perlunifaq.1

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

.Ve
.PP
Or if you already have an open filehandle:
.PP
.Vb 1
\&    binmode $fh, \*(Aq:encoding(UTF\-8)\*(Aq;
.Ve
.PP
Some database drivers for \s-1DBI\s0 can also automatically encode and decode, but
that is sometimes limited to the \s-1UTF\-8\s0 encoding.
.Sh "What if I don't know which encoding was used?"
.IX Subsection "What if I don't know which encoding was used?"
Do whatever you can to find out, and if you have to: guess. (Don't forget to
document your guess with a comment.)
.PP
You could open the document in a web browser, and change the character set or
character encoding until you can visually confirm that all characters look the
way they should.
.PP
There is no way to reliably detect the encoding automatically, so if people
keep sending you data without charset indication, you may have to educate them.
.Sh "Can I use Unicode in my Perl sources?"
.IX Subsection "Can I use Unicode in my Perl sources?"
Yes, you can! If your sources are \s-1UTF\-8\s0 encoded, you can indicate that with the
\&\f(CW\*(C`use utf8\*(C'\fR pragma.
.PP
.Vb 1
\&    use utf8;
.Ve
.PP
This doesn't do anything to your input, or to your output. It only influences
the way your sources are read. You can use Unicode in string literals, in
identifiers (but they still have to be \*(L"word characters\*(R" according to \f(CW\*(C`\ew\*(C'\fR),
and even in custom delimiters.
.Sh "Data::Dumper doesn't restore the \s-1UTF8\s0 flag; is it broken?"
.IX Subsection "Data::Dumper doesn't restore the UTF8 flag; is it broken?"
No, Data::Dumper's Unicode abilities are as they should be. There have been
some complaints that it should restore the \s-1UTF8\s0 flag when the data is read
again with \f(CW\*(C`eval\*(C'\fR. However, you should really not look at the flag, and
nothing indicates that Data::Dumper should break this rule.
.PP
Here's what happens: when Perl reads in a string literal, it sticks to 8 bit
encoding as long as it can. (But perhaps originally it was internally encoded
as \s-1UTF\-8\s0, when you dumped it.) When it has to give that up because other
characters are added to the text string, it silently upgrades the string to
\&\s-1UTF\-8\s0.
.PP
If you properly encode your strings for output, none of this is of your
concern, and you can just \f(CW\*(C`eval\*(C'\fR dumped data as always.
.Sh "Why do regex character classes sometimes match only in the \s-1ASCII\s0 range?"
.IX Subsection "Why do regex character classes sometimes match only in the ASCII range?"
.Sh "Why do some characters not uppercase or lowercase correctly?"
.IX Subsection "Why do some characters not uppercase or lowercase correctly?"
It seemed like a good idea at the time, to keep the semantics the same for
standard strings, when Perl got Unicode support. While it might be repaired
in the future, we now have to deal with the fact that Perl treats equal
strings differently, depending on the internal state.
.PP
Affected are \f(CW\*(C`uc\*(C'\fR, \f(CW\*(C`lc\*(C'\fR, \f(CW\*(C`ucfirst\*(C'\fR, \f(CW\*(C`lcfirst\*(C'\fR, \f(CW\*(C`\eU\*(C'\fR, \f(CW\*(C`\eL\*(C'\fR, \f(CW\*(C`\eu\*(C'\fR, \f(CW\*(C`\el\*(C'\fR,
\&\f(CW\*(C`\ed\*(C'\fR, \f(CW\*(C`\es\*(C'\fR, \f(CW\*(C`\ew\*(C'\fR, \f(CW\*(C`\eD\*(C'\fR, \f(CW\*(C`\eS\*(C'\fR, \f(CW\*(C`\eW\*(C'\fR, \f(CW\*(C`/.../i\*(C'\fR, \f(CW\*(C`(?i:...)\*(C'\fR,
\&\f(CW\*(C`/[[:posix:]]/\*(C'\fR.
.PP
To force Unicode semantics, you can upgrade the internal representation to
by doing \f(CW\*(C`utf8::upgrade($string)\*(C'\fR. This does not change strings that were
already upgraded.
.PP
For a more detailed discussion, see Unicode::Semantics on \s-1CPAN\s0.
.Sh "How can I determine if a string is a text string or a binary string?"
.IX Subsection "How can I determine if a string is a text string or a binary string?"
You can't. Some use the \s-1UTF8\s0 flag for this, but that's misuse, and makes well
behaved modules like Data::Dumper look bad. The flag is useless for this
purpose, because it's off when an 8 bit encoding (by default \s-1ISO\-8859\-1\s0) is
used to store the string.
