perl -pe 'chomp; $s=!$s,next if s/^\s*<\/?pre>\s*$//i; $_="

" if $s; $_.="\n";' out ##

##

in:              out:
----------------------------------
           
...etc...        ...etc...
            Line 1
Line 1           Line 2
Line 2           ...etc...
...etc...        Line n
Line n           ...etc...
           
...etc...