IO::HTML(3)

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.

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