perl (most popular) Tcl (easiest for beginners) Python (new, Java-like, best for large programs)

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Perl Example 1

#!/usr/local/bin/perl## Program to do the obvious#print 'Hello world.'; # Print a message

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Comments on “Hello, World”

Comments are # to end of line But the first line, #!/usr/local/bin/perl, tells where to find

the Perl compiler on your system Perl statements end with semicolons Perl is case-sensitive Perl is compiled and run in a single operation

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Perl Example 2

#!/ex2/usr/bin/perl# Remove blank lines from a file# Usage: singlespace < oldfile > newfile

while ($line = <STDIN>) { if ($line eq "\n") { next; } print "$line";}

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More Perl notes

On the UNIX command line; < filename means to get input from this file > filename means to send output to this file

In Perl, <STDIN> is the input file, <STDOUT> is the output file

Scalar variables start with $ Scalar variables hold strings or numbers, and they are

interchangeable Examples:

$priority = 9; $priority = '9';

Array variables start with @

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Perl Example 3

#!/usr/local/bin/perl# Usage: fixm <filenames># Replace \r with \n -- replaces input files

foreach $file (@ARGV) { print "Processing $file\n"; if (-e "fixm_temp") { die "*** File fixm_temp already exists!\n"; } if (! -e $file) { die "*** No such file: $file!\n"; } open DOIT, "| tr \'\\015' \'\\012' < $file > fixm_temp" or die "*** Can't: tr '\015' '\012' < $infile > $outfile\n"; close DOIT; open DOIT, "| mv -f fixm_temp $file"

or die "*** Can't: mv -f fixm_temp $file\n"; close DOIT;}

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Comments on example 3

In # Usage: fixm <filenames>, the angle brackets just mean to supply a list of file names here

In UNIX text editors, the \r (carriage return) character usually shows up as ^M (hence the name fixm_temp)

The UNIX command tr '\015' '\012' replaces all \015 characters (\r) with \012 (\n) characters

The format of the open and close commands is: open fileHandle, fileName close fileHandle, fileName

"| tr \'\\015' \'\\012' < $file > fixm_temp" says: Take input from $file, pipe it to the tr command, put the output on fixm_temp

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Arithmetic in Perl

$a = 1 + 2; # Add 1 and 2 and store in $a$a = 3 - 4; # Subtract 4 from 3 and store in $a$a = 5 * 6; # Multiply 5 and 6$a = 7 / 8; # Divide 7 by 8 to give 0.875$a = 9 ** 10; # Nine to the power of 10, that is, 910

$a = 5 % 2; # Remainder of 5 divided by 2++$a; # Increment $a and then return it$a++; # Return $a and then increment it--$a; # Decrement $a and then return it$a--; # Return $a and then decrement it

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String and assignment operators

$a = $b . $c; # Concatenate $b and $c$a = $b x $c; # $b repeated $c times

$a = $b; # Assign $b to $a$a += $b; # Add $b to $a$a -= $b; # Subtract $b from $a$a .= $b; # Append $b onto $a

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Single and double quotes

$a = 'apples'; $b = 'bananas'; print $a . ' and ' . $b;

prints: apples and bananas print '$a and $b';

prints: $a and $b print "$a and $b";

prints: apples and bananas

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Arrays

@food = ("apples", "bananas", "cherries"); But… print $food[1];

prints "bananas" @morefood = ("meat", @food);

@morefood == ("meat", "apples", "bananas", "cherries");

($a, $b, $c) = (5, 10, 20);

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push and pop

push adds one or more things to the end of a list push (@food, "eggs", "bread"); push returns the new length of the list

pop removes and returns the last element $sandwich = pop(@food);

$len = @food; # $len gets length of @food $#food # returns index of last element

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foreach

# Visit each item in turn and call it $morsel

foreach $morsel (@food){ print "$morsel\n"; print "Yum yum\n"; }

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Tests

“Zero” is false. This includes:0, '0', "0", '', ""

Anything not false is true Use == and != for numbers, eq and ne for strings &&, ||, and ! are and, or, and not, respectively.

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for loops

for loops are just as in C or Java

for ($i = 0; $i < 10; ++$i){ print "$i\n";}

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while loops

#!/usr/local/bin/perlprint "Password? ";$a = <STDIN>;chop $a; # Remove the newline at endwhile ($a ne "fred"){ print "sorry. Again? "; $a = <STDIN>; chop $a;}

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do..while and do..until loops

#!/usr/local/bin/perldo{ print "Password? "; $a = <STDIN>; chop $a;}while ($a ne "fred");

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if statements

if ($a){ print "The string is not empty\n";}else{ print "The string is empty\n";

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if - elsif statements

if (!$a) { print "The string is empty\n"; }elsif (length($a) == 1) { print "The string has one character\n"; }elsif (length($a) == 2) { print "The string has two characters\n"; }else { print "The string has many characters\n"; }

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Why Perl?

Two factors make Perl important: Pattern matching/string manipulation

Based on regular expressions (REs) REs are similar in power to those in Formal Languages… …but have many convenience features

Ability to execute UNIX commands Less useful outside a UNIX environment

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Basic pattern matching

$sentence =~ /the/ True if $sentence contains "the"

$sentence = "The dog bites.";if ($sentence =~ /the/) # is false …because Perl is case-sensitive

!~ is "does not contain"

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RE special characters

. # Any single character except a newline

^ # The beginning of the line or string

$ # The end of the line or string

* # Zero or more of the last character

+ # One or more of the last character

? # Zero or one of the last character

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RE examples

^.*$ # matches the entire string

hi.*bye # matches from "hi" to "bye" inclusive

x +y # matches x, one or more blanks, and y

^Dear # matches "Dear" only at beginning

bags? # matches "bag" or "bags"

hiss+ # matches "hiss", "hisss", "hissss", etc.

