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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>).