Reference a variable within variable - hashes? - perl

Question

I have been looking for a solution to my problem and Hashes seem to be the answer after reading several posts but I am unsure how to implement this for my requirement, can anyone suggest how or even a better option?
In my code the variable $host is being set from values in a database. I loop through these values changing the value of $host each time.
I want to discard some types of host names, and to determine which hosts to discard I read in a user-configurable file which holds the Perl regex for that exclude. i.e. the config file has a line
EXCLUDE=\d+DAT\d+,\d+INF\d+
I then want to build up the Perl regexp match (logical OR), i.e.
if ( $host =~ m/\d+DAT\d+/ || $host =~ m/\d+INF\d+/ ) {
# do something
}
At the moment my code is hard wired as in the above example, but how can I dynamically construct the Perl regex after reading in the config file?
I have read the config file into an array and will start from there. The code above needs to end up like this:
if ($exclude clause) {
# do something
}
This is how I set about achieving that reading from the array:
for ($i = 1; $i < #conf; $i++) {
$exclude_clause .= "$host =~/" . #conf[$i] . "/ || ";
}
$exclude_clause =~ s/ \|\| $//;
The problem is referencing $host within the $exclude_clause. My regex string is built OK apart from the $host.

Answer 1

I would suggest a different approach that doesn't require you to build up a big Regex string and then evaluate it. Instead, what about using the List::MoreUtils module's any function, which accepts a block of code, evaluates it for each member of a list, and returns true once the block returns true for at least one entry in the list. For example:
use List::MoreUtils qw{ any };
if ( any { $host =~ $_ } #conf ) {
# do something
}
In the code block passed to any, the temp variable $_ contains the current entry in the list. That way you can avoid constructing a Regex in the first place.

Answer 2

I think you should store the complete regex in the configuration file, but it can be be a set of comma separated alternatives if need be.
You would use the qr// construct to build the regex:
my $exc1 = "\d+DAT\d+"; # Read from configuration file
my $ecc2 = "\d+INF\d+";
my $rex1 = qr/$exc1/;
my $rex2 = qr/$exc2/;
...populate $host...
if ($host =~ $rex1 || $host =~ $rex2)
{
...exclude $host...
}
else
{
...include $host...
}
Alternatively, you can build a single regex:
my $exc1 = "\d+DAT\d+"; # Read from configuration file
my $ecc2 = "\d+INF\d+";
my $rex = qr/$exc1|$exc2/;
...populate $host...
if ($host =~ $rex)
{
...exclude $host...
}
else
{
...include $host...
}
The single regex can be built from as many alternative exclusionary regex fragments as you like. Of course, if the value in the file is:
EXCLUDE=\d+DAT\d+|\d+INF\d+
then your code simplifies once more, assuming the regex string is read into $exc:
my $exc = "\d+DAT\d+|\d+INF\d+"; # Read from file
my $rex = qr/$exc/;
...populate $host...
if ($host =~ $rex)
{
...exclude $host...
}
else
{
...include $host...
}