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Square brackets

[qjk] # Either q or j or k

[^qjk] # Neither q nor j nor k

[a-z] # Anything from a to z inclusive

[^a-z] # No lower case letters

[a-zA-Z] # Any letter

[a-z]+ # Any non-zero sequence of # lower case letters

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More examples

[aeiou]+ # matches one or more vowels

[^aeiou]+ # matches one or more nonvowels

[0-9]+ # matches an unsigned integer

[0-9A-F] # matches a single hex digit

[a-zA-Z] # matches any letter

[a-zA-Z0-9_]+ # matches identifiers

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More special characters

\n # A newline\t # A tab\w # Any alphanumeric; same as [a-zA-Z0-9_]\W # Any non-word char; same as [^a-zA-Z0-9_]\d # Any digit. The same as [0-9]\D # Any non-digit. The same as [^0-9]\s # Any whitespace character\S # Any non-whitespace character\b # A word boundary, outside [] only\B # No word boundary

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Quoting special characters

\| # Vertical bar\[ # An open square bracket\) # A closing parenthesis\* # An asterisk\^ # A carat symbol\/ # A slash\\ # A backslash

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Alternatives and parentheses

jelly|cream # Either jelly or cream

(eg|le)gs # Either eggs or legs

(da)+ # Either da or dada or # dadada or...

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Substitution

=~ is a test, as in: $sentence =~ /the/ !~ is the negated test, as in:

$sentence !~ /the/ =~ is also used for replacement, as in:

$sentence =~ /london/London/ This is an expression, whose value is the number of

substitutions made (0 or 1)

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The $_ variable

Often we want to process one string repeatedly The $_ variable holds the current string If a subject is omitted, $_ is assumed Hence, the following are equivalent:

if ($sentence =~ /under/) … $_ = $sentence; if (/under/) ...

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Global substitutions

s/london/London/ substitutes London for the first occurrence of london in $_

s/london/London/g substitutes London for each occurrence of london in $_

The value of a substitution expression is the number of substitutions actually made

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Case-insensitive substitutions

s/london/London/i case-insensitive substitution; will replace london,

LONDON, London, LoNDoN, etc. You can combine global substitution with case-

insensitive substitution s/london/London/gi

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Remembering patterns

Any part of the pattern enclosed in parentheses is assigned to the special variables $1, $2, $3, …, $9

Numbers are assigned according to the left (opening) parentheses

"The moon is high" =~ /The (.*) is (.*)/ Afterwards, $1 = "moon" and $2 = "high"

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Dynamic matching

During the match, an early part of the match that is tentatively assigned to $1, $2, etc. can be referred to by \1, \2, etc.

Example: \b.+\b matches a single word /(\b.+\b) \1/ matches repeated words "Now is the the time" =~ /(\b.+\b) \1/ Afterwards, $1 = "the"

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tr

tr does character-by-character translation tr returns the number of substitutions made $sentence =~ tr/abc/edf/;

replaces a with e, b with d, c with f $count = ($sentence =~ tr/*/*/);

counts asterisks tr/a-z/A-Z/;

converts to all uppercase

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split

split breaks a string into parts

$info = "Caine:Michael:Actor:14, Leafy Drive";@personal = split(/:/, $info);

@personal = ("Caine", "Michael", "Actor", "14, Leafy Drive");

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Associative arrays

Associative arrays allow lookup by name rather than by index

Associative array names begin with % Example:

%fruit = ("apples", "red", "bananas", "yellow", "cherries", "red");

Now, $fruit{"bananas"} returns "yellow" Note: braces, not parentheses

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Associative Arrays II

Can be converted to normal arrays:@food = %fruit;

You cannot index an associative array, but you can use the keys and values functions:foreach $f (keys %fruit){ print ("The color of $f is " . $fruit{$f} . "\n");}

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Associative Arrays III

The function each gets key-value pairs while (($f, $c) = each(%fruit))

{ print "$f is $c\n";}

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Calling subroutines

Assume you have a subroutine printargs that just prints out its arguments

Subroutine calls:

&printargs("perly", "king"); Prints: "perly king"

&printargs("frog", "and", "toad"); Prints: "frog and toad"

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Defining subroutines

Here's the definition of printargs: sub printargs

{ print "@_\n"; }

Where are the parameters? Parameters are put in the array @_ which has nothing to

do with $_

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Returning a result

The value of a subroutine is the value of the last expression that was evaluated

sub maximum{ if ($_[0] > $_[1]) { $_[0]; } else { $_[1]; }}

$biggest = &maximum(37, 24);

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Local variables

@_ is local to the subroutine, and… …so are $_[0], $_[1], $_[2], … local creates local variables

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Example subroutine

sub inside{ local($a, $b); # Make local variables ($a, $b) = ($_[0], $_[1]); # Assign values $a =~ s/ //g; # Strip spaces from $b =~ s/ //g; # local variables ($a =~ /$b/ || $b =~ /$a/); # Is $b inside $a # or $a inside $b?}&inside("lemon", "dole money"); # true

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Perl V

There are only a few differences between Perl 4 and Perl 5 Perl 5 has modules Perl 5 modules can be treated as classes Perl 5 has “auto” variables

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The End