perlunicode.pod

[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.

=over 4

=item Input and Output Disciplines

A filehandle can be marked as containing perl's internal Unicode
encoding (UTF-8 or UTF-EBCDIC) by opening it 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 mark the Perl source itself as being in a particular encoding,
see L<encoding>.

=item Regular Expressions

The regular expression compiler produces polymorphic opcodes.  That is,
the pattern adapts to the data and automatically switch to the Unicode
character scheme when presented with Unicode data, or a traditional
byte scheme when presented with byte data.

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

The C<utf8> pragma implements the tables used for Unicode support.
However, these tables are automatically loaded on demand, so the
C<utf8> pragma should not normally be used.

As a compatibility measure, this pragma must be explicitly used to
enable recognition of UTF-8 in the Perl scripts themselves on ASCII
based machines or recognize UTF-EBCDIC on EBCDIC based machines.
B<NOTE: this should be the only place where an explicit C<use utf8>
is needed>.

You can also use the C<encoding> pragma to change the default encoding
of the data in your script; see L<encoding>.

=back

=head2 Byte and Character semantics

Beginning with version 5.6, Perl uses logically wide characters to
represent strings internally.  This internal representation of strings
uses either the UTF-8 or the UTF-EBCDIC encoding.

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

However, as strictly 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 is characters, Perl now 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.

This behavior preserves compatibility with earlier versions of Perl,
which allowed byte semantics in Perl operations, but only as long as
none of the program's inputs are marked as being as 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.

On Windows platforms, if the C<-C> command line switch is used, (or the
${^WIDE_SYSTEM_CALLS} global flag is set to C<1>), all system calls
will use the corresponding wide character APIs.  Note that this is
currently only implemented on Windows since other platforms lack an
API standard on this area.

Regardless of the above, the C<bytes> pragma can always be used to
force byte semantics in a particular lexical scope.  See L<bytes>.

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 until a future version of Perl
in which character semantics will become the default.  This pragma may
then become a no-op.  See L<utf8>.

Unless mentioned otherwise, Perl operators will use character semantics
when they are dealing with Unicode data, and byte semantics otherwise.
Thus, character semantics for these operations apply transparently; if
the input data came from a Unicode source (for example, by adding a
character encoding discipline to the filehandle whence it came, or a
literal UTF-8 string constant in the program), character semantics
apply; otherwise, byte semantics are in effect.  To force byte semantics
on Unicode data, the C<bytes> pragma should be used.

Notice that if you concatenate strings with byte semantics and strings
with Unicode character data, the bytes will by default be upgraded
I<as if they were ISO 8859-1 (Latin-1)> (or if in EBCDIC, after a
translation to ISO 8859-1).  To change this, use the C<encoding>
pragma, see L<encoding>.

Under character semantics, many operations that formerly operated on
bytes change to operating on characters.  For ASCII data this makes no
difference, because UTF-8 stores ASCII in single bytes, but for any
character greater than C<chr(127)>, the character B<may> be stored in
a sequence of two or more bytes, all of which have the high bit set.

For C1 controls or Latin 1 characters on an EBCDIC platform the
character may be stored in a UTF-EBCDIC multi byte sequence.  But by
and large, the user need not worry about this, because Perl hides it
from the user.  A character in Perl is logically just a number ranging
from 0 to 2**32 or so.  Larger characters encode to longer sequences
of bytes internally, but again, this is just an internal detail which
is hidden at the Perl level.

=head2 Effects of character semantics

Character semantics have the following effects:

=over 4

=item *

Strings and patterns may contain characters that have an ordinal value
larger than 255.

Presuming you use a Unicode editor to edit your program, such
characters will typically occur directly within the literal strings as
UTF-8 (or UTF-EBCDIC on EBCDIC platforms) characters, but you can also
specify a particular character with an extension of the C<\x>
notation.  UTF-X characters are specified by putting the hexadecimal
code within curlies after the C<\x>.  For instance, a Unicode smiley
face is C<\x{263A}>.

