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NAME

       perlunicode - Unicode support in Perl

DESCRIPTION

       If you haven't already, before reading this document, you should become familiar with both
       perlunitut and perluniintro.

       Unicode aims to UNI-fy the en-CODE-ings of all the world's character sets into a single
       Standard.   For quite a few of the various coding standards that existed when Unicode was
       first created, converting from each to Unicode essentially meant adding a constant to each
       code point in the original standard, and converting back meant just subtracting that same
       constant.  For ASCII and ISO-8859-1, the constant is 0.  For ISO-8859-5, (Cyrillic) the
       constant is 864; for Hebrew (ISO-8859-8), it's 1488; Thai (ISO-8859-11), 3424; and so
       forth.  This made it easy to do the conversions, and facilitated the adoption of Unicode.

       And it worked; nowadays, those legacy standards are rarely used.  Most everyone uses
       Unicode.

       Unicode is a comprehensive standard.  It specifies many things outside the scope of Perl,
       such as how to display sequences of characters.  For a full discussion of all aspects of
       Unicode, see <http://www.unicode.org>.

   Important Caveats
       Even though some of this section may not be understandable to you on first reading, we
       think it's important enough to highlight some of the gotchas before delving further, so
       here goes:

       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.

       Also, the use of Unicode may present security issues that aren't obvious, see "Security
       Implications of Unicode".

       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 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
           Use the ":encoding(...)" layer  to read from and write to filehandles using the
           specified encoding.  (See open.)

       You should convert your non-ASCII, non-UTF-8 Perl scripts to be UTF-8.
           See encoding.

       "use utf8" still needed to enable UTF-8 in scripts
           If your Perl script is itself encoded in UTF-8, the "use utf8" pragma must be
           explicitly included to enable recognition of that (in string or regular expression
           literals, or in identifier names).  This is the only time when an explicit "use utf8"
           is needed.  (See utf8).

           If a Perl script begins with the bytes that form the UTF-8 encoding of the Unicode
           BYTE ORDER MARK ("BOM", see "Unicode Encodings"), those bytes are completely ignored.

       UTF-16 scripts autodetected
           If a Perl script begins with the Unicode "BOM" (UTF-16LE, UTF16-BE), or if the script
           looks like non-"BOM"-marked UTF-16 of either endianness, Perl will correctly read in
           the script as the appropriate Unicode encoding.

   Byte and Character Semantics
       Before Unicode, most encodings used 8 bits (a single byte) to encode each character.  Thus
       a character was a byte, and a byte was a character, and there could be only 256 or fewer
       possible characters.  "Byte Semantics" in the title of this section refers to this
       behavior.  There was no need to distinguish between "Byte" and "Character".

       Then along comes Unicode which has room for over a million characters (and Perl allows for
       even more).  This means that a character may require more than a single byte to represent
       it, and so the two terms are no longer equivalent.  What matter are the characters as
       whole entities, and not usually the bytes that comprise them.  That's what the term
       "Character Semantics" in the title of this section refers to.

       Perl had to change internally to decouple "bytes" from "characters".  It is important that
       you too change your ideas, if you haven't already, so that "byte" and "character" no
       longer mean the same thing in your mind.

       The basic building block of Perl strings has always been a "character".  The changes
       basically come down to that the implementation no longer thinks that a character is always
       just a single byte.

       There are various things to note:

       ·   String handling functions, for the most part, continue to operate in terms of
           characters.  "length()", for example, returns the number of characters in a string,
           just as before.  But that number no longer is necessarily the same as the number of
           bytes in the string (there may be more bytes than characters).  The other such
           functions include "chop()", "chomp()", "substr()", "pos()", "index()", "rindex()",
           "sort()", "sprintf()", and "write()".

           The exceptions are:

           ·   the bit-oriented "vec"

                

           ·   the byte-oriented "pack"/"unpack" "C" format

               However, the "W" specifier does operate on whole characters, as does the "U"
               specifier.

           ·   some operators that interact with the platform's operating system

               Operators dealing with filenames are examples.

           ·   when the functions are called from within the scope of the "use bytes" pragma

               Likely, you should use this only for debugging anyway.

