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HTML::Tree::Scanning(3)              User Contributed Perl Documentation             HTML::Tree::Scanning(3)



NAME
       HTML::Tree::Scanning -- article: "Scanning HTML"

SYNOPSIS
         # This an article, not a module.

DESCRIPTION
       The following article by Sean M. Burke first appeared in The Perl Journal #19 and is copyright 2000
       The Perl Journal. It appears courtesy of Jon Orwant and The Perl Journal.  This document may be
       distributed under the same terms as Perl itself.

       (Note that this is discussed in chapters 6 through 10 of the book Perl and LWP
       <http://lwp.interglacial.com/> which was written after the following documentation, and which is
       available free online.)

Scanning HTML
       -- Sean M. Burke

       In The Perl Journal issue 17, Ken MacFarlane's article "Parsing HTML with HTML::Parser" describes how
       the HTML::Parser module scans HTML source as a stream of start-tags, end-tags, text, comments, etc.
       In TPJ #18, my "Trees" article kicked around the idea of tree-shaped data structures.  Now I'll try
       to tie it together, in a discussion of HTML trees.

       The CPAN module HTML::TreeBuilder takes the tags that HTML::Parser picks out, and builds a parse tree
       -- a tree-shaped network of objects...

           Footnote: And if you need a quick explanation of objects, see my TPJ17 article "A User's View of
           Object-Oriented Modules"; or go whole hog and get Damian Conway's excellent book Object-Oriented
           Perl, from Manning Publications.

       ...representing the structured content of the HTML document.  And once the document is parsed as a
       tree, you'll find the common tasks of extracting data from that HTML document/tree to be quite
       straightforward.

   HTML::Parser, HTML::TreeBuilder, and HTML::Element
       You use HTML::TreeBuilder to make a parse tree out of an HTML source file, by simply saying:

         use HTML::TreeBuilder;
         my $tree = HTML::TreeBuilder->new();
         $tree->parse_file('foo.html');

       and then $tree contains a parse tree built from the HTML source from the file "foo.html".  The way
       this parse tree is represented is with a network of objects -- $tree is the root, an element with
       tag-name "html", and its children typically include a "head" and "body" element, and so on.  Elements
       in the tree are objects of the class HTML::Element.

       So, if you take this source:

         <html><head><title>Doc 1</title></head>
         <body>
         Stuff <hr> 2000-08-17
         </body></html>

       and feed it to HTML::TreeBuilder, it'll return a tree of objects that looks like this:

                      html
                    /      \
                head        body
               /          /   |  \
            title    "Stuff"  hr  "2000-08-17"
              |
           "Doc 1"

       This is a pretty simple document, but if it were any more complex, it'd be a bit hard to draw in that
       style, since it's sprawl left and right.  The same tree can be represented a bit more easily
       sideways, with indenting:

         . html
            . head
               . title
                  . "Doc 1"
            . body
               . "Stuff"
               . hr
               . "2000-08-17"

       Either way expresses the same structure.  In that structure, the root node is an object of the class
       HTML::Element

           Footnote: Well actually, the root is of the class HTML::TreeBuilder, but that's just a subclass
           of HTML::Element, plus the few extra methods like "parse_file" that elaborate the tree

       , with the tag name "html", and with two children: an HTML::Element object whose tag names are "head"
       and "body".  And each of those elements have children, and so on down.  Not all elements (as we'll
       call the objects of class HTML::Element) have children -- the "hr" element doesn't.  And note all
       nodes in the tree are elements -- the text nodes ("Doc 1", "Stuff", and "2000-08-17") are just
       strings.

       Objects of the class HTML::Element each have three noteworthy attributes:

       "_tag" -- (best accessed as "$e->tag") this element's tag-name, lowercased (e.g., "em" for an "em"
       element).
               Footnote: Yes, this is misnamed.  In proper SGML terminology, this is instead called a "GI",
               short for "generic identifier"; and the term "tag" is used for a token of SGML source that
               represents either the start of an element (a start-tag like "<em lang='fr'>") or the end of
               an element (an end-tag like "</em>".  However, since more people claim to have been abducted
               by aliens than to have ever seen the SGML standard, and since both encounters typically
               involve a feeling of "missing time", it's not surprising that the terminology of the SGML
               standard is not closely followed.

