perlunicode


DESCRIPTION
   Important Caveats
       Unicode support is an extensive requirement. While Perl does not
       implement the Unicode standard or the accompanying technical reports
       from cover to cover, Perl does support many Unicode features.

       People who want to learn to use Unicode in Perl, should probably read
       the Perl Unicode tutorial, perlunitut and perluniintro, before reading
       this reference document.

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

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

           This pragma doesn't affect I/O, and 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

       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" is in effect (which overrides "use feature
       'unicode_strings'" in the same scope), Perl uses the semantics
       associated with the current locale.  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.  On
       EBCDIC platforms, this is almost seamless, as the EBCDIC code pages
       that Perl handles are equivalent to Unicode's first 256 code points.
       (The exception is that EBCDIC regular expression case-insensitive
       matching rules are not as as robust as Unicode's.)   But on ASCII
       platforms, Perl uses US-ASCII (or Basic Latin in Unicode terminology)
       byte semantics, meaning that characters whose ordinal numbers are in
       the range 128 - 255 are undefined except for their ordinal numbers.
       This means that none have case (upper and lower), nor are any a member
       of character classes, like "[:alpha:]" or "\w".  (But all do belong to
       the "\W" class or the Perl regular expression extension "[:^alpha:]".)

       This behavior preserves compatibility with earlier versions of Perl,
       which allowed byte semantics in Perl operations only if none of the
       program's inputs were marked as being a source of Unicode character
       data.  Such data may come from filehandles, from calls to external
       programs, from information provided by the system (such as %ENV), or
       from literals and constants in the source text.

       The "utf8" pragma is primarily a compatibility device that enables
       recognition of UTF-(8|EBCDIC) in literals encountered by the parser.
       Note that this pragma is only required while Perl defaults to byte
       semantics; when character semantics become the default, this pragma may
       become a no-op.  See utf8.

       If strings operating under byte semantics and strings with Unicode
       character data are concatenated, the new string will have character
       semantics.  This can cause surprises: See "BUGS", below.  You can
       choose to be warned when this happens.  See encoding::warnings.

       Under character semantics, many operations that formerly operated on
       bytes now operate on characters. A character in Perl is logically just
       a number ranging from 0 to 2**31 or so. Larger characters may encode
       into longer sequences of bytes internally, but this internal detail is
       mostly hidden for Perl code.  See perluniintro for more.

   Effects of Character Semantics
       Character semantics have the following effects:

       o   Strings--including hash keys--and regular expression patterns may
           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.

           Additionally, if you

              use charnames ':full';

           you can use the "\N{...}" notation and put the official Unicode
           character name within the braces, such as "\N{WHITE SMILING FACE}".
           See charnames.

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

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

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

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

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

       o   The special pattern "\X" matches a logical character, an "extended
           grapheme cluster" in Standardese.  In Unicode what appears to the
           user to be a single character, for example an accented "G", may in
           fact be composed of a sequence of characters, in this case a "G"
           followed by an accent character.  "\X" will match the entire
           sequence.

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

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

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

       o   The "chr()" and "ord()" functions work on characters, similar to
           "pack("W")" and "unpack("W")", not "pack("C")" and "unpack("C")".
           "pack("C")" and "unpack("C")" are methods for emulating byte-
           oriented "chr()" and "ord()" on Unicode strings.  While these
           methods reveal the internal encoding of Unicode strings, that is
           not something one normally needs to care about at all.

       o   The bit string operators, "& | ^ ~", can operate on character data.
           However, for backward compatibility, such as when using bit string
           operations when characters are all less than 256 in ordinal value,
           one should not use "~" (the bit complement) with characters of both
           values less than 256 and values greater than 256.  Most
           importantly, DeMorgan's laws ("~($x|$y) eq ~$x&~$y" and "~($x&$y)
           eq ~$x|~$y") will not hold.  The reason for this mathematical faux
           pas is that the complement cannot return both the 8-bit (byte-wide)
           bit complement and the full character-wide bit complement.

       o   You can define your own mappings to be used in "lc()", "lcfirst()",
           "uc()", and "ucfirst()" (or their double-quoted string inlined
           versions such as "\U"). See User-Defined Case-Mappings for more
           details.