       ·   Strings--including hash keys--and regular expression patterns may contain characters
           that have ordinal values 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 "use utf8", the latter may require a "BOM".)

           "Creating Unicode" in perluniintro gives other ways to place non-ASCII characters in
           your strings.

       ·   The "chr()" and "ord()" functions work on whole characters.

       ·   Regular expressions match whole characters.  For example, "." matches a whole
           character instead of only a single byte.

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

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

       ·   The bit string operators, "& | ^ ~" and (starting in v5.22) "&. |. ^.  ~." can operate
           on characters that don't fit into a byte.  However, the current behavior is likely to
           change.  You should not use these operators on strings that are encoded in UTF-8.  If
           you're not sure about the encoding of a string, downgrade it before using any of these
           operators; you can use "utf8::utf8_downgrade()".

       The bottom line is that Perl has always practiced "Character Semantics", but with the
       advent of Unicode, that is now different than "Byte Semantics".

   ASCII Rules versus Unicode Rules
       Before Unicode, when a character was a byte was a character, Perl knew only about the 128
       characters defined by ASCII, code points 0 through 127 (except for under "use locale").
       That left the code points 128 to 255 as unassigned, and available for whatever use a
       program might want.  The only semantics they have is their ordinal numbers, and that they
       are members of none of the non-negative character classes.  None are considered to match
       "\w" for example, but all match "\W".

       Unicode, of course, assigns each of those code points a particular meaning (along with
       ones above 255).  To preserve backward compatibility, Perl only uses the Unicode meanings
       when there is some indication that Unicode is what is intended; otherwise the non-ASCII
       code points remain treated as if they are unassigned.

       Here are the ways that Perl knows that a string should be treated as Unicode:

       ·   Within the scope of "use utf8"

           If the whole program is Unicode (signified by using 8-bit Unicode Transformation
           Format), then all strings within it must be Unicode.

       ·   Within the scope of "use feature 'unicode_strings'"

           This pragma was created so you can explicitly tell Perl that operations executed
           within its scope are to use Unicode rules.  More operations are affected with newer
           perls.  See "The "Unicode Bug"".

       ·   Within the scope of "use 5.012" or higher

           This implicitly turns on "use feature 'unicode_strings'".

       ·   Within the scope of "use locale 'not_characters'", or "use locale" and the current
           locale is a UTF-8 locale.

           The former is defined to imply Unicode handling; and the latter indicates a Unicode
           locale, hence a Unicode interpretation of all strings within it.

       ·   When the string contains a Unicode-only code point

           Perl has never accepted code points above 255 without them being Unicode, so their use
           implies Unicode for the whole string.

       ·   When the string contains a Unicode named code point "\N{...}"

           The "\N{...}" construct explicitly refers to a Unicode code point, even if it is one
           that is also in ASCII.  Therefore the string containing it must be Unicode.

       ·   When the string has come from an external source marked as Unicode

           The "-C" command line option can specify that certain inputs to the program are
           Unicode, and the values of this can be read by your Perl code, see "${^UNICODE}" in
           perlvar.

       ·   When the string has been upgraded to UTF-8

           The function "utf8::utf8_upgrade()" can be explicitly used to permanently (unless a
           subsequent "utf8::utf8_downgrade()" is called) cause a string to be treated as
           Unicode.

       ·   There are additional methods for regular expression patterns

           A pattern that is compiled with the "/u" or "/a" modifiers is treated as Unicode
           (though there are some restrictions with "/a").  Under the "/d" and "/l" modifiers,
           there are several other indications for Unicode; see "Character set modifiers" in
           perlre.

       Note that all of the above are overridden within the scope of "use bytes"; but you should
       be using this pragma only for debugging.

       Note also that some interactions with the platform's operating system never use Unicode
       rules.

       When Unicode rules are in effect:

       ·   Case translation operators use the Unicode case translation tables.

           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 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.)

       ·   Character classes in regular expressions match based on the character properties
           specified in the Unicode properties database.

           "\w" can be used to match a Japanese ideograph, for instance; and "[[:digit:]]" a
           Bengali number.