       "_parent" -- (best accessed as "$e->parent") the element that is $obj's parent, or undef if this
       element is the root of its tree.
       "_content" -- (best accessed as "$e->content_list") the list of nodes (i.e., elements or text
       segments) that are $e's children.

       Moreover, if an element object has any attributes in the SGML sense of the word, then those are
       readable as "$e->attr('name')" -- for example, with the object built from having parsed "<a
       id='foo'>bar</a>", "$e->attr('id')" will return the string "foo".  Moreover, "$e->tag" on that object
       returns the string "a", "$e->content_list" returns a list consisting of just the single scalar "bar",
       and "$e->parent" returns the object that's this node's parent -- which may be, for example, a "p"
       element.

       And that's all that there is to it -- you throw HTML source at TreeBuilder, and it returns a tree
       built of HTML::Element objects and some text strings.

       However, what do you do with a tree of objects?  People code information into HTML trees not for the
       fun of arranging elements, but to represent the structure of specific text and images -- some text is
       in this "li" element, some other text is in that heading, some images are in that other table cell
       that has those attributes, and so on.

       Now, it may happen that you're rendering that whole HTML tree into some layout format.  Or you could
       be trying to make some systematic change to the HTML tree before dumping it out as HTML source again.
       But, in my experience, by far the most common programming task that Perl programmers face with HTML
       is in trying to extract some piece of information from a larger document.  Since that's so common
       (and also since it involves concepts that are basic to more complex tasks), that is what the rest of
       this article will be about.

   Scanning HTML trees
       Suppose you have a thousand HTML documents, each of them a press release.  They all start out:

         [...lots of leading images and junk...]
         <h1>ConGlomCo to Open New Corporate Office in Ougadougou</h1>
         BAKERSFIELD, CA, 2000-04-24 -- ConGlomCo's vice president in charge
         of world conquest, Rock Feldspar, announced today the opening of a
         new office in Ougadougou, the capital city of Burkino Faso, gateway
         to the bustling "Silicon Sahara" of Africa...
         [...etc...]

       ...and what you've got to do is, for each document, copy whatever text is in the "h1" element, so
       that you can, for example, make a table of contents of it.  Now, there are three ways to do this:

          You can just use a regexp to scan the file for a text pattern.

           For many very simple tasks, this will do fine.  Many HTML documents are, in practice, very
           consistently formatted as far as placement of linebreaks and whitespace, so you could just get
           away with scanning the file like so:

             sub get_heading {
               my $filename = $_[0];
               local *HTML;
               open(HTML, $filename)
                 or die "Couldn't open $filename);
               my $heading;
              Line:
               while(<HTML>) {
                 if( m{<h1>(.*?)</h1>}i ) {  # match it!
                   $heading = $1;
                   last Line;
                 }
               }
               close(HTML);
               warn "No heading in $filename?"
                unless defined $heading;
               return $heading;
             }

           This is quick and fast, but awfully fragile -- if there's a newline in the middle of a heading's
           text, it won't match the above regexp, and you'll get an error.  The regexp will also fail if the
           "h1" element's start-tag has any attributes.  If you have to adapt your code to fit more kinds of
           start-tags, you'll end up basically reinventing part of HTML::Parser, at which point you should
           probably just stop, and use HTML::Parser itself:

          You can use HTML::Parser to scan the file for an "h1" start-tag token, then capture all the text
           tokens until the "h1" close-tag.  This approach is extensively covered in the Ken MacFarlane's
           TPJ17 article "Parsing HTML with HTML::Parser".  (A variant of this approach is to use
           HTML::TokeParser, which presents a different and rather handier interface to the tokens that
           HTML::Parser picks out.)