.PP
This is something you, the programmer, has to keep track of; sorry. You could
consider adopting a kind of \*(L"Hungarian notation\*(R" to help with this.
.Sh "How do I convert from encoding \s-1FOO\s0 to encoding \s-1BAR\s0?"
.IX Subsection "How do I convert from encoding FOO to encoding BAR?"
By first converting the FOO-encoded byte string to a text string, and then the
text string to a BAR-encoded byte string:
.PP
.Vb 2
\&    my $text_string = decode(\*(AqFOO\*(Aq, $foo_string);
\&    my $bar_string  = encode(\*(AqBAR\*(Aq, $text_string);
.Ve
.PP
or by skipping the text string part, and going directly from one binary
encoding to the other:
.PP
.Vb 2
\&    use Encode qw(from_to);
\&    from_to($string, \*(AqFOO\*(Aq, \*(AqBAR\*(Aq);  # changes contents of $string
.Ve
.PP
or by letting automatic decoding and encoding do all the work:
.PP
.Vb 3
\&    open my $foofh, \*(Aq<:encoding(FOO)\*(Aq, \*(Aqexample.foo.txt\*(Aq;
\&    open my $barfh, \*(Aq>:encoding(BAR)\*(Aq, \*(Aqexample.bar.txt\*(Aq;
\&    print { $barfh } $_ while <$foofh>;
.Ve
.ie n .Sh "What are ""decode_utf8""\fP and \f(CW""encode_utf8""?"
.el .Sh "What are \f(CWdecode_utf8\fP and \f(CWencode_utf8\fP?"
.IX Subsection "What are decode_utf8 and encode_utf8?"
These are alternate syntaxes for \f(CW\*(C`decode(\*(Aqutf8\*(Aq, ...)\*(C'\fR and \f(CW\*(C`encode(\*(Aqutf8\*(Aq,
\&...)\*(C'\fR.
.ie n .Sh "What is a ""wide character""?"
.el .Sh "What is a ``wide character''?"
.IX Subsection "What is a wide character?"
This is a term used both for characters with an ordinal value greater than 127,
characters with an ordinal value greater than 255, or any character occupying
than one byte, depending on the context.
.PP
The Perl warning \*(L"Wide character in ...\*(R" is caused by a character with an
ordinal value greater than 255. With no specified encoding layer, Perl tries to
fit things in \s-1ISO\-8859\-1\s0 for backward compatibility reasons. When it can't, it
emits this warning (if warnings are enabled), and outputs \s-1UTF\-8\s0 encoded data
instead.
.PP
To avoid this warning and to avoid having different output encodings in a single
stream, always specify an encoding explicitly, for example with a PerlIO layer:
.PP
.Vb 1
\&    binmode STDOUT, ":encoding(UTF\-8)";
.Ve
.SH "INTERNALS"
.IX Header "INTERNALS"
.ie n .Sh "What is ""the \s-1UTF8\s0 flag""?"
.el .Sh "What is ``the \s-1UTF8\s0 flag''?"
.IX Subsection "What is the UTF8 flag?"
Please, unless you're hacking the internals, or debugging weirdness, don't
think about the \s-1UTF8\s0 flag at all. That means that you very probably shouldn't
use \f(CW\*(C`is_utf8\*(C'\fR, \f(CW\*(C`_utf8_on\*(C'\fR or \f(CW\*(C`_utf8_off\*(C'\fR at all.
.PP
The \s-1UTF8\s0 flag, also called SvUTF8, is an internal flag that indicates that the
current internal representation is \s-1UTF\-8\s0. Without the flag, it is assumed to be
\&\s-1ISO\-8859\-1\s0. Perl converts between these automatically.
.PP
One of Perl's internal formats happens to be \s-1UTF\-8\s0. Unfortunately, Perl can't
keep a secret, so everyone knows about this. That is the source of much
confusion. It's better to pretend that the internal format is some unknown
encoding, and that you always have to encode and decode explicitly.
.ie n .Sh "What about the ""use bytes"" pragma?"
.el .Sh "What about the \f(CWuse bytes\fP pragma?"
.IX Subsection "What about the use bytes pragma?"
Don't use it. It makes no sense to deal with bytes in a text string, and it
makes no sense to deal with characters in a byte string. Do the proper
conversions (by decoding/encoding), and things will work out well: you get
character counts for decoded data, and byte counts for encoded data.
.PP
\&\f(CW\*(C`use bytes\*(C'\fR is usually a failed attempt to do something useful. Just forget
about it.
.ie n .Sh "What about the ""use encoding"" pragma?"
.el .Sh "What about the \f(CWuse encoding\fP pragma?"
