use strict;
my $str = '/media/catalog/product/cache/1/thumbnail/543x403/db978388cfd007780066eaab38556cef/l/e/legal_slide01.jpg
# check if match with above:
/pub/media/catalog/product/cache/543x403/l/e/legal_slide01.jpg
/media/catalog/product/cache/1/thumbnail/543x403/db978388cfd007780066eaab38556cef/n/u/number1.png
# check if match with above:
/pub/media/catalog/product/cache/543x403/n/u/number1.png';
my $regex = qr@^/(media/catalog/product/cache)/\d+/thumbnail/(\d+x\d+)/[a-z0-9]+/(\S+)@mp;
my $subst = '/pub/$1/$2/$3';
my $result = $str =~ s@$regex@$subst@rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html