This article is a summary of Perl's basic syntax 2022 to learn Perl quickly.
The original article is The Fastest Master of Perl Basic Synta | Perl ABC
Perl Basic Syntax
Syntax Check
Be sure to write the following two lines first.
use strict;
use warnings;
use strict makes syntax check strict.
use warnings displays warnings to prevent mistakes.
print function
The print function prints a string to the standard output.
print "Hello world";
Comments
Perl's comment.
# comment
Variable Declarations
The my keyword declares a variable.
# Scalar variables
my $num;
# Array variables
my @students
# Hash variables
my %month_num;
Execution of Perl Programs
The perl command executes Perl programs.
perl script.pl
If you want to print the output to a file, you can use redirection.
perl script.pl > file.txt
These are executed on CUI envrinements, for example:
- Command Prompt on Windows
- Shell on Mac/Linux/UNIX
Compile check
You can do only compile check without runing the script.
perl -c script.pl
Perl Debugger
Perl has the debugger. To start the debugger, use -d option with the perl command.
perl -d script.pl
Numbers
Numbers
You can assign a number to a scalar variable. The number is a integral number or a floating point number.
my $num = 1;
my $num = 1.234;
If the digits is large, underscores can be used as delimiters.
my $num = 100_000_000;
Numbers written in source codes are called numeric literals.
Arithmetic Operations
These are basic arithmetic operations.
# Addition
$num = 1 + 1;
# Subtruction
$num = 1 - 1;
# Multiplication
$num = 1 * 2;
# Division
$num = 1 / 2;
To culcurate the quotient, perform division and then extract the integer part using the int function.
# Quotient
$quo = int(3 / 2);
The % operator calculates the remainder.
# Remainder
$mod = 3 % 2;
Increment and decrement
# Increment
$i++
# Decrement
$i--
Strings
Single Quoted Strings
This is a single quoted string. You can assign it to a scalar varialbe.
my $str1 = 'abc';
Double Quoted Strings
This is a double quoted string. You can assign it to a scalar varialbe. In double quoted strings, you can use escape sequences such as \t(tab), \n(line break) or etc.
my $str2 = "def";
my $str3 = "a\tbc\n";
You can also use variable expansions in double quoted strings.
# Variable expansion - The result is "abc def"
my $str4 = "$str1 def";
String Operators and Functions
Often used string operators and functions.
- String Concatnation Operator
- join Function
- split Function
- length Function
- substr Function
- index Function
Examples:
# Concat two strings
my $join1 = 'aaa' . ' Bbb';
# Concat strings with a delimiter
my $join2 = join(',','aaa', 'bbb', 'ccc');
# Split
my @record = split(/,/, 'aaa,bbb,ccc');
# Length
my $length = length 'abcdef';
# Cut - The result is "ab"
my $substr = substr('abcd', 0, 2);
# Search - Returns the found position, otherwise returns -1
my $result = index('abcd', 'cd');
Arrays
Explains Perl arrays. Arrays are data structures to have multiple values.
Array Declarations
This is an array declaration and assignment. A list can be assigned to an array.
# Array declarations
my @array;
# Assignment to the array
@array = (1, 2, 3);
Getting and Setting an Element of the Array
Set and get an element of the array.
# Get an element of the array
$array[0];
$array[1];
# Set an element of the array
$array[0] = 1;
$array[1] = 2;
The Length of the Array
To get the length of the array, By evaluating the array in a scalar context, you can get the length of the array.
# Get the legnth of the array
my $array_length = @array;
Array Functions
Often used array functions.
# Cut off the first element
my $first = shift @array;
# Add an element at the beginning of the array
unshift @array, 5;
# Cut off the last element
my $last = pop @array;
# Add an element at the end of the array
push @array, 9;
Hashes
Explains Perl hashes. Hashes are data structures that have key-value pairs.
Hash Declarations
This is a hash declaration. A list can be assigned to a hash.
# Hash declaration
my %hash;
# Assignment values to the hash
%hash = (a => 1, b => 2);
Getting and Setting a Value of the Hash
Get and set a value of the hash.
# Get a value of the hash
$hash{a};
$hash{b};
# Set a value of the hash
$hash{a} = 5;
$hash{b} = 7;
If the key of the hash is not consists of "a-zA-Z0-9_", it must be enclosed in single or double quotes.
$hash{'some-key'} = 5;
Hash Functions
Often used hash functions.
# Get all keys
my @keys = keys %hash;
# Get all values
my @values = values %hash;
# Check if the key exists
exists $hash{a};
# Delete a key of the hash
delete $hash{a};
Conditional Branches
Explains conditional branches.
if Statements
For conditional branches, You can use if statements.
if ($condition) {
# If the condition is ture, the statements in this block are executed.
}
if-else statement
This is a if-else statement.
if ($condition) {
# If the condition is ture, the statements in this block are executed.
}
else {
# If the condition is false, the statements in this block are executed.
}
if-elsif Statements
This is a if-elsif statement.
if ($condition1) {
# If the condition 1 is ture, the statements in this block are executed.
}
elsif ($condition1) {
# If the condition 2 is ture, the statements in this block are executed.
}
Comparison operator
The list of Perl comparison operators. Perl has numeric comparison operators and string comparison operators.