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

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

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

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

       More formally, "\p{Uppercase}" matches any single character whose
       Unicode Uppercase property value is True, and "\P{Uppercase}" matches
       any character whose Uppercase property value is False, and they could
       have been written as "\p{Uppercase=True}" and "\p{Uppercase=False}",
       respectively.

       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 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.)

       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>).

           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:
           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 property gives what script a given character is in,
       and the property can be specified with the compound form like
       "\p{Script=Hebrew}" (short: "\p{sc=hebr}").  Perl furnishes shortcuts
       for all script names.  You can omit everything up through the equals
       (or colon), and simply write "\p{Latin}" or "\P{Cyrillic}".

       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. The digits 0-9 and
       similar groups, like punctuation, are in the script called "Common".
       There is also a script called "Inherited" for characters that modify
       other characters, and inherit the script value of the controlling
       character.  (Note that there are several different sets of digits in
       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 synonyms
       for compound-form Unicode properties (for those, 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{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".

       "\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]".

           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.
           Mnemonic: Space, as modified by Perl.  (It doesn't include the
           vertical tab which both the Posix standard and Unicode consider
           white space.)

       "\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 subroutines can
       be defined in any package.  The user-defined properties can be used in
       the regular expression "\p" and "\P" constructs; if you are using a
       user-defined property from a package other than the one you are in, you
       must specify its package in the "\p" or "\P" construct.

           # assuming property Is_Foreign defined in Lang::
           package main;  # property package name required
           if ($txt =~ /\p{Lang::IsForeign}+/) { ... }

           package Lang;  # property package name not required
           if ($txt =~ /\p{IsForeign}+/) { ... }

       Note that the effect is compile-time and immutable once defined.
       However, the subroutines are passed a single parameter, which is 0 if
       case-sensitive matching is in effect and non-zero if caseless matching
       is in effect.  The subroutine may return different values depending on
       the value of the flag, and one set of values will immutably be in
       effect for all case-sensitive matches, and the other set for all case-
       insensitive matches.

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

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

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

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

       o   Something to negate, prefixed "!": an existing character property
           (prefixed by "utf8::") or a user-defined character property, to
           represent all the characters in that property; two hexadecimal code
           points for a range; or a single hexadecimal code point.

       o   Something to intersect with, prefixed by "&": an existing character
           property (prefixed by "utf8::") or a user-defined character
           property, for all the characters except the characters in the
           property; two hexadecimal code points for a range; or a single
           hexadecimal code point.

       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
           }


   User-Defined Case Mappings (for serious hackers only)
       This featured is deprecated and is scheduled to be removed in Perl
       5.16.  The CPAN module Unicode::Casing provides better functionality
       without the drawbacks described below.

       You can define your own mappings to be used in "lc()", "lcfirst()",
       "uc()", and "ucfirst()" (or their string-inlined versions, "\L", "\l",
       "\U", and "\u").  The mappings are currently only valid on strings
       encoded in UTF-8, but see below for a partial workaround for this
       restriction.

       The principle is similar to that of user-defined character properties:
       define subroutines that do the mappings.  "ToLower" is used for "lc()",
       "\L", "lcfirst()", and "\l"; "ToTitle" for "ucfirst()" and "\u"; and
       "ToUpper" for "uc()" and "\U".

       "ToUpper()" should look something like this:

           sub ToUpper {
               return <<END;
           0061\t007A\t0041
           0101\t\t0100
           END
           }

       This sample "ToUpper()" has the effect of mapping "a-z" to "A-Z", 0x101
       to 0x100, and all other characters map to themselves.  The first
       returned line means to map the code point at 0x61 ("a") to 0x41 ("A"),
       the code point at 0x62 ("b") to 0x42 ("B"),  ..., 0x7A ("z") to 0x5A
       ("Z").  The second line maps just the code point 0x101 to 0x100.  Since
       there are no other mappings defined, all other code points map to
       themselves.

