HTML::TreeBuilder(3pm)User Contributed Perl DocumentatioHTML::TreeBuilder(3pm)NAMEHTML::TreeBuilder - Parser that builds a HTML syntax tree
VERSION
This document describes version 5.03 of HTML::TreeBuilder, released
September 22, 2012 as part of HTML-Tree.
SYNOPSIS
use HTML::TreeBuilder 5 -weak; # Ensure weak references in use
foreach my $file_name (@ARGV) {
my $tree = HTML::TreeBuilder->new; # empty tree
$tree->parse_file($file_name);
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 = $tree->delete; # Not required with weak references
}
DESCRIPTION
(This class is part of the HTML::Tree dist.)
This class is for HTML syntax trees that get built out of HTML source.
The way to use it is to:
1. start a new (empty) HTML::TreeBuilder object,
2. then use one of the methods from HTML::Parser (presumably with
"$tree->parse_file($filename)" for files, or with
"$tree->parse($document_content)" and "$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
involving traversing it looking for some bit of information in it,
4. previous versions of HTML::TreeBuilder required you to call
"$tree->delete()" to erase the contents of the tree from memory when
you're done with the tree. This is not normally required anymore. See
"Weak References" in HTML::Element for details.
ATTRIBUTES
Most of the following attributes native to HTML::TreeBuilder control
how parsing takes place; they should be set before you try parsing into
the given object. You can set the attributes by passing a TRUE or
FALSE value as argument. E.g., "$root->implicit_tags" returns the
current setting for the "implicit_tags" option,
"$root->implicit_tags(1)" turns that option on, and
"$root->implicit_tags(0)" turns it off.
implicit_tags
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" in HTML::Element attribute set.
implicit_body_p_tag
This controls an aspect of implicit element behavior, if
"implicit_tags" is on: If a text element (PCDATA) or a phrasal element
(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 implicated -- the PCDATA
or phrasal element is simply placed under "<body>". Default is false.
no_expand_entities
This attribute controls whether entities are decoded during the initial
parse of the source. Enable this if you don't want entities decoded to
their character value. e.g. '&' is decoded to '&' by default, but
will be unchanged if this is enabled. Default is false (entities will
be decoded.)
ignore_unknown
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.)
ignore_text
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.
ignore_ignorable_whitespace
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.)
no_space_compacting
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.
p_strict
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>" (although the latter may be
removed in future versions).
For example, when going thru this snippet of code,
<p>stuff
<ul>
TreeBuilder will normally (with "p_strict" false) put the "<ul>"
element 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 special 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 experience.
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
parsing.
store_comments
This determines whether TreeBuilder will normally store comments found
while parsing content into $root. Currently, this is off by default.
store_declarations
This determines whether TreeBuilder will normally store markup
declarations found while parsing content into $root. This is on by
default.
store_pis
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
correctly). 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.
warn
This determines whether syntax errors during parsing should generate
warnings, emitted via Perl's "warn" function.
This is off (false) by default.
METHODS
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::TreeBuilder) 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.
new_from_file
$root = HTML::TreeBuilder->new_from_file($filename_or_filehandle);
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_comments" (for that, call "new", and then
set options, before calling "parse_file"). See the notes (below) on
parameters to "parse_file".
If HTML::TreeBuilder is unable to read the file, then "new_from_file"
dies. The error can also be found in $!. (This behavior is new in
HTML-Tree 5. Previous versions returned a tree with only implicit
elements.)
new_from_content
$root = HTML::TreeBuilder->new_from_content(...);
This "shortcut" constructor merely combines constructing a new object
(with the "new" method, below), and calling "for(...){$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").
Example usages: "HTML::TreeBuilder->new_from_content(@lines)", or
"HTML::TreeBuilder->new_from_content($content)".
new_from_url
$root = HTML::TreeBuilder->new_from_url($url)
This "shortcut" constructor combines constructing a new object (with
the "new" method, below), loading LWP::UserAgent, fetching the
specified URL, and calling "$new->parse( $response->decoded_content)"
and "$new->eof" on it. Returns the new object. Note that this
provides no way of setting any parse options like "store_comments".
If LWP is unable to fetch the URL, or the response is not HTML (as
determined by "content_is_html" in HTTP::Headers), then "new_from_url"
dies, and the HTTP::Response object is found in
$HTML::TreeBuilder::lwp_response.
You must have installed LWP::UserAgent for this method to work. LWP is
not installed automatically, because it's a large set of modules and
you might not need it.
new
$root = HTML::TreeBuilder->new();
This creates a new HTML::TreeBuilder object. This method takes no
attributes.
parse_file
$root->parse_file(...)
[An important method inherited from HTML::Parser, which see. Current
versions of HTML::Parser can take a filespec, or a filehandle object,
like *FOO, or some object from class IO::Handle, IO::File, IO::Socket)
or the like. I think you should check that a given file exists before
calling "$root->parse_file($filespec)".]
When you pass a filename to "parse_file", HTML::Parser opens it in
binary mode, which means it's interpreted as Latin-1 (ISO-8859-1). If
the file is in another encoding, like UTF-8 or UTF-16, this will not do
the right thing.
One solution is to open the file yourself using the proper ":encoding"
layer, and pass the filehandle to "parse_file". You can automate this
process by using "html_file" in IO::HTML, which will use the HTML5
encoding sniffing algorithm to automatically determine the proper
":encoding" layer and apply it.
In the next major release of HTML-Tree, I plan to have it use IO::HTML
automatically. If you really want your file opened in binary mode, you
should open it yourself and pass the filehandle to "parse_file".
