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PERLUNICODE(1perl)     Perl Programmers Reference Guide	    PERLUNICODE(1perl)

NAME
       perlunicode - Unicode support in Perl

DESCRIPTION
   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 Perl Unicode tutorial, perlunitut and perluniintro, before reading
       this reference document.

       Also, the use of Unicode may present security issues that aren't
       obvious.	 Read Unicode Security Considerations
       <http://www.unicode.org/reports/tr36>.

       Safest if you "use feature 'unicode_strings'"
	   In order to preserve backward compatibility, Perl does not turn on
	   full internal Unicode support unless the pragma "use feature
	   'unicode_strings'" is specified.  (This is automatically selected
	   if you use "use 5.012" or higher.)  Failure to do this can trigger
	   unexpected surprises.  See "The "Unicode Bug"" below.

	   This pragma doesn't affect I/O.  Nor does it change the internal
	   representation of strings, only their interpretation.  There are
	   still several places where Unicode isn't fully supported, such as
	   in filenames.

       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 ":encoding(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 open.

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

       "use utf8" still needed to enable UTF-8/UTF-EBCDIC in scripts
	   As a compatibility measure, the "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.  These are the only times when an
	   explicit "use utf8" is needed.  See utf8.

       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.)

       "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 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 "Byte and Character Semantics" for more details.

   Byte and Character Semantics
       Perl uses logically-wide characters to represent strings internally.

       Starting in Perl 5.14, Perl-level operations work with characters
       rather than bytes within the scope of a "use feature 'unicode_strings'"
       (or equivalently "use 5.012" or higher).	 (This is not true if bytes
       have been explicitly requested by "use bytes", nor necessarily true for
       interactions with the platform's operating system.)

       For earlier Perls, and when "unicode_strings" is not in effect, Perl
       provides a fairly safe environment that can handle both types of
       semantics in 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.

       When "use locale" (but not "use locale ':not_characters'") is in
       effect, Perl uses the semantics associated with the current locale.
       ("use locale" overrides "use feature 'unicode_strings'" in the same
       scope; while "use locale ':not_characters'" effectively also selects
       "use feature 'unicode_strings'" in its scope; see perllocale.)
       Otherwise, Perl uses the platform's native byte semantics for
       characters whose code points are less than 256, and Unicode semantics
       for those greater than 255.  That means that non-ASCII characters 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
       "[:alpha:]" or "\w".  (But all do belong to the "\W" class or the Perl
       regular expression extension "[:^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 "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 utf8.

       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 "BUGS", below.	 You can
       choose to be warned when this happens.  See 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 perluniintro for more.

   Effects of Character Semantics
       Character semantics have the following effects:

       ·   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 "use utf8", the
	   latter requires a BOM.)

	   Unicode characters can also be added to a string by using the
	   "\N{U+...}" notation.  The Unicode code for the desired character,
	   in hexadecimal, should be placed in the braces, after the "U". For
	   instance, a smiley face is "\N{U+263A}".

	   Alternatively, you can use the "\x{...}" notation for characters
	   0x100 and above.  For characters below 0x100 you may get byte
	   semantics instead of character semantics;  see "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, thus it is more portable to use "\N{U+...}" instead.

	   Additionally, you can use the "\N{...}" notation and put the
	   official Unicode character name within the braces, such as
	   "\N{WHITE SMILING FACE}".  This automatically loads the charnames
	   module with the ":full" and ":short" options.  If you prefer
	   different options for this module, you can instead, before the
	   "\N{...}", explicitly load it with your desired options; for
	   example,

	      use charnames ':loose';

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

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

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

       ·   Named Unicode properties, scripts, and block ranges may be used
	   (like bracketed character classes) by using the "\p{}" "matches
	   property" construct and the "\P{}" negation, "doesn't match
	   property".  See "Unicode Character Properties" for more details.

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

       ·   The special pattern "\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 "G", may in
	   fact be composed of a sequence of characters, in this case a "G"
	   followed by an accent character.  "\X" will match the entire
	   sequence.