=item *

Identifiers within the Perl script may contain Unicode alphanumeric
characters, including ideographs.  (You are currently on your own when
it comes to using the canonical forms of characters--Perl doesn't
(yet) attempt to canonicalize variable names for you.)

=item *

Regular expressions match characters instead of bytes.  For instance,
"." matches a character instead of a byte.  (However, the C<\C> pattern
is provided to force a match a single byte ("C<char>" in C, hence C<\C>).)

=item *

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

=item *

Named Unicode properties and block ranges may be used as character
classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't
match property) constructs.  For instance, C<\p{Lu}> matches any
character with the Unicode uppercase property, while C<\p{M}> matches
any mark character.  Single letter properties may omit the brackets,
so that can be written C<\pM> also.  Many predefined character classes
are available, such as C<\p{IsMirrored}> and C<\p{InTibetan}>.

The C<\p{Is...}> test for "general properties" such as "letter",
"digit", while the C<\p{In...}> test for Unicode scripts and blocks.

The official Unicode script and block names have spaces and dashes as
separators, but for convenience you can have dashes, spaces, and
underbars at every word division, and you need not care about correct
casing.  It is recommended, however, that for consistency you use the
following naming: the official Unicode script, block, or property name
(see below for the additional rules that apply to block names),
with whitespace and dashes replaced with underbar, and the words
"uppercase-first-lowercase-rest".  That is, "Latin-1 Supplement"
becomes "Latin_1_Supplement".

You can also negate both C<\p{}> and C<\P{}> by introducing a caret
(^) between the first curly and the property name: C<\p{^In_Tamil}> is
equal to C<\P{In_Tamil}>.

The C<In> and C<Is> can be left out: C<\p{Greek}> is equal to
C<\p{In_Greek}>, C<\P{Pd}> is equal to C<\P{Pd}>.

    Short       Long

    L           Letter
    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
    Nl          Letter_Number
    No          Other_Number

    P           Punctuation
    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
    Cf          Format
    Cs          Surrogate
    Co          Private_Use
    Cn          Unassigned

There's also C<L&> which is an alias for C<Ll>, C<Lu>, and C<Lt>.

The following reserved ranges have C<In> tests:

    CJK_Ideograph_Extension_A
    CJK_Ideograph
    Hangul_Syllable
    Non_Private_Use_High_Surrogate
    Private_Use_High_Surrogate
    Low_Surrogate
    Private_Surrogate
    CJK_Ideograph_Extension_B
    Plane_15_Private_Use
    Plane_16_Private_Use

For example C<"\x{AC00}" =~ \p{HangulSyllable}> will test true.
(Handling of surrogates is not implemented yet, because Perl
uses UTF-8 and not UTF-16 internally to represent Unicode.
So you really can't use the "Cs" category.)

Additionally, because scripts differ in their directionality
(for example Hebrew is written right to left), all characters
have their directionality defined:

    BidiL       Left-to-Right
    BidiLRE     Left-to-Right Embedding
    BidiLRO     Left-to-Right Override
    BidiR       Right-to-Left
    BidiAL      Right-to-Left Arabic
    BidiRLE     Right-to-Left Embedding
    BidiRLO     Right-to-Left Override
    BidiPDF     Pop Directional Format
    BidiEN      European Number
    BidiES      European Number Separator
    BidiET      European Number Terminator
    BidiAN      Arabic Number
    BidiCS      Common Number Separator
    BidiNSM     Non-Spacing Mark
    BidiBN      Boundary Neutral
    BidiB       Paragraph Separator
    BidiS       Segment Separator
    BidiWS      Whitespace
    BidiON      Other Neutrals

=back

=head2 Scripts

The scripts available for C<\p{In...}> and C<\P{In...}>, for example
C<\p{InLatin}> or \p{InCyrillic>, are as follows:

    Arabic
    Armenian
    Bengali
    Bopomofo
    Canadian-Aboriginal
    Cherokee
    Cyrillic
    Deseret
    Devanagari
    Ethiopic
    Georgian
    Gothic
    Greek
    Gujarati
    Gurmukhi
    Han
    Hangul
    Hebrew
    Hiragana
    Inherited
    Kannada
    Katakana
    Khmer
    Lao
    Latin
    Malayalam
    Mongolian
    Myanmar
    Ogham
    Old-Italic
    Oriya
    Runic
    Sinhala
    Syriac
    Tamil
    Telugu
    Thaana
    Thai
    Tibetan
    Yi

There are also extended property classes that supplement the basic
properties, defined by the F<PropList> Unicode database:

    ASCII_Hex_Digit
    Bidi_Control
    Dash
    Diacritic
    Extender
    Hex_Digit
    Hyphen
    Ideographic
    Join_Control
    Noncharacter_Code_Point
    Other_Alphabetic
    Other_Lowercase
    Other_Math
    Other_Uppercase
    Quotation_Mark
    White_Space

and further derived properties:

    Alphabetic      Lu + Ll + Lt + Lm + Lo + Other_Alphabetic
    Lowercase       Ll + Other_Lowercase
    Uppercase       Lu + Other_Uppercase
    Math            Sm + Other_Math

    ID_Start        Lu + Ll + Lt + Lm + Lo + Nl
    ID_Continue     ID_Start + Mn + Mc + Nd + Pc

    Any             Any character
    Assigned        Any non-Cn character
    Common          Any character (or unassigned code point)
                    not explicitly assigned to a script

=head2 Blocks

In addition to B<scripts>, Unicode also defines B<blocks> of
characters.  The difference between scripts and blocks is that the
scripts concept is closer to natural languages, while the blocks
concept is more an artificial grouping based on groups of 256 Unicode
characters.  For example, the C<Latin> script contains letters from
many blocks.  On the other hand, the C<Latin> script 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
other similar groups, like punctuation, are in a category called
C<Common>.

For more about scripts, see the UTR #24:

   http://www.unicode.org/unicode/reports/tr24/

For more about blocks, see:

   http://www.unicode.org/Public/UNIDATA/Blocks.txt

Because there are overlaps in naming (there are, for example, both
a script called C<Katakana> and a block called C<Katakana>, the block
version has C<Block> appended to its name, C<\p{InKatakanaBlock}>.

Notice that this definition was introduced in Perl 5.8.0: in Perl
5.6 only the blocks were used; in Perl 5.8.0 scripts became the
preferential Unicode character class definition; this meant that
the definitions of some character classes changed (the ones in the
below list that have the C<Block> appended).

   Alphabetic Presentation Forms
   Arabic Block
   Arabic Presentation Forms-A
   Arabic Presentation Forms-B
   Armenian Block
   Arrows
   Basic Latin
   Bengali Block
   Block Elements
   Bopomofo Block
   Bopomofo Extended
   Box Drawing
   Braille Patterns
   Byzantine Musical Symbols
   CJK Compatibility
   CJK Compatibility Forms
   CJK Compatibility Ideographs
   CJK Compatibility Ideographs Supplement
   CJK Radicals Supplement
   CJK Symbols and Punctuation
   CJK Unified Ideographs
   CJK Unified Ideographs Extension A
   CJK Unified Ideographs Extension B
   Cherokee Block
   Combining Diacritical Marks
   Combining Half Marks
   Combining Marks for Symbols
   Control Pictures
   Currency Symbols
   Cyrillic Block
   Deseret Block
   Devanagari Block
   Dingbats
   Enclosed Alphanumerics
   Enclosed CJK Letters and Months
   Ethiopic Block
   General Punctuation
   Geometric Shapes
   Georgian Block
   Gothic Block
   Greek Block
   Greek Extended
   Gujarati Block
   Gurmukhi Block
   Halfwidth and Fullwidth Forms
   Hangul Compatibility Jamo
   Hangul Jamo
   Hangul Syllables
   Hebrew Block
   High Private Use Surrogates
   High Surrogates
   Hiragana Block
   IPA Extensions
   Ideographic Description Characters
   Kanbun
   Kangxi Radicals
   Kannada Block
   Katakana Block
   Khmer Block
   Lao Block
   Latin 1 Supplement
   Latin Extended Additional
   Latin Extended-A
   Latin Extended-B
   Letterlike Symbols
   Low Surrogates
   Malayalam Block
   Mathematical Alphanumeric Symbols
   Mathematical Operators
   Miscellaneous Symbols
   Miscellaneous Technical
   Mongolian Block
   Musical Symbols
   Myanmar Block
   Number Forms
   Ogham Block
   Old Italic Block
   Optical Character Recognition
   Oriya Block
   Private Use
   Runic Block
   Sinhala Block
   Small Form Variants
   Spacing Modifier Letters
   Specials
   Superscripts and Subscripts
   Syriac Block
   Tags
   Tamil Block
   Telugu Block
   Thaana Block
   Thai Block
   Tibetan Block
   Unified Canadian Aboriginal Syllabics
   Yi Radicals
   Yi Syllables

