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HTML::TreeBuilder(3)  User Contributed Perl Documentation HTML::TreeBuilder(3)

       HTML::TreeBuilder - Parser that builds a HTML syntax tree

	 foreach my $file_name (@ARGV) {
	   my $tree = HTML::TreeBuilder->new; # empty tree
	   print "Hey, here's a dump of the parse tree of $file_name:\n";
	   $tree->dump; # a method we inherit from HTML::Element
	   print "And here it is, bizarrely rerendered as HTML:\n",
	     $tree->as_HTML, "\n";

	   # Now that we're done with it, we must destroy it.
	   $tree = $tree->delete;

       (This class is part of the HTML::Tree dist.)

       This class is for HTML syntax trees that get built out of HTML source.
       The way to use it is to:

       1. start a new (empty) HTML::TreeBuilder object,

       2. then use one of the methods from HTML::Parser (presumably with
       $tree->parse_file($filename) for files, or with $tree->parse($docu‐
       ment_content) and $tree->eof if you've got the content in a string) to
       parse the HTML document into the tree $tree.

       (You can combine steps 1 and 2 with the "new_from_file" or
       "new_from_content" methods.)

       2b. call $root->elementify() if you want.

       3. do whatever you need to do with the syntax tree, presumably involv‐
       ing traversing it looking for some bit of information in it,

       4. and finally, when you're done with the tree, call $tree->delete() to
       erase the contents of the tree from memory.  This kind of thing usually
       isn't necessary with most Perl objects, but it's necessary for Tree‐
       Builder objects.	 See HTML::Element for a more verbose explanation of
       why this is the case.

       Objects of this class inherit the methods of both HTML::Parser and
       HTML::Element.  The methods inherited from HTML::Parser are used for
       building the HTML tree, and the methods inherited from HTML::Element
       are what you use to scrutinize the tree.	 Besides this (HTML::Tree‐
       Builder) documentation, you must also carefully read the HTML::Element
       documentation, and also skim the HTML::Parser documentation -- probably
       only its parse and parse_file methods are of interest.

       Most of the following methods native to HTML::TreeBuilder control how
       parsing takes place; they should be set before you try parsing into the
       given object.  You can set the attributes by passing a TRUE or FALSE
       value as argument.  E.g., $root->implicit_tags returns the current set‐
       ting for the implicit_tags option, $root->implicit_tags(1) turns that
       option on, and $root->implicit_tags(0) turns it off.

       $root = HTML::TreeBuilder->new_from_file(...)
	   This "shortcut" constructor merely combines constructing a new
	   object (with the "new" method, below), and calling
	   $new->parse_file(...) on it.	 Returns the new object.  Note that
	   this provides no way of setting any parse options like store_com‐
	   ments (for that, call new, and then set options, before calling
	   parse_file).	 See the notes (below) on parameters to parse_file.

       $root = HTML::TreeBuilder->new_from_content(...)
	   This "shortcut" constructor merely combines constructing a new
	   object (with the "new" method, below), and calling
	   for(...){$new->parse($_)} and $new->eof on it.  Returns the new
	   object.  Note that this provides no way of setting any parse
	   options like store_comments (for that, call new, and then set
	   options, before calling parse_file).	 Example usages: HTML::Tree‐
	   Builder->new_from_content(@lines), or HTML::Tree‐

       $root = HTML::TreeBuilder->new()
	   This creates a new HTML::TreeBuilder object.	 This method takes no

	   [An important method inherited from HTML::Parser, which see.	 Cur‐
	   rent versions of HTML::Parser can take a filespec, or a filehandle
	   object, like *FOO, or some object from class IO::Handle, IO::File,
	   IO::Socket) or the like.  I think you should check that a given
	   file exists before calling $root->parse_file($filespec).]

	   [A important method inherited from HTML::Parser, which see.	See
	   the note below for $root->eof().]

	   This signals that you're finished parsing content into this tree;
	   this runs various kinds of crucial cleanup on the tree.  This is
	   called for you when you call $root->parse_file(...), but not when
	   you call $root->parse(...).	So if you call $root->parse(...), then
	   you must call $root->eof() once you've finished feeding all the
	   chunks to parse(...), and before you actually start doing anything
	   else with the tree in $root.

	   Basically a happly alias for "$root->parse(...); $root->eof".
	   Takes the exact same arguments as "$root->parse()".

	   [An important method inherited from HTML::Element, which see.]

	   This changes the class of the object in $root from HTML::Tree‐
	   Builder to the class used for all the rest of the elements in that
	   tree (generally HTML::Element).  Returns $root.

	   For most purposes, this is unnecessary, but if you call this after
	   (after!!)  you've finished building a tree, then it keeps you from
	   accidentally trying to call anything but HTML::Element methods on
	   it.	(I.e., if you accidentally call "$root->parse_file(...)" on
	   the already-complete and elementified tree, then instead of charg‐
	   ing ahead and wreaking havoc, it'll throw a fatal error -- since
	   $root is now an object just of class HTML::Element which has no
	   "parse_file" method.