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

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

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

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

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

       ·   The "chr()" and "ord()" functions work on characters, similar to
	   "pack("W")" and "unpack("W")", not "pack("C")" and "unpack("C")".
	   "pack("C")" and "unpack("C")" are methods for emulating byte-
	   oriented "chr()" and "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.

       ·   The bit string operators, "& | ^ ~", 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 "~" (the bit complement) with characters of both
	   values less than 256 and values greater than 256.  Most
	   importantly, DeMorgan's laws ("~($x|$y) eq ~$x&~$y" and "~($x&$y)
	   eq ~$x|~$y") will not hold.	The reason for this mathematical faux
	   pas is that the complement cannot return both the 8-bit (byte-wide)
	   bit complement and the full character-wide bit complement.

       ·   There is a CPAN module, Unicode::Casing, which allows you to define
	   your own mappings to be used in "lc()", "lcfirst()", "uc()",
	   "ucfirst()", and "fc" (or their double-quoted string inlined
	   versions such as "\U").  (Prior to Perl 5.16, this functionality
	   was partially provided in the Perl core, but suffered from a number
	   of insurmountable drawbacks, so the CPAN module was written
	   instead.)

       ·   And finally, "scalar reverse()" reverses by character rather than
	   by byte.

   Unicode Character Properties
       (The only time that Perl considers a sequence of individual code points
       as a single logical character is in the "\X" construct, already
       mentioned above.	  Therefore "character" in this discussion means a
       single Unicode code point.)

       Very nearly all Unicode character properties are accessible through
       regular expressions by using the "\p{}" "matches property" construct
       and the "\P{}" "doesn't match property" for its negation.

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

       More formally, "\p{Uppercase}" matches any single character whose
       Unicode Uppercase property value is True, and "\P{Uppercase}" matches
       any character whose Uppercase property value is False, and they could
       have been written as "\p{Uppercase=True}" and "\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 "Bidirectional Character Types" below), can take on
       several different values, such as Left, Right, Whitespace, and others.
       To match these, one needs to specify both 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
       "\p{Bidi_Class: Left}".

       All Unicode-defined character properties may be written in these
       compound forms of "\p{property=value}" or "\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 that is more descriptive and hence easier to understand.  Thus the
       "L" and "Letter" properties above are equivalent and can be used
       interchangeably.	 Likewise, "Upper" is a synonym for "Uppercase", and
       we could have written "\p{Uppercase}" equivalently as "\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 perluniprops.

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

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

       Almost all properties are immune to case-insensitive matching.  That
       is, adding a "/i" regular expression modifier does not change what they
       match.  There are two sets that are affected.  The first set is
       "Uppercase_Letter", "Lowercase_Letter", and "Titlecase_Letter", all of
       which match "Cased_Letter" under "/i" matching.	And the second set is
       "Uppercase", "Lowercase", and "Titlecase", all of which match "Cased"
       under "/i" matching.  This set also includes its subsets "PosixUpper"
       and "PosixLower" both of which under "/i" matching match "PosixAlpha".
       (The difference between these sets is that some things, such as Roman
       numerals, come in both upper and lower case so they are "Cased", but
       aren't considered letters, so they aren't "Cased_Letter"s.)

       The result is undefined if you try to match a non-Unicode code point
       (that is, one above 0x10FFFF) against a Unicode property.  Currently, a
       warning is raised, and the match will fail.  In some cases, this is
       counterintuitive, as both these fail:

	chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails.
	chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Fails!

       General_Category

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

       The compound way of writing these is like "\p{General_Category=Number}"
       (short, "\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 "\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
	   Co	       Private_Use
	   Cn	       Unassigned

       Single-letter properties match all characters in any of the two-letter
       sub-properties starting with the same letter.  "LC" and "L&" are
       special: both are aliases for the set consisting of everything matched
       by "Ll", "Lu", and "Lt".

       Bidirectional Character Types

       Because scripts differ in their directionality (Hebrew and Arabic are
       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,
       "\p{Bidi_Class:R}" matches characters that are normally written right
       to left.

