Though many Monks use vi, SOPW is not the appropriate place for a question strictly about vi.

If this were a perl question... and if your output illustration reflects your actual desire ("# 1 aba baba Cheers" does not match "how can i write regular exp to add new line in between # and 1."):

#!/usr/bin/perl
use strict;
use warnings;
# 800208 

my $test;
while ( <DATA> ) {
    $test = $_;
    if ( $test =~ /#1 aba baba/ ) {
        chomp $test;
        print "$test Cheers\n";
    } else {
        print "$test";
    }
}

__DATA__
#1 aba baba
#1 bsdb dbbsd
#1 sdh hsds
#1 aba baba
#2 aba baba
[download]

I think you need something like:

%s/^#1/#\r1/g
[download]

Hi!

vi has perl regular expressions?

If you compile vim with perl support, you can directly execute perl commands inside vim

see also: http://vimdoc.sourceforge.net/htmldoc/if_perl.html

Have fun!

Update Your command would be:

:perldo $_ =~ s/#1/# 1/g;
[download]

with :perldo executing the given command on each line of your file.

You could simply do:

$test = '#1 aba baba';
$test =~ s/#1/# 1/g;
print "$test\n";
[download]