Guard open.t against Encode not being available

[perl5.git] / pod / perlunicode.pod

=head1 NAME

perlunicode - Unicode support in Perl

=head1 DESCRIPTION

=head2 Important Caveats

Unicode support is an extensive requirement. While Perl does not
implement the Unicode standard or the accompanying technical reports
from cover to cover, Perl does support many Unicode features.

People who want to learn to use Unicode in Perl, should probably read
L<the Perl Unicode tutorial, perlunitut|perlunitut>, before reading
this reference document.

=over 4

=item Input and Output Layers

Perl knows when a filehandle uses Perl's internal Unicode encodings
(UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with
the ":utf8" layer.  Other encodings can be converted to Perl's
encoding on input or from Perl's encoding on output by use of the
":encoding(...)"  layer.  See L<open>.

To indicate that Perl source itself is in UTF-8, use C<use utf8;>.

=item Regular Expressions

The regular expression compiler produces polymorphic opcodes.  That is,
the pattern adapts to the data and automatically switches to the Unicode
character scheme when presented with data that is internally encoded in
UTF-8 -- or instead uses a traditional byte scheme when presented with
byte data.

=item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts

As a compatibility measure, the C<use utf8> pragma must be explicitly
included to enable recognition of UTF-8 in the Perl scripts themselves
(in string or regular expression literals, or in identifier names) on
ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based
machines.  B<These are the only times when an explicit C<use utf8>
is needed.>  See L<utf8>.

=item BOM-marked scripts and UTF-16 scripts autodetected

If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE,
or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either
endianness, Perl will correctly read in the script as Unicode.
(BOMless UTF-8 cannot be effectively recognized or differentiated from
ISO 8859-1 or other eight-bit encodings.)

=item C<use encoding> needed to upgrade non-Latin-1 byte strings

By default, there is a fundamental asymmetry in Perl's Unicode model:
implicit upgrading from byte strings to Unicode strings assumes that
they were encoded in I<ISO 8859-1 (Latin-1)>, but Unicode strings are
downgraded with UTF-8 encoding.  This happens because the first 256
codepoints in Unicode happens to agree with Latin-1.

See L</"Byte and Character Semantics"> for more details.

=back

=head2 Byte and Character Semantics

Beginning with version 5.6, Perl uses logically-wide characters to
represent strings internally.

In future, Perl-level operations will be expected to work with
characters rather than bytes.

However, as an interim compatibility measure, Perl aims to
provide a safe migration path from byte semantics to character
semantics for programs.  For operations where Perl can unambiguously
decide that the input data are characters, Perl switches to
character semantics.  For operations where this determination cannot
be made without additional information from the user, Perl decides in
favor of compatibility and chooses to use byte semantics.

Under byte semantics, when C<use locale> is in effect, Perl uses the
semantics associated with the current locale.  Absent a C<use locale>, and
absent a C<use feature 'unicode_strings'> pragma, Perl currently uses US-ASCII
(or Basic Latin in Unicode terminology) byte semantics, meaning that characters
whose ordinal numbers are in the range 128 - 255 are undefined except for their
ordinal numbers.  This means that none have case (upper and lower), nor are any
a member of character classes, like C<[:alpha:]> or C<\w>.  (But all do belong
to the C<\W> class or the Perl regular expression extension C<[:^alpha:]>.)

This behavior preserves compatibility with earlier versions of Perl,
which allowed byte semantics in Perl operations only if
none of the program's inputs were marked as being a source of Unicode
character data.  Such data may come from filehandles, from calls to
external programs, from information provided by the system (such as %ENV),
or from literals and constants in the source text.

The C<bytes> pragma will always, regardless of platform, force byte
semantics in a particular lexical scope.  See L<bytes>.

The C<use feature 'unicode_strings'> pragma is intended to always, regardless
of platform, force Unicode semantics in a particular lexical scope.  In
release 5.12, it is partially implemented, applying only to case changes.
See L</The "Unicode Bug"> below.

The C<utf8> pragma is primarily a compatibility device that enables
recognition of UTF-(8|EBCDIC) in literals encountered by the parser.
Note that this pragma is only required while Perl defaults to byte
semantics; when character semantics become the default, this pragma
may become a no-op.  See L<utf8>.

Unless explicitly stated, Perl operators use character semantics
for Unicode data and byte semantics for non-Unicode data.
The decision to use character semantics is made transparently.  If
input data comes from a Unicode source--for example, if a character
encoding layer is added to a filehandle or a literal Unicode
string constant appears in a program--character semantics apply.
Otherwise, byte semantics are in effect.  The C<bytes> pragma should
be used to force byte semantics on Unicode data, and the C<use feature
'unicode_strings'> pragma to force Unicode semantics on byte data (though in
5.12 it isn't fully implemented).

If strings operating under byte semantics and strings with Unicode
character data are concatenated, the new string will have
character semantics.  This can cause surprises: See L</BUGS>, below

Under character semantics, many operations that formerly operated on
bytes now operate on characters. A character in Perl is
logically just a number ranging from 0 to 2**31 or so. Larger
characters may encode into longer sequences of bytes internally, but
this internal detail is mostly hidden for Perl code.
See L<perluniintro> for more.

=head2 Effects of Character Semantics

Character semantics have the following effects:

=over 4

=item *

Strings--including hash keys--and regular expression patterns may
contain characters that have an ordinal value larger than 255.

If you use a Unicode editor to edit your program, Unicode characters may
occur directly within the literal strings in UTF-8 encoding, or UTF-16.
(The former requires a BOM or C<use utf8>, the latter requires a BOM.)

Unicode characters can also be added to a string by using the C<\x{...}>
notation.  The Unicode code for the desired character, in hexadecimal,
should be placed in the braces. For instance, a smiley face is
C<\x{263A}>.  This encoding scheme works for all characters, but
for characters under 0x100, note that Perl may use an 8 bit encoding
internally, for optimization and/or backward compatibility.

Additionally, if you

   use charnames ':full';

you can use the C<\N{...}> notation and put the official Unicode
character name within the braces, such as C<\N{WHITE SMILING FACE}>.

=item *

If an appropriate L<encoding> is specified, identifiers within the
Perl script may contain Unicode alphanumeric characters, including
ideographs.  Perl does not currently attempt to canonicalize variable
names.

=item *

Regular expressions match characters instead of bytes.  "." matches
a character instead of a byte.

=item *

Character classes in regular expressions match characters instead of
bytes and match against the character properties specified in the
Unicode properties database.  C<\w> can be used to match a Japanese
ideograph, for instance.

=item *

Named Unicode properties, scripts, and block ranges may be used like
character classes via the C<\p{}> "matches property" construct and
the C<\P{}> negation, "doesn't match property".
See L</"Unicode Character Properties"> for more details.

You can define your own character properties and use them
in the regular expression with the C<\p{}> or C<\P{}> construct.
See L</"User-Defined Character Properties"> for more details.

=item *

The special pattern C<\X> matches a logical character, an "extended grapheme
cluster" in Standardese.  In Unicode what appears to the user to be a single
character, for example an accented C<G>, may in fact be composed of a sequence
of characters, in this case a C<G> followed by an accent character.  C<\X>
will match the entire sequence.

=item *

The C<tr///> operator translates characters instead of bytes.  Note
that the C<tr///CU> functionality has been removed.  For similar
functionality see pack('U0', ...) and pack('C0', ...).