       Scripts

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

       The Unicode Script and Script_Extensions properties give what script a
       given character is in.  Either property can be specified with the
       compound form like "\p{Script=Hebrew}" (short: "\p{sc=hebr}"), or
       "\p{Script_Extensions=Javanese}" (short: "\p{scx=java}").  In addition,
       Perl furnishes shortcuts for all "Script" property names.  You can omit
       everything up through the equals (or colon), and simply write
       "\p{Latin}" or "\P{Cyrillic}".  (This is not true for
       "Script_Extensions", which is required to be written in the compound
       form.)

       The difference between these two properties involves characters that
       are used in multiple scripts.  For example the digits '0' through '9'
       are used in many parts of the world.  These are placed in a script
       named "Common".	Other characters are used in just a few scripts.  For
       example, the "KATAKANA-HIRAGANA DOUBLE HYPHEN" is used in both Japanese
       scripts, Katakana and Hiragana, but nowhere else.  The "Script"
       property places all characters that are used in multiple scripts in the
       "Common" script, while the "Script_Extensions" property places those
       that are used in only a few scripts into each of those scripts; while
       still using "Common" for those used in many scripts.  Thus both these
       match:

	"0" =~ /\p{sc=Common}/	   # Matches
	"0" =~ /\p{scx=Common}/	   # Matches

       and only the first of these match:

	"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common}	 # Matches
	"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match

       And only the last two of these match:

	"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana}  # No match
	"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana}  # No match
	"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
	"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches

       "Script_Extensions" is thus an improved "Script", in which there are
       fewer characters in the "Common" script, and correspondingly more in
       other scripts.  It is new in Unicode version 6.0, and its data are
       likely to change significantly in later releases, as things get sorted
       out.

       (Actually, besides "Common", the "Inherited" script, contains
       characters that are used in multiple scripts.  These are modifier
       characters which modify other characters, and inherit the script value
       of the controlling character.  Some of these are used in many scripts,
       and so go into "Inherited" in both "Script" and "Script_Extensions".
       Others are used in just a few scripts, so are in "Inherited" in
       "Script", but not in "Script_Extensions".)

       It is worth stressing that there are several different sets of digits
       in Unicode that are equivalent to 0-9 and are matchable by "\d" in a
       regular expression.  If they are used in a single language only, they
       are in that language's "Script" and "Script_Extension".	If they are
       used in more than one script, they will be in "sc=Common", but only if
       they are used in many scripts should they be in "scx=Common".

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

       Use of "Is" Prefix

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

       Blocks

       In addition to scripts, Unicode also defines 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 as well as several other blocks, 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 the digits 0-9,
       because those digits are shared across many scripts, and hence are in
       the "Common" script.

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

       The "Script" or "Script_Extensions" properties are likely to be the
       ones you want to use when processing natural language; the Block
       property may occasionally be useful in working with the nuts and bolts
       of Unicode.

       Block names are matched in the compound form, like "\p{Block: Arrows}"
       or "\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 "\p{In_Arrows}" or
       "\p{In_Hebrew}".	 For backwards compatibility, the "In" prefix may be
       omitted if there is no naming conflict with a script or any other
       property, and you can even use an "Is" prefix instead in those cases.
       But it is not a good idea to do this, for a couple reasons:

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

       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 "\p{Hebrew}" would have
	   matched the block Hebrew; now it doesn't.

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

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

       Other Properties

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

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

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

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

       "\p{Alnum}"
	   This matches any "\p{Alphabetic}" or "\p{Decimal_Number}"
	   character.

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

       "\p{ASCII}"
	   This matches any of the 128 characters in the US-ASCII character
	   set, which is a subset of Unicode.

       "\p{Assigned}"
	   This matches any assigned code point; that is, any code point whose
	   general category is not Unassigned (or equivalently, not Cn).

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

       "\p{Decomposition_Type: Non_Canonical}"	  (Short: "\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, etc., above, below, to one side
	   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 these can be
	   variously combined 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 "\X" regular
	   expression construct to match such sequences.

