Narrowing In (Perl & LWP)

8.5. Narrowing In

Now, we could try excluding every kind of thing we know we don't want. We could exclude the mailto: link by excluding all URLs that start with mailto:; we could exclude the guest bio URLs by excluding URLs that contain guestinfo; we could exclude the "Previous" and "Next" links by ignoring any URLs with dayFA in them; and we could think of a way to exclude the image URLs. However, tomorrow the people at Fresh Air might add this to their general template:

<a href="buynow.html"><img alt="Buy the Terry Gross mug"
  src="/mug.jpg" width=450 weight=90></a>

Because that isn't explicitly excluded, it would make its way through and appear as a segment link in every program listed.

It is a valid approach to come up with criteria for the kinds of things we don't want to see, but it's usually easier to come up with criteria to capture what we do want to see. So this is what we'll do.

We could characterize the links we're after in several ways:

These links all contain a <font...> ...  sequence and a  ...  sequence.
They all have an <a ...> tag with an href attribute pointing to a URL.
The URL they point to looks like http://www.npr.org/ramfiles/fa/20010702.fa.ram.
Notably, the URL's scheme is http, it's on the server www.npr.org, its path includes ramfiles, and it ends in .ram.
The (trimmed) link text up to /a always begins with Listen to .

Now, of these, the first criterion is most reminiscent of the sort of things we did earlier with the BBC news extractor. But in this case, it's actually sort of a bother, because we can't specify that the next token after the <a ...> start-tag is a <font...> tag.

If, by this first criterion, we simply mean that calling $x->get_tag('/a', 'font', 'b') should give you <font...> or  before you hit </a>, well, this is true. But in either case, you'll have skipped over all the tokens between the current point in the stream and the next tag you find, and once you've skipped them, you can't get them back. In this case, we can get away with throwing out the content of <a ...>...</a> sequences that don't meet this one criterion, but in many situations you run into, you won't have that luxury. Moreover, in jumping from the <a ...> start-tag to the first <font...> tag, we may be jumping over text that we want but will never be able to get.

We could try implementing this all with the same approach we used with the BBC extractor in Chapter 7, "HTML Processing with Tokens", where we cook up several patterns (such as an <a href...> start-tag, a text token Listen to , a <font...> start-tag, some whitespace, and a  start-tag) and base our pattern matcher on get_token( ) so we can always call unget_token( ) on tokens that don't match the pattern. This is feasible, but it's sounding like the hardest of the criteria to formalize, at least under HTML::TokeParser. (But testing whether a tag sequence contains another is easy with HTML::TreeBuilder, as we see in later chapters.) So we'll try to make do without this one criterion and consider it a last resort.

Winding irrevocably past things is a problem not just with get_tag( ). It's also a problem with get_text( ) and get_trimmed_text( ). Once you use any of these methods to skip past tags and/or comments, they're gone for good. Unless you did something particularly perverse, such as read a huge chunk of the stream with get_token( ) and then stuffed it back in with unget_token( ) while still keeping a copy around. If you're even contemplating something like that, it's a definite sign that your program is outgrowing what you can do with HTML::TokeParser, and you should either write a new searcher method that's like get_text( ) but that can restore tokens to the buffer, or more likely move on to a parsing model based on HTML::TreeBuilder.

The next criteria (numbers 3 and 4 in the list above) are easy to formalize. These involve characteristics of the URL. We simply add a line to our while loop, like so:

while(my $a_tag = $stream->get_tag('a')) {
  my $url = $a_tag->[1]{'href'} || next;
  next unless $url =~ m{^http:}s and $url =~ m/www\.npr\.org/i
   and $url =~ m{/ramfiles/} and $url =~ m/\.ram$/;
  #  (There's many other ways of doing the above.)
  my $text = $stream->get_trimmed_text('/a');
  printf "%s\n  %s\n", $text, $url;
}

But this raises a point on which many programmers will, legitimately, diverge. Currently, we can say "it's interesting only if the URL ends in .ram," like so:

next unless $url =~ m/\.ram$/;

It works! But what if, tomorrow, some code like the following is added to the normal template?

<a href="/stuff/holiday_greets.ram">Happy Holidays
 from Terry Gross!</a>
<!-- just a short RA file of Terry saying "Happy NATO Day!" -->

We'll be annoyed we didn't make our link extractor check $url =~ m/www\.npr\.org/i and $url =~ m{/ramfiles/}. On the other hand, if we do check those additional facts about the URL, and tomorrow all the .ram files are moved off of www.npr.org and onto archive.npr.org, or onto terrygross.com or whatever, then it'll look like there were no links for this program! Then we'll be annoyed that we did make our link extractor check those additional things about the URL. Moreover, tomorrow NPR could switch to a better audio format than RealAudio, and all the .ram files could turn into something else, such that even m/\.ram$/ is no longer true. It could even be something served across a protocol other than HTTP! In other words, no part of the URL is reliably stable. On one hand, National Public Radio is not normally characterized by lavish budgets for web design (and redesign, and re-redesign), so you can expect some measure of stability. But on the other hand, you never know!


8.4. First Code		8.6. Rewrite for Features