.IX Subsection "What about the use encoding pragma?"
Don't use it. Unfortunately, it assumes that the programmer's environment and
that of the user will use the same encoding. It will use the same encoding for
the source code and for \s-1STDIN\s0 and \s-1STDOUT\s0. When a program is copied to another
machine, the source code does not change, but the \s-1STDIO\s0 environment might.
.PP
If you need non-ASCII characters in your source code, make it a \s-1UTF\-8\s0 encoded
file and \f(CW\*(C`use utf8\*(C'\fR.
.PP
If you need to set the encoding for \s-1STDIN\s0, \s-1STDOUT\s0, and \s-1STDERR\s0, for example
based on the user's locale, \f(CW\*(C`use open\*(C'\fR.
.ie n .Sh "What is the difference between "":encoding""\fP and \f(CW"":utf8""?"
.el .Sh "What is the difference between \f(CW:encoding\fP and \f(CW:utf8\fP?"
.IX Subsection "What is the difference between :encoding and :utf8?"
Because \s-1UTF\-8\s0 is one of Perl's internal formats, you can often just skip the
encoding or decoding step, and manipulate the \s-1UTF8\s0 flag directly.
.PP
Instead of \f(CW\*(C`:encoding(UTF\-8)\*(C'\fR, you can simply use \f(CW\*(C`:utf8\*(C'\fR, which skips the
encoding step if the data was already represented as \s-1UTF8\s0 internally. This is
widely accepted as good behavior when you're writing, but it can be dangerous
when reading, because it causes internal inconsistency when you have invalid
byte sequences. Using \f(CW\*(C`:utf8\*(C'\fR for input can sometimes result in security
breaches, so please use \f(CW\*(C`:encoding(UTF\-8)\*(C'\fR instead.
.PP
Instead of \f(CW\*(C`decode\*(C'\fR and \f(CW\*(C`encode\*(C'\fR, you could use \f(CW\*(C`_utf8_on\*(C'\fR and \f(CW\*(C`_utf8_off\*(C'\fR,
but this is considered bad style. Especially \f(CW\*(C`_utf8_on\*(C'\fR can be dangerous, for
the same reason that \f(CW\*(C`:utf8\*(C'\fR can.
.PP
There are some shortcuts for oneliners; see \f(CW\*(C`\-C\*(C'\fR in perlrun.
.ie n .Sh "What's the difference between ""UTF\-8""\fP and \f(CW""utf8""?"
.el .Sh "What's the difference between \f(CWUTF\-8\fP and \f(CWutf8\fP?"
.IX Subsection "What's the difference between UTF-8 and utf8?"
\&\f(CW\*(C`UTF\-8\*(C'\fR is the official standard. \f(CW\*(C`utf8\*(C'\fR is Perl's way of being liberal in
what it accepts. If you have to communicate with things that aren't so liberal,
you may want to consider using \f(CW\*(C`UTF\-8\*(C'\fR. If you have to communicate with things
that are too liberal, you may have to use \f(CW\*(C`utf8\*(C'\fR. The full explanation is in
Encode.
.PP
\&\f(CW\*(C`UTF\-8\*(C'\fR is internally known as \f(CW\*(C`utf\-8\-strict\*(C'\fR. The tutorial uses \s-1UTF\-8\s0
consistently, even where utf8 is actually used internally, because the
distinction can be hard to make, and is mostly irrelevant.
.PP
For example, utf8 can be used for code points that don't exist in Unicode, like
9999999, but if you encode that to \s-1UTF\-8\s0, you get a substitution character (by
default; see \*(L"Handling Malformed Data\*(R" in Encode for more ways of dealing with
this.)
.PP
Okay, if you insist: the \*(L"internal format\*(R" is utf8, not \s-1UTF\-8\s0. (When it's not
some other encoding.)
.Sh "I lost track; what encoding is the internal format really?"
.IX Subsection "I lost track; what encoding is the internal format really?"
It's good that you lost track, because you shouldn't depend on the internal
format being any specific encoding. But since you asked: by default, the
internal format is either \s-1ISO\-8859\-1\s0 (latin\-1), or utf8, depending on the
history of the string. On \s-1EBCDIC\s0 platforms, this may be different even.
.PP
Perl knows how it stored the string internally, and will use that knowledge
when you \f(CW\*(C`encode\*(C'\fR. In other words: don't try to find out what the internal
encoding for a certain string is, but instead just encode it into the encoding
that you want.
.SH "AUTHOR"
.IX Header "AUTHOR"
Juerd Waalboer <#####@juerd.nl>
.SH "SEE ALSO"
.IX Header "SEE ALSO"
perlunicode, perluniintro, Encode