	   But Unicode's intent is to unify the existing character set
	   standards and practices, and several 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 the character "LATIN CAPITAL LETTER E" followed by
	   the character "COMBINING ACUTE ACCENT".

	   "LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed"
	   character, and its 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
	   somewhat 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
	   <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".

	   For your convenience, Perl has added the "Non_Canonical"
	   decomposition type to mean any of the several compatibility
	   decompositions.

       "\p{Graph}"
	   Matches any character that is graphic.  Theoretically, this means a
	   character that on a printer would cause ink to be used.

       "\p{HorizSpace}"
	   This is the same as "\h" and "\p{Blank}":  a character that changes
	   the spacing horizontally.

       "\p{In=*}"
	   This is a synonym for "\p{Present_In=*}"

       "\p{PerlSpace}"
	   This is the same as "\s", restricted to ASCII, namely "[ \f\n\r\t]"
	   and starting in Perl v5.18, experimentally, a vertical tab.

	   Mnemonic: Perl's (original) space

       "\p{PerlWord}"
	   This is the same as "\w", restricted to ASCII, namely
	   "[A-Za-z0-9_]"

	   Mnemonic: Perl's (original) word.

       "\p{Posix...}"
	   There are several of these, which are equivalents using the "\p"
	   notation for Posix classes and are described in "POSIX Character
	   Classes" in perlrecharclass.

       "\p{Present_In: *}"    (Short: "\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 1.1 or 4.0; or the "*" can also be "Unassigned".  This
	   property will match the code points whose final disposition has
	   been settled as of the Unicode release given by the version number;
	   "\p{Present_In: Unassigned}" will match those code points whose
	   meaning has yet to be assigned.

	   For example, "U+0041" "LATIN CAPITAL LETTER A" was present in the
	   very first Unicode release available, which is 1.1, so this
	   property is true for all valid "*" versions.	 On the other hand,
	   "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 "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 "U+0041" would match only 1.1; and
	   "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 in which the decision to make them so was made.
	   For example, "U+FDD0" is to be permanently unassigned to a
	   character, and the decision to do that was made in version 3.1, so
	   "\p{Age=3.1}" matches this character, as also does "\p{Present_In:
	   3.1}" and up.

       "\p{Print}"
	   This matches any character that is graphical or blank, except
	   controls.

       "\p{SpacePerl}"
	   This is the same as "\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
	   white space.)

       "\p{Title}" and	"\p{Titlecase}"
	   Under case-sensitive matching, these both match the same code
	   points as "\p{General Category=Titlecase_Letter}" ("\p{gc=lt}").
	   The difference is that under "/i" caseless matching, these match
	   the same as "\p{Cased}", whereas "\p{gc=lt}" matches
	   "\p{Cased_Letter").

       "\p{VertSpace}"
	   This is the same as "\v":  A character that changes the spacing
	   vertically.

       "\p{Word}"
	   This is the same as "\w", including over 100_000 characters beyond
	   ASCII.

       "\p{XPosix...}"
	   There are several of these, which are the standard Posix classes
	   extended to the full Unicode range.	They are described in "POSIX
	   Character Classes" in perlrecharclass.

   User-Defined Character Properties
       You can define your own binary character properties by defining
       subroutines whose names begin with "In" or "Is".	 (The experimental
       feature "(?[ ])" in perlre provides an alternative which allows more
       complex definitions.)  The subroutines can be defined in any package.
       The user-defined properties can be used in the regular expression "\p"
       and "\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 "\p" or "\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.
       However, the subroutines are passed a single parameter, which is 0 if
       case-sensitive matching is in effect and non-zero if caseless matching
       is in effect.  The subroutine may return different values depending on
       the value of the flag, and one set of values will immutably be in
       effect for all case-sensitive matches, and the other set for all case-
       insensitive matches.

       Note that if the regular expression is tainted, then Perl will die
       rather than calling the subroutine, where the name of the subroutine is
       determined by the tainted data.

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

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

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

       ·   Something to include, prefixed by "+": a built-in character
	   property (prefixed by "utf8::") or a fully qualified (including
	   package name) 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.

