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+=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
+the L<Perl Unicode tutorial, perlunitut|perlunitut>, before reading
+this reference document.
+
+Also, the use of Unicode may present security issues that aren't obvious.
+Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
+
+=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 character (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.
+You can choose to be warned when this happens. See L<encoding::warnings>.
+
+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<\N{U+...}>
+notation. The Unicode code for the desired character, in hexadecimal,
+should be placed in the braces, after the C<U>. For instance, a smiley face is
+C<\N{U+263A}>.
+
+Alternatively, you can use the C<\x{...}> notation for characters 0x100 and
+above. For characters below 0x100 you may get byte semantics instead of
+character semantics; see L</The "Unicode Bug">. On EBCDIC machines there is
+the additional problem that the value for such characters gives the EBCDIC
+character rather than the Unicode one.
+
+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}>.
+See L<charnames>.
+
+=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 *
+
+Bracketed 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 bracketed
+character classes) by using 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 C<lc()>,
+C<lcfirst()>, C<uc()>, and C<ucfirst()> (or their double-quoted string inlined
+versions such as C<\U>).
+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 bracketed character classes) by using the C<\p{}> "matches
+property" construct and the C<\P{}> negation, "doesn't match property".
+
+Note that the only time that Perl considers a sequence of individual code
+points as a single logical character is in the C<\X> construct, already
+mentioned above. Therefore "character" in this discussion means a single
+Unicode code point.
+
+For instance, C<\p{Uppercase}> matches any single 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 property names, so C<\p{L}> is equivalent to C<\pL>.
+
+More formally, C<\p{Uppercase}> matches any single 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, 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 both aliases for the set consisting of everything matched by 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 the property can be specified 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", 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, 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 blank, except controls.
+
+=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 the 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 ordinals 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($string), or to
+use the standard module L<Encode>. Also, a scalar that has any characters
+whose ordinal is above 0x100, or which were specified using either of the
+C<\N{...}> notations will automatically have character semantics.
+
+=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
+
+Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
+there are situations where you simply need to force a byte
+string into UTF-8, or vice versa. The low-level calls
+utf8::upgrade($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