Majority of the contents of this tutorial section were written by Nik Silver, at the School of Computer Studies, University of Leeds, UK. Assuming working knowledge of any programming language, we will now try to see what Perl programs look like.
Ever since Kernighan and Ritchie came out with C programming language, people have started learning almost any programming language with the obligatory "Hello World" program. Let us do the same!
Here is the basic perl program that we'll use to get started.
#! /usr/local/bin/perl
#
# prints a greeting.
#
print 'Hello world.'; # Print a message
A common Perl-pitfall is to write cryptic code. In that context, Perl do provide for comments, albeit not very flexible. Perl treats any thing from a hash # to the end of line as a comment. Block comments are not possible. So, if you want to have a block of comments, you must ensure that each line starts with #.
Everything other than comments are Perl statements, which must end with a semicolon, like the last line above. Unlike C, you need not put a wrapping character \ for long statements. A Perl statement always ends with a semicolon.
Type in the example program using a text editor, and save it. The first line of the program is a typical shell construct, which will make the shell start the interpreter and feed the remaining lines of the file as an input to the interpreter.
After you've entered and saved the program make sure the file is executable by using the command
chmod u+x progname
perl progname
progname
progname
perl -w progname
perl -d progname
Perl supports 3 basic types of variables, viz., scalars, lists and hashes. We will explore each of these little more.
The most basic kind of variable in Perl is the scalar variable. Scalar variables hold both strings and numbers, and are remarkable in that strings and numbers are completely interchangeable. For example, the statement
$age = 27;
$age = 'Twenty Seven';
$priority = '9';
$default = '0009';
$age = 'Twenty Seven';
$age = $age + 10;
In general variable names consists of numbers, letters and underscores, but they should not start with a number and the variable $_ is special, as we'll see later. Also, Perl is case sensitive, so $a and $A are different.
Perl uses all the usual C arithmetic operators:
$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
$a = 5 % 2; # Remainder of 5 divided by 2
++$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
$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
Other operators can be found on the perlop manual page. Type man perlop at the prompt.
The following code prints apples and pears using concatenation:
$a = 'apples';
$b = 'pears';
print $a.' and '.$b;
print '$a and $b';
print "$a and $b";
This exercise is to rewrite the Hello world program so that (a) the string is assigned to a variable and (b) this variable is then printed with a newline character. Use the double quotes and don't use the concatenation operator.
A slightly more interesting kind of variable is the list variable which is an array of scalars (i.e. numbers and strings). From now on, we will use the terms list and array interchangeably.
Array variables have the same format as scalar variables except that they are prefixed by an @ symbol. The statement
@food = ("apples", "pears", "eels");
@music = ("whistle", "flute");
The array is accessed by using indices starting from 0, and square brackets are used to specify the index. The expression
$food[2]
As in all of Perl, the same expression in a different context can produce a different result. The first assignment below explodes the @music variable so that it is equivalent to the second assignment.
@moremusic = ("organ", @music, "harp");
@moremusic = ("organ", "whistle", "flute", "harp");
push(@food, "eggs");
push(@food, "eggs", "lard");
push(@food, ("eggs", "lard"));
push(@food, @morefood);
To remove the last item from a list and return it use the pop function. From our original list the pop function returns eels and @food now has two elements:
$grub = pop(@food); # Now $grub = "eels"
It is also possible to assign an array to a scalar variable. As usual context is important. The line
$f = @food;
$f = "@food";
When you get overloaded with oddity, use the English module which lets you name these variables in more user-friendly (i.e. to English-speaking people) way.
Arrays can also be used to make multiple assignments to scalar variables:
($a, $b) = ($c, $d); # Same as $a=$c; $b=$d;
($a, $b) = @food; # $a and $b are the first two
# items of @food.
($a, @somefood) = @food; # $a is the first item of @food
# @somefood is a list of the
# others.
(@somefood, $a) = @food; # @somefood is @food and
# $a is undefined.
Finally, you may want to find the index of the last element of a list. To do this for the @food array use the expression
$#food
Since context is important, it shouldn't be too surprising that the following all produce different results:
print @food; # By itself
print "@food"; # Embedded in double quotes
print @food.""; # In a scalar context
Ordinary list arrays allow us to access their element by number. The first element of array @food is $food[0]. The second element is $food[1], and so on. But Perl also allows us to create arrays which are accessed by string. These are called associative arrays or hashes.