=item *

Case translation operators use the Unicode case translation tables
when character input is provided.  Note that C<uc()>, or C<\U> in
interpolated strings, translates to uppercase, while C<ucfirst>,
or C<\u> in interpolated strings, translates to titlecase in languages
that make the distinction (which is equivalent to uppercase in languages
without the distinction).

=item *

Most operators that deal with positions or lengths in a string will
automatically switch to using character positions, including
C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
C<sprintf()>, C<write()>, and C<length()>.  An operator that
specifically does not switch is C<vec()>.  Operators that really don't
care include operators that treat strings as a bucket of bits such as
C<sort()>, and operators dealing with filenames.

=item *

The C<pack()>/C<unpack()> letter C<C> does I<not> change, since it is often
used for byte-oriented formats.  Again, think C<char> in the C language.

There is a new C<U> specifier that converts between Unicode characters
and code points. There is also a C<W> specifier that is the equivalent of
C<chr>/C<ord> and properly handles character values even if they are above 255.

=item *

The C<chr()> and C<ord()> functions work on characters, similar to
C<pack("W")> and C<unpack("W")>, I<not> C<pack("C")> and
C<unpack("C")>.  C<pack("C")> and C<unpack("C")> are methods for
emulating byte-oriented C<chr()> and C<ord()> on Unicode strings.
While these methods reveal the internal encoding of Unicode strings,
that is not something one normally needs to care about at all.

=item *

The bit string operators, C<& | ^ ~>, can operate on character data.
However, for backward compatibility, such as when using bit string
operations when characters are all less than 256 in ordinal value, one
should not use C<~> (the bit complement) with characters of both
values less than 256 and values greater than 256.  Most importantly,
DeMorgan's laws (C<~($x|$y) eq ~$x&~$y> and C<~($x&$y) eq ~$x|~$y>)
will not hold.  The reason for this mathematical I<faux pas> is that
the complement cannot return B<both> the 8-bit (byte-wide) bit
complement B<and> the full character-wide bit complement.

=item *

You can define your own mappings to be used in lc(),
lcfirst(), uc(), and ucfirst() (or their string-inlined versions).
See L</"User-Defined Case Mappings"> for more details.

=back

=over 4

=item *

And finally, C<scalar reverse()> reverses by character rather than by byte.

=back

=head2 Unicode Character Properties

Most Unicode character properties are accessible by using regular expressions.
They are used like character classes via the C<\p{}> "matches property"
construct and the C<\P{}> negation, "doesn't match property".

For instance, C<\p{Uppercase}> matches any character with the Unicode
"Uppercase" property, while C<\p{L}> matches any character with a
General_Category of "L" (letter) property.  Brackets are not
required for single letter properties, so C<\p{L}> is equivalent to C<\pL>.

More formally, C<\p{Uppercase}> matches any character whose Unicode Uppercase
property value is True, and C<\P{Uppercase}> matches any character whose
Uppercase property value is False, and they could have been written as
C<\p{Uppercase=True}> and C<\p{Uppercase=False}>, respectively

This formality is needed when properties are not binary, that is if they can
take on more values than just True and False.  For example, the Bidi_Class (see
L</"Bidirectional Character Types"> below), can take on a number of different
values, such as Left, Right, Whitespace, and others.  To match these, one needs
to specify the property name (Bidi_Class), and the value being matched against
(Left, Right, I<etc.>).  This is done, as in the examples above, by having the
two components separated by an equal sign (or interchangeably, a colon), like
C<\p{Bidi_Class: Left}>.

All Unicode-defined character properties may be written in these compound forms
of C<\p{property=value}> or C<\p{property:value}>, but Perl provides some
additional properties that are written only in the single form, as well as
single-form short-cuts for all binary properties and certain others described
below, in which you may omit the property name and the equals or colon
separator.

Most Unicode character properties have at least two synonyms (or aliases if you
prefer), a short one that is easier to type, and a longer one which is more
descriptive and hence it is easier to understand what it means.  Thus the "L"
and "Letter" above are equivalent and can be used interchangeably.  Likewise,
"Upper" is a synonym for "Uppercase", and we could have written
C<\p{Uppercase}> equivalently as C<\p{Upper}>.  Also, there are typically
various synonyms for the values the property can be.   For binary properties,
"True" has 3 synonyms: "T", "Yes", and "Y"; and "False has correspondingly "F",
"No", and "N".  But be careful.  A short form of a value for one property may
not mean the same thing as the same short form for another.  Thus, for the
General_Category property, "L" means "Letter", but for the Bidi_Class property,
"L" means "Left".  A complete list of properties and synonyms is in
L<perluniprops>.

Upper/lower case differences in the property names and values are irrelevant,
thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
Similarly, you can add or subtract underscores anywhere in the middle of a
word, so that these are also equivalent to C<\p{U_p_p_e_r}>.  And white space
is irrelevant adjacent to non-word characters, such as the braces and the equals
or colon separators so C<\p{   Upper  }> and C<\p{ Upper_case : Y }> are
equivalent to these as well.  In fact, in most cases, white space and even
hyphens can be added or deleted anywhere.  So even C<\p{ Up-per case = Yes}> is
equivalent.  All this is called "loose-matching" by Unicode.  The few places
where stricter matching is employed is in the middle of numbers, and the Perl
extension properties that begin or end with an underscore.  Stricter matching
cares about white space (except adjacent to the non-word characters) and
hyphens, and non-interior underscores.

You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
(^) between the first brace and the property name: C<\p{^Tamil}> is
equal to C<\P{Tamil}>.

=head3 B<General_Category>

Every Unicode character is assigned a general category, which is the "most
usual categorization of a character" (from
L<http://www.unicode.org/reports/tr44>).

The compound way of writing these is like C<\p{General_Category=Number}>
(short, C<\p{gc:n}>).  But Perl furnishes shortcuts in which everything up
through the equal or colon separator is omitted.  So you can instead just write
C<\pN>.

Here are the short and long forms of the General Category properties:

    Short       Long

    L           Letter
    LC, L&      Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
    Lu          Uppercase_Letter
    Ll          Lowercase_Letter
    Lt          Titlecase_Letter
    Lm          Modifier_Letter
    Lo          Other_Letter

    M           Mark
    Mn          Nonspacing_Mark
    Mc          Spacing_Mark
    Me          Enclosing_Mark

    N           Number
    Nd          Decimal_Number (also Digit)
    Nl          Letter_Number
    No          Other_Number

    P           Punctuation (also Punct)
    Pc          Connector_Punctuation
    Pd          Dash_Punctuation
    Ps          Open_Punctuation
    Pe          Close_Punctuation
    Pi          Initial_Punctuation
                (may behave like Ps or Pe depending on usage)
    Pf          Final_Punctuation
                (may behave like Ps or Pe depending on usage)
    Po          Other_Punctuation

    S           Symbol
    Sm          Math_Symbol
    Sc          Currency_Symbol
    Sk          Modifier_Symbol
    So          Other_Symbol

    Z           Separator
    Zs          Space_Separator
    Zl          Line_Separator
    Zp          Paragraph_Separator

    C           Other
    Cc          Control	(also Cntrl)
    Cf          Format
    Cs          Surrogate   (not usable)
    Co          Private_Use
    Cn          Unassigned

Single-letter properties match all characters in any of the
two-letter sub-properties starting with the same letter.
C<LC> and C<L&> are special cases, which are aliases for the set of
C<Ll>, C<Lu>, and C<Lt>.

Because Perl hides the need for the user to understand the internal
representation of Unicode characters, there is no need to implement
the somewhat messy concept of surrogates. C<Cs> is therefore not
supported.

