I'm not quite sure what you're asking for. A regular expression for a substitution? Something like this?

#!/usr/bin/perl -w
use strict;
use warnings;

my $str = "this.is.example.text";
print "Before: $str\n";
$str =~ s/^(\w+\.\w+\.)(\w+\.\w+)$/[$1] [$2]/;
print "After : $str\n";

__END__
Before: this.is.example.text
After : [this.is.] [example.text]
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If you're looking for something more sophisticated, then you should show us the code you wrote to solve your problem, and explain why that's not working for you.

If you are looking for something as simple as a regular expression, and I may be wrong about this, but you seem to be asking some very basic questions in your previous threads, then I cannot overstate the usefulness of a good Perl book, like Programming Perl.

You can split on the full-stops (which are regular expression metacharacters so need to be escaped) but use capturing parentheses in the regex so that they are retained in the resulting array; (have a look at perlretut, perlreref and perlre). Then you can construct your new string by interpolating array slices (Slices) in a double-quoted string, first having set the list separator (see $LIST_SEPARATOR in perlvar) to an empty string so that each element abuts the next.

$ perl -E '
> $text = q{this.is.example.text};
> @items = split m{(\.)}, $text;
> say for @items;
> $new = do                 
>    {
>        local $" = q{};
>        qq{[@items[ 0 .. 3 ]] [@items[ 4 .. 6 ]]};
>    };
> say $new;'
this
.
is
.
example
.
text
[this.is.] [example.text]
$
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I hope this is helpful.

This will identify bracketed expressions in sentence 2 and bracket them in sentence 1.

use strict;
use warnings;

my $snt1 = 'this.is.example.text';
my $snt2 = '[this.is.] [example.text]';

foreach ($snt2 =~ /\[(.+?)\]/g) {
  $snt1 =~ s/\Q$_\E/[$_]/;
}

print $snt1;
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