=over 4

=item *

The special pattern C<\X> match matches any extended Unicode sequence
(a "combining character sequence" in Standardese), where the first
character is a base character and subsequent characters are mark
characters that apply to the base character.  It is equivalent to
C<(?:\PM\pM*)>.

=item *

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

=item *

Case translation operators use the Unicode case translation tables
when provided character input.  Note that C<uc()> (also known as C<\U>
in doublequoted strings) translates to uppercase, while C<ucfirst>
(also known as C<\u> in doublequoted strings) translates to titlecase
(for languages that make the distinction).  Naturally the
corresponding backslash sequences have the same semantics.

=item *

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

=item *

The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change,
since they're often used for byte-oriented formats.  (Again, think
"C<char>" in the C language.)  However, there is a new "C<U>" specifier
that will convert between UTF-8 characters and integers.  (It works
outside of the utf8 pragma too.)

=item *

The C<chr()> and C<ord()> functions work on characters.  This is like
C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and
C<unpack("C")>.  In fact, the latter are how you now emulate
byte-oriented C<chr()> and C<ord()> for Unicode strings.
(Note that this reveals the internal UTF-8 encoding of strings and
you are not supposed to do that unless you know what you are doing.)

=item *

The bit string operators C<& | ^ ~> can operate on character data.
However, for backward compatibility reasons (bit string operations
when the characters all are less than 256 in ordinal value) one should
not mix C<~> (the bit complement) and characters both less than 256 and
equal or greater than 256.  Most importantly, the DeMorgan's laws
(C<~($x|$y) eq ~$x&~$y>, C<~($x&$y) eq ~$x|~$y>) won't hold.
Another way to look at this is that the complement cannot return
B<both> the 8-bit (byte) wide bit complement B<and> the full character
wide bit complement.

=item *

lc(), uc(), lcfirst(), and ucfirst() work for the following cases:

=over 8

=item *

the case mapping is from a single Unicode character to another
single Unicode character

=item *

the case mapping is from a single Unicode character to more
than one Unicode character

=back

What doesn't yet work are the following cases:

=over 8

=item *

the "final sigma" (Greek)

=item *

anything to with locales (Lithuanian, Turkish, Azeri)

=back

See the Unicode Technical Report #21, Case Mappings, for more details.

=item *

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

=back

=head2 Character encodings for input and output

See L<Encode>.

=head1 CAVEATS

As of yet, there is no method for automatically coercing input and
output to some encoding other than UTF-8 or UTF-EBCDIC.  This is planned 
in the near future, however.

Whether an arbitrary piece of data will be treated as "characters" or
"bytes" by internal operations cannot be divined at the current time.

Use of locales with utf8 may lead to odd results.  Currently there is
some attempt to apply 8-bit locale info to characters in the range
0..255, but this is demonstrably incorrect for locales that use
characters above that range (when mapped into Unicode).  It will also
tend to run slower.  Avoidance of locales is strongly encouraged.