       ·   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.

   Extended Grapheme Clusters (Logical characters)
       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.  The Unicode standard calls these
       "extended grapheme clusters" (which is an improved version of the no-longer much used
       "grapheme cluster"); Perl furnishes the "\X" regular expression construct to match such
       sequences in their entirety.

       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, like ISO-8859-1, which has quite a few of them. For example, "LATIN
       CAPITAL LETTER E WITH ACUTE" was already in this standard when Unicode came along.
       Unicode therefore added it to its repertoire as that single character.  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 "E" and the "COMBINING ACCENT" 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.  A string may
       be comprised as much as possible of precomposed characters, or it may be comprised of
       entirely decomposed characters.  Unicode calls these respectively, "Normalization Form
       Composed" (NFC) and "Normalization Form Decomposed".  The "Unicode::Normalize" module
       contains functions that convert between the two.  A string may also have both composed
       characters and decomposed characters; this module can be used to make it all one or the
       other.

       You may be presented with strings in any of these equivalent forms.  There is currently
       nothing in Perl 5 that ignores the differences.  So you'll have to specially hanlde it.
       The usual advice is to convert your inputs to "NFD" before processing further.

       For more detailed information, see <http://unicode.org/reports/tr15/>.

   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 (see "General_Category" below).  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" property (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" 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.)

       See "Beyond Unicode code points" for special considerations when matching Unicode
       properties against non-Unicode code points.

       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 values the "General Category" property can have:

           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 a "Bidi_Class" property.  Some of the values this
       property can have are:

           Value       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.  Unlike the "General_Category"
       property, this property can have more values added in a future Unicode release.  Those
       listed above comprised the complete set for many Unicode releases, but others were added
       in Unicode 6.3; you can always find what the current ones are in perluniprops.  And
       <http://www.unicode.org/reports/tr9/> describes how to use them.

       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.  The "Script_Extensions" property is an improved version of "Script", as
       demonstrated below.  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_Extensions"
       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", which is required to
       be written in the compound form.  Prior to Perl v5.26, the single form returned the plain
       old "Script" version, but was changed because "Script_Extensions" gives better results.)

       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.  New code should probably be using "Script_Extensions" and not plain
       "Script".  If you compile perl with a Unicode release that doesn't have
       "Script_Extensions", the single form Perl extensions will instead refer to the plain
       "Script" property.  If you compile with a version of Unicode that doesn't have the
       "Script" property, these extensions will not be defined at all.

       (Actually, besides "Common", the "Inherited" script, contains characters that are used in
       multiple scripts.  These are modifier characters which 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_Extensions".  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".

       The explanation above has omitted some detail; refer to UAX#24 "Unicode Script Property":
       <http://www.unicode.org/reports/tr24>.

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

       Use of the "Is" Prefix

       For backward compatibility (with Perl 5.6), all properties writable without using the
       compound form 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
       the 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_Extensions" or "Script" 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.

       Perl also defines single form synonyms for the block property in cases where these do not
       conflict with something else.  But don't use any of these, because they are unstable.
       Since these are Perl extensions, they are subordinate to official Unicode property names;
       Unicode doesn't know nor care about Perl's extensions.  It may happen that a name that
       currently means the Perl extension will later be changed without warning to mean a
       different Unicode property in a future version of the perl interpreter that uses a later
       Unicode release, and your code would no longer work.  The extensions are mentioned here
       for completeness:  Take the block name and prefix it with one of: "In" (for example
       "\p{Blk=Arrows}" can currently be written as "\p{In_Arrows}"); or sometimes "Is" (like
       "\p{Is_Arrows}"); or sometimes no prefix at all ("\p{Arrows}").  As of this writing
       (Unicode 9.0) there are no conflicts with using the "In_" prefix, but there are plenty
       with the other two forms.  For example, "\p{Is_Hebrew}" and "\p{Hebrew}" mean
       "\p{Script_Extensions=Hebrew}" which is NOT the same thing as "\p{Blk=Hebrew}".  Our
       advice used to be to use the "In_" prefix as a single form way of specifying a block.  But
       Unicode 8.0 added properties whose names begin with "In", and it's now clear that it's
       only luck that's so far prevented a conflict.  Using "In" is only marginally less typing
       than "Blk:", and the latter's meaning is clearer anyway, and guaranteed to never conflict.
       So don't take chances.  Use "\p{Blk=foo}" for new code.  And be sure that block is what
       you really really want to do.  In most cases scripts are what you want instead.

