perlintro.pod

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conditional blocks more English like:

    # the traditional way
    if ($zippy) {
        print "Yow!";
    }

    # the Perlish post-condition way
    print "Yow!" if $zippy;
    print "We have no bananas" unless $bananas;

=item while

    while ( condition ) {
        ...
    }

There's also a negated version, for the same reason we have C<unless>:

    until ( condition ) {
        ...
    }

You can also use C<while> in a post-condition:

    print "LA LA LA\n" while 1;          # loops forever

=item for

Exactly like C:

    for ($i = 0; $i <= $max; $i++) {
        ...
    }

The C style for loop is rarely needed in Perl since Perl provides
the more friendly list scanning C<foreach> loop.

=item foreach

    foreach (@array) {
        print "This element is $_\n";
    }

    print $list[$_] foreach 0 .. $max;

    # you don't have to use the default $_ either...
    foreach my $key (keys %hash) {
        print "The value of $key is $hash{$key}\n";
    }

=back

For more detail on looping constructs (and some that weren't mentioned in
this overview) see L<perlsyn>.

=head2 Builtin operators and functions

Perl comes with a wide selection of builtin functions.  Some of the ones
we've already seen include C<print>, C<sort> and C<reverse>.  A list of
them is given at the start of L<perlfunc> and you can easily read
about any given function by using C<perldoc -f I<functionname>>.

Perl operators are documented in full in L<perlop>, but here are a few
of the most common ones:

=over 4

=item Arithmetic

    +   addition
    -   subtraction
    *   multiplication
    /   division

=item Numeric comparison

    ==  equality
    !=  inequality
    <   less than
    >   greater than
    <=  less than or equal
    >=  greater than or equal

=item String comparison

    eq  equality
    ne  inequality
    lt  less than
    gt  greater than
    le  less than or equal
    ge  greater than or equal

(Why do we have separate numeric and string comparisons?  Because we don't
have special variable types, and Perl needs to know whether to sort
numerically (where 99 is less than 100) or alphabetically (where 100 comes
before 99).

=item Boolean logic

    &&  and
    ||  or
    !   not

(C<and>, C<or> and C<not> aren't just in the above table as descriptions
of the operators -- they're also supported as operators in their own
right.  They're more readable than the C-style operators, but have
different precedence to C<&&> and friends.  Check L<perlop> for more
detail.)

=item Miscellaneous

    =   assignment
    .   string concatenation
    x   string multiplication
    ..  range operator (creates a list of numbers)

=back

Many operators can be combined with a C<=> as follows:

    $a += 1;        # same as $a = $a + 1
    $a -= 1;        # same as $a = $a - 1
    $a .= "\n";     # same as $a = $a . "\n";

=head2 Files and I/O

You can open a file for input or output using the C<open()> function.
It's documented in extravagant detail in L<perlfunc> and L<perlopentut>,
but in short:

    open(my $in,  "<",  "input.txt")  or die "Can't open input.txt: $!";
    open(my $out, ">",  "output.txt") or die "Can't open output.txt: $!";
    open(my $log, ">>", "my.log")     or die "Can't open my.log: $!";

You can read from an open filehandle using the C<< <> >> operator.  In
scalar context it reads a single line from the filehandle, and in list
context it reads the whole file in, assigning each line to an element of
the list:

    my $line  = <$in>;
    my @lines = <$in>;

Reading in the whole file at one time is called slurping. It can
be useful but it may be a memory hog. Most text file processing
can be done a line at a time with Perl's looping constructs.

The C<< <> >> operator is most often seen in a C<while> loop:

    while (<$in>) {     # assigns each line in turn to $_
        print "Just read in this line: $_";
    }

We've already seen how to print to standard output using C<print()>.
However, C<print()> can also take an optional first argument specifying
which filehandle to print to:

    print STDERR "This is your final warning.\n";
    print $out $record;
    print $log $logmessage;

When you're done with your filehandles, you should C<close()> them
(though to be honest, Perl will clean up after you if you forget):

    close $in or die "$in: $!";

=head2 Regular expressions

Perl's regular expression support is both broad and deep, and is the
subject of lengthy documentation in L<perlrequick>, L<perlretut>, and
elsewhere.  However, in short:

=over 4

=item Simple matching

    if (/foo/)       { ... }  # true if $_ contains "foo"
    if ($a =~ /foo/) { ... }  # true if $a contains "foo"

The C<//> matching operator is documented in L<perlop>.  It operates on
C<$_> by default, or can be bound to another variable using the C<=~>
binding operator (also documented in L<perlop>).