To define an associative array we use the usual parenthesis notation, but the array itself is prefixed by a % sign. Suppose we want to create an array of people and their ages. It would look like this:
%ages = ("Michael Caine", 39,
"Dirty Den", 34,
"Angie", 27,
"Willy", "21 in dog years",
"The Queen Mother", 108);
$ages{"Michael Caine"}; # Returns 39
$ages{"Dirty Den"}; # Returns 34
$ages{"Angie"}; # Returns 27
$ages{"Willy"}; # Returns "21 in dog years"
$ages{"The Queen Mother"}; # Returns 108
An associative array can be converted back into a list array just by assigning it to a list array variable. A list array can be converted into an associative array by assigning it to an associative array variable. Ideally the list array will have an even number of elements:
@info = %ages; # @info is a list array. It
# now has 10 elements
$info[5]; # Returns the value 27 from
# the list array @info
%moreages = @info; # %moreages is an associative
# array. It is the same as %ages
Associative arrays do not have any order to their elements (they are just like hash tables) but is it possible to access all the elements in turn using the keys function and the values function:
foreach $person (keys %ages)
{
print "I know the age of $person\n";
}
foreach $age (values %ages)
{
print "Somebody is $age\n";
}
When keys and values are called in a scalar context they return the number of key/value pairs in the associative array.
There is also a function each which returns a two element list of a key and its value. Every time each is called it returns another key/value pair:
while (($person, $age) = each(%ages))
{
print "$person is $age\n";
}
When you run a perl program, or any script in UNIX, there will be certain environment variables set. These will be things like USER which contains your username and DISPLAY which specifies which screen your graphics will go to. When you run a perl CGI script on the World Wide Web there are environment variables which hold other useful information. All these variables and their values are stored in the associative %ENV array in which the keys are the variable names. Try the following in a perl program:
print "You are called $ENV{'USER'} and you are ";
print "using display $ENV{'DISPLAY'}\n";
More interesting possibilities arise when we introduce control structures and looping. Perl supports lots of different kinds of control structures which tend to be like those in C, but are very similar to Pascal, too. Here we discuss a few of them.
To go through each line of an array or other list-like structure (such as lines in a file) Perl uses the foreach structure. This has the form
foreach $morsel (@food) # Visit each item in turn
# and call it $morsel
{
print "$morsel\n"; # Print the item
print "Yum yum\n"; # That was nice
}
The next few structures rely on a test being true or false. In Perl any non-zero number and non-empty string is counted as true. The number zero, zero by itself in a string, and the empty string are counted as false. Here are some tests on numbers and strings.
$a == $b # Is $a numerically equal to $b?
# Beware: Don't use the = operator.
$a != $b # Is $a numerically unequal to $b?
$a eq $b # Is $a string-equal to $b?
$a ne $b # Is $a string-unequal to $b?
You can also use logical and, or and not:
($a && $b) # Is $a and $b true?
($a || $b) # Is either $a or $b true?
!($a) # is $a false?
Perl has a for structure that mimics that of C. It has the form
for (initialise; test; inc)
{
first_action;
second_action;
etc
}
First of all the statement initialise is executed. Then while test is true the block of actions is executed. After each time the block is executed inc takes place. Here is an example for loop to print out the numbers 0 to 9.
for ($i = 0; $i < 10; ++$i) # Start with $i = 1
# Do it while $i < 10
# Increment $i before repeating
{
print "$i\n";
}
Here is a program that reads some input from the keyboard and won't continue until it is the correct password
#!/usr/local/bin/perl
print "Password? "; # Ask for input
$a = ; # Get input
chop $a; # Remove the newline at end
while ($a ne "fred") # While input is wrong...
{
print "sorry. Again? "; # Ask again
$a = ; # Get input again
chop $a; # Chop off newline again
}
To test the opposite thing we can use the until statement in just the same way. This executes the block repeatedly until the expression is true, not while it is true.
Another useful technique is putting the while or until check at the end of the statement block rather than at the beginning. This will require the presence of the do operator to mark the beginning of the block and the test at the end. If we forgo the sorry. Again message in the above password program then it could be written like this.
#!/usr/local/bin/perl
do
{
print "Password? "; # Ask for input
$a = ; # Get input
chop $a; # Chop off newline
}
while ($a ne "fred") # Redo while wrong input
Modify the program from the previous exercise so that each line of the file is read in one by one and is output with a line number at the beginning. You should get something like:
1 root:oYpYXm/qRO6N2:0:0:Super-User:/:/bin/csh
2 sysadm:*:0:0:System V Administration:/usr/admin:/bin/sh
3 diag:*:0:996:Hardware Diagnostics:/usr/diags:/bin/csh
etc
while ($line = <INFO>)
{
...
}
When you have done this see if you can alter it so that line numbers are printed as 001, 002, ..., 009, 010, 011, 012, etc. To do this you should only need to change one line by inserting an extra four characters. Perl's clever like that.
