Ok, now I get it.

Your idea is clever but smells a bit like a hack. Whenever the config file changes minimally (for example an additional space or empty line somewhere) your method breaks. And as you have already found out it can't cope with a variable number of lines.

With a bit more complexity you might still get this to work, by doing the parsing piece by piece (aka chunk by chunk). Your end user would have to split the static lines from the dynamic and give you something like this:

Chunk 1: 
  (mpls ldp)
Chunk 2: 
  (router-id) (__REGEX__)
...
Chunk 6: 
 !
 interface (__REGEX__)
[download]

Your end user would have to know that Chunk 6 has to include the line with the '!' because it is part of the repeating pattern.

Then your script should match chunk after chunk. And try to repeat matching any chunks with __REGEX__ in them until they don't match anymore. And naturally it should warn if it doesn't get at least one match