=head3 B<Bidirectional Character Types>

Because scripts differ in their directionality--Hebrew is
written right to left, for example--Unicode supplies these properties in
the Bidi_Class class:

    Property    Meaning

    L           Left-to-Right
    LRE         Left-to-Right Embedding
    LRO         Left-to-Right Override
    R           Right-to-Left
    AL          Arabic Letter
    RLE         Right-to-Left Embedding
    RLO         Right-to-Left Override
    PDF         Pop Directional Format
    EN          European Number
    ES          European Separator
    ET          European Terminator
    AN          Arabic Number
    CS          Common Separator
    NSM         Non-Spacing Mark
    BN          Boundary Neutral
    B           Paragraph Separator
    S           Segment Separator
    WS          Whitespace
    ON          Other Neutrals

This property is always written in the compound form.
For example, C<\p{Bidi_Class:R}> matches characters that are normally
written right to left.

=head3 B<Scripts>

The world's languages are written in a number of scripts.  This sentence
(unless you're reading it in translation) is written in Latin, while Russian is
written in Cyrllic, and Greek is written in, well, Greek; Japanese mainly in
Hiragana or Katakana.  There are many more.

The Unicode Script property gives what script a given character is in,
and can be matched with the compound form like C<\p{Script=Hebrew}> (short:
C<\p{sc=hebr}>).  Perl furnishes shortcuts for all script names.  You can omit
everything up through the equals (or colon), and simply write C<\p{Latin}> or
C<\P{Cyrillic}>.

A complete list of scripts and their shortcuts is in L<perluniprops>.

=head3 B<Use of "Is" Prefix>

For backward compatibility (with Perl 5.6), all properties mentioned
so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
C<\p{Arabic}>.

=head3 B<Blocks>

In addition to B<scripts>, Unicode also defines B<blocks> of
characters.  The difference between scripts and blocks is that the
concept of scripts is closer to natural languages, while the concept
of blocks is more of an artificial grouping based on groups of Unicode
characters with consecutive ordinal values. For example, the "Basic Latin"
block is all characters whose ordinals are between 0 and 127, inclusive, in
other words, the ASCII characters.  The "Latin" script contains some letters
from this block as well as several more, like "Latin-1 Supplement",
"Latin Extended-A", I<etc.>, but it does not contain all the characters from
those blocks. It does not, for example, contain digits, because digits are
shared across many scripts. Digits and similar groups, like punctuation, are in
the script called C<Common>.  There is also a script called C<Inherited> for
characters that modify other characters, and inherit the script value of the
controlling character.

For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
L<http://www.unicode.org/reports/tr24>

The Script property is likely to be the one you want to use when processing
natural language; the Block property may be useful in working with the nuts and
bolts of Unicode.

Block names are matched in the compound form, like C<\p{Block: Arrows}> or
C<\p{Blk=Hebrew}>.  Unlike most other properties only a few block names have a
Unicode-defined short name.  But Perl does provide a (slight) shortcut:  You
can say, for example C<\p{In_Arrows}> or C<\p{In_Hebrew}>.  For backwards
compatibility, the C<In> prefix may be omitted if there is no naming conflict
with a script or any other property, and you can even use an C<Is> prefix
instead in those cases.  But it is not a good idea to do this, for a couple
reasons:

=over 4

=item 1

It is confusing.  There are many naming conflicts, and you may forget some.
For example, C<\p{Hebrew}> means the I<script> Hebrew, and NOT the I<block>
Hebrew.  But would you remember that 6 months from now?

=item 2

It is unstable.  A new version of Unicode may pre-empt the current meaning by
creating a property with the same name.  There was a time in very early Unicode
releases when C<\p{Hebrew}> would have matched the I<block> Hebrew; now it
doesn't.

=back

Some people just prefer to always use C<\p{Block: foo}> and C<\p{Script: bar}>
instead of the shortcuts, for clarity, and because they can't remember the
difference between 'In' and 'Is' anyway (or aren't confident that those who
eventually will read their code will know).

A complete list of blocks and their shortcuts is in L<perluniprops>.

=head3 B<Other Properties>

There are many more properties than the very basic ones described here.
A complete list is in L<perluniprops>.

Unicode defines all its properties in the compound form, so all single-form
properties are Perl extensions.  A number of these are just synonyms for the
Unicode ones, but some are genunine extensions, including a couple that are in
the compound form.  And quite a few of these are actually recommended by Unicode
(in L<http://www.unicode.org/reports/tr18>).

This section gives some details on all the extensions that aren't synonyms for
compound-form Unicode properties (for those, you'll have to refer to the
L<Unicode Standard|http://www.unicode.org/reports/tr44>.

=over

=item B<C<\p{All}>>

This matches any of the 1_114_112 Unicode code points.  It is a synonym for
C<\p{Any}>.

=item B<C<\p{Alnum}>>

This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character.

=item B<C<\p{Any}>>

This matches any of the 1_114_112 Unicode code points.  It is a synonym for
C<\p{All}>.

=item B<C<\p{Assigned}>>

This matches any assigned code point; that is, any code point whose general
category is not Unassigned (or equivalently, not Cn).

=item B<C<\p{Blank}>>

This is the same as C<\h> and C<\p{HorizSpace}>:  A character that changes the
spacing horizontally.

=item B<C<\p{Decomposition_Type: Non_Canonical}>>    (Short: C<\p{Dt=NonCanon}>)

Matches a character that has a non-canonical decomposition.

To understand the use of this rarely used property=value combination, it is
necessary to know some basics about decomposition.
Consider a character, say H.  It could appear with various marks around it,
such as an acute accent, or a circumflex, or various hooks, circles, arrows,
I<etc.>, above, below, to one side and/or the other, I<etc.>  There are many
possibilities among the world's languages.  The number of combinations is
astronomical, and if there were a character for each combination, it would
soon exhaust Unicode's more than a million possible characters.  So Unicode
took a different approach: there is a character for the base H, and a
character for each of the possible marks, and they can be combined variously
to get a final logical character.  So a logical character--what appears to be a
single character--can be a sequence of more than one individual characters.
This is called an "extended grapheme cluster".  (Perl furnishes the C<\X>
construct to match such sequences.)

But Unicode's intent is to unify the existing character set standards and
practices, and a number of pre-existing standards have single characters that
mean the same thing as some of these combinations.  An example is ISO-8859-1,
which has quite a few of these in the Latin-1 range, an example being "LATIN
CAPITAL LETTER E WITH ACUTE".  Because this character was in this pre-existing
standard, Unicode added it to its repertoire.  But this character is considered
by Unicode to be equivalent to the sequence consisting of first the character
"LATIN CAPITAL LETTER E", then the character "COMBINING ACUTE ACCENT".

"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and
the equivalence with the sequence is called canonical equivalence.  All
pre-composed characters are said to have a decomposition (into the equivalent
sequence) and the decomposition type is also called canonical.

However, many more characters have a different type of decomposition, a
"compatible" or "non-canonical" decomposition.  The sequences that form these
decompositions are not considered canonically equivalent to the pre-composed
character.  An example, again in the Latin-1 range, is the "SUPERSCRIPT ONE".
It is kind of like a regular digit 1, but not exactly; its decomposition
into the digit 1 is called a "compatible" decomposition, specifically a
"super" decomposition.  There are several such compatibility
decompositions (see L<http://www.unicode.org/reports/tr44>), including one
called "compat" which means some miscellaneous type of decomposition
that doesn't fit into the decomposition categories that Unicode has chosen. 