Of course Perl also allows if/then/else statements. These are of the following form:
if ($a)
{
print "The string is not empty\n";
}
else
{
print "The string is empty\n";
}
It is also possible to include more alternatives in a conditional statement:
if (!$a) # The ! is the not operator
{
print "The string is empty\n";
}
elsif (length($a) == 1) # If above fails, try this
{
print "The string has one character\n";
}
elsif (length($a) == 2) # If that fails, try this
{
print "The string has two characters\n";
}
else # Now, everything has failed
{
print "The string has lots of characters\n";
}
Sometimes, it is more readable to use unless instead of if (!...) . The switch-case statement familiar to C programmers are not available in Perl. You can simulate it in other ways. See the manual pages.
From the previous exercise you should have a program which prints out the password file with line numbers. Change it so that works with the text file. Now alter the program so that line numbers aren't printed or counted with blank lines, but every line is still printed, including the blank ones. Remember that when a line of the file is read in it will still include its newline character at the end.
Here is the basic perl program which does the same as the UNIX cat command on a certain file.
#!/usr/local/bin/perl
#
# Program to open the password file, read it in,
# print it, and close it again.
$file = '/etc/passwd'; # Name the file
open(INFO, $file); # Open the file
@lines = <INFO>; # Read it into an array
close(INFO); # Close the file
print @lines; # Print the array
The open function opens a file for input (i.e. for reading). The first parameter is the filehandle which allows Perl to refer to the file in future. The second parameter is an expression denoting the filename. If the filename was given in quotes then it is taken literally without shell expansion. So the expression '~/notes/todolist' will not be interpreted successfully. If you want to force shell expansion then use angled brackets: that is, use <~/notes/todolist> instead.
The close function tells Perl to finish with that file.
There are a few useful points to add to this discussion on file-handling. First, the open statement can also specify a file for output and for appending as well as for input. To do this, prefix the filename with a > for output and a >> for appending:
open(INFO, $file); # Open for input
open(INFO, ">$file"); # Open for output
open(INFO, ">>$file"); # Open for appending
open(INFO, "<$file"); # Also open for input
Second, if you want to print something to a file you've already opened for output then you can use the print statement with an extra parameter. To print a string to the file with the INFO filehandle use
print INFO "This line goes to the file.\n";
Third, you can use the following to open the standard input (usually the keyboard) and standard output (usually the screen) respectively:
open(INFO, '-'); # Open standard input
open(INFO, '>-'); # Open standard output
In the above program the information is read from a file. The file is the INFO file and to read from it Perl uses angled brackets. So the statement
@lines = <INFO>;
Modify the above program so that the entire file is printed with a # symbol at the beginning of each line. You should only have to add one line and modify another. Use the $" variable. Unexpected things can happen with files, so you may find it helpful to use the -w option.
You can very easily substitute reading a file to reading a pipe. The following example shows reading the ouput of the ps command.
open(PS,"ps -aef|") or die "Cannot open ps \n";
while(){
print ;
}
close(PS);
One of the most useful features of Perl (if not the most useful feature) is its powerful string manipulation facilities. At the heart of this is the regular expression (RE) which is shared by many other UNIX utilities.
A regular expression is contained in slashes, and matching occurs with the =~ operator. The following expression is true if the string the appears in variable $sentence.
$sentence =~ /the/
$sentence = "The quick brown fox";
$sentence !~ /the/
We could use a conditional as
if ($sentence =~ /under/)
{
print "We're talking about rugby\n";
}
$sentence = "Up and under";
$sentence = "Best winkles in Sunderland";
if (/under/)
{
print "We're talking about rugby\n";
}
In an RE there are plenty of special characters, and it is these that both give them their power and make them appear very complicated. It's best to build up your use of REs slowly; their creation can be something of an art form.
Here are some special RE characters and their meaning
. # 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
t.e # t followed by anthing followed by e
# This will match the
# tre
# tle
# but not te
# tale
^f # f at the beginning of a line
^ftp # ftp at the beginning of a line
e$ # e at the end of a line
tle$ # tle at the end of a line
und* # un followed by zero or more d characters
# This will match un
# und
# undd
# unddd (etc)
.* # Any string without a newline. This is because
# the . matches anything except a newline and
# the * means zero or more of these.
^$ # A line with nothing in it.
There are even more options. Square brackets are used to match any one of the characters inside them. Inside square brackets a - indicates "between" and a ^ at the beginning means "not":
[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
A vertical bar | represents an "or" and parentheses (...) can be used to group things together:
jelly|cream # Either jelly or cream
(eg|le)gs # Either eggs or legs
(da)+ # Either da or dada or dadada or...
Here are some more special characters:
\n # A newline
\t # A tab
\w # Any alphanumeric (word) character.
