Forget complicated regex. For this you can use rindex / substr:

use strict;
use warnings;

my $regex = '\default\main\TSDEMO\WORKAREA\tsdemo_intranet\TSDEMO\imag
+es\corner';
my $website = 'TSDEMO';

my $parent = substr($regex, rindex($regex, $website));
print $parent;
[download]

(note that this gives TSDEMO\images\corner with no leading \, since you don't have $website as \TSDEMO)

Or if you really MUST have case insensitive matching - which doesn't make sense because file paths are sensitive - you can make uppercase copies of the original string and use those:

use strict;
use warnings;

my $regex = '\default\main\TSDEMO\WORKAREA\tsdemo_intranet\TSDEMO\imag
+es\corner';
my $website = 'TSDEMO';

my $tregex = uc($regex);
my $parent = substr($regex, rindex(uc($regex), uc($website)));
print $parent;
[download]

BTW, what you're asking for is the third match, not the second. With case insensitive matching, tsdemo from tsdemo_intranet also matches.