       This mechanism is not well behaved as far as affecting other packages
       and scopes.  All non-threaded programs have exactly one uppercasing
       behavior, one lowercasing behavior, and one titlecasing behavior in
       effect for utf8-encoded strings for the duration of the program.  Each
       of these behaviors is irrevocably determined the first time the
       corresponding function is called to change a utf8-encoded string's
       case.  If a corresponding "To-" function has been defined in the
       package that makes that first call, the mapping defined by that
       function will be the mapping used for the duration of the program's
       execution across all packages and scopes.  If no corresponding "To-"
       function has been defined in that package, the standard official
       mapping will be used for all packages and scopes, and any corresponding
       "To-" function anywhere will be ignored.  Threaded programs have
       similar behavior.  If the program's casing behavior has been decided at
       the time of a thread's creation, the thread will inherit that behavior.
       But, if the behavior hasn't been decided, the thread gets to decide for
       itself, and its decision does not affect other threads nor its creator.

       As shown by the example above, you have to furnish a complete mapping;
       you can't just override a couple of characters and leave the rest
       thing into your subroutine.  But this will only be valid on Perls that
       use the same Unicode version.  Another option would be to have your
       subroutine read the official mapping files and overwrite the affected
       code points.

       If you have only a few mappings to change, starting in 5.14 you can use
       the following trick, here illustrated for Turkish.

           use Config;
           use charnames ":full";

           sub ToUpper {
               my $official = do "$Config{privlib}/unicore/To/Upper.pl";
               $utf8::ToSpecUpper{'i'} =
                                  "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
               return $official;
           }

       This takes the official mappings and overrides just one, for "LATIN
       SMALL LETTER I".  The keys to the hash must be the bytes that form the
       UTF-8 (on EBCDIC platforms, UTF-EBCDIC) of the character, as
       illustrated by the inverse function.

           sub ToLower {
               my $official = do $lower;
               $utf8::ToSpecLower{"\xc4\xb0"} = "i";
               return $official;
           }

       This example is for an ASCII platform, and "\xc4\xb0" is the string of
       bytes that together form the UTF-8 that represents "\N{LATIN CAPITAL
       LETTER I WITH DOT ABOVE}", "U+0130".  You can avoid having to figure
       out these bytes, and at the same time make it work on all platforms by
       instead writing:

           sub ToLower {
               my $official = do $lower;
               my $sequence = "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
               utf8::encode($sequence);
               $utf8::ToSpecLower{$sequence} = "i";
               return $official;
           }

       This works because "utf8::encode()" takes the single character and
       converts it to the sequence of bytes that constitute it.  Note that we
       took advantage of the fact that "i" is the same in UTF-8 or UTF_EBCIDIC
       as not; otherwise we would have had to write

               $utf8::ToSpecLower{$sequence} = "\N{LATIN SMALL LETTER I}";

       in the ToLower example, and in the ToUpper example, use

               my $sequence = "\N{LATIN SMALL LETTER I}";
               utf8::encode($sequence);

       advisable, you can cause the mappings to be used globally by importing
       into "CORE::GLOBAL" (see CORE).

       You can partially get around the restriction that the source strings
       must be in utf8 by using "use subs" (or by importing into
       "CORE::GLOBAL") by:

        use subs qw(uc ucfirst lc lcfirst);

        sub uc($) {
            my $string = shift;
            utf8::upgrade($string);
            return CORE::uc($string);
        }

        sub lc($) {
            my $string = shift;
            utf8::upgrade($string);

            # Unless an I is before a dot_above, it turns into a dotless i.
            # (The character class with the combining classes matches non-above
            # marks following the I.  Any number of these may be between the 'I' and
            # the dot_above, and the dot_above will still apply to the 'I'.
            use charnames ":full";
            $string =~
                    s/I
                      (?! [^\p{ccc=0}\p{ccc=Above}]* \N{COMBINING DOT ABOVE} )
                     /\N{LATIN SMALL LETTER DOTLESS I}/gx;

            # But when the I is followed by a dot_above, remove the
            # dot_above so the end result will be i.
            $string =~ s/I
                           ([^\p{ccc=0}\p{ccc=Above}]* )
                           \N{COMBINING DOT ABOVE}
                        /i$1/gx;
            return CORE::lc($string);
        }

       These examples (also for Turkish) make sure the input is in UTF-8, and
       then call the corresponding official function, which will use the
       "ToUpper()" and "ToLower()" functions you have defined.  (For Turkish,
       there are other required functions: "ucfirst", "lcfirst", and
       "ToTitle". These are very similar to the ones given above.)