Note that most Unicode characters don't have a decomposition, so their
decomposition type is "None".

Perl has added the C<Non_Canonical> type, for your convenience, to mean any of
the compatibility decompositions.

=item B<C<\p{Graph}>>

Matches any character that is graphic.  Theoretically, this means a character
that on a printer would cause ink to be used.

=item B<C<\p{HorizSpace}>>

This is the same as C<\h> and C<\p{Blank}>:  A character that changes the
spacing horizontally.

=item B<C<\p{In=*}>> 

This is a synonym for C<\p{Present_In=*}>

=item B<C<\p{PerlSpace}>>

This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>.

Mnemonic: Perl's (original) space

=item B<C<\p{PerlWord}>>

This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]>

Mnemonic: Perl's (original) word.

=item B<C<\p{PosixAlnum}>>

This matches any alphanumeric character in the ASCII range, namely
C<[A-Za-z0-9]>.

=item B<C<\p{PosixAlpha}>>

This matches any alphabetic character in the ASCII range, namely C<[A-Za-z]>.

=item B<C<\p{PosixBlank}>>

This matches any blank character in the ASCII range, namely C<S<[ \t]>>.

=item B<C<\p{PosixCntrl}>>

This matches any control character in the ASCII range, namely C<[\x00-\x1F\x7F]>

=item B<C<\p{PosixDigit}>>

This matches any digit character in the ASCII range, namely C<[0-9]>.

=item B<C<\p{PosixGraph}>>

This matches any graphical character in the ASCII range, namely C<[\x21-\x7E]>.

=item B<C<\p{PosixLower}>>

This matches any lowercase character in the ASCII range, namely C<[a-z]>.

=item B<C<\p{PosixPrint}>>

This matches any printable character in the ASCII range, namely C<[\x20-\x7E]>.
These are the graphical characters plus SPACE.

=item B<C<\p{PosixPunct}>>

This matches any punctuation character in the ASCII range, namely
C<[\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]>.  These are the
graphical characters that aren't word characters.  Note that the Posix standard
includes in its definition of punctuation, those characters that Unicode calls
"symbols."

=item B<C<\p{PosixSpace}>>

This matches any space character in the ASCII range, namely
C<S<[ \f\n\r\t\x0B]>> (the last being a vertical tab).

=item B<C<\p{PosixUpper}>>

This matches any uppercase character in the ASCII range, namely C<[A-Z]>.

=item B<C<\p{Present_In: *}>>    (Short: C<\p{In=*}>)

This property is used when you need to know in what Unicode version(s) a
character is.

The "*" above stands for some two digit Unicode version number, such as
C<1.1> or C<4.0>; or the "*" can also be C<Unassigned>.  This property will
match the code points whose final disposition has been settled as of the
Unicode release given by the version number; C<\p{Present_In: Unassigned}>
will match those code points whose meaning has yet to be assigned.

For example, C<U+0041> "LATIN CAPITAL LETTER A" was present in the very first
Unicode release available, which is C<1.1>, so this property is true for all
valid "*" versions.  On the other hand, C<U+1EFF> was not assigned until version
5.1 when it became "LATIN SMALL LETTER Y WITH LOOP", so the only "*" that
would match it are 5.1, 5.2, and later.

Unicode furnishes the C<Age> property from which this is derived.  The problem
with Age is that a strict interpretation of it (which Perl takes) has it
matching the precise release a code point's meaning is introduced in.  Thus
C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1.  This is not usually what
you want.

Some non-Perl implementations of the Age property may change its meaning to be
the same as the Perl Present_In property; just be aware of that.

Another confusion with both these properties is that the definition is not
that the code point has been assigned, but that the meaning of the code point
has been determined.  This is because 66 code points will always be
unassigned, and, so the Age for them is the Unicode version the decision to
make them so was made in.  For example, C<U+FDD0> is to be permanently
unassigned to a character, and the decision to do that was made in version 3.1,
so C<\p{Age=3.1}> matches this character and C<\p{Present_In: 3.1}> and up
matches as well.

=item B<C<\p{Print}>>

This matches any character that is graphical or is space, but not a control.

=item B<C<\p{SpacePerl}>>

This is the same as C<\s>, including beyond ASCII.

Mnemonic: Space, as modified by Perl.  (It doesn't include vertical tab which
both the Posix standard and Unicode consider to be space.)

=item B<C<\p{VertSpace}>>

This is the same as C<\v>:  A character that changes the spacing vertically.

=item B<C<\p{Word}>>

This is the same as C<\w>, including beyond ASCII.

=back

=head2 User-Defined Character Properties

You can define your own binary character properties by defining subroutines
whose names begin with "In" or "Is".  The subroutines can be defined in any
package.  The user-defined properties can be used in the regular expression
C<\p> and C<\P> constructs; if you are using a user-defined property from a
package other than the one you are in, you must specify its package in the
C<\p> or C<\P> construct.

    # assuming property Is_Foreign defined in Lang::
    package main;  # property package name required
    if ($txt =~ /\p{Lang::IsForeign}+/) { ... }

    package Lang;  # property package name not required
    if ($txt =~ /\p{IsForeign}+/) { ... }


Note that the effect is compile-time and immutable once defined.

The subroutines must return a specially-formatted string, with one
or more newline-separated lines.  Each line must be one of the following:

=over 4

=item *

A single hexadecimal number denoting a Unicode code point to include.

=item *

Two hexadecimal numbers separated by horizontal whitespace (space or
tabular characters) denoting a range of Unicode code points to include.

=item *

Something to include, prefixed by "+": a built-in character
property (prefixed by "utf8::") or a user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.

=item *

Something to exclude, prefixed by "-": an existing character
property (prefixed by "utf8::") or a user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.

=item *

Something to negate, prefixed "!": an existing character
property (prefixed by "utf8::") or a user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.

=item *

Something to intersect with, prefixed by "&": an existing character
property (prefixed by "utf8::") or a user-defined character property,
for all the characters except the characters in the property; two
hexadecimal code points for a range; or a single hexadecimal code point.

=back

For example, to define a property that covers both the Japanese
syllabaries (hiragana and katakana), you can define

    sub InKana {
	return <<END;
    3040\t309F
    30A0\t30FF
    END
    }

Imagine that the here-doc end marker is at the beginning of the line.
Now you can use C<\p{InKana}> and C<\P{InKana}>.

You could also have used the existing block property names:

    sub InKana {
	return <<'END';
    +utf8::InHiragana
    +utf8::InKatakana
    END
    }

Suppose you wanted to match only the allocated characters,
not the raw block ranges: in other words, you want to remove
the non-characters:

    sub InKana {
	return <<'END';
    +utf8::InHiragana
    +utf8::InKatakana
    -utf8::IsCn
    END
    }

The negation is useful for defining (surprise!) negated classes.

    sub InNotKana {
	return <<'END';
    !utf8::InHiragana
    -utf8::InKatakana
    +utf8::IsCn
    END
    }

Intersection is useful for getting the common characters matched by
two (or more) classes.

    sub InFooAndBar {
        return <<'END';
    +main::Foo
    &main::Bar
    END
    }

It's important to remember not to use "&" for the first set -- that
would be intersecting with nothing (resulting in an empty set).

=head2 User-Defined Case Mappings

You can also define your own mappings to be used in the lc(),
lcfirst(), uc(), and ucfirst() (or their string-inlined versions).
The principle is similar to that of user-defined character
properties: to define subroutines
with names like C<ToLower> (for lc() and lcfirst()), C<ToTitle> (for
the first character in ucfirst()), and C<ToUpper> (for uc(), and the
rest of the characters in ucfirst()).