       A complete list of blocks 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 every possible code point.  It is equivalent to "qr/./s".  Unlike all the
           other non-user-defined "\p{}" property matches, no warning is ever generated if this
           is property is matched against a non-Unicode code point (see "Beyond Unicode code
           points" below).

       "\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{Unicode}".

       "\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.

           The "Extended Grapheme Clusters (Logical characters)" section above talked about
           canonical decompositions.  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 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 other 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, 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 until
           v5.18, 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{Unicode}"
           This matches any of the 1_114_112 Unicode code points.  "\p{Any}".

       "\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 when 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 code point to include.

       ·   Two hexadecimal numbers separated by horizontal whitespace (space or tabular
           characters) denoting a range of 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 unassigned 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.

       Unlike non-user-defined "\p{}" property matches, no warning is ever generated if these
       properties are matched against a non-Unicode code point (see "Beyond Unicode code points"
       below).

   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 UTS#18 "Unicode Regular Expressions"
       <http://www.unicode.org/reports/tr18>, version 13, November 2013.

       Level 1 - Basic Unicode Support

        RL1.1   Hex Notation                     - Done          [1]
        RL1.2   Properties                       - Done          [2]
        RL1.2a  Compatibility Properties         - Done          [3]
        RL1.3   Subtraction and Intersection     - Experimental  [4]
        RL1.4   Simple Word Boundaries           - Done          [5]
        RL1.5   Simple Loose Matches             - Done          [6]
        RL1.6   Line Boundaries                  - Partial       [7]
        RL1.7   Supplementary Code Points        - Done          [8]

       [1] "\N{U+...}" and "\x{...}"
       [2] "\p{...}" "\P{...}".  This requirement is for a minimal list of properties.  Perl
       supports these and all other Unicode character properties, as R2.7 asks (see "Unicode
       Character Properties" above).
       [3] Perl has "\d" "\D" "\s" "\S" "\w" "\W" "\X" "[:prop:]" "[:^prop:]", plus all the
       properties specified by <http://www.unicode.org/reports/tr18/#Compatibility_Properties>.
       These are described above in "Other Properties"
       [4] The experimental feature "(?[...])" starting 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 lookahead

               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

       [5] "\b" "\B" meet most, but not all, the details of this requirement, but "\b{wb}" and
       "\B{wb}" do, as well as the stricter R2.3.
       [6] Note that Perl does Full case-folding in matching, 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.

       [7] The reason this is considered to be only partially implemented is that Perl has
           "qr/\b{lb}/" and "Unicode::LineBreak" that are conformant with UAX#14 "Unicode Line
           Breaking Algorithm" <http://www.unicode.org/reports/tr14>.  The regular expression
           construct provides default behavior, while the heavier-weight module provides
           customizable line breaking.

           But Perl treats "\n" as the start- and end-line delimiter, whereas Unicode specifies
           more characters that should be so-interpreted.

           These are:

            VT   U+000B  (\v in C)
            FF   U+000C  (\f)
            CR   U+000D  (\r)
            NEL  U+0085
            LS   U+2028
            PS   U+2029

           "^" and "$" in regular expression patterns are supposed to match all these, but don't.
           These characters also don't, but should, affect "<>" $., and script line numbers.

           Also, lines should not be split within "CRLF" (i.e. there is no empty line between
           "\r" and "\n").  For "CRLF", try the ":crlf" layer (see PerlIO).