=head1 UNICODE REGULAR EXPRESSION SUPPORT LEVEL

The following list of Unicode regular expression support describes
feature by feature the Unicode support implemented in Perl as of Perl
5.8.0.  The "Level N" and the section numbers refer to the Unicode
Technical Report 18, "Unicode Regular Expression Guidelines".

=over 4

=item *

Level 1 - Basic Unicode Support

        2.1 Hex Notation                        - done          [1]
                Named Notation                  - done          [2]
        2.2 Categories                          - done          [3][4]
        2.3 Subtraction                         - MISSING       [5][6]
        2.4 Simple Word Boundaries              - done          [7]
        2.5 Simple Loose Matches                - done          [8]
        2.6 End of Line                         - MISSING       [9][10]

        [ 1] \x{...}
        [ 2] \N{...}
        [ 3] . \p{Is...} \P{Is...}
        [ 4] now scripts (see UTR#24 Script Names) in addition to blocks
        [ 5] have negation
        [ 6] can use look-ahead to emulate subtraction (*)
        [ 7] include Letters in word characters
        [ 8] see UTR#21 Case Mappings: Perl implements 1:1 mappings
        [ 9] see UTR#13 Unicode Newline Guidelines
        [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029})
             (should also affect <>, $., and script line numbers)
  
(*) Instead of [\u0370-\u03FF-[{UNASSIGNED}]] as suggested by the TR
18 you can use negated lookahead: to match currently assigned modern
Greek characters use for example

		/(?!\p{Cn})[\x{0370}-\x{03ff}]/

In other words: the matched character must not be a non-assigned
character, but it must be in the block of modern Greek characters.

=item *

Level 2 - Extended Unicode Support

        3.1 Surrogates                          - MISSING
        3.2 Canonical Equivalents               - MISSING       [11][12]
        3.3 Locale-Independent Graphemes        - MISSING       [13]
        3.4 Locale-Independent Words            - MISSING       [14]
        3.5 Locale-Independent Loose Matches    - MISSING       [15]

        [11] see UTR#15 Unicode Normalization
        [12] have Unicode::Normalize but not integrated to regexes
        [13] have \X but at this level . should equal that
        [14] need three classes, not just \w and \W
        [15] see UTR#21 Case Mappings

=item *

Level 3 - Locale-Sensitive Support

        4.1 Locale-Dependent Categories         - MISSING
        4.2 Locale-Dependent Graphemes          - MISSING       [16][17]
        4.3 Locale-Dependent Words              - MISSING
        4.4 Locale-Dependent Loose Matches      - MISSING
        4.5 Locale-Dependent Ranges             - MISSING

        [16] see UTR#10 Unicode Collation Algorithms
        [17] have Unicode::Collate but not integrated to regexes

=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 the encoding used internally by Perl.  UTF-8 is a variable
length (1 to 6 bytes, current character allocations require 4 bytes), 
byteorder independent encoding.  For ASCII, UTF-8 is transparent
(and we really do mean 7-bit ASCII, not any 8-bit encoding).

The following table is from Unicode 3.1.

 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+FFFF       E1..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

Or, another way to look at it, as 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 C<10>, and the
leading bits of the start byte tells how many bytes the are in the
encoded character.

=item UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)

UTF-16 is a 2 or 4 byte encoding.  The Unicode code points
0x0000..0xFFFF are stored in two 16-bit units, and the code points
0x010000..0x10FFFF in four 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 0x01000..0x10FFFF
range of Unicode code points in pairs of 16-bit units.  The I<high
surrogates> are the range 0xD800..0xDBFF, and the I<low surrogates>
are the range 0xDC00..0xDFFFF.  The surrogate encoding is

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

and the decoding is

	$uni = 0x10000 + ($hi - 0xD8000) * 0x400 + ($lo - 0xDC00);

If you try to generate surrogates (for example by using chr()), you
will get an error because firstly a surrogate on its own is
meaningless, and secondly because Perl encodes its Unicode characters
in UTF-8 (not 16-bit numbers), which makes the encoded character doubly
illegal.