           Using HTML::Parser is less fragile than our first approach, since it's not sensitive to the exact
           internal formatting of the start-tag (much less whether it's split across two lines).  However,
           when you need more information about the context of the "h1" element, or if you're having to deal
           with any of the tricky bits of HTML, such as parsing of tables, you'll find out the flat list of
           tokens that HTML::Parser returns isn't immediately useful.  To get something useful out of those
           tokens, you'll need to write code that knows some things about what elements take no content (as
           with "hr" elements), and that a "</p>" end-tags are omissible, so a "<p>" will end any currently
           open paragraph -- and you're well on your way to pointlessly reinventing much of the code in
           HTML::TreeBuilder

               Footnote: And, as the person who last rewrote that module, I can attest that it wasn't
               terribly easy to get right!  Never underestimate the perversity of people coding HTML.

           , at which point you should probably just stop, and use HTML::TreeBuilder itself:

          You can use HTML::Treebuilder, and scan the tree of element objects that you get back.

       The last approach, using HTML::TreeBuilder, is the diametric opposite of first approach:  The first
       approach involves just elementary Perl and one regexp, whereas the TreeBuilder approach involves
       being at home with the concept of tree-shaped data structures and modules with object-oriented
       interfaces, as well as with the particular interfaces that HTML::TreeBuilder and HTML::Element
       provide.

       However, what the TreeBuilder approach has going for it is that it's the most robust, because it
       involves dealing with HTML in its "native" format -- it deals with the tree structure that HTML code
       represents, without any consideration of how the source is coded and with what tags omitted.

       So, to extract the text from the "h1" elements of an HTML document:

         sub get_heading {
           my $tree = HTML::TreeBuilder->new;
           $tree->parse_file($_[0]);   # !
           my $heading;
           my $h1 = $tree->look_down('_tag', 'h1');  # !
           if($h1) {
             $heading = $h1->as_text;   # !
           } else {
             warn "No heading in $_[0]?";
           }
           $tree->delete; # clear memory!
           return $heading;
         }

       This uses some unfamiliar methods that need explaining.  The "parse_file" method that we've seen
       before, builds a tree based on source from the file given.  The "delete" method is for marking a
       tree's contents as available for garbage collection, when you're done with the tree.  The "as_text"
       method returns a string that contains all the text bits that are children (or otherwise descendants)
       of the given node -- to get the text content of the $h1 object, we could just say:

         $heading = join '', $h1->content_list;

       but that will work only if we're sure that the "h1" element's children will be only text bits -- if
       the document contained:

         <h1>Local Man Sees <cite>Blade</cite> Again</h1>

       then the sub-tree would be:

         . h1
           . "Local Man Sees "
           . cite
             . "Blade"
           . " Again'

       so "join '', $h1->content_list" will be something like:

         Local Man Sees HTML::Element=HASH(0x15424040) Again

       whereas "$h1->as_text" would yield:

         Local Man Sees Blade Again

       and depending on what you're doing with the heading text, you might want the "as_HTML" method
       instead.  It returns the (sub)tree represented as HTML source.  "$h1->as_HTML" would yield:

         <h1>Local Man Sees <cite>Blade</cite> Again</h1>

       However, if you wanted the contents of $h1 as HTML, but not the $h1 itself, you could say:

         join '',
           map(
             ref($_) ? $_->as_HTML : $_,
             $h1->content_list
           )

       This "map" iterates over the nodes in $h1's list of children; and for each node that's just a text
       bit (as "Local Man Sees " is), it just passes through that string value, and for each node that's an
       actual object (causing "ref" to be true), "as_HTML" will used instead of the string value of the
       object itself (which would be something quite useless, as most object values are).  So that "as_HTML"
       for the "cite" element will be the string "<cite>Blade</cite>".  And then, finally, "join" just puts
       into one string all the strings that the "map" returns.

       Last but not least, the most important method in our "get_heading" sub is the "look_down" method.
       This method looks down at the subtree starting at the given object ($h1), looking for elements that
       meet criteria you provide.