       [8] 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           - Retracted     [9]
                                                  by Unicode
        RL2.2   Extended Grapheme Clusters      - Partial       [10]
        RL2.3   Default Word Boundaries         - Done          [11]
        RL2.4   Default Case Conversion         - Done
        RL2.5   Name Properties                 - Done
        RL2.6   Wildcard Properties             - Missing
        RL2.7   Full Properties                 - Done

       [9] Unicode has rewritten this portion of UTS#18 to say that getting canonical equivalence
       (see UAX#15 "Unicode Normalization Forms" <http://www.unicode.org/reports/tr15>) is
       basically to be done at the programmer level.  Use NFD to write both your regular
       expressions and text to match them against (you can use Unicode::Normalize).
       [10] Perl has "\X" and "\b{gcb}" but we don't have a "Grapheme Cluster Mode".
       [11] see UAX#29 "Unicode Text Segmentation" <http://www.unicode.org/reports/tr29>,

       Level 3 - Tailored Support

        RL3.1   Tailored Punctuation            - Missing
        RL3.2   Tailored Grapheme Clusters      - Missing       [12]
        RL3.3   Tailored Word Boundaries        - Missing
        RL3.4   Tailored Loose Matches          - Retracted by Unicode
        RL3.5   Tailored Ranges                 - Retracted by Unicode
        RL3.6   Context Matching                - Missing       [13]
        RL3.7   Incremental Matches             - Missing
        RL3.8   Unicode Set Sharing             - Unicode is proposing
                                                  to retract this
        RL3.9   Possible Match Sets             - Missing
        RL3.10  Folded Matching                 - Retracted by Unicode
        RL3.11  Submatchers                     - Missing

       [12] Perl has Unicode::Collate, but it isn't integrated with regular expressions.  See
       UTS#10 "Unicode Collation Algorithms" <http://www.unicode.org/reports/tr10>.
       [13] Perl has "(?<=x)" and "(?=x)", but lookaheads or lookbehinds should see outside of
       the target substring

   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.  In most
           of Perl's documentation, including elsewhere in this document, the term "UTF-8" means
           also "UTF-EBCDIC".  But in this section, "UTF-8" refers only to the encoding used on
           ASCII platforms.  It is a superset of 7-bit US-ASCII, so anything encoded in ASCII has
           the identical representation when encoded in UTF-8.

           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.  In addition, it is deprecated to use a code point
           larger than what a signed integer variable on your system can hold.  On 32-bit ASCII
           systems, this means "0x7FFF_FFFF" is the legal maximum going forward (much higher on
           64-bit systems).

       ·   UTF-EBCDIC

           Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.  This means that all
           the basic characters (which includes all those that have ASCII equivalents (like "A",
           "0", "%", etc.)  are the same in both EBCDIC and UTF-EBCDIC.)

           UTF-EBCDIC is used on EBCDIC platforms.  It generally requires more bytes to represent
           a given code point than UTF-8 does; the largest Unicode code points take 5 bytes to
           represent (instead of 4 in UTF-8), and, extended for 64-bit words, it uses 14 bytes
           instead of 13 bytes in UTF-8.

       ·   UTF-16, UTF-16BE, UTF-16LE, Surrogates, and "BOM"'s (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 "BOM"'s, 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 ASCII platform 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, except 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.

   Noncharacter code points
       66 code points are set aside in Unicode as "noncharacter code points".  These all have the
       "Unassigned" ("Cn") "General_Category", and no character will ever be assigned to any of
       them.  They are the 32 code points between "U+FDD0" and "U+FDEF" inclusive, and the 34
       code points:

        U+FFFE   U+FFFF
        U+1FFFE  U+1FFFF
        U+2FFFE  U+2FFFF
        ...
        U+EFFFE  U+EFFFF
        U+FFFFE  U+FFFFF
        U+10FFFE U+10FFFF

       Until Unicode 7.0, the noncharacters were "forbidden for use in open interchange of
       Unicode text data", so that code that processed those streams could use these code points
       as sentinels that could be mixed in with character data, and would always be
       distinguishable from that data.  (Emphasis above and in the next paragraph are added in
       this document.)