The string returned by the subroutines needs to be two hexadecimal numbers
separated by two tabulators: the two numbers being, respectively, the source
code point and the destination code point.  For example:

    sub ToUpper {
	return <<END;
    0061\t\t0041
    END
    }

defines an uc() mapping that causes only the character "a"
to be mapped to "A"; all other characters will remain unchanged.

(For serious hackers only)  The above means you have to furnish a complete
mapping; you can't just override a couple of characters and leave the rest
unchanged.  You can find all the mappings in the directory
C<$Config{privlib}>/F<unicore/To/>.  The mapping data is returned as the
here-document, and the C<utf8::ToSpecFoo> are special exception mappings
derived from <$Config{privlib}>/F<unicore/SpecialCasing.txt>.  The "Digit" and
"Fold" mappings that one can see in the directory are not directly
user-accessible, one can use either the C<Unicode::UCD> module, or just match
case-insensitively (that's when the "Fold" mapping is used).

The mappings will only take effect on scalars that have been marked as having
Unicode characters, for example by using C<utf8::upgrade()>.
Old byte-style strings are not affected.

The mappings are in effect for the package they are defined in.

=head2 Character Encodings for Input and Output

See L<Encode>.

=head2 Unicode Regular Expression Support Level

The following list of Unicode support for regular expressions describes
all the features currently supported.  The references to "Level N"
and the section numbers refer to the Unicode Technical Standard #18,
"Unicode Regular Expressions", version 11, in May 2005.

=over 4

=item *

Level 1 - Basic Unicode Support

        RL1.1   Hex Notation                        - done          [1]
        RL1.2   Properties                          - done          [2][3]
        RL1.2a  Compatibility Properties            - done          [4]
        RL1.3   Subtraction and Intersection        - MISSING       [5]
        RL1.4   Simple Word Boundaries              - done          [6]
        RL1.5   Simple Loose Matches                - done          [7]
        RL1.6   Line Boundaries                     - MISSING       [8]
        RL1.7   Supplementary Code Points           - done          [9]

        [1]  \x{...}
        [2]  \p{...} \P{...}
	[3]  supports not only minimal list, but all Unicode character
	     properties (see L</Unicode Character Properties>)
        [4]  \d \D \s \S \w \W \X [:prop:] [:^prop:]
        [5]  can use regular expression look-ahead [a] or
             user-defined character properties [b] to emulate set operations
        [6]  \b \B
	[7]  note that Perl does Full case-folding in matching (but with bugs),
	     not Simple: for example U+1F88 is equivalent to U+1F00 U+03B9,
             not with 1F80.  This difference matters mainly for certain Greek
             capital letters with certain modifiers: the Full case-folding
             decomposes the letter, while the Simple case-folding would map
             it to a single character.
        [8]  should do ^ and $ also on U+000B (\v in C), FF (\f), CR (\r),
             CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS (U+2029);
             should also affect <>, $., and script line numbers;
             should not split lines within CRLF [c] (i.e. there is no empty
             line between \r and \n)
        [9]  UTF-8/UTF-EBDDIC used in perl allows not only U+10000 to U+10FFFF
             but also beyond U+10FFFF [d]

[a] You can mimic class subtraction using lookahead.
For example, what UTS#18 might write as

    [{Greek}-[{UNASSIGNED}]]

in Perl can be written as:

    (?!\p{Unassigned})\p{InGreekAndCoptic}
    (?=\p{Assigned})\p{InGreekAndCoptic}

But in this particular example, you probably really want

    \p{GreekAndCoptic}

which will match assigned characters known to be part of the Greek script.

Also see the Unicode::Regex::Set module, it does implement the full
UTS#18 grouping, intersection, union, and removal (subtraction) syntax.

[b] '+' for union, '-' for removal (set-difference), '&' for intersection
(see L</"User-Defined Character Properties">)

[c] Try the C<:crlf> layer (see L<PerlIO>).

[d] U+FFFF will currently generate a warning message if 'utf8' warnings are
    enabled

=item *

Level 2 - Extended Unicode Support

        RL2.1   Canonical Equivalents           - MISSING       [10][11]
        RL2.2   Default Grapheme Clusters       - MISSING       [12]
        RL2.3   Default Word Boundaries         - MISSING       [14]
        RL2.4   Default Loose Matches           - MISSING       [15]
        RL2.5   Name Properties                 - MISSING       [16]
        RL2.6   Wildcard Properties             - MISSING

        [10] see UAX#15 "Unicode Normalization Forms"
        [11] have Unicode::Normalize but not integrated to regexes
        [12] have \X but we don't have a "Grapheme Cluster Mode"
        [14] see UAX#29, Word Boundaries
        [15] see UAX#21 "Case Mappings"
        [16] have \N{...} but neither compute names of CJK Ideographs
             and Hangul Syllables nor use a loose match [e]

[e] C<\N{...}> allows namespaces (see L<charnames>).

=item *

Level 3 - Tailored Support

        RL3.1   Tailored Punctuation            - MISSING
        RL3.2   Tailored Grapheme Clusters      - MISSING       [17][18]
        RL3.3   Tailored Word Boundaries        - MISSING
        RL3.4   Tailored Loose Matches          - MISSING
        RL3.5   Tailored Ranges                 - MISSING
        RL3.6   Context Matching                - MISSING       [19]
        RL3.7   Incremental Matches             - MISSING
      ( RL3.8   Unicode Set Sharing )
        RL3.9   Possible Match Sets             - MISSING
        RL3.10  Folded Matching                 - MISSING       [20]
        RL3.11  Submatchers                     - MISSING

        [17] see UAX#10 "Unicode Collation Algorithms"
        [18] have Unicode::Collate but not integrated to regexes
        [19] have (?<=x) and (?=x), but look-aheads or look-behinds should see
             outside of the target substring
        [20] need insensitive matching for linguistic features other than case;
             for example, hiragana to katakana, wide and narrow, simplified Han
             to traditional Han (see UTR#30 "Character Foldings")

=back

=head2 Unicode Encodings

Unicode characters are assigned to I<code points>, which are abstract
numbers.  To use these numbers, various encodings are needed.

=over 4

=item *

UTF-8

UTF-8 is a variable-length (1 to 6 bytes, current character allocations
require 4 bytes), byte-order independent encoding. For ASCII (and we
really do mean 7-bit ASCII, not another 8-bit encoding), UTF-8 is
transparent.

The following table is from Unicode 3.2.

 Code Points		1st Byte  2nd Byte  3rd Byte  4th Byte

   U+0000..U+007F	00..7F
   U+0080..U+07FF     * C2..DF    80..BF
   U+0800..U+0FFF	E0      * A0..BF    80..BF
   U+1000..U+CFFF       E1..EC    80..BF    80..BF
   U+D000..U+D7FF       ED        80..9F    80..BF
   U+D800..U+DFFF       +++++++ utf16 surrogates, not legal utf8 +++++++
   U+E000..U+FFFF       EE..EF    80..BF    80..BF
  U+10000..U+3FFFF	F0      * 90..BF    80..BF    80..BF
  U+40000..U+FFFFF	F1..F3    80..BF    80..BF    80..BF
 U+100000..U+10FFFF	F4        80..8F    80..BF    80..BF

Note the gaps before several of the byte entries above marked by '*'.  These are
caused by legal UTF-8 avoiding non-shortest encodings: it is technically
possible to UTF-8-encode a single code point in different ways, but that is
explicitly forbidden, and the shortest possible encoding should always be used
(and that is what Perl does).