       The reason this is only a partial fix is that it doesn't affect the
       "\l", "\L", "\u", and "\U" case-change operations in regular
       expressions, which still require the source to be encoded in utf8 (see
       "The "Unicode Bug""). (Again, use Unicode::Casing instead.)

       The "lc()" example shows how you can add context-dependent casing. Note
       that context-dependent casing suffers from the problem that the string
       passed to the casing function may not have sufficient context to make
       the proper choice. Also, it will not be called for "\l", "\L", "\u",
       and "\U".

                   RL1.1   Hex Notation                     - done          [1]
                   RL1.2   Properties                       - done          [2][3]
                   RL1.2a  Compatibility Properties         - done          [4]
                   RL1.3   Subtraction and Intersection     - MISSING       [5]
                   RL1.4   Simple Word Boundaries           - done          [6]
                   RL1.5   Simple Loose Matches             - done          [7]
                   RL1.6   Line Boundaries                  - MISSING       [8][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 L</Unicode Character Properties>)
                   [4]  \d \D \s \S \w \W \X [:prop:] [:^prop:]
                   [5]  can use regular expression look-ahead [a] or
                        user-defined character properties [b] to emulate set
                        operations
                   [6]  \b \B
                   [7]  note that Perl does Full case-folding in matching (but with
                        bugs), not Simple: for example U+1F88 is equivalent to
                        U+1F00 U+03B9, not with 1F80.  This difference matters
                        mainly for certain Greek capital letters with certain
                        modifiers: the Full case-folding decomposes the letter,
                        while the Simple case-folding would map it to a single
                        character.
                   [8]  should do ^ and $ also on U+000B (\v in C), FF (\f), CR
                        (\r), CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS
                        (U+2029); should also affect <>, $., and script line
                        numbers; should not split lines within CRLF [c] (i.e. there
                        is no empty line between \r and \n)
                   [9]  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

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

               [{Greek}-[{UNASSIGNED}]]

           in Perl can be written as:

               (?!\p{Unassigned})\p{InGreekAndCoptic}
               (?=\p{Assigned})\p{InGreekAndCoptic}

           But in this particular example, you probably really want

               \p{GreekAndCoptic}

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

                   RL2.2   Default Grapheme Clusters       - MISSING       [12]
                   RL2.3   Default Word Boundaries         - MISSING       [14]
                   RL2.4   Default Loose Matches           - MISSING       [15]
                   RL2.5   Name Properties                 - MISSING       [16]
                   RL2.6   Wildcard Properties             - MISSING

                   [10] see UAX#15 "Unicode Normalization Forms"
                   [11] have Unicode::Normalize but not integrated to regexes
                   [12] have \X but we don't have a "Grapheme Cluster Mode"
                   [14] see UAX#29, Word Boundaries
                   [15] see UAX#21 "Case Mappings"
                   [16] missing loose match [e]

           [e] "\N{...}" allows namespaces (see charnames).

       o   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.

       o   UTF-8

           UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
           encoding. For ASCII (and we really do mean 7-bit ASCII, not another
           8-bit encoding), UTF-8 is transparent.

           The following table is from Unicode 3.2.

            Code Points            1st Byte  2nd Byte  3rd Byte  4th Byte

              U+0000..U+007F       00..7F
              U+0080..U+07FF     * C2..DF    80..BF

           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".

       o   UTF-EBCDIC

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

       o   UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)

           The followings items are mostly for reference and general Unicode
           knowledge, Perl doesn't use these constructs internally.