       The criteria are specified in the method's argument list.  Each criterion can consist of two scalars,
       a key and a value, which express that you want elements that have that attribute (like "_tag", or
       "src") with the given value ("h1"); or the criterion can be a reference to a subroutine that, when
       called on the given element, returns true if that is a node you're looking for.  If you specify
       several criteria, then that's taken to mean that you want all the elements that each satisfy all the
       criteria.  (In other words, there's an "implicit AND".)

       And finally, there's a bit of an optimization -- if you call the "look_down" method in a scalar
       context, you get just the first node (or undef if none) -- and, in fact, once "look_down" finds that
       first matching element, it doesn't bother looking any further.

       So the example:

         $h1 = $tree->look_down('_tag', 'h1');

       returns the first element at-or-under $tree whose "_tag" attribute has the value "h1".

   Complex Criteria in Tree Scanning
       Now, the above "look_down" code looks like a lot of bother, with barely more benefit than just
       grepping the file!  But consider if your criteria were more complicated -- suppose you found that
       some of the press releases that you were scanning had several "h1" elements, possibly before or after
       the one you actually want.  For example:

         <h1><center>Visit Our Corporate Partner
          <br><a href="/dyna/clickthru"
            ><img src="/dyna/vend_ad"></a>
         </center></h1>
         <h1><center>ConGlomCo President Schreck to Visit Regional HQ
          <br><a href="/photos/Schreck_visit_large.jpg"
            ><img src="/photos/Schreck_visit.jpg"></a>
         </center></h1>

       Here, you want to ignore the first "h1" element because it contains an ad, and you want the text from
       the second "h1".  The problem is in formalizing the way you know that it's an ad.  Since ad banners
       are always entreating you to "visit" the sponsoring site, you could exclude "h1" elements that
       contain the word "visit" under them:

         my $real_h1 = $tree->look_down(
           '_tag', 'h1',
           sub {
             $_[0]->as_text !~ m/\bvisit/i
           }
         );

       The first criterion looks for "h1" elements, and the second criterion limits those to only the ones
       whose text content doesn't match "m/\bvisit/".  But unfortunately, that won't work for our example,
       since the second "h1" mentions "ConGlomCo President Schreck to Visit Regional HQ".

       Instead you could try looking for the first "h1" element that doesn't contain an image:

         my $real_h1 = $tree->look_down(
           '_tag', 'h1',
           sub {
             not $_[0]->look_down('_tag', 'img')
           }
         );

       This criterion sub might seem a bit odd, since it calls "look_down" as part of a larger "look_down"
       operation, but that's fine.  Note that when considered as a boolean value, a "look_down" in a scalar
       context value returns false (specifically, undef) if there's no matching element at or under the
       given element; and it returns the first matching element (which, being a reference and object, is
       always a true value), if any matches.  So, here,

         sub {
           not $_[0]->look_down('_tag', 'img')
         }

       means "return true only if this element has no 'img' element as descendants (and isn't an 'img'
       element itself)."

       This correctly filters out the first "h1" that contains the ad, but it also incorrectly filters out
       the second "h1" that contains a non-advertisement photo besides the headline text you want.

       There clearly are detectable differences between the first and second "h1" elements -- the only
       second one contains the string "Schreck", and we could just test for that:

         my $real_h1 = $tree->look_down(
           '_tag', 'h1',
           sub {
             $_[0]->as_text =~ m{Schreck}
           }
         );

       And that works fine for this one example, but unless all thousand of your press releases have
       "Schreck" in the headline, that's just not a general solution.  However, if all the ads-in-"h1"s that
       you want to exclude involve a link whose URL involves "/dyna/", then you can use that:

         my $real_h1 = $tree->look_down(
           '_tag', 'h1',
           sub {
             my $link = $_[0]->look_down('_tag','a');
             return 1 unless $link;
               # no link means it's fine
             return 0 if $link->attr('href') =~ m{/dyna/};
               # a link to there is bad
             return 1; # otherwise okay
           }
         );

       Or you can look at it another way and say that you want the first "h1" element that either contains
       no images, or else whose image has a "src" attribute whose value contains "/photos/":

         my $real_h1 = $tree->look_down(
           '_tag', 'h1',
           sub {
             my $img = $_[0]->look_down('_tag','img');
             return 1 unless $img;
               # no image means it's fine
             return 1 if $img->attr('src') =~ m{/photos/};
               # good if a photo
             return 0; # otherwise bad
           }
         );

