IO::HTML - Open an HTML file with automatic charset detection
This document describes version 1.004 of
IO::HTML, released September 26, 2020.
use IO::HTML; # exports html_file by default
use HTML::TreeBuilder;
my $tree = HTML::TreeBuilder->new_from_file(
html_file('foo.html')
);
# Alternative interface:
open(my $in, '<:raw', 'bar.html');
my $encoding = IO::HTML::sniff_encoding($in, 'bar.html');
IO::HTML provides an easy way to open a file containing HTML while
automatically determining its encoding. It uses the HTML5 encoding
sniffing algorithm specified in section 8.2.2.2 of the draft standard.
The algorithm as implemented here is:
-
If the file begins with a byte order mark indicating UTF-16LE,
UTF-16BE, or UTF-8, then that is the encoding.
-
If the first
$bytes_to_check bytes of the file contain a <meta> tag that
indicates the charset, and Encode recognizes the specified charset
name, then that is the encoding. (This portion of the algorithm is
implemented by find_charset_in .)
The <meta> tag can be in one of two formats:
<meta charset="...">
<meta http-equiv="Content-Type" content="...charset=...">
The search is case-insensitive, and the order of attributes within the
tag is irrelevant. Any additional attributes of the tag are ignored.
The first matching tag with a recognized encoding ends the search.
-
If the first
$bytes_to_check bytes of the file are valid UTF-8 (with at least 1
non-ASCII character), then the encoding is UTF-8.
-
If all else fails, use the default character encoding. The HTML5
standard suggests the default encoding should be locale dependent, but
currently it is always
cp1252 unless you set
$IO::HTML::default_encoding to a different value. Note:
sniff_encoding does not apply this step; only html_file does
that.
$filehandle = html_file($filename, \%options);
This function (exported by default) is the primary entry point. It
opens the file specified by $filename for reading, uses
sniff_encoding to find a suitable encoding layer, and applies it.
It also applies the :crlf layer. If the file begins with a BOM,
the filehandle is positioned just after the BOM.
The optional second argument is a hashref containing options. The
possible keys are described under find_charset_in .
If sniff_encoding is unable to determine the encoding, it defaults
to $IO::HTML::default_encoding , which is set to cp1252
(a.k.a. Windows-1252) by default. According to the standard, the
default should be locale dependent, but that is not currently
implemented.
It dies if the file cannot be opened, or if sniff_encoding cannot
determine the encoding and $IO::HTML::default_encoding has been set
to undef .
($filehandle, $encoding, $bom)
= html_file_and_encoding($filename, \%options);
This function (exported only by request) is just like html_file ,
but returns more information. In addition to the filehandle, it
returns the name of the encoding used, and a flag indicating whether a
byte order mark was found (if $bom is true, the file began with a
BOM). This may be useful if you want to write the file out again
(especially in conjunction with the html_outfile function).
The optional second argument is a hashref containing options. The
possible keys are described under find_charset_in .
It dies if the file cannot be opened, or if sniff_encoding cannot
determine the encoding and $IO::HTML::default_encoding has been set
to undef .
The result of calling html_file_and_encoding in scalar context is undefined
(in the C sense of there is no guarantee what you'll get).
$filehandle = html_outfile($filename, $encoding, $bom);
This function (exported only by request) opens $filename for output
using $encoding , and writes a BOM to it if $bom is true.
If $encoding is undef , it defaults to $IO::HTML::default_encoding .
$encoding may be either an encoding name or an Encode::Encoding object.
It dies if the file cannot be opened, or if both $encoding and
$IO::HTML::default_encoding are undef .
($encoding, $bom) = sniff_encoding($filehandle, $filename, \%options);
This function (exported only by request) runs the HTML5 encoding
sniffing algorithm on $filehandle (which must be seekable, and
should have been opened in :raw mode). $filename is used only
for error messages (if there's a problem using the filehandle), and
defaults to ``file'' if omitted. The optional third argument is a
hashref containing options. The possible keys are described under
find_charset_in .