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

           Surrogates are code points set aside to encode the
           "U+10000..U+10FFFF" range of Unicode code points in pairs of 16-bit
           units.  The high surrogates are the range "U+D800..U+DBFF" and the
           low surrogates are the range "U+DC00..U+DFFF".  The surrogate
           encoding is

               $hi = ($uni - 0x10000) / 0x400 + 0xD800;
               $lo = ($uni - 0x10000) % 0x400 + 0xDC00;

           and the decoding is
           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.)

       o   UTF-32, UTF-32BE, UTF-32LE

           The UTF-32 family is pretty much like the UTF-16 family, expect
           that the units are 32-bit, and therefore the surrogate scheme is
           not needed.  UTF-32 is a fixed-width encoding.  The BOM signatures
           are "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00 0x00" for LE.

       o   UCS-2, UCS-4

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

       o   UTF-7

           A seven-bit safe (non-eight-bit) encoding, which is useful if the
           transport or storage is not eight-bit safe.  Defined by RFC 2152.

   Non-character code points
       66 code points are set aside in Unicode as "non-character code points".
       These all have the Unassigned (Cn) General Category, and they never
       will be assigned.  These are never supposed to be in legal Unicode
       input streams, so that code can use them as sentinels that can be mixed
       in with character data, and they always will be distinguishable from

   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:

       o   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.

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

       As discussed elsewhere, Perl has one foot (two hooves?) planted in each
       of two worlds: the old world of bytes and the new world of characters,
       upgrading from bytes to characters when necessary.  If your legacy code
       does not explicitly use Unicode, no automatic switch-over to characters
       should happen.  Characters shouldn't get downgraded to bytes, either.
       It is possible to accidentally mix bytes and characters, however (see
       perluniintro), in which case "\w" in regular expressions might start
       behaving differently (unless the "/a" modifier is in effect).  Review
       your code.  Use warnings and the "strict" pragma.

   Unicode in Perl on EBCDIC
       The way Unicode is handled on EBCDIC platforms is still experimental.
       On such platforms, references to UTF-8 encoding in this document and
       elsewhere should be read as meaning the UTF-EBCDIC specified in Unicode
       Technical Report 16, unless ASCII vs. EBCDIC issues are specifically
       discussed. There is no "utfebcdic" pragma or ":utfebcdic" layer;
       rather, "utf8" and ":utf8" are reused to mean the platform's "natural"
       8-bit encoding of Unicode. See perlebcdic for more discussion of the
       issues.

       byte strings both as arguments and results, or UTF-8 strings if the
       (problematic) "encoding" pragma has been used.

       One reason that Perl does not attempt to resolve the role of Unicode in
       these situations is that the answers are highly dependent on the
       operating system and the file system(s).  For example, whether
       filenames can be in Unicode and in exactly what kind of encoding, is
       not exactly a portable concept.  Similarly for "qx" and "system": how
       well will the "command-line interface" (and which of them?) handle
       Unicode?

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

       o   %ENV

       o   glob (aka the <*>)

       o   open, opendir, sysopen

       o   qx (aka the backtick operator), system

       o   readdir, readlink

   The "Unicode Bug"
       The term, the "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.  (The lesson here
       is to specify "unicode_strings" to avoid the headaches.)

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

       In byte semantics, they are considered to be unassigned characters,
       meaning that the only semantics they have is their ordinal numbers, and
       that they are not members of various character classes.  None are
       considered to match "\w" for example, but all match "\W".

       The behavior is known to have effects on these areas:

       o   Changing the case of a scalar, that is, using "uc()", "ucfirst()",
           "lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in regular
           expression substitutions.

       o   Using caseless ("/i") regular expression matching

       o   Matching any of several properties in regular expressions, namely
           "\b", "\B", "\s", "\S", "\w", "\W", and all the Posix character
           classes except "[[:ascii:]]".

       o   In "quotemeta" or its inline equivalent "\Q", no characters code
       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.

       Starting in Perl 5.14, "use feature 'unicode_strings'" can be used to
       cause Perl to use Unicode semantics on all string operations within the
       scope of the feature subpragma.  Regular expressions compiled in its
       scope retain that behavior even when executed or compiled into larger
       regular expressions outside the scope.  (The pragma does not, however,
       affect the "quotemeta" behavior.  Nor does it affect the deprecated
       user-defined case changing operations--these still require a UTF-8
       encoded string to operate.)