       Recall that this use of "look_down" in a scalar context means to return the first element at or under
       $tree that matches all the criteria.  But if you notice that you can formulate criteria that'll match
       several possible "h1" elements, some of which may be bogus but the last one of which is always the
       one you want, then you can use "look_down" in a list context, and just use the last element of that
       list:

         my @h1s = $tree->look_down(
           '_tag', 'h1',
           ...maybe more criteria...
         );
         die "What, no h1s here?" unless @h1s;
         my $real_h1 = $h1s[-1]; # last or only

   A Case Study: Scanning Yahoo News's HTML
       The above (somewhat contrived) case involves extracting data from a bunch of pre-existing HTML files.
       In that sort of situation, if your code works for all the files, then you know that the code works --since -since
       since the data it's meant to handle won't go changing or growing; and, typically, once you've used
       the program, you'll never need to use it again.

       The other kind of situation faced in many data extraction tasks is where the program is used
       recurringly to handle new data -- such as from ever-changing Web pages.  As a real-world example of
       this, consider a program that you could use (suppose it's crontabbed) to extract headline-links from
       subsections of Yahoo News ("http://dailynews.yahoo.com/").

       Yahoo News has several subsections:

       http://dailynews.yahoo.com/h/tc/ for technology news
       http://dailynews.yahoo.com/h/sc/ for science news
       http://dailynews.yahoo.com/h/hl/ for health news
       http://dailynews.yahoo.com/h/wl/ for world news
       http://dailynews.yahoo.com/h/en/ for entertainment news

       and others.  All of them are built on the same basic HTML template -- and a scarily complicated
       template it is, especially when you look at it with an eye toward making up rules that will select
       where the real headline-links are, while screening out all the links to other parts of Yahoo, other
       news services, etc.  You will need to puzzle over the HTML source, and scrutinize the output of
       "$tree->dump" on the parse tree of that HTML.

       Sometimes the only way to pin down what you're after is by position in the tree. For example,
       headlines of interest may be in the third column of the second row of the second table element in a
       page:

         my $table = ( $tree->look_down('_tag','table') )[1];
         my $row2  = ( $table->look_down('_tag', 'tr' ) )[1];
         my $col3  = ( $row2->look-down('_tag', 'td')   )[2];
         ...then do things with $col3...

       Or they may be all the links in a "p" element that has at least three "br" elements as children:

         my $p = $tree->look_down(
           '_tag', 'p',
           sub {
             2 < grep { ref($_) and $_->tag eq 'br' }
                      $_[0]->content_list
           }
         );
         @links = $p->look_down('_tag', 'a');

       But almost always, you can get away with looking for properties of the of the thing itself, rather
       than just looking for contexts.  Now, if you're lucky, the document you're looking through has clear
       semantic tagging, such is as useful in CSS -- note the class="headlinelink" bit here:

         <a href="...long_news_url..." class="headlinelink">Elvis
         seen in tortilla</a>

       If you find anything like that, you could leap right in and select links with:

         @links = $tree->look_down('class','headlinelink');

       Regrettably, your chances of seeing any sort of semantic markup principles really being followed with
       actual HTML are pretty thin.

           Footnote: In fact, your chances of finding a page that is simply free of HTML errors are even
           thinner.  And surprisingly, sites like Amazon or Yahoo are typically worse as far as quality of
           code than personal sites whose entire production cycle involves simply being saved and uploaded
           from Netscape Composer.

       The code may be sort of "accidentally semantic", however -- for example, in a set of pages I was
       scanning recently, I found that looking for "td" elements with a "width" attribute value of "375" got
       me exactly what I wanted.  No-one designing that page ever conceived of "width=375" as meaning "this
       is a headline", but if you impute it to mean that, it works.