       ·   Something to exclude, prefixed by "-": an existing character
	   property (prefixed by "utf8::") or a fully qualified (including
	   package name) 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.

       ·   Something to negate, prefixed "!": an existing character property
	   (prefixed by "utf8::") or a fully qualified (including package
	   name) 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.

       ·   Something to intersect with, prefixed by "&": an existing character
	   property (prefixed by "utf8::") or a fully qualified (including
	   package name) 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.

       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 "\p{InKana}" and "\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
	   }

       This will match all non-Unicode code points, since every one of them is
       not in Kana.  You can use intersection to exclude these, if desired, as
       this modified example shows:

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

       &utf8::Any must be the last line in the definition.

       Intersection is used generally for getting the common characters
       matched by two (or more) classes.  It's important to remember not to
       use "&" for the first set; that would be intersecting with nothing,
       resulting in an empty set.

       (Note that official Unicode properties differ from these in that they
       automatically exclude non-Unicode code points and a warning is raised
       if a match is attempted on one of those.)

   User-Defined Case Mappings (for serious hackers only)
       This feature has been removed as of Perl 5.16.  The CPAN module
       Unicode::Casing provides better functionality without the drawbacks
       that this feature had.  If you are using a Perl earlier than 5.16, this
       feature was most fully documented in the 5.14 version of this pod:
       <http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>

   Character Encodings for Input and Output
       See Encode.

   Unicode Regular Expression Support Level
       The following list of Unicode supported features for regular
       expressions describes all features currently directly supported by core
       Perl.  The references to "Level N" and the section numbers refer to the
       Unicode Technical Standard #18, "Unicode Regular Expressions", version
       13, from August 2008.

       ·   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     - experimental  [5]
	    RL1.4   Simple Word Boundaries	     - done	     [6]
	    RL1.5   Simple Loose Matches	     - done	     [7]
	    RL1.6   Line Boundaries		     - MISSING	     [8][9]
	    RL1.7   Supplementary Code Points	     - done	     [10]

	   [1] \x{...}

	   [2] \p{...} \P{...}

	   [3] supports not only minimal list, but all Unicode character
	       properties (see Unicode Character Properties above)

	   [4] \d \D \s \S \w \W \X [:prop:] [:^prop:]

	   [5] The experimental feature in v5.18 "(?[...])" accomplishes this.
	       See "(?[ ])" in perlre.	If you don't want to use an
	       experimental feature, you can use one of the following:

	       ·   Regular expression look-ahead

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

		       [{Block=Greek}-[{UNASSIGNED}]]

		   in Perl can be written as:

		       (?!\p{Unassigned})\p{Block=Greek}
		       (?=\p{Assigned})\p{Block=Greek}

		   But in this particular example, you probably really want

		       \p{Greek}

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

	       ·   CPAN module Unicode::Regex::Set

		   It does implement the full UTS#18 grouping, intersection,
		   union, and removal (subtraction) syntax.

	       ·   "User-Defined Character Properties"

		   '+' for union, '-' for removal (set-difference), '&' for
		   intersection

	   [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, instead of just U+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 (i.e. there is no empty line between \r and
	       \n).  For CRLF, try the ":crlf" layer (see PerlIO).

	   [9] Linebreaking conformant with UAX#14 "Unicode Line Breaking
	       Algorithm" is available through the Unicode::LineBreaking
	       module.

	   [10]
	       UTF-8/UTF-EBDDIC used in Perl allows not only U+10000 to
	       U+10FFFF but also beyond U+10FFFF

       ·   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		    - DONE
	    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] This is covered in Chapter 3.13 (in Unicode 6.0)

       ·   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")

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

       ·   UTF-8

	   UTF-8 is a variable-length (1 to 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 marked by "*" before several of the byte entries
	   above.  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.

	   The original UTF-8 specification allowed up to 6 bytes, to allow
	   encoding of numbers up to 0x7FFF_FFFF.  Perl continues to allow
	   those, and has extended that up to 13 bytes to encode code points
	   up to what can fit in a 64-bit word.	 However, Perl will warn if
	   you output any of these as being non-portable; and under strict
	   UTF-8 input protocols, they are forbidden.

