How do you parse a file and pull a block of markup?

[eluser]nubianxp[/eluser]
I have a remote file for example, which contains the ff:

Code:

...bunch of markups...
<div id="block-i-need-to-get">
  <div>some content</div>
  <div>some more content</div>
</div>
...more bunch of markups...

what i need is that particular code block only, and i understand that there's a preg_match() function that can be used for this, the problem is that, i don't know how to construct the code that will do it... any tips/links/snippets?

thanks in advance.