CONTENTS

• NAME
• VERSION
• SYNOPSIS
• DESCRIPTION
• SUBROUTINES
  • html_file
  • html_file_and_encoding
  • html_outfile
  • sniff_encoding
  • find_charset_in
• EXPORTS
• SEE ALSO
• DIAGNOSTICS
• CONFIGURATION AND ENVIRONMENT
• DEPENDENCIES
• INCOMPATIBILITIES
• BUGS AND LIMITATIONS
• AUTHOR
• COPYRIGHT AND LICENSE
• DISCLAIMER OF WARRANTY

#NAME

IO::HTML - Open an HTML file with automatic charset detection

#VERSION

This document describes version 1.004 of IO::HTML, released September 26, 2020.

#SYNOPSIS

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');

#DESCRIPTION

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:

1. If the file begins with a byte order mark indicating UTF-16LE, UTF-16BE, or UTF-8, then that is the encoding.

2. 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.

3. 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.

4. 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.

#SUBROUTINES

#html_file

$filehandle = html_file($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.

#html_file_and_encoding

($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).

#html_outfile

$filehandle = html_outfile($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.

#sniff_encoding

($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.)

#find_charset_in

$encoding = find_charset_in($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 Encode::Encoding 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.

#EXPORTS

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.

#SEE ALSO

The HTML5 specification, section 8.2.2.2 Determining the character encoding: http://www.w3.org/TR/html5/syntax.html#determining-the-character-encoding

#DIAGNOSTICS

#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.

#CONFIGURATION AND ENVIRONMENT

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).

#DEPENDENCIES

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.

#INCOMPATIBILITIES

None reported.

#BUGS AND LIMITATIONS

No bugs have been reported.

#AUTHOR

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?Queue=IO-HTML.

You can follow or contribute to IO-HTML's development at https://github.com/madsen/io-html.

#COPYRIGHT AND LICENSE

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.

#DISCLAIMER OF WARRANTY

BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENSE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

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