Numeric Comparison Operators
Numeric comparison compares the values as numbers.
| Operators | Meanings |
|---|---|
| $x == $y | $x is equal to $y |
| $x != $y | $x is not equal to $y |
| $x < $y | $x is less than $y |
| $x > $y | $x is greater than $y |
| $x <= $y | $x is less than or equal to $y |
| $x >= $y | $x is greater than or equal to $y |
String Comparison Operators
String comparison operators compares the values as strings. The values are compared with the dictionary order.
| Operators | Meanings |
|---|---|
| $x eq $y | $x is equal to $y |
| $x ne $y | $x is not equal to $y |
| $x lt $y | $x is less than $y |
| $x gt $y | $x is greater than $y |
| $x le $y | $x is less than or equal to $y |
| $x ge $y | $x is greater than or equal to $y |
Loop Syntax
Explains loop syntax such as while/for.
The while statement
This is a while statement.
my $i = 0;
while ($i < 5) {
# Do something
$i++;
}
The for statement
This is a for statement.
for (my $i = 0; $i < 5; $i++) {
# Do something
}
The foreach statement
This is a foreach statement to iterate each element of the array.
foreach my $num (@nums) {
# Do something
}
In Perl, the foreach statement is alias for the for statement.
# Same as the above foreach statement
for my $num (@nums) {
# Do something
}
Subroutines
Explaines subroutines.
Subroutine Definition
This is a subroutine defintiion. A subroutine recieves the arguments, execute the statements, and return the return values.
sub sum {
# Revices arguments
my ($num1, $num2) = @_;
# Execute the statements
my $total = $num1 + $num2;
# Return the return values
return $total;
}
Calling Subroutines
Call a subroutine.
# Call a subroutine
my $sum = sum(1, 2);
File Input/Output
Explains file Input/Output.
# Open file
open my $fh, '<', $file
or die "Cannot open'$file':$!";
while (my $line = <$fh>) {
...
}
close $fh;
The open function opens a file. The "<" means the read mode.
The <$fh> is a line input operator.
$! is a predefined variable to be set to the error that the operating system returns.
Often Used Features
Explains Perl often used features.
Perl True and False Values
Explains Perl true and false values.
False Values
These are false values in Perl.
- undef
- 0
- ""
- "0"
- ()
True Values
True values are all values except for the above false values.
The defined Function
The defined function checks if the value is defined.
defined $num;
Command Line Arguments
@ARGV is command line arguments.
my ($args0, $args1, $args2) = @ARGV;
Scalar Context and List Context
Perl has context that the value is evaluated.
These are examples that the return values are different corresponding to scalar context and list context.
# Scalar context
my $time_str = localtime();
# List contex
my @datetime = localtime();
The unless Statement
Perl has the unless statement.
unless ($condition) {
...
}
The above unless statement is same as the following if statement.
if (!$condition) {
...
}
Postfix if, Postfix unless
Perl has the postfix if statement and the postfix unless statement.
# Postfix if
print $num if $num > 3;
# Postfix unless
die "error" unless $num;
Postfix for
Per has the postfix for statement.
# Postfix for
print $_ for @nums;
Each element of the array is assigned to $_.
$_ is called default variables
Array Slices and Hash Slices
Perl has array slices and hash slices syntax to get the specified elements.
# Array slice
my @select = @array[1, 4, 5];
# Hash slice
my @select = @hash{'a', 'b', 'd'};
The map Function
Perl has the map function to process each element of the array. Each element is assigned to $_.
my @mapped = map {$_ * 2} @array;
The above result of the map funtion is same as the result of the following for statement.
my @mapped;
for my $elem (@array) {
my $new_elem = $elem * 2;
push @mapped, $new_elem;
}
The grep function
Perl has the grep function to get the elements that the condition match. Each element is assigned to $_.
my @select = grep {$_ =~ /cat/} @array;
The above result of the grep function is same as the result of the following for statement.
my @select;
for my $elem (@array) {
if ($elem =~ /cat/) {
push @select, $elem;
}
}
List Assignment
This is the syntax of list assignment.
# List assignment
my ($num1, $num2) = ($num3, $num4);
The Range Operator
The range operator creates a list that has a range of integers.
# Range operator
my @numes = (0 .. 5);
This is same as the following code.
my @numes = (0, 1, 2, 3, 4, 5);
The string list operator
Perl has the string list operator to create a string list easily.
# String list operator
my @strs = qw(aaa bbb ccc);
This is same as the following code.
my @strs = ('aaa', 'bbb', 'ccc');
Return Statments that has No Operand
The return statments that has no operand returns undef in scalar context, or returns an empty list in list context.
sub foo {
return;
}
# undef
my $ret = foo();
# ()
my @ret = foo();
Exception Handling
The die function throws an exception.
# Throw an exception
die "Error message";
The eval block catch exceptions. If an exception occurs, the exception message is assinged to the predefined variable "$@".
# Catch exceptions
eval {
# Do something
};
# The exception message
if ($@) {
...
}
Read Whole Content from a File
To read whole content from a file, you can use the following syntax.
# Read whole content from a file
my $content = do { local $/; <$fh> };
The Ternary Operator
Perl has Ternary operator.
# Ternary operator
my $num = $flag ? 1 : 2;
||=
Perl has special assign operator "||=".
$num ||= 2;
This is same as the following code.
$num = $num || 2;
//=
Perl has the defined-or operator and the special assign operator.
$num //= 2;
This is same as the following code.
$num = $num // 2;
Module Loading
The use function loads a module.
use SomeModule;
Write a configuration file in Perl
The do function read the configuration file written by Perl.
use FindBind;
my $conf_file = "$FindBin::Bin/app.conf";
my $conf = do $conf_file
or die "Can't load config file \"$conf_file\":$! $@";
Examples of config file:
{
name =>'Foo',
number => 9
}
Multiple Line Comments
Although Perl has not the syntax of multiple line comments, You can use POD syntax to write multiple line comments.
=pod
Comment1
Comment2
Comment3
=cut
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