perlunicode


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

       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:

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

           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';

       o   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.

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

       o   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.

       o   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.

       o   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.

       o   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', ...).

       o   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).

           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.

       o   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.

       o   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.

       o   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.)

       o   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.
       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
       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

           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
        "\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

       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
           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

           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
           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:

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

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

       o   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.

       o   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
           &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.

       o   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
                       (?=\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.

               o   CPAN module Unicode::Regex::Set

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

               o   "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

       o   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"
                 ( 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.

       o   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

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

       o   UTF-EBCDIC

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

       o   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-
           match case-insensitively, or to output them.  (But don't try this
           on Perls before 5.14.)

       o   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.

       o   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).

       o   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
           about non-shortest length UTF-8 along with other malformations,
           such as the surrogates, which are not Unicode code points valid for
           interchange.

       o   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?

       o   chdir, chmod, chown, chroot, exec, link, lstat, mkdir, rename,

   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:

       o   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.

       o   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.

       o   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.

       o   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
        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.

       o   "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.

       o   "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
           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).

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

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

       o   "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.

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

       o   "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.

       o   "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.

       o   "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
       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;

       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.

       o  A filehandle that should read or write UTF-8

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

       o  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
            }

       o  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:

       o  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;
                  }
                }
              }
            }

       o  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>).
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