       Unicode 7.0 changed the wording so that they are "not recommended for use in open
       interchange of Unicode text data".  The 7.0 Standard goes on to say:

           "If a noncharacter is received in open interchange, an application is not required to
           interpret it in any way.  It is good practice, however, to recognize it as a
           noncharacter and to take appropriate action, such as replacing it with "U+FFFD"
           replacement character, to indicate the problem in the text.  It is not recommended to
           simply delete noncharacter code points from such text, because of the potential
           security issues caused by deleting uninterpreted characters.  (See conformance clause
           C7 in Section 3.2, Conformance Requirements, and Unicode Technical Report #36,
           "Unicode Security Considerations"
           <http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."

       This change was made because it was found that various commercial tools like editors, or
       for things like source code control, had been written so that they would not handle
       program files that used these code points, effectively precluding their use almost
       entirely!  And that was never the intent.  They've always been meant to be usable within
       an application, or cooperating set of applications, at will.

       If you're writing code, such as an editor, that is supposed to be able to handle any
       Unicode text data, then you shouldn't be using these code points yourself, and instead
       allow them in the input.  If you need sentinels, they should instead be something that
       isn't legal Unicode.  For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
       they never appear in well-formed UTF-8.  (There are equivalents for UTF-EBCDIC).  You can
       also store your Unicode code points in integer variables and use negative values as
       sentinels.

       If you're not writing such a tool, then whether you accept noncharacters as input is up to
       you (though the Standard recommends that you not).  If you do strict input stream checking
       with Perl, these code points continue to be forbidden.  This is to maintain backward
       compatibility (otherwise potential security holes could open up, as an unsuspecting
       application that was written assuming the noncharacters would be filtered out before
       getting to it, could now, without warning, start getting them).  To do strict checking,
       you can use the layer ":encoding('UTF-8')".

       Perl continues to warn (using the warning category "nonchar", which is a sub-category of
       "utf8") if an attempt is made to output noncharacters.

   Beyond Unicode code points
       The maximum Unicode code point is "U+10FFFF", and Unicode only defines operations on code
       points up through that.  But Perl works on 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 any are output.

       Since Unicode rules are not defined on these code points, if a Unicode-defined operation
       is done on them, Perl uses what we believe are sensible rules, while generally warning,
       using the "non_unicode" category.  For example, "uc("\x{11_0000}")" will generate such a
       warning, returning the input parameter as its result, since Perl defines the uppercase of
       every non-Unicode code point to be the code point itself.  (All the case changing
       operations, not just uppercasing, work this way.)

       The situation with matching Unicode properties in regular expressions, the "\p{}" and
       "\P{}" constructs, against these code points is not as clear cut, and how these are
       handled has changed as we've gained experience.

       One possibility is to treat any match against these code points as undefined.  But since
       Perl doesn't have the concept of a match being undefined, it converts this to failing or
       "FALSE".  This is almost, but not quite, what Perl did from v5.14 (when use of these code
       points became generally reliable) through v5.18.  The difference is that Perl treated all
       "\p{}" matches as failing, but all "\P{}" matches as succeeding.

       One problem with this is that it leads to unexpected, and confusing results in some cases:

        chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Failed on <= v5.18
        chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Failed! on <= v5.18

       That is, it treated both matches as undefined, and converted that to false (raising a
       warning on each).  The first case is the expected result, but the second is likely
       counterintuitive: "How could both be false when they are complements?"  Another problem
       was that the implementation optimized many Unicode property matches down to already
       existing simpler, faster operations, which don't raise the warning.  We chose to not forgo
       those optimizations, which help the vast majority of matches, just to generate a warning
       for the unlikely event that an above-Unicode code point is being matched against.

       As a result of these problems, starting in v5.20, what Perl does is to treat non-Unicode
       code points as just typical unassigned Unicode characters, and matches accordingly.
       (Note: Unicode has atypical unassigned code points.  For example, it has noncharacter code
       points, and ones that, when they do get assigned, are destined to be written Right-to-
       left, as Arabic and Hebrew are.  Perl assumes that no non-Unicode code point has any
       atypical properties.)

       Perl, in most cases, will raise a warning when matching an above-Unicode code point
       against a Unicode property when the result is "TRUE" for "\p{}", and "FALSE" for "\P{}".
       For example:

        chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails, no warning
        chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Succeeds, with warning

       In both these examples, the character being matched is non-Unicode, so Unicode doesn't
       define how it should match.  It clearly isn't an ASCII hex digit, so the first example
       clearly should fail, and so it does, with no warning.  But it is arguable that the second
       example should have an undefined, hence "FALSE", result.  So a warning is raised for it.