Another way to look at it is via bits:

 Code Points                    1st Byte   2nd Byte  3rd Byte  4th Byte

                    0aaaaaaa     0aaaaaaa
            00000bbbbbaaaaaa     110bbbbb  10aaaaaa
            ccccbbbbbbaaaaaa     1110cccc  10bbbbbb  10aaaaaa
  00000dddccccccbbbbbbaaaaaa     11110ddd  10cccccc  10bbbbbb  10aaaaaa

As you can see, the continuation bytes all begin with "10", and the
leading bits of the start byte tell how many bytes there are in the
encoded character.

=item *

UTF-EBCDIC

Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.

=item *

UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)

The followings items are mostly for reference and general Unicode
knowledge, Perl doesn't use these constructs internally.

UTF-16 is a 2 or 4 byte encoding.  The Unicode code points
C<U+0000..U+FFFF> are stored in a single 16-bit unit, and the code
points C<U+10000..U+10FFFF> in two 16-bit units.  The latter case is
using I<surrogates>, the first 16-bit unit being the I<high
surrogate>, and the second being the I<low surrogate>.

Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
range of Unicode code points in pairs of 16-bit units.  The I<high
surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
are the range C<U+DC00..U+DFFF>.  The surrogate encoding is

	$hi = ($uni - 0x10000) / 0x400 + 0xD800;
	$lo = ($uni - 0x10000) % 0x400 + 0xDC00;

and the decoding is

	$uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);

If you try to generate surrogates (for example by using chr()), you
will get a warning, if warnings are turned on, because those code
points are not valid for a Unicode character.

Because of the 16-bitness, UTF-16 is byte-order dependent.  UTF-16
itself can be used for in-memory computations, but if storage or
transfer is required either UTF-16BE (big-endian) or UTF-16LE
(little-endian) encodings must be chosen.

This introduces another problem: what if you just know that your data
is UTF-16, but you don't know which endianness?  Byte Order Marks, or
BOMs, are a solution to this.  A special character has been reserved
in Unicode to function as a byte order marker: the character with the
code point C<U+FEFF> is the BOM.

The trick is that if you read a BOM, you will know the byte order,
since if it was written on a big-endian platform, you will read the
bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
you will read the bytes C<0xFF 0xFE>.  (And if the originating platform
was writing in UTF-8, you will read the bytes C<0xEF 0xBB 0xBF>.)

The way this trick works is that the character with the code point
C<U+FFFE> is guaranteed not to be a valid Unicode character, so the
sequence of bytes C<0xFF 0xFE> is unambiguously "BOM, represented in
little-endian format" and cannot be C<U+FFFE>, represented in big-endian
format".  (Actually, C<U+FFFE> is legal for use by your program, even for
input/output, but better not use it if you need a BOM.  But it is "illegal for
interchange", so that an unsuspecting program won't get confused.)

=item *

UTF-32, UTF-32BE, UTF-32LE

The UTF-32 family is pretty much like the UTF-16 family, expect that
the units are 32-bit, and therefore the surrogate scheme is not
needed.  The BOM signatures will be C<0x00 0x00 0xFE 0xFF> for BE and
C<0xFF 0xFE 0x00 0x00> for LE.

=item *

UCS-2, UCS-4

Encodings defined by the ISO 10646 standard.  UCS-2 is a 16-bit
encoding.  Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
because it does not use surrogates.  UCS-4 is a 32-bit encoding,
functionally identical to UTF-32.

=item *

UTF-7

A seven-bit safe (non-eight-bit) encoding, which is useful if the
transport or storage is not eight-bit safe.  Defined by RFC 2152.

=back

=head2 Security Implications of Unicode

Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
Also, note the following:

=over 4

=item *

Malformed UTF-8

Unfortunately, the specification of UTF-8 leaves some room for
interpretation of how many bytes of encoded output one should generate
from one input Unicode character.  Strictly speaking, the shortest
possible sequence of UTF-8 bytes should be generated,
because otherwise there is potential for an input buffer overflow at
the receiving end of a UTF-8 connection.  Perl always generates the
shortest length UTF-8, and with warnings on, Perl will warn about
non-shortest length UTF-8 along with other malformations, such as the
surrogates, which are not real Unicode code points.

=item *

Regular expressions behave slightly differently between byte data and
character (Unicode) data.  For example, the "word character" character
class C<\w> will work differently depending on if data is eight-bit bytes
or Unicode.

In the first case, the set of C<\w> characters is either small--the
default set of alphabetic characters, digits, and the "_"--or, if you
are using a locale (see L<perllocale>), the C<\w> might contain a few
more letters according to your language and country.

In the second case, the C<\w> set of characters is much, much larger.
Most importantly, even in the set of the first 256 characters, it will
probably match different characters: unlike most locales, which are
specific to a language and country pair, Unicode classifies all the
characters that are letters I<somewhere> as C<\w>.  For example, your
locale might not think that LATIN SMALL LETTER ETH is a letter (unless
you happen to speak Icelandic), but Unicode does.

As discussed elsewhere, Perl has one foot (two hooves?) planted in
each of two worlds: the old world of bytes and the new world of
characters, upgrading from bytes to characters when necessary.
If your legacy code does not explicitly use Unicode, no automatic
switch-over to characters should happen.  Characters shouldn't get
downgraded to bytes, either.  It is possible to accidentally mix bytes
and characters, however (see L<perluniintro>), in which case C<\w> in
regular expressions might start behaving differently.  Review your
code.  Use warnings and the C<strict> pragma.

=back

=head2 Unicode in Perl on EBCDIC

The way Unicode is handled on EBCDIC platforms is still
experimental.  On such platforms, references to UTF-8 encoding in this
document and elsewhere should be read as meaning the UTF-EBCDIC
specified in Unicode Technical Report 16, unless ASCII vs. EBCDIC issues
are specifically discussed. There is no C<utfebcdic> pragma or
":utfebcdic" layer; rather, "utf8" and ":utf8" are reused to mean
the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
for more discussion of the issues.

=head2 Locales

Usually locale settings and Unicode do not affect each other, but
there are a couple of exceptions:

=over 4

=item *

You can enable automatic UTF-8-ification of your standard file
handles, default C<open()> layer, and C<@ARGV> by using either
the C<-C> command line switch or the C<PERL_UNICODE> environment
variable, see L<perlrun> for the documentation of the C<-C> switch.

=item *

Perl tries really hard to work both with Unicode and the old
byte-oriented world. Most often this is nice, but sometimes Perl's
straddling of the proverbial fence causes problems.

=back

=head2 When Unicode Does Not Happen

While Perl does have extensive ways to input and output in Unicode,
and few other 'entry points' like the @ARGV which can be interpreted
as Unicode (UTF-8), there still are many places where Unicode (in some
encoding or another) could be given as arguments or received as
results, or both, but it is not.

The following are such interfaces.  Also, see L</The "Unicode Bug">.
For all of these interfaces Perl
currently (as of 5.8.3) simply assumes byte strings both as arguments
and results, or UTF-8 strings if the C<encoding> pragma has been used.

One reason why Perl does not attempt to resolve the role of Unicode in
these cases is that the answers are highly dependent on the operating
system and the file system(s).  For example, whether filenames can be
in Unicode, and in exactly what kind of encoding, is not exactly a
portable concept.  Similarly for the qx and system: how well will the
'command line interface' (and which of them?) handle Unicode?