	   Note that elementify currently deletes all the private attributes
	   of $root except for "_tag", "_parent", "_content", "_pos", and
	   "_implicit".	 If anyone requests that I change this to leave in yet
	   more private attributes, I might do so, in future versions.

       @nodes = $root->guts()
       $parent_for_nodes = $root->guts()
	   In list context (as in the first case), this method returns the
	   topmost non-implicit nodes in a tree.  This is useful when you're
	   parsing HTML code that you know doesn't expect an HTML document,
	   but instead just a fragment of an HTML document.  For example, if
	   you wanted the parse tree for a file consisting of just this:

	     <li>I like pie!

	   Then you would get that with "@nodes = $root->guts();".  It so hap‐
	   pens that in this case, @nodes will contain just one element
	   object, representing the "li" node (with "I like pie!" being its
	   text child node).  However, consider if you were parsing this:


	   In that case, "$root->guts()" would return three items: an element
	   object for the first "hr", a text string "Hooboy!", and another
	   "hr" element object.

	   For cases where you want definitely one element (so you can treat
	   it as a "document fragment", roughly speaking), call "guts()" in
	   scalar context, as in "$parent_for_nodes = $root->guts()". That
	   works like "guts()" in list context; in fact, "guts()" in list con‐
	   text would have returned exactly one value, and if it would have
	   been an object (as opposed to a text string), then that's what
	   "guts" in scalar context will return.  Otherwise, if "guts()" in
	   list context would have returned no values at all, then "guts()" in
	   scalar context returns undef.  In all other cases, "guts()" in
	   scalar context returns an implicit 'div' element node, with chil‐
	   dren consisting of whatever nodes "guts()" in list context would
	   have returned.  Note that that may detach those nodes from $root's

       @nodes = $root->disembowel()
       $parent_for_nodes = $root->disembowel()
	   The "disembowel()" method works just like the "guts()" method,
	   except that disembowel definitively destroys the tree above the
	   nodes that are returned.  Usually when you want the guts from a
	   tree, you're just going to toss out the rest of the tree anyway, so
	   this saves you the bother.  (Remember, "disembowel" means "remove
	   the guts from".)

	   Setting this attribute to true will instruct the parser to try to
	   deduce implicit elements and implicit end tags.  If it is false you
	   get a parse tree that just reflects the text as it stands, which is
	   unlikely to be useful for anything but quick and dirty parsing.
	   (In fact, I'd be curious to hear from anyone who finds it useful to
	   have implicit_tags set to false.)  Default is true.

	   Implicit elements have the implicit() attribute set.

	   This controls an aspect of implicit element behavior, if
	   implicit_tags is on:	 If a text element (PCDATA) or a phrasal ele‐
	   ment (such as "<em>") is to be inserted under "<body>", two things
	   can happen: if implicit_body_p_tag is true, it's placed under a
	   new, implicit "<p>" tag.  (Past DTDs suggested this was the only
	   correct behavior, and this is how past versions of this module
	   behaved.)  But if implicit_body_p_tag is false, nothing is impli‐
	   cated -- the PCDATA or phrasal element is simply placed under
	   "<body>".  Default is false.

	   This attribute controls whether unknown tags should be represented
	   as elements in the parse tree, or whether they should be ignored.
	   Default is true (to ignore unknown tags.)

	   Do not represent the text content of elements.  This saves space if
	   all you want is to examine the structure of the document.  Default
	   is false.

	   If set to true, TreeBuilder will try to avoid creating ignorable
	   whitespace text nodes in the tree.  Default is true.	 (In fact, I'd
	   be interested in hearing if there's ever a case where you need this
	   off, or where leaving it on leads to incorrect behavior.)

	   This determines whether TreeBuilder compacts all whitespace strings
	   in the document (well, outside of PRE or TEXTAREA elements), or
	   leaves them alone.  Normally (default, value of 0), each string of
	   contiguous whitespace in the document is turned into a single
	   space.  But that's not done if no_space_compacting is set to 1.

	   Setting no_space_compacting to 1 might be useful if you want to
	   read in a tree just to make some minor changes to it before writing
	   it back out.

	   This method is experimental.	 If you use it, be sure to report any
	   problems you might have with it.

	   If set to true (and it defaults to false), TreeBuilder will take a
	   narrower than normal view of what can be under a "p" element; if it
	   sees a non-phrasal element about to be inserted under a "p", it
	   will close that "p".	 Otherwise it will close p elements only for
	   other "p"'s, headings, and "form" (altho the latter may be removed
	   in future versions).

	   For example, when going thru this snippet of code,


	   TreeBuilder will normally (with "p_strict" false) put the "ul" ele‐
	   ment under the "p" element.	However, with "p_strict" set to true,
	   it will close the "p" first.