       In Perl 5.12, the subpragma affected casing changes, but not regular
       expressions.  See "lc" in perlfunc for details on how this pragma works
       in combination with various others for casing.

       For earlier Perls, 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.
           characters in the scalar.  What the "UTF8" flag means is that the
           sequence of octets in the representation of the scalar is the
           sequence of UTF-8 encoded code points of the characters of a
           string.  The "UTF8" flag being off means that each octet in this
           representation encodes a single character with code point 0..255
           within the string.  Perl's Unicode model is not to use UTF-8 until
           it is absolutely necessary.

       o   "uvchr_to_utf8(buf, chr)" writes a Unicode character code point
           into a buffer encoding the code point as UTF-8, and returns a
           pointer pointing after the UTF-8 bytes.  It works appropriately on
           EBCDIC machines.

       o   "utf8_to_uvchr(buf, lenp)" reads UTF-8 encoded bytes from a buffer
           and returns the Unicode character code point and, optionally, the
           length of the UTF-8 byte sequence.  It works appropriately on
           EBCDIC machines.

       o   "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.

       o   "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).

       o   is_utf8_char(s) returns true if the pointer points to a valid UTF-8
           character.

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

       o   "UTF8SKIP(buf)" will return the number of bytes in the UTF-8
           encoded character in the buffer.  "UNISKIP(chr)" will return the
           number of bytes required to UTF-8-encode the Unicode character code
           point.  "UTF8SKIP()" is useful for example for iterating over the
           characters of a UTF-8 encoded buffer; "UNISKIP()" is useful, for
           example, in computing the size required for a UTF-8 encoded buffer.

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

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


       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).

       It is even possible to copy the built files to a different directory,
       and then change utf8_heavy.pl in the directory $Config{privlib} to
       point to the new directory, or maybe make a copy of that directory
       before making the change, and using @INC or the "-I" run-time flag to
       switch between versions at will (but because of caching, not in the
       middle of a process), but all this is beyond the scope of these
       instructions.

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
       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

       o   A scalar that is going to be passed to some extension

           Be it Compress::Zlib, Apache::Request or any extension that has no
           mention of Unicode in the manpage, you need to make sure that the
           UTF8 flag is stripped off. Note that at the time of this writing
           (October 2002) the mentioned modules are not UTF-8-aware. Please
           check the documentation to verify if this is still true.

             if ($] > 5.007) {
               require Encode;
               $val = Encode::encode_utf8($val); # make octets
             }

       o   A scalar we got back from an extension

           If you believe the scalar comes back as UTF-8, you will most likely
           want the UTF8 flag restored:

             if ($] > 5.007) {
               require Encode;
               $val = Encode::decode_utf8($val);
             }

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

             if ($] > 5.007) {
               require Encode;
               Encode::_utf8_on($val);
             }

       o   A wrapper for fetchrow_array and fetchrow_hashref

           When the database contains only UTF-8, a wrapper function or method
           is a convenient way to replace all your fetchrow_array and
           fetchrow_hashref calls. A wrapper function will also make it easier
           to adapt to future enhancements in your database driver. Note that
           at the time of this writing (October 2002), the DBI has no
           standardized way to deal with UTF-8 data. Please check the
           documentation to verify if that is still true.

             sub fetchrow {
               # $what is one of fetchrow_{array,hashref}
               my($self, $sth, $what) = @_;
               if ($] < 5.007) {
                 return $sth->$what;
               } else {
                 require Encode;
                 if (wantarray) {
                   my @arr = $sth->$what;
                   for (@arr) {
                     defined && /[^\000-\177]/ && Encode::_utf8_on($_);
                   }
                   return @arr;
                   }
                 }
               }
             }

       o   A large scalar that you know can only contain ASCII

           Scalars that contain only ASCII and are marked as UTF-8 are
           sometimes a drag to your program. If you recognize such a
           situation, just remove the UTF8 flag:

             utf8::downgrade($val) if $] > 5.007;

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



perl v5.14.2                      2011-09-26                    PERLUNICODE(1)
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