=over 4

=item *

chdir, chmod, chown, chroot, exec, link, lstat, mkdir,
rename, rmdir, stat, symlink, truncate, unlink, utime, -X

=item *

%ENV

=item *

glob (aka the <*>)

=item *

open, opendir, sysopen

=item *

qx (aka the backtick operator), system

=item *

readdir, readlink

=back

=head2 The "Unicode Bug"

The term, the "Unicode bug" has been applied to an inconsistency with the
Unicode characters whose code points are in the Latin-1 Supplement block, that
is, between 128 and 255.  Without a locale specified, unlike all other
characters or code points, these characters have very different semantics in
byte semantics versus character semantics.

In character semantics they are interpreted as Unicode code points, which means
they have the same semantics as Latin-1 (ISO-8859-1).

In byte semantics, they are considered to be unassigned characters, meaning
that the only semantics they have is their ordinal numbers, and that they are
not members of various character classes.  None are considered to match C<\w>
for example, but all match C<\W>.  (On EBCDIC platforms, the behavior may
be different from this, depending on the underlying C language library
functions.)

The behavior is known to have effects on these areas:

=over 4

=item *

Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in regular expression
substitutions.

=item *

Using caseless (C</i>) regular expression matching

=item *

Matching a number of properties in regular expressions, such as C<\w>

=item *

User-defined case change mappings.  You can create a C<ToUpper()> function, for
example, which overrides Perl's built-in case mappings.  The scalar must be
encoded in utf8 for your function to actually be invoked.

=back

This behavior can lead to unexpected results in which a string's semantics
suddenly change if a code point above 255 is appended to or removed from it,
which changes the string's semantics from byte to character or vice versa.  As
an example, consider the following program and its output:

 $ perl -le'
     $s1 = "\xC2";
     $s2 = "\x{2660}";
     for ($s1, $s2, $s1.$s2) {
         print /\w/ || 0;
     }
 '
 0
 0
 1

If there's no C<\w> in C<s1> or in C<s2>, why does their concatenation have one?

This anomaly stems from Perl's attempt to not disturb older programs that
didn't use Unicode, and hence had no semantics for characters outside of the
ASCII range (except in a locale), along with Perl's desire to add Unicode
support seamlessly.  The result wasn't seamless: these characters were
orphaned.

Work is being done to correct this, but only some of it was complete in time
for the 5.12 release.  What has been finished is the important part of the case
changing component.  Due to concerns, and some evidence, that older code might
have come to rely on the existing behavior, the new behavior must be explicitly
enabled by the feature C<unicode_strings> in the L<feature> pragma, even though
no new syntax is involved.

See L<perlfunc/lc> for details on how this pragma works in combination with
various others for casing.  Even though the pragma only affects casing
operations in the 5.12 release, it is planned to have it affect all the
problematic behaviors in later releases: you can't have one without them all.

In the meantime, a workaround is to always call utf8::upgrade($string), or to
use the standard modules L<Encode> or L<charnames>.

=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)

Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
there are situations where you simply need to force a byte
string into UTF-8, or vice versa.  The low-level calls
utf8::upgrade($bytestring) and utf8::downgrade($utf8string[, FAIL_OK]) are
the answers.

Note that utf8::downgrade() can fail if the string contains characters
that don't fit into a byte.

Calling either function on a string that already is in the desired state is a
no-op.

=head2 Using Unicode in XS

If you want to handle Perl Unicode in XS extensions, you may find the
following C APIs useful.  See also L<perlguts/"Unicode Support"> for an
explanation about Unicode at the XS level, and L<perlapi> for the API
details.

=over 4

=item *

C<DO_UTF8(sv)> returns true if the C<UTF8> flag is on and the bytes
pragma is not in effect.  C<SvUTF8(sv)> returns true if the C<UTF8>
flag is on; the bytes pragma is ignored.  The C<UTF8> flag being on
does B<not> mean that there are any characters of code points greater
than 255 (or 127) in the scalar or that there are even any characters
in the scalar.  What the C<UTF8> flag means is that the sequence of
octets in the representation of the scalar is the sequence of UTF-8
encoded code points of the characters of a string.  The C<UTF8> flag
being off means that each octet in this representation encodes a
single character with code point 0..255 within the string.  Perl's
Unicode model is not to use UTF-8 until it is absolutely necessary.

=item *

C<uvchr_to_utf8(buf, chr)> writes a Unicode character code point into
a buffer encoding the code point as UTF-8, and returns a pointer
pointing after the UTF-8 bytes.  It works appropriately on EBCDIC machines.

=item *

C<utf8_to_uvchr(buf, lenp)> reads UTF-8 encoded bytes from a buffer and
returns the Unicode character code point and, optionally, the length of
the UTF-8 byte sequence.  It works appropriately on EBCDIC machines.

=item *

C<utf8_length(start, end)> returns the length of the UTF-8 encoded buffer
in characters.  C<sv_len_utf8(sv)> returns the length of the UTF-8 encoded
scalar.

=item *

C<sv_utf8_upgrade(sv)> converts the string of the scalar to its UTF-8
encoded form.  C<sv_utf8_downgrade(sv)> does the opposite, if
possible.  C<sv_utf8_encode(sv)> is like sv_utf8_upgrade except that
it does not set the C<UTF8> flag.  C<sv_utf8_decode()> does the
opposite of C<sv_utf8_encode()>.  Note that none of these are to be
used as general-purpose encoding or decoding interfaces: C<use Encode>
for that.  C<sv_utf8_upgrade()> is affected by the encoding pragma
but C<sv_utf8_downgrade()> is not (since the encoding pragma is
designed to be a one-way street).

=item *

C<is_utf8_char(s)> returns true if the pointer points to a valid UTF-8
character.

=item *

C<is_utf8_string(buf, len)> returns true if C<len> bytes of the buffer
are valid UTF-8.

=item *

C<UTF8SKIP(buf)> will return the number of bytes in the UTF-8 encoded
character in the buffer.  C<UNISKIP(chr)> will return the number of bytes
required to UTF-8-encode the Unicode character code point.  C<UTF8SKIP()>
is useful for example for iterating over the characters of a UTF-8
encoded buffer; C<UNISKIP()> is useful, for example, in computing
the size required for a UTF-8 encoded buffer.

=item *

C<utf8_distance(a, b)> will tell the distance in characters between the
two pointers pointing to the same UTF-8 encoded buffer.

=item *

C<utf8_hop(s, off)> will return a pointer to a UTF-8 encoded buffer
that is C<off> (positive or negative) Unicode characters displaced
from the UTF-8 buffer C<s>.  Be careful not to overstep the buffer:
C<utf8_hop()> will merrily run off the end or the beginning of the
buffer if told to do so.

=item *

C<pv_uni_display(dsv, spv, len, pvlim, flags)> and
C<sv_uni_display(dsv, ssv, pvlim, flags)> are useful for debugging the
output of Unicode strings and scalars.  By default they are useful
only for debugging--they display B<all> characters as hexadecimal code
points--but with the flags C<UNI_DISPLAY_ISPRINT>,
C<UNI_DISPLAY_BACKSLASH>, and C<UNI_DISPLAY_QQ> you can make the
output more readable.

=item *

C<ibcmp_utf8(s1, pe1, l1, u1, s2, pe2, l2, u2)> can be used to
compare two strings case-insensitively in Unicode.  For case-sensitive
comparisons you can just use C<memEQ()> and C<memNE()> as usual.