The return value is "undef" if there's an error opening the file. In
that case, the error will be in $!.
parse
$root->parse(...)
[A important method inherited from HTML::Parser, which see. See the
note below for "$root->eof()".]
eof
$root->eof();
This signals that you're finished parsing content into this tree; this
runs various kinds of crucial cleanup on the tree. This is called for
you when you call "$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.
parse_content
$root->parse_content(...);
Basically a handy alias for "$root->parse(...); $root->eof". Takes the
exact same arguments as "$root->parse()".
delete
$root->delete();
[A previously important method inherited from HTML::Element, which
see.]
elementify
$root->elementify();
This changes the class of the object in $root from HTML::TreeBuilder 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 charging 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.
guts
@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 = $root->guts();". It so happens
that in this case, @nodes will contain just one element object,
representing the "<li>" node (with "I like pie!" being its text child
node). However, consider if you were parsing this:
<hr>Hooboy!<hr>
In that case, "$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 context 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 children consisting of whatever
nodes "guts()" in list context would have returned. Note that that may
detach those nodes from $root's tree.
disembowel
@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".)
INTERNAL METHODS
You should not need to call any of the following methods directly.
element_class
$classname = $h->element_class;
This method returns the class which will be used for new elements. It
defaults to HTML::Element, but can be overridden by subclassing or
esoteric means best left to those will will read the source and then
not complain when those esoteric means change. (Just subclass.)
comment
Accept a "here's a comment" signal from HTML::Parser.
declaration
Accept a "here's a markup declaration" signal from HTML::Parser.
done
TODO: document
end
Either: Acccept an end-tag signal from HTML::Parser Or: Method for
closing currently open elements in some fairly complex way, as used by
other methods in this class.
TODO: Why is this hidden?
process
Accept a "here's a PI" signal from HTML::Parser.
start
Accept a signal from HTML::Parser for start-tags.
TODO: Why is this hidden?
stunt
TODO: document
stunted
TODO: document
text
Accept a "here's a text token" signal from HTML::Parser.
TODO: Why is this hidden?
tighten_up
Legacy
Redirects to "delete_ignorable_whitespace" in HTML::Element.
warning
Wrapper for CORE::warn
TODO: why not just use carp?
SUBROUTINES
DEBUG
Are we in Debug mode? This is a constant subroutine, to allow compile-
time optimizations. To control debug mode, set
$HTML::TreeBuilder::DEBUG before loading HTML::TreeBuilder.
HTML AND ITS DISCONTENTS
HTML is rather harder to parse than people who write it generally
suspect.
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
implicitly 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
interpreted 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
difficult if not impossible to correctly parse with nearly any set of
straightforward SGML rules. That's why the internals of
HTML::TreeBuilder consist of lots and lots of special cases -- instead
of being just a generic SGML parser with HTML DTD rules plugged in.
TRANSLATIONS?
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
foresee getting around to implementing them in any other language any
time soon.
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
implementation/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
comparison.) Some of the complexity comes from little-used features,
and some of it comes from having the HTML tokenizer (HTML::Parser)
being a separate 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-recovery -- because, as Roseanne Rosannadanna once said, "it's
always something".
Users looking to compare several HTML parsers should look at the source
for Raggett's Tidy ("<http://www.w3.org/People/Raggett/tidy/>"),
Mozilla ("<http://www.mozilla.org/>"), and possibly root around the
browsers section of Yahoo to find the various open-source ones
("<http://dir.yahoo.com/Computers_and_Internet/Software/Internet/World_Wide_Web/Browsers/>").
BUGS
* 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
objectionable 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 $tree itself (which will, oddly enough, be an implicit
"<html>" element). This seems counter-intuitive and problematic; but
seeing as how almost no HTML ever parses correctly with "implicit_tags"
off, this interface oddity seems the least of your problems.
BUG REPORTS
When a document parses in a way different from how you think it should,
I ask that you report this to me as a bug. The first thing you should
do is copy the document, trim out as much of it as you can while still
producing the bug in question, and then email me that mini-document and
the code you're using to parse it, to the HTML::Tree bug queue at
"<bug-html-tree at rt.cpan.org>".
Include a note as to how it parses (presumably including its
"$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.
SEE ALSO
For more information about the HTML-Tree distribution: HTML::Tree.
Modules used by HTML::TreeBuilder: HTML::Parser, HTML::Element,
HTML::Tagset.
For converting between XML::DOM::Node, HTML::Element, and XML::Element
trees: HTML::DOMbo.
For opening a HTML file with automatic charset detection: IO::HTML.
AUTHOR
Current maintainers:
· Christopher J. Madsen "<perl AT cjmweb.net>"
· Jeff Fearn "<jfearn AT cpan.org>"
Original HTML-Tree author:
· Gisle Aas
Former maintainers:
· Sean M. Burke
· Andy Lester
· Pete Krawczyk "<petek AT cpan.org>"
You can follow or contribute to HTML-Tree's development at
<http://github.com/madsen/HTML-Tree>.
COPYRIGHT AND LICENSE
Copyright 1995-1998 Gisle Aas, 1999-2004 Sean M. Burke, 2005 Andy
Lester, 2006 Pete Krawczyk, 2010 Jeff Fearn, 2012 Christopher J.
Madsen.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
The programs in this library are distributed in the hope that they will
be useful, but without any warranty; without even the implied warranty
of merchantability or fitness for a particular purpose.
perl v5.18.1 2013-10-07 HTML::TreeBuilder(3pm)