# The same as [a-zA-Z0-9_]
\W # Any non-word character.
# The 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: space,
# tab, newline, etc
\S # Any non-whitespace character
\b # A word boundary, outside [] only
\B # No word boundary
Clearly characters like $, |, [, ), \, / and so on are peculiar cases in regular expressions. If you want to match for one of those then you have to preceed it by a backslash. So:
\| # Vertical bar
\[ # An open square bracket
\) # A closing parenthesis
\* # An asterisk
\^ # A carat symbol
\/ # A slash
\\ # A backslash
As was mentioned earlier, it's probably best to build up your use of regular expressions slowly. Here are a few examples. Remember that to use them for matching they should be put in /.../ slashes
[01] # Either "0" or "1"
\/0 # A division by zero: "/0"
\/ 0 # A division by zero with a space: "/ 0"
\/\s0 # A division by zero with a whitespace:
# "/ 0" where the space may be a tab etc.
\/ *0 # A division by zero with possibly some
# spaces: "/0" or "/ 0" or "/ 0" etc.
\/\s*0 # A division by zero with possibly some
# whitespace.
\/\s*0\.0* # As the previous one, but with decimal
# point and maybe some 0s after it. Accepts
# "/0." and "/0.0" and "/0.00" etc and
# "/ 0." and "/ 0.0" and "/ 0.00" etc.
# Check for valid currency value
^([0-9]+|[0-9]{1,3}(,[0-9]{3})*)(\.[0-9]{1,2})?$
# Check for valid email address
^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*$
Previously your program counted non-empty lines. Alter it so that instead of counting non-empty lines it counts only lines with
Just like the sed and tr utilities in Unix, you have s/// and tr/// in Perl. The former is for substitution and the later is for translation.
$bar =~ s/this/that/g; # change this to that in $bar
$path =~ s|/usr/bin|/usr/local/bin|;
s/\bgreen\b/mauve/g; # don't change wintergreen
s/Login: $foo/Login: $bar/; # run-time pattern
$count = ($paragraph =~ s/Mister\b/Mrg); # get change-count
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx; # Delete (most) C comments.
s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
for ($variable) { # trim white space in $variable, cheap
s/^\s+//;
s/\s+$//;
}
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
#Note the use of $ instead of \ in the last example. Unlike sed,
#we use the \ form in only the left hand side.
#Anywhere else it's $.
$myname = "BABU";
$myname =~ tr/[A-Z]/[a-z]/ ; # yields babu
Perl provides a split function to split strings, based on REs. The syntax is
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split
# process the password file
open(PASSWD, '/etc/passwd');
while () {
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
# note that $shell still has a new line.
# use chop or chomp to remove the newline
#...
($login, $passwd, $remainder) = split(/:/, $_, 3);
# here we use LIMIT to set the number of fields
}
Like any good programming language Perl allows the user to define their own functions, called subroutines. They may be placed anywhere in your program but it's probably best to put them all at the beginning or all at the end. A subroutine has the form
sub mysubroutine
{
print "Not a very interesting routine\n";
print "This does the same thing every time\n";
}
&mysubroutine; # Call the subroutine
&mysubroutine($_); # Call it with a parameter
&mysubroutine(1+2, $_); # Call it with two parameters
In the above case the parameters are acceptable but ignored. When the subroutine is called any parameters are passed as a list in the special @_ list array variable. This variable has absolutely nothing to do with the $_ scalar variable. The following subroutine merely prints out the list that it was called with. It is followed by a couple of examples of its use.
sub printargs
{
print "@_\n";
}
&printargs("perly", "king"); # Example prints "perly king"
&printargs("frog", "and", "toad"); # Prints "frog and toad"
sub printfirsttwo
{
print "Your first argument was $_[0]\n";
print "and $_[1] was your second\n";
}
Result of a subroutine is always the last thing evaluated. This subroutine returns the maximum of two input parameters. An example of its use follows.
sub maximum
{
if ($_[0] > $_[1])
{
$_[0];
}
else
{
$_[1];
}
}
$biggest = &maximise(37, 24); # Now $biggest is 37
The @_ variable is local to the current subroutine, and so of course are $_[0], $_[1], $_[2], and so on. Other variables can be made local too, and this is useful if we want to start altering the input parameters. The following subroutine tests to see if one string is inside another, spaces not withstanding. An example follows.
sub inside
{
local($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 $b?
}
&inside("lemon", "dole money"); # true
local($a, $b) = ($_[0], $_[1]);
Only a very brief of Perl is covered in this tutorial. The easiest way to lern Perl is to look at existing code. The Perl manual pages and FAQ's are really superb and will help you a lot. Unless until you are sure, run Perl with the -w switch!
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