	   In theory, there should be strictness options like this for
	   other/all elements besides just "p"; but I treat this as a specal
	   case simply because of the fact that "p" occurs so frequently and
	   its end-tag is omitted so often; and also because application of
	   strictness rules at parse-time across all elements often makes tiny
	   errors in HTML coding produce drastically bad parse-trees, in my

	   If you find that you wish you had an option like this to enforce
	   content-models on all elements, then I suggest that what you want
	   is content-model checking as a stage after TreeBuilder has finished

	   This determines whether TreeBuilder will normally store comments
	   found while parsing content into $root.  Currently, this is off by

	   This determines whether TreeBuilder will normally store markup dec‐
	   larations found while parsing content into $root.  This is on by

	   This determines whether TreeBuilder will normally store processing
	   instructions found while parsing content into $root -- assuming a
	   recent version of HTML::Parser (old versions won't parse PIs cor‐
	   rectly).  Currently, this is off (false) by default.

	   It is somewhat of a known bug (to be fixed one of these days, if
	   anyone needs it?) that PIs in the preamble (before the "html"
	   start-tag) end up actually under the "html" element.

	   This determines whether syntax errors during parsing should gener‐
	   ate warnings, emitted via Perl's "warn" function.

	   This is off (false) by default.

       HTML is rather harder to parse than people who write it generally sus‐

       Here's the problem: HTML is a kind of SGML that permits "minimization"
       and "implication".  In short, this means that you don't have to close
       every tag you open (because the opening of a subsequent tag may implic‐
       itly close it), and if you use a tag that can't occur in the context
       you seem to using it in, under certain conditions the parser will be
       able to realize you mean to leave the current context and enter the new
       one, that being the only one that your code could correctly be inter‐
       preted in.

       Now, this would all work flawlessly and unproblematically if: 1) all
       the rules that both prescribe and describe HTML were (and had been)
       clearly set out, and 2) everyone was aware of these rules and wrote
       their code in compliance to them.

       However, it didn't happen that way, and so most HTML pages are diffi‐
       cult if not impossible to correctly parse with nearly any set of
       straightforward SGML rules.  That's why the internals of HTML::Tree‐
       Builder consist of lots and lots of special cases -- instead of being
       just a generic SGML parser with HTML DTD rules plugged in.

       The techniques that HTML::TreeBuilder uses to perform what I consider
       very robust parses on everyday code are not things that can work only
       in Perl.	 To date, the algorithms at the center of HTML::TreeBuilder
       have been implemented only in Perl, as far as I know; and I don't fore‐
       see getting around to implementing them in any other language any time

       If, however, anyone is looking for a semester project for an applied
       programming class (or if they merely enjoy extra-curricular masochism),
       they might do well to see about choosing as a topic the implementa‐
       tion/adaptation of these routines to any other interesting programming
       language that you feel currently suffers from a lack of robust
       HTML-parsing.  I welcome correspondence on this subject, and point out
       that one can learn a great deal about languages by trying to translate
       between them, and then comparing the result.

       The HTML::TreeBuilder source may seem long and complex, but it is
       rather well commented, and symbol names are generally self-explanatory.
       (You are encouraged to read the Mozilla HTML parser source for compari‐
       son.)  Some of the complexity comes from little-used features, and some
       of it comes from having the HTML tokenizer (HTML::Parser) being a sepa‐
       rate module, requiring somewhat of a different interface than you'd
       find in a combined tokenizer and tree-builder.  But most of the length
       of the source comes from the fact that it's essentially a long list of
       special cases, with lots and lots of sanity-checking, and sanity-recov‐
       ery -- because, as Roseanne Rosannadanna once said, "it's always some‐

       Users looking to compare several HTML parsers should look at the source
       for Raggett's Tidy ("<>"),
       Mozilla ("<>"), and possibly root around the
       browsers section of Yahoo to find the various open-source ones

       * Framesets seem to work correctly now.	Email me if you get a strange
       parse from a document with framesets.

       * Really bad HTML code will, often as not, make for a somewhat objec‐
       tionable parse tree.  Regrettable, but unavoidably true.

       * If you're running with implicit_tags off (God help you!), consider
       that $tree->content_list probably contains the tree or grove from the
       parse, and not $tree itself (which will, oddly enough, be an implicit
       'html' element).	 This seems counter-intuitive and problematic; but
       seeing as how almost no HTML ever parses correctly with implicit_tags
       off, this interface oddity seems the least of your problems.

       When a document parses in a way different from how you think it should,
       I ask that you report this to me as a bug.  The first thing you should
       do is copy the document, trim out as much of it as you can while still
       producing the bug in question, and then email me that mini-document and
       the code you're using to parse it, to the HTML::Tree bug queue at
       "bug-html-tree at".

       Include a note as to how it parses (presumably including its
       $tree->dump output), and then a careful and clear explanation of where
       you think the parser is going astray, and how you would prefer that it
       work instead.

       HTML::Tree; HTML::Parser, HTML::Element, HTML::Tagset


       Copyright 1995-1998 Gisle Aas, 1999-2004 Sean M. Burke, 2005 Andy
       Lester, 2006 Pete Krawczyk.

       This library is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

       This program is distributed in the hope that it will be useful, but
       without any warranty; without even the implied warranty of mer‐
       chantability or fitness for a particular purpose.

       Currently maintained by Pete Krawczyk "<>"

       Original authors: Gisle Aas, Sean Burke and Andy Lester.

perl v5.8.8			  2006-08-04		  HTML::TreeBuilder(3)

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