Perl - Syntax Overview
Perl borrows syntax and concepts from many languages: awk, sed, C,
Bourne Shell, Smalltalk, Lisp and even English. However, there are some
definite differences between the languages. This chapter is designd to
quickly get you up to speed on the syntax that is expected in Perl.
A Perl program consists of a sequence of declarations and statements,
which run from the top to the bottom. Loops, subroutines, and other
control structures allow you to jump around within the code. Every
simple statement must end with a semicolon (;).
Perl is a free-form language: you can format and indent it however you
like. Whitespace serves mostly to separate tokens, unlike languages like
Python where it is an important part of the syntax, or Fortran where it
is immaterial.
First Perl Program
Interactive Mode Programming
You can use Perl interpreter with -e option
at command line, which lets you execute Perl statements from the
command line. Let's try something at $ prompt as follows −
$perl -e 'print "Hello World\n"'
This execution will produce the following result −
Hello, world
Script Mode Programming
Assuming you are already on $ prompt, let's open a text file hello.pl
using vi or vim editor and put the following lines inside your file.
#!/usr/bin/perl # This will print "Hello, World" print "Hello, world\n";
Here /usr/bin/perl is the actual
perl interpreter binary. Before you execute your script, be sure to
change the mode of the script file and give execution priviledge,
generally a setting of 0755 works perfectly and finally you execute the
above script as follows −
$chmod 0755 hello.pl $./hello.pl
This execution will produce the following result −
Hello, world
You can use parentheses for functions arguments or omit them according
to your personal taste. They are only required occasionally to clarify
the issues of precedence. Following two statements produce the same
result.
print("Hello, world\n"); print "Hello, world\n";
This will produce the following result −
Hello, world Hello, world
Perl File Extension
A Perl script can be created inside of any normal simple-text editor
program. There are several programs available for every type of
platform. There are many programs designd for programmers available for
download on the web.
As a Perl convention, a Perl file must be saved with a .pl or .PL file
extension in order to be recognized as a functioning Perl script. File
names can contain numbers, symbols, and letters but must not contain a
space. Use an underscore (_) in places of spaces.
Comments in Perl
Comments in any programming language are friends of developers. Comments
can be used to make program user friendly and they are simply skipped
by the interpreter without impacting the code functionality. For
example, in the above program, a line starting with hash #is a comment.
Simply saying comments in Perl start with a hash symbol and run to the end of the line −
# This is a comment in perl
Lines starting with = are interpreted as the start of a section of
embedded documentation (pod), and all subsequent lines until the next
=cut are ignored by the compiler. Following is the example −
#!/usr/bin/perl # This is a single line comment print "Hello, world\n"; =begin comment This is all part of multiline comment. You can use as many lines as you like These comments will be ignored by the compiler until the next =cut is encountered. =cut
This will produce the following result −
Hello, world
Whitespaces in Perl
A Perl program does not care about whitespaces. Following program works perfectly fine −
#!/usr/bin/perl print "Hello, world\n";
This will produce the following result −
Hello, world
But if spaces are inside the quoted strings, then they would be printed as is. For example −
#!/usr/bin/perl # This would print with a line break in the middle print "Hello world\n";
This will produce the following result −
Hello
world
All types of whitespace like spaces, tabs, newlines, etc. are equivalent
for the interpreter when they are used outside of the quotes. A line
containing only whitespace, possibly with a comment, is known as a blank
line, and Perl totally ignores it.
Single and Double Quotes in Perl
You can use double quotes or single quotes around literal strings as follows −
#!/usr/bin/perl print "Hello, world\n"; print 'Hello, world\n';
This will produce the following result −
Hello, world Hello, world\n
There is an important difference in single and double quotes. Only double quotes interpolate variables
and special characters such as newlines \n, where as single quote does
not interpolate any variable or special character. Check below example
where we are using $a as a variable to store a value and later printing
that value −
#!/usr/bin/perl $a = 10; print "Value of a = $a\n"; print 'Value of a = $a\n';
This will produce the following result −
Value of a = 10 Value of a = $a\n
"Here" Documents
You can store or print multiline text with a great comfort. Even you can
make use of variables inside the "here" document. Below is a simple
syntax, check carefully there must be no space between the << and
the identifier.
An identifier may be either a bare word or some quoted text like we used
EOF below. If identifier is quoted, the type of quote you use
determines the treatment of the text inside the here document, just as
in regular quoting. An unquoted identifier works like double quotes.
#!/usr/bin/perl $a = 10; $var = <<"EOF"; This is the syntax for here document and it will continue until it encounters a EOF in the first line. This is case of double quote so variable value will be interpolated. For example value of a = $a EOF print "$var\n"; $var = <<'EOF'; This is case of single quote so variable value will not be interpolated. For example value of a = $a EOF print "$var\n";
This will produce the following result −
This is the syntax for here document and it will continue until it encounters a EOF in the first line. This is case of double quote so variable value will be interpolated. For example value of a = 10 This is case of single quote so variable value will be interpolated. For example value of a = $a
Escaping Characters
Perl uses the backslash (\) character to escape any type of character
that might interfere with our code. Let's take one example where we want
to print double quote and $ sign −
#!/usr/bin/perl $result = "This is \"number\""; print "$result\n"; print "\$result\n";
This will produce the following result −
This is "number" $result
Perl Identifiers
A Perl identifier is a name used to identify a variable, function,
class, module, or other object. A Perl variable name starts with either
$, @ or % followed by zero or more letters, underscores, and digits (0
to 9).
Perl does not allow punctuation characters such as @, $, and % within identifiers. Perl is a case sensitive programming language. Thus $Manpower and $manpower are two different identifiers in Perl.