It returns Perl's canonical name for the encoding, which is not
necessarily the same as the MIME or IANA charset name. It returns
undef if the encoding cannot be determined. $bom is true if the
file began with a byte order mark. In scalar context, it returns only
$encoding .
The filehandle's position is restored to its original position
(normally the beginning of the file) unless $bom is true. In that
case, the position is immediately after the BOM.
Tip: If you want to run sniff_encoding on a file you've already
loaded into a string, open an in-memory file on the string, and pass
that handle:
($encoding, $bom) = do {
open(my $fh, '<', \$string); sniff_encoding($fh)
};
(This only makes sense if $string contains bytes, not characters.)
$encoding = find_charset_in($string_containing_HTML, \%options);
This function (exported only by request) looks for charset information
in a <meta> tag in a possibly-incomplete HTML document using
the ``two step'' algorithm specified by HTML5. It does not look for a BOM.
The <meta> tag must begin within the first $IO::HTML::bytes_to_check
bytes of the string.
It returns Perl's canonical name for the encoding, which is not
necessarily the same as the MIME or IANA charset name. It returns
undef if no charset is specified or if the specified charset is not
recognized by the Encode module.
The optional second argument is a hashref containing options. The
following keys are recognized:
- encoding
-
If true, return the the Encode::Encoding manpage object instead of its name.
Defaults to false.
- need_pragma
-
If true (the default), follow the HTML5 spec and examine the
content attribute only of <meta http-equiv="Content-Type" .
If set to 0, relax the HTML5 spec, and look for ``charset='' in the
content attribute of every meta tag.
By default, only html_file is exported. Other functions may be
exported on request.
For people who prefer not to export functions, all functions beginning
with html_ have an alias without that prefix (e.g. you can call
IO::HTML::file(...) instead of IO::HTML::html_file(...) . These
aliases are not exportable.
The following export tags are available:
- :all
-
All exportable functions.
- :rw
-
html_file , html_file_and_encoding , html_outfile .
The HTML5 specification, section 8.2.2.2 Determining the character encoding:
http://www.w3.org/TR/html5/syntax.html#determining-the-character-encoding
- Could not read %s: %s
-
The specified file could not be read from for the reason specified by
$! .
- Could not seek %s: %s
-
The specified file could not be rewound for the reason specified by
$! .
- Failed to open %s: %s
-
The specified file could not be opened for reading for the reason
specified by
$! .
- No default encoding specified
-
The
sniff_encoding algorithm didn't find an encoding to use, and
you set $IO::HTML::default_encoding to undef .
There are two global variables that affect IO::HTML. If you need to
change them, you should do so using local if possible:
my $file = do {
# This file may define the charset later in the header
local $IO::HTML::bytes_to_check = 4096;
html_file(...);
};
- $bytes_to_check
-
This is the number of bytes that
sniff_encoding will read from the
stream. It is also the number of bytes that find_charset_in will
search for a <meta> tag containing charset information.
It must be a positive integer.
The HTML 5 specification recommends using the default value of 1024,
but some pages do not follow the specification.
- $default_encoding
-
This is the encoding that
html_file and html_file_and_encoding
will use if no encoding can be detected by sniff_encoding .
The default value is cp1252 (a.k.a. Windows-1252).
Setting it to undef will cause the file subroutines to croak if
sniff_encoding fails to determine the encoding. (sniff_encoding
itself does not use $default_encoding ).
IO::HTML has no non-core dependencies for Perl 5.8.7+. With earlier
versions of Perl 5.8, you need to upgrade Encode to at least
version 2.10, and
you may need to upgrade Exporter to at least version
5.57.
None reported.
No bugs have been reported.
Christopher J. Madsen <perl AT cjmweb.net>
Please report any bugs or feature requests
to <bug-IO-HTML AT rt.cpan.org>
or through the web interface at
http://rt.cpan.org/Public/Bug/Report.html.
You can follow or contribute to IO-HTML's development at
https://github.com/madsen/io-html.
This software is copyright (c) 2020 by Christopher J. Madsen.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
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