	   The Unicode non-character code points are also disallowed in UTF-8
	   in "open interchange".  See "Non-character code points".

       ·   UTF-EBCDIC

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

       ·   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.

	   Like UTF-8, UTF-16 is a variable-width encoding, but where UTF-8
	   uses 8-bit code units, UTF-16 uses 16-bit code units.  All code
	   points occupy either 2 or 4 bytes in UTF-16: code points
	   "U+0000..U+FFFF" are stored in a single 16-bit unit, and code
	   points "U+10000..U+10FFFF" in two 16-bit units.  The latter case is
	   using surrogates, the first 16-bit unit being the high surrogate,
	   and the second being the low surrogate.

	   Surrogates are code points set aside to encode the
	   "U+10000..U+10FFFF" range of Unicode code points in pairs of 16-bit
	   units.  The high surrogates are the range "U+D800..U+DBFF" and the
	   low surrogates are the range "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);

	   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 "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 "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
	   "U+FFFE" is not supposed to be in input streams, so the sequence of
	   bytes "0xFF 0xFE" is unambiguously "BOM, represented in little-
	   endian format" and cannot be "U+FFFE", represented in big-endian
	   format".

	   Surrogates have no meaning in Unicode outside their use in pairs to
	   represent other code points.	 However, Perl allows them to be
	   represented individually internally, for example by saying
	   "chr(0xD801)", so that all code points, not just those valid for
	   open interchange, are representable.	 Unicode does define semantics
	   for them, such as their General Category is "Cs".  But because
	   their use is somewhat dangerous, Perl will warn (using the warning
	   category "surrogate", which is a sub-category of "utf8") if an
	   attempt is made to do things like take the lower case of one, or
	   match case-insensitively, or to output them.	 (But don't try this
	   on Perls before 5.14.)

       ·   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.	UTF-32 is a fixed-width encoding.  The BOM signatures
	   are "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00 0x00" for LE.

       ·   UCS-2, UCS-4

	   Legacy, fixed-width encodings defined by the ISO 10646 standard.
	   UCS-2 is a 16-bit encoding.	Unlike UTF-16, UCS-2 is not extensible
	   beyond "U+FFFF", because it does not use surrogates.	 UCS-4 is a
	   32-bit encoding, functionally identical to UTF-32 (the difference
	   being that UCS-4 forbids neither surrogates nor code points larger
	   than 0x10_FFFF).

       ·   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.

   Non-character code points
       66 code points are set aside in Unicode as "non-character code points".
       These all have the Unassigned (Cn) General Category, and they never
       will be assigned.  These are never supposed to be in legal Unicode
       input streams, so that code can use them as sentinels that can be mixed
       in with character data, and they always will be distinguishable from
       that data.  To keep them out of Perl input streams, strict UTF-8 should
       be specified, such as by using the layer ":encoding('UTF-8')".  The
       non-character code points are the 32 between U+FDD0 and U+FDEF, and the
       34 code points U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE,
       U+10FFFF.  Some people are under the mistaken impression that these are
       "illegal", but that is not true.	 An application or cooperating set of
       applications can legally use them at will internally; but these code
       points are "illegal for open interchange".  Therefore, Perl will not
       accept these from input streams unless lax rules are being used, and
       will warn (using the warning category "nonchar", which is a sub-
       category of "utf8") if an attempt is made to output them.

   Beyond Unicode code points
       The maximum Unicode code point is U+10FFFF.  But Perl accepts code
       points up to the maximum permissible unsigned number available on the
       platform.  However, Perl will not accept these from input streams
       unless lax rules are being used, and will warn (using the warning
       category "non_unicode", which is a sub-category of "utf8") if an
       attempt is made to operate on or output them.  For example,
       "uc(0x11_0000)" will generate this warning, returning the input
       parameter as its result, as the upper case of every non-Unicode code
       point is the code point itself.

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

       ·   Malformed UTF-8

	   Unfortunately, the original 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 Unicode code points valid for
	   interchange.