Because of the 16-bitness, UTF-16 is byteorder 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) 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
(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 0xFEFF 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 0xFE 0xFF, but if it was written on a little endian platform,
you will read the bytes 0xFF 0xFE.  (And if the originating platform
was writing in UTF-8, you will read the bytes 0xEF 0xBB 0xBF.)

The way this trick works is that the character with the code point
0xFFFE is guaranteed not to be a valid Unicode character, so the
sequence of bytes 0xFF 0xFE is unambiguously "BOM, represented in
little-endian format" and cannot be "0xFFFE, represented in big-endian
format".

=item UTF-32, UTF-32BE, UTF32-LE

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 0x00 0x00 0xFE 0xFF for BE and
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, UCS-4 is a 32-bit encoding.  Unlike UTF-16, UCS-2
is not extensible beyond 0xFFFF, because it does not use surrogates.

=item UTF-7

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

=back

=head2 Security Implications of 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, one is supposed
to always generate the shortest possible sequence of UTF-8 bytes,
because otherwise there is potential for input buffer overflow at the
receiving end of a UTF-8 connection.  Perl always generates the shortest
length UTF-8, and with warnings on (C<-w> or C<use warnings;>) Perl will
warn about non-shortest length UTF-8 (and other malformations, too,
such as the surrogates, which are not real character code points.)

=head2 Unicode in Perl on EBCDIC

The way Unicode is handled on EBCDIC platforms is still rather
experimental.  On such a platform, references to UTF-8 encoding in this
document and elsewhere should be read as meaning UTF-EBCDIC as
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 re-used to mean
the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
for more discussion of the issues.

=head2 Using Unicode in XS

If you want to handle Perl Unicode in XS extensions, you may find
the following C APIs useful:

=over 4

=item *

DO_UTF8(sv) returns true if the UTF8 flag is on and the bytes
pragma is not in effect.  SvUTF8(sv) returns true is the UTF8
flag is on, the bytes pragma is ignored.  Remember that UTF8
flag being on does not mean that there would be any characters
of code points greater than 255 or 127 in the scalar, or that
there even are any characters in the scalar.  The UTF8 flag
means that any characters added to the string will be encoded
in UTF8 if the code points of the characters are greater than
255.  Not "if greater than 127", since Perl's Unicode model
is not to use UTF-8 until it's really necessary.

=item *

uvuni_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.

=item *

utf8_to_uvuni(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).

=item *

utf8_length(s, len) returns the length of the UTF-8 encoded buffer in
characters.  sv_len_utf8(sv) returns the length of the UTF-8 encoded
scalar.

=item *

sv_utf8_upgrade(sv) converts the string of the scalar to its UTF-8
encoded form.  sv_utf8_downgrade(sv) does the opposite (if possible).
sv_utf8_encode(sv) is like sv_utf8_upgrade but the UTF8 flag does not
get turned on.  sv_utf8_decode() does the opposite of sv_utf8_encode().

=item *

is_utf8_char(buf) returns true if the buffer points to valid UTF-8.

=item *

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

=item *

UTF8SKIP(buf) will return the number of bytes in the UTF-8 encoded
character in the buffer.  UNISKIP(chr) will return the number of bytes
required to UTF-8-encode the Unicode character code point.

=item *

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

=item *

utf8_hop(s, off) will return a pointer to an UTF-8 encoded buffer that
is C<off> (positive or negative) Unicode characters displaced from the
UTF-8 buffer C<s>.

=item *

pv_uni_display(dsv, spv, len, pvlim, flags) and sv_uni_display(dsv,
ssv, pvlim, flags) are useful for debug output of Unicode strings and
scalars (only for debug: they display B<all> characters as hexadecimal
code points).

=item *

ibcmp_utf8(s1, u1, len1, s2, u2, len2) can be used to compare two
strings case-insensitively in Unicode.  (For case-sensitive
comparisons you can just use memEQ() and memNE() as usual.)

=back

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

=head1 SEE ALSO

L<perluniintro>, L<encoding>, L<Encode>, L<open>, L<utf8>, L<bytes>,
L<perlretut>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">

=cut