       An approach like this happens to work for the Yahoo News code, because the headline-links are
       distinguished by the fact that they (and they alone) contain a "b" element:

         <a href="...long_news_url..."><b>Elvis seen in tortilla</b></a>

       or, diagrammed as a part of the parse tree:

         . a  [href="...long_news_url..."]
           . b
             . "Elvis seen in tortilla"

       A rule that matches these can be formalized as "look for any 'a' element that has only one daugher
       node, which must be a 'b' element".  And this is what it looks like when cooked up as a "look_down"
       expression and prefaced with a bit of code that retrieves the text of the given Yahoo News page and
       feeds it to TreeBuilder:

         use strict;
         use HTML::TreeBuilder 2.97;
         use LWP::UserAgent;
         sub get_headlines {
           my $url = $_[0] || die "What URL?";

           my $response = LWP::UserAgent->new->request(
             HTTP::Request->new( GET => $url )
           );
           unless($response->is_success) {
             warn "Couldn't get $url: ", $response->status_line, "\n";
             return;
           }

           my $tree = HTML::TreeBuilder->new();
           $tree->parse($response->content);
           $tree->eof;

           my @out;
           foreach my $link (
             $tree->look_down(   # !
               '_tag', 'a',
               sub {
                 return unless $_[0]->attr('href');
                 my @c = $_[0]->content_list;
                 @c == 1 and ref $c[0] and $c[0]->tag eq 'b';
               }
             )
           ) {
             push @out, [ $link->attr('href'), $link->as_text ];
           }

           warn "Odd, fewer than 6 stories in $url!" if @out < 6;
           $tree->delete;
           return @out;
         }

       ...and add a bit of code to actually call that routine and display the results...

         foreach my $section (qw[tc sc hl wl en]) {
           my @links = get_headlines(
             "http://dailynews.yahoo.com/h/$section/"
           );
           print
             $section, ": ", scalar(@links), " stories\n",
             map(("  ", $_->[0], " : ", $_->[1], "\n"), @links),
             "\n";
         }

       And we've got our own headline-extractor service!  This in and of itself isn't no amazingly useful
       (since if you want to see the headlines, you can just look at the Yahoo News pages), but it could
       easily be the basis for quite useful features like filtering the headlines for matching certain
       keywords of interest to you.

       Now, one of these days, Yahoo News will decide to change its HTML template.  When this happens, this
       will appear to the above program as there being no links that meet the given criteria; or, less
       likely, dozens of erroneous links will meet the criteria.  In either case, the criteria will have to
       be changed for the new template; they may just need adjustment, or you may need to scrap them and
       start over.

   Regardez, duvet!
       It's often quite a challenge to write criteria to match the desired parts of an HTML parse tree.
       Very often you can pull it off with a simple "$tree->look_down('_tag', 'h1')", but sometimes you do
       have to keep adding and refining criteria, until you might end up with complex filters like what I've
       shown in this article.  The benefit to learning how to deal with HTML parse trees is that one main
       search tool, the "look_down" method, can do most of the work, making simple things easy, while still
       making hard things possible.

       [end body of article]

   [Author Credit]
       Sean M. Burke ("sburke@cpan.org") is the current maintainer of "HTML::TreeBuilder" and
       "HTML::Element", both originally by Gisle Aas.

       Sean adds: "I'd like to thank the folks who listened to me ramble incessantly about HTML::TreeBuilder
       and HTML::Element at this year's Yet Another Perl Conference and O'Reilly Open Source Software
       Convention."

BACK
       Return to the HTML::Tree docs.



perl v5.12.5                                     2013-08-25                          HTML::Tree::Scanning(3)

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Способ сообщить о проблеме с этой страницей руководства зависит от типа проблемы:

Ошибки содержания
Ошибки отчета в содержании этой документации к проекту Perl. (См. perlbug (1) для инструкций представления.)
Отчеты об ошибках
Сообщите об ошибках в функциональности описанного инструмента или API к Apple через Генератор отчетов Ошибки и к проекту Perl, использующему perlbug (1).
Форматирование проблем
Отчет, форматирующий ошибки в интерактивной версии этих страниц со ссылками на отзыв ниже.