       ·   Regular expression pattern matching may surprise you if you're not
	   accustomed to Unicode.  Starting in Perl 5.14, several pattern
	   modifiers are available to control this, called the character set
	   modifiers.  Details are given in "Character set modifiers" in
	   perlre.

       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
       perluniintro), in which case "\w" in regular expressions might start
       behaving differently (unless the "/a" modifier is in effect).  Review
       your code.  Use warnings and the "strict" pragma.

   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 "utfebcdic" pragma or ":utfebcdic" layer;
       rather, "utf8" and ":utf8" are reused to mean the platform's "natural"
       8-bit encoding of Unicode. See perlebcdic for more discussion of the
       issues.

   Locales
       See "Unicode and UTF-8" in perllocale

   When Unicode Does Not Happen
       While Perl does have extensive ways to input and output in Unicode, and
       a few other "entry points" like the @ARGV array (which can sometimes be
       interpreted as UTF-8), there are still 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 "The "Unicode Bug"".  For
       all of these interfaces Perl currently (as of v5.16.0) simply assumes
       byte strings both as arguments and results, or UTF-8 strings if the
       (problematic) "encoding" pragma has been used.

       One reason that Perl does not attempt to resolve the role of Unicode in
       these situations 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 "qx" and "system": how
       well will the "command-line interface" (and which of them?) handle
       Unicode?

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

       ·   %ENV

       ·   glob (aka the <*>)

       ·   open, opendir, sysopen

       ·   qx (aka the backtick operator), system

       ·   readdir, readlink

   The "Unicode Bug"
       The term, "Unicode bug" has been applied to an inconsistency on ASCII
       platforms with the Unicode code points 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, unless "use
       feature 'unicode_strings'" is specified, directly or indirectly.	 (It
       is indirectly specified by a "use v5.12" or higher.)

       In character semantics these upper-Latin1 characters are interpreted as
       Unicode code points, which means they have the same semantics as
       Latin-1 (ISO-8859-1).

       In byte semantics (without "unicode_strings"), 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 "\w" for example, but
       all match "\W".

       Perl 5.12.0 added "unicode_strings" to force character semantics on
       these code points in some circumstances, which fixed portions of the
       bug; Perl 5.14.0 fixed almost all of it; and Perl 5.16.0 fixed the
       remainder (so far as we know, anyway).  The lesson here is to enable
       "unicode_strings" to avoid the headaches described below.

       The old, problematic behavior affects these areas:

       ·   Changing the case of a scalar, that is, using "uc()", "ucfirst()",
	   "lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in double-
	   quotish contexts, such as regular expression substitutions.	Under
	   "unicode_strings" starting in Perl 5.12.0, character semantics are
	   generally used.  See "lc" in perlfunc for details on how this works
	   in combination with various other pragmas.

       ·   Using caseless ("/i") regular expression matching.  Starting in
	   Perl 5.14.0, regular expressions compiled within the scope of
	   "unicode_strings" use character semantics even when executed or
	   compiled into larger regular expressions outside the scope.

       ·   Matching any of several properties in regular expressions, namely
	   "\b", "\B", "\s", "\S", "\w", "\W", and all the Posix character
	   classes except "[[:ascii:]]".  Starting in Perl 5.14.0, regular
	   expressions compiled within the scope of "unicode_strings" use
	   character semantics even when executed or compiled into larger
	   regular expressions outside the scope.

       ·   In "quotemeta" or its inline equivalent "\Q", no code points above
	   127 are quoted in UTF-8 encoded strings, but in byte encoded
	   strings, code points between 128-255 are always quoted.  Starting
	   in Perl 5.16.0, consistent quoting rules are used within the scope
	   of "unicode_strings", as described in "quotemeta" in perlfunc.

       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'
	    no feature 'unicode_strings';
	    $s1 = "\xC2";
	    $s2 = "\x{2660}";
	    for ($s1, $s2, $s1.$s2) {
		print /\w/ || 0;
	    }
	'
	0
	0
	1

       If there's no "\w" in "s1" or in "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.