       Thus the warning is raised for many fewer cases than in earlier Perls, and only when what
       the result is could be arguable.  It turns out that none of the optimizations made by Perl
       (or are ever likely to be made) cause the warning to be skipped, so it solves both
       problems of Perl's earlier approach.  The most commonly used property that is affected by
       this change is "\p{Unassigned}" which is a short form for
       "\p{General_Category=Unassigned}".  Starting in v5.20, all non-Unicode code points are
       considered "Unassigned".  In earlier releases the matches failed because the result was
       considered undefined.

       The only place where the warning is not raised when it might ought to have been is if
       optimizations cause the whole pattern match to not even be attempted.  For example, Perl
       may figure out that for a string to match a certain regular expression pattern, the string
       has to contain the substring "foobar".  Before attempting the match, Perl may look for
       that substring, and if not found, immediately fail the match without actually trying it;
       so no warning gets generated even if the string contains an above-Unicode code point.

       This behavior is more "Do what I mean" than in earlier Perls for most applications.  But
       it catches fewer issues for code that needs to be strictly Unicode compliant.  Therefore
       there is an additional mode of operation available to accommodate such code.  This mode is
       enabled if a regular expression pattern is compiled within the lexical scope where the
       "non_unicode" warning class has been made fatal, say by:

        use warnings FATAL => "non_unicode"

       (see warnings).  In this mode of operation, Perl will raise the warning for all matches
       against a non-Unicode code point (not just the arguable ones), and it skips the
       optimizations that might cause the warning to not be output.  (It currently still won't
       warn if the match isn't even attempted, like in the "foobar" example above.)

       In summary, Perl now normally treats non-Unicode code points as typical Unicode unassigned
       code points for regular expression matches, raising a warning only when it is arguable
       what the result should be.  However, if this warning has been made fatal, it isn't
       skipped.

       There is one exception to all this.  "\p{All}" looks like a Unicode property, but it is a
       Perl extension that is defined to be true for all possible code points, Unicode or not, so
       no warning is ever generated when matching this against a non-Unicode code point.  (Prior
       to v5.20, it was an exact synonym for "\p{Any}", matching code points 0 through 0x10FFFF.)

   Security Implications of Unicode
       First, read Unicode Security Considerations <http://www.unicode.org/reports/tr36>.

       Also, note the following:

       ·   Malformed UTF-8

           UTF-8 is very structured, so many combinations of bytes are invalid.  In the past,
           Perl tried to soldier on and make some sense of invalid combinations, but this can
           lead to security holes, so now, if the Perl core needs to process an invalid
           combination, it will either raise a fatal error, or will replace those bytes by the
           sequence that forms the Unicode REPLACEMENT CHARACTER, for which purpose Unicode
           created it.

           Every code point can be represented by more than one possible syntactically valid
           UTF-8 sequence.  Early on, both Unicode and Perl considered any of these to be valid,
           but now, all sequences longer than the shortest possible one are considered to be
           malformed.

           Unicode considers many code points to be illegal, or to be avoided.  Perl generally
           accepts them, once they have passed through any input filters that may try to exclude
           them.  These have been discussed above (see "Surrogates" under UTF-16 in "Unicode
           Encodings", "Noncharacter code points", and "Beyond Unicode code points").

       ·   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 ASCII and single-byte locales, and the new world of Unicode, upgrading when
       necessary.  If your legacy code does not explicitly use Unicode, no automatic switch-over
       to Unicode should happen.

   Unicode in Perl on EBCDIC
       Unicode is supported on EBCDIC platforms.  See perlebcdic.

       Unless ASCII vs. EBCDIC issues are specifically being discussed, references to UTF-8
       encoding in this document and elsewhere should be read as meaning UTF-EBCDIC on EBCDIC
       platforms.  See "Unicode and UTF" in perlebcdic.

       Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly hidden from you;
       "use utf8" (and NOT something like "use utfebcdic") declares the the script is in the
       platform's "native" 8-bit encoding of Unicode.  (Similarly for the ":utf8" layer.)

   Locales
       See "Unicode and UTF-8" in perllocale

   When Unicode Does Not Happen
       There are still many places where Unicode (in some encoding or another) could be given as
       arguments or received as results, or both in Perl, but it is not, in spite of Perl having
       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).

       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 (deprecated) "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 with the 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 can have very different
       semantics depending on the rules in effect.  (Characters whose code points are above 255
       force Unicode rules; whereas the rules for ASCII characters are the same under both ASCII
       and Unicode rules.)

       Under Unicode rules, these upper-Latin1 characters are interpreted as Unicode code points,
       which means they have the same semantics as Latin-1 (ISO-8859-1) and C1 controls.

       As explained in "ASCII Rules versus Unicode Rules", under ASCII rules, they are considered
       to be unassigned characters.

       This can lead to unexpected results.  For example, a string's semantics can suddenly
       change if a code point above 255 is appended to it, which changes the rules from ASCII to
       Unicode.  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" nor 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, along with Perl's desire to add Unicode support seamlessly.  But the result
       turned out to not be seamless.  (By the way, you can choose to be warned when things like
       this happen.  See "encoding::warnings".)

       "use feature 'unicode_strings'" was added, starting in Perl v5.12, to address this
       problem.  It affects these things:

       ·   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, Unicode rules 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 Unicode rules even when executed or compiled into larger regular
           expressions outside the scope.

       ·   Matching any of several properties in regular expressions.

           These properties are "\b" (without braces), "\B" (without braces), "\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 Unicode rules even when executed or compiled into larger regular
           expressions outside the scope.

       ·   In "quotemeta" or its inline equivalent "\Q".

           Starting in Perl 5.16.0, consistent quoting rules are used within the scope of
           "unicode_strings", as described in "quotemeta" in perlfunc.  Prior to that, or outside
           its scope, 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.

       ·   In the ".." or range operator.

           Starting in Perl 5.26.0, the range operator on strings treats their lengths
           consistently within the scope of "unicode_strings". Prior to that, or outside its
           scope, it could produce strings whose length in characters exceeded that of the right-
           hand side, where the right-hand side took up more bytes than the correct range
           endpoint.

       You can see from the above that the effect of "unicode_strings" increased over several
       Perl releases.  (And Perl's support for Unicode continues to improve; it's best to use the
       latest available release in order to get the most complete and accurate results possible.)
       Note that "unicode_strings" is automatically chosen if you "use 5.012" or higher.

       For Perls earlier than those described above, or when a string is passed to a function
       outside the scope of "unicode_strings", see the next section.

   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 standard
       module Encode can be used for this, or the low-level calls "utf8::upgrade($bytestring)"
       and "utf8::downgrade($utf8string[, FAIL_OK])".

       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.

       "ASCII Rules versus Unicode Rules" gives all the ways that a string is made to use Unicode
       rules.

   Using Unicode in XS
       See "Unicode Support" in perlguts for an introduction to Unicode at the XS level, and
       "Unicode Support" in perlapi for the API details.

   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 the goal is
       to allow you to change to use any earlier one.  In Perls v5.20 and v5.22, however, the
       earliest usable version is Unicode 5.1.  Perl v5.18 and v5.24 are able to handle all
       earlier versions.

       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).

   Porting code from perl-5.6.X
       Perls starting in 5.8 have 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(UTF-8)";
            }

       ·  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("UTF-8", $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("UTF-8", $val);
            }

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

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

       ·  A wrapper for DBI "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 DBI 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;

BUGS

       See also "The "Unicode Bug"" above.

   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("UTF-8", Foo::Bar::escape_html(
                                            Encode::encode("UTF-8", $what)));
           }

       Sometimes, when the extension does not convert data but just stores and retrieves it, 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 improved the situation.  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 "[0-9]" (then again, there are hundreds of Unicode characters matching "Nd" compared
       with the 10 ASCII characters matching "[0-9]").

SEE ALSO

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