use strict;
my $str = '1
00:00:00,000 --> 00:00:16,700
to use 2 languages.
2
00:00:16,700 --> 00:00:19,600
I was saying that we are going to use 2 languages
3
00:00:19,600 --> 00:00:24,700
...I myself will continue to speak because of time';
my $regex = qr/(?:^|\n)\d+\n[\d\:\,\s\->]+/p;
my $subst = '\\n';
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