useful include, file renaming, file copying and file compression.


=head2 Behind The Scenes

To help explain what C<File::GlobMapper> does, consider what code you
would write if you wanted to rename all files in the current directory
that ended in C<.tar.gz> to C<.tgz>. So say these files are in the
current directory

    alpha.tar.gz
    beta.tar.gz
    gamma.tar.gz

and they need renamed to this

    alpha.tgz
    beta.tgz
    gamma.tgz

Below is a possible implementation of a script to carry out the rename
(error cases have been omitted)

    foreach my $old ( glob "*.tar.gz" )
    {
        my $new = $old;
        $new =~ s#(.*)\.tar\.gz$#$1.tgz# ;

        rename $old => $new 
            or die "Cannot rename '$old' to '$new': $!\n;
    }

Notice that a file glob pattern C<*.tar.gz> was used to match the
C<.tar.gz> files, then a fairly similar regular expression was used in
the substitute to allow the new filename to be created.

Given that the file glob is just a cut-down regular expression and that it
has already done a lot of the hard work in pattern matching the filenames,
wouldn't it be handy to be able to use the patterns in the fileglob to
drive the new filename?

Well, that's I<exactly> what C<File::GlobMapper> does. 

Here is same snippet of code rewritten using C<globmap>

    for my $pair (globmap '<*.tar.gz>' => '<#1.tgz>' )
    {
        my ($from, $to) = @$pair;
        rename $from => $to 
            or die "Cannot rename '$old' to '$new': $!\n;
    }

So how does it work?

Behind the scenes the C<globmap> function does a combination of a
file glob to match existing filenames followed by a substitute
to create the new filenames. 

Notice how both parameters to C<globmap> are strings that are delimited by <>.
This is done to make them look more like file globs - it is just syntactic
sugar, but it can be handy when you want the strings to be visually
distinctive. The enclosing <> are optional, so you don't have to use them - in
fact the first thing globmap will do is remove these delimiters if they are
present.

The first parameter to C<globmap>, C<*.tar.gz>, is an I<Input File Glob>. 
Once the enclosing "< ... >" is removed, this is passed (more or
less) unchanged to C<File::Glob> to carry out a file match.

Next the fileglob C<*.tar.gz> is transformed behind the scenes into a
full Perl regular expression, with the additional step of wrapping each
transformed wildcard metacharacter sequence in parenthesis.

In this case the input fileglob C<*.tar.gz> will be transformed into
this Perl regular expression 

    ([^/]*)\.tar\.gz

Wrapping with parenthesis allows the wildcard parts of the Input File
Glob to be referenced by the second parameter to C<globmap>, C<#1.tgz>,
the I<Output File Glob>. This parameter operates just like the replacement
part of a substitute command. The difference is that the C<#1> syntax
is used to reference sub-patterns matched in the input fileglob, rather
than the C<$1> syntax that is used with perl regular expressions. In
this case C<#1> is used to refer to the text matched by the C<*> in the
Input File Glob. This makes it easier to use this module where the
parameters to C<globmap> are typed at the command line.

The final step involves passing each filename matched by the C<*.tar.gz>
file glob through the derived Perl regular expression in turn and
expanding the output fileglob using it.

The end result of all this is a list of pairs of filenames. By default
that is what is returned by C<globmap>. In this example the data structure
returned will look like this

     ( ['alpha.tar.gz' => 'alpha.tgz'],
       ['beta.tar.gz'  => 'beta.tgz' ],
       ['gamma.tar.gz' => 'gamma.tgz']
     )