=back

For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
in the Perl source code distribution.

=head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)

Perl by default comes with the latest supported Unicode version built in, but
you can change to use any earlier one.

Download the files in the version of Unicode that you want from the Unicode web
site L<http://www.unicode.org>).  These should replace the existing files in
C<\$Config{privlib}>/F<unicore>.  (C<\%Config> is available from the Config
module.)  Follow the instructions in F<README.perl> in that directory to change
some of their names, and then run F<make>.

It is even possible to download them to a different directory, and then change
F<utf8_heavy.pl> in the directory C<\$Config{privlib}> to point to the new
directory, or maybe make a copy of that directory before making the change, and
using C<@INC> or the C<-I> run-time flag to switch between versions at will
(but because of caching, not in the middle of a process), but all this is
beyond the scope of these instructions.

=head1 BUGS

=head2 Interaction with Locales

Use of locales with Unicode data may lead to odd results.  Currently,
Perl attempts to attach 8-bit locale info to characters in the range
0..255, but this technique is demonstrably incorrect for locales that
use characters above that range when mapped into Unicode.  Perl's
Unicode support will also tend to run slower.  Use of locales with
Unicode is discouraged.

=head2 Problems with characters in the Latin-1 Supplement range

See L</The "Unicode Bug">

=head2 Problems with case-insensitive regular expression matching

There are problems with case-insensitive matches, including those involving
character classes (enclosed in [square brackets]), characters whose fold
is to multiple characters (such as the single character LATIN SMALL LIGATURE
FFL matches case-insensitively with the 3-character string C<ffl>), and
characters in the Latin-1 Supplement.

=head2 Interaction with Extensions

When Perl exchanges data with an extension, the extension should be
able to understand the UTF8 flag and act accordingly. If the
extension doesn't know about the flag, it's likely that the extension
will return incorrectly-flagged data.

So if you're working with Unicode data, consult the documentation of
every module you're using if there are any issues with Unicode data
exchange. If the documentation does not talk about Unicode at all,
suspect the worst and probably look at the source to learn how the
module is implemented. Modules written completely in Perl shouldn't
cause problems. Modules that directly or indirectly access code written
in other programming languages are at risk.

For affected functions, the simple strategy to avoid data corruption is
to always make the encoding of the exchanged data explicit. Choose an
encoding that you know the extension can handle. Convert arguments passed
to the extensions to that encoding and convert results back from that
encoding. Write wrapper functions that do the conversions for you, so
you can later change the functions when the extension catches up.

To provide an example, let's say the popular Foo::Bar::escape_html
function doesn't deal with Unicode data yet. The wrapper function
would convert the argument to raw UTF-8 and convert the result back to
Perl's internal representation like so:

    sub my_escape_html ($) {
      my($what) = shift;
      return unless defined $what;
      Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
    }

Sometimes, when the extension does not convert data but just stores
and retrieves them, you will be in a position to use the otherwise
dangerous Encode::_utf8_on() function. Let's say the popular
C<Foo::Bar> extension, written in C, provides a C<param> method that
lets you store and retrieve data according to these prototypes:

    $self->param($name, $value);            # set a scalar
    $value = $self->param($name);           # retrieve a scalar

If it does not yet provide support for any encoding, one could write a
derived class with such a C<param> method:

    sub param {
      my($self,$name,$value) = @_;
      utf8::upgrade($name);     # make sure it is UTF-8 encoded
      if (defined $value) {
        utf8::upgrade($value);  # make sure it is UTF-8 encoded
        return $self->SUPER::param($name,$value);
      } else {
        my $ret = $self->SUPER::param($name);
        Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
        return $ret;
      }
    }

Some extensions provide filters on data entry/exit points, such as
DB_File::filter_store_key and family. Look out for such filters in
the documentation of your extensions, they can make the transition to
Unicode data much easier.

=head2 Speed

Some functions are slower when working on UTF-8 encoded strings than
on byte encoded strings.  All functions that need to hop over
characters such as length(), substr() or index(), or matching regular
expressions can work B<much> faster when the underlying data are
byte-encoded.

In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
a caching scheme was introduced which will hopefully make the slowness
somewhat less spectacular, at least for some operations.  In general,
operations with UTF-8 encoded strings are still slower. As an example,
the Unicode properties (character classes) like C<\p{Nd}> are known to
be quite a bit slower (5-20 times) than their simpler counterparts
like C<\d> (then again, there 268 Unicode characters matching C<Nd>
compared with the 10 ASCII characters matching C<d>).

=head2 Problems on EBCDIC platforms

There are a number of known problems with Perl on EBCDIC platforms.  If you
want to use Perl there, send email to perlbug@perl.org.

In earlier versions, when byte and character data were concatenated,
the new string was sometimes created by
decoding the byte strings as I<ISO 8859-1 (Latin-1)>, even if the
old Unicode string used EBCDIC.

If you find any of these, please report them as bugs.

=head2 Porting code from perl-5.6.X

Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
was required to use the C<utf8> pragma to declare that a given scope
expected to deal with Unicode data and had to make sure that only
Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to
your code. The examples are written such that the code will continue
to work under 5.6, so you should be safe to try them out.

=over 4

=item *

A filehandle that should read or write UTF-8

  if ($] > 5.007) {
    binmode $fh, ":encoding(utf8)";
  }

=item *

A scalar that is going to be passed to some extension

Be it Compress::Zlib, Apache::Request or any extension that has no
mention of Unicode in the manpage, you need to make sure that the
UTF8 flag is stripped off. Note that at the time of this writing
(October 2002) the mentioned modules are not UTF-8-aware. Please
check the documentation to verify if this is still true.

  if ($] > 5.007) {
    require Encode;
    $val = Encode::encode_utf8($val); # make octets
  }

=item *

A scalar we got back from an extension

If you believe the scalar comes back as UTF-8, you will most likely
want the UTF8 flag restored:

  if ($] > 5.007) {
    require Encode;
    $val = Encode::decode_utf8($val);
  }

=item *

Same thing, if you are really sure it is UTF-8

  if ($] > 5.007) {
    require Encode;
    Encode::_utf8_on($val);
  }

=item *

A wrapper for fetchrow_array and fetchrow_hashref

When the database contains only UTF-8, a wrapper function or method is
a convenient way to replace all your fetchrow_array and
fetchrow_hashref calls. A wrapper function will also make it easier to
adapt to future enhancements in your database driver. Note that at the
time of this writing (October 2002), the DBI has no standardized way
to deal with UTF-8 data. Please check the documentation to verify if
that is still true.

  sub fetchrow {
    my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref}
    if ($] < 5.007) {
      return $sth->$what;
    } else {
      require Encode;
      if (wantarray) {
        my @arr = $sth->$what;
        for (@arr) {
          defined && /[^\000-\177]/ && Encode::_utf8_on($_);
        }
        return @arr;
      } else {
        my $ret = $sth->$what;
        if (ref $ret) {
          for my $k (keys %$ret) {
            defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret->{$k};
          }
          return $ret;
        } else {
          defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
          return $ret;
        }
      }
    }
  }


=item *

A large scalar that you know can only contain ASCII

Scalars that contain only ASCII and are marked as UTF-8 are sometimes
a drag to your program. If you recognize such a situation, just remove
the UTF8 flag:

  utf8::downgrade($val) if $] > 5.007;

=back

=head1 SEE ALSO

L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
L<perlretut>, L<perlvar/"${^UNICODE}">
L<http://www.unicode.org/reports/tr44>).

=cut