       For Perls earlier than those described above, or when a string is
       passed to a function outside the subpragma's scope, a workaround is to
       always call "utf8::upgrade($string)", or to use the standard module
       Encode.	 Also, a scalar that has any characters whose ordinal is above
       0x100, or which were specified using either of the "\N{...}" notations,
       will automatically have character semantics.

   Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
       Sometimes (see "When Unicode Does Not Happen" or "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.

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

       ·   "DO_UTF8(sv)" returns true if the "UTF8" flag is on and the bytes
	   pragma is not in effect.  "SvUTF8(sv)" returns true if the "UTF8"
	   flag is on; the bytes pragma is ignored.  The "UTF8" flag being on
	   does 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 "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 "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.

       ·   "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.

       ·   "utf8_to_uvchr_buf(buf, bufend, 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.

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

       ·   "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 except that
	   it does not set the "UTF8" flag.  "sv_utf8_decode()" does the
	   opposite of "sv_utf8_encode()".  Note that none of these are to be
	   used as general-purpose encoding or decoding interfaces: "use
	   Encode" for that.  "sv_utf8_upgrade()" is affected by the encoding
	   pragma but "sv_utf8_downgrade()" is not (since the encoding pragma
	   is designed to be a one-way street).

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

       ·   "is_utf8_char_buf(buf, buf_end)" returns true if the pointer points
	   to a valid UTF-8 character.

       ·   "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.  "UTF8SKIP()" is useful for example for iterating over the
	   characters of a UTF-8 encoded buffer; "UNISKIP()" is useful, for
	   example, in computing the size required for a UTF-8 encoded buffer.

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

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

       ·   "pv_uni_display(dsv, spv, len, pvlim, flags)" and
	   "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 all characters as
	   hexadecimal code points--but with the flags "UNI_DISPLAY_ISPRINT",
	   "UNI_DISPLAY_BACKSLASH", and "UNI_DISPLAY_QQ" you can make the
	   output more readable.

       ·   "foldEQ_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 "memEQ()" and "memNE()" as
	   usual, except if one string is in utf8 and the other isn't.

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

   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 desired version of Unicode from the Unicode
       web site <http://www.unicode.org>).  These should replace the existing
       files in lib/unicore in the Perl source tree.  Follow the instructions
       in README.perl in that directory to change some of their names, and
       then build perl (see INSTALL).

BUGS
   Interaction with Locales
       See "Unicode and UTF-8" in perllocale

   Problems with characters in the Latin-1 Supplement range
       See "The "Unicode Bug""

   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 recognize that 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 able to use the otherwise dangerous
       Encode::_utf8_on() function. Let's say the popular "Foo::Bar"
       extension, written in C, provides a "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 "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.

   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 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 "\p{Nd}" are known to
       be quite a bit slower (5-20 times) than their simpler counterparts like
       "\d" (then again, there are hundreds of Unicode characters matching
       "Nd" compared with the 10 ASCII characters matching "d").

   Problems on EBCDIC platforms
       There are several 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
       ISO 8859-1 (Latin-1), even if the old Unicode string used EBCDIC.

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

   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 "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.

       ·  A filehandle that should read or write UTF-8

	    if ($] > 5.008) {
	      binmode $fh, ":encoding(utf8)";
	    }

       ·  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
	  (January 2012) the mentioned modules are not UTF-8-aware. Please
	  check the documentation to verify if this is still true.

	    if ($] > 5.008) {
	      require Encode;
	      $val = Encode::encode_utf8($val); # make octets
	    }

       ·  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 ($] > 5.008) {
	      require Encode;
	      $val = Encode::decode_utf8($val);
	    }

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

	    if ($] > 5.008) {
	      require Encode;
	      Encode::_utf8_on($val);
	    }

       ·  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 (January 2012), 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 {
	      # $what is one of fetchrow_{array,hashref}
	      my($self, $sth, $what) = @_;
	      if ($] < 5.008) {
		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;
		  }
		}
	      }
	    }

       ·  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 $] > 5.008;

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

perl v5.18.2			  2014-01-06		    PERLUNICODE(1perl)
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