Here's one regex (plus while loop) for it:

  while ( s{ ^ (.*?)
               (\s* \S+)_DEL
               (.*) $ }
           {$1$3\n$1$2$3}xmg )
  {}
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I am trying to cook up one using (?{ ... }) constructs, but not having as much luck.

Update: Oops, left off the empty loop body...

Update (at 2004-04-29 23:49 -0700): Here is a more procedural solution that uses the same basic idea as above. It might be a bit more useful, as it returns a list of variants instead of one big string. (Would be interesting to benchmark them...)

sub del_selective
{
    my $n = shift;
    if ( $n =~ m/(.*?)(\s*\S+)_DEL(.*)/s )
    {
        my $s1 = $1.$3;
        my $s2 = $1.$2.$3;
        return ( del_selective( $s1 ),
                 del_selective( $s2 ) );
    }
    else
    {
        return ( $n );
    }
}
[download]