=item Simple substitution

    s/foo/bar/;               # replaces foo with bar in $_
    $a =~ s/foo/bar/;         # replaces foo with bar in $a
    $a =~ s/foo/bar/g;        # replaces ALL INSTANCES of foo with bar in $a

The C<s///> substitution operator is documented in L<perlop>.

=item More complex regular expressions

You don't just have to match on fixed strings.  In fact, you can match
on just about anything you could dream of by using more complex regular
expressions.  These are documented at great length in L<perlre>, but for
the meantime, here's a quick cheat sheet:

    .                   a single character
    \s                  a whitespace character (space, tab, newline, ...)
    \S                  non-whitespace character
    \d                  a digit (0-9)
    \D                  a non-digit
    \w                  a word character (a-z, A-Z, 0-9, _)
    \W                  a non-word character
    [aeiou]             matches a single character in the given set
    [^aeiou]            matches a single character outside the given set
    (foo|bar|baz)       matches any of the alternatives specified

    ^                   start of string
    $                   end of string

Quantifiers can be used to specify how many of the previous thing you
want to match on, where "thing" means either a literal character, one
of the metacharacters listed above, or a group of characters or
metacharacters in parentheses.

    *                   zero or more of the previous thing
    +                   one or more of the previous thing
    ?                   zero or one of the previous thing
    {3}                 matches exactly 3 of the previous thing
    {3,6}               matches between 3 and 6 of the previous thing
    {3,}                matches 3 or more of the previous thing

Some brief examples:

    /^\d+/              string starts with one or more digits
    /^$/                nothing in the string (start and end are adjacent)
    /(\d\s){3}/         a three digits, each followed by a whitespace
                        character (eg "3 4 5 ")
    /(a.)+/             matches a string in which every odd-numbered letter
                        is a (eg "abacadaf")

    # This loop reads from STDIN, and prints non-blank lines:
    while (<>) {
        next if /^$/;
        print;
    }

=item Parentheses for capturing

As well as grouping, parentheses serve a second purpose.  They can be
used to capture the results of parts of the regexp match for later use.
The results end up in C<$1>, C<$2> and so on.

    # a cheap and nasty way to break an email address up into parts

    if ($email =~ /([^@]+)@(.+)/) {
        print "Username is $1\n";
        print "Hostname is $2\n";
    }

=item Other regexp features

Perl regexps also support backreferences, lookaheads, and all kinds of
other complex details.  Read all about them in L<perlrequick>,
L<perlretut>, and L<perlre>.

=back

=head2 Writing subroutines

Writing subroutines is easy:

    sub logger {
        my $logmessage = shift;
        open my $logfile, ">>", "my.log" or die "Could not open my.log: $!";
        print $logfile $logmessage;
    }

Now we can use the subroutine just as any other built-in function:

    logger("We have a logger subroutine!");

What's that C<shift>?  Well, the arguments to a subroutine are available
to us as a special array called C<@_> (see L<perlvar> for more on that).
The default argument to the C<shift> function just happens to be C<@_>.
So C<my $logmessage = shift;> shifts the first item off the list of
arguments and assigns it to C<$logmessage>.

We can manipulate C<@_> in other ways too:

    my ($logmessage, $priority) = @_;       # common
    my $logmessage = $_[0];                 # uncommon, and ugly

Subroutines can also return values:

    sub square {
        my $num = shift;
        my $result = $num * $num;
        return $result;
    }

Then use it like:

    $sq = square(8);

For more information on writing subroutines, see L<perlsub>.

=head2 OO Perl

OO Perl is relatively simple and is implemented using references which
know what sort of object they are based on Perl's concept of packages.
However, OO Perl is largely beyond the scope of this document.
Read L<perlboot>, L<perltoot>, L<perltooc> and L<perlobj>.

As a beginning Perl programmer, your most common use of OO Perl will be
in using third-party modules, which are documented below.

=head2 Using Perl modules

Perl modules provide a range of features to help you avoid reinventing
the wheel, and can be downloaded from CPAN ( http://www.cpan.org/ ).  A
number of popular modules are included with the Perl distribution
itself.

Categories of modules range from text manipulation to network protocols
to database integration to graphics.  A categorized list of modules is
also available from CPAN.

To learn how to install modules you download from CPAN, read
L<perlmodinstall>

To learn how to use a particular module, use C<perldoc I<Module::Name>>.
Typically you will want to C<use I<Module::Name>>, which will then give
you access to exported functions or an OO interface to the module.

L<perlfaq> contains questions and answers related to many common
tasks, and often provides suggestions for good CPAN modules to use.

L<perlmod> describes Perl modules in general.  L<perlmodlib> lists the
modules which came with your Perl installation.

If you feel the urge to write Perl modules, L<perlnewmod> will give you
good advice.

=head1 AUTHOR

Kirrily "Skud" Robert <[e-mail]>