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NAME
    Unicode::Collate - Unicode Collation Algorithm

SYNOPSIS
      use Unicode::Collate;

      #construct
      $Collator = Unicode::Collate->new(%tailoring);

      #sort
      @sorted = $Collator->sort(@not_sorted);

      #compare
      $result = $Collator->cmp($a, $b); # returns 1, 0, or -1.

    Note: Strings in @not_sorted, $a and $b are interpreted according to
    Perl's Unicode support. See perlunicode, perluniintro, perlunitut,
    perlunifaq, utf8. Otherwise you can use "preprocess" or should decode
    them before.

DESCRIPTION
    This module is an implementation of Unicode Technical Standard #10
    (a.k.a. UTS #10) - Unicode Collation Algorithm (a.k.a. UCA).

  Constructor and Tailoring
    The "new" method returns a collator object. If new() is called with no
    parameters, the collator should do the default collation.

       $Collator = Unicode::Collate->new(
          UCA_Version => $UCA_Version,
          alternate => $alternate, # alias for 'variable'
          backwards => $levelNumber, # or \@levelNumbers
          entry => $element,
          hangul_terminator => $term_primary_weight,
          highestFFFF => $bool,
          identical => $bool,
          ignoreName => qr/$ignoreName/,
          ignoreChar => qr/$ignoreChar/,
          ignore_level2 => $bool,
          katakana_before_hiragana => $bool,
          level => $collationLevel,
          long_contraction => $bool,
          minimalFFFE => $bool,
          normalization  => $normalization_form,
          overrideCJK => \&overrideCJK,
          overrideHangul => \&overrideHangul,
          preprocess => \&preprocess,
          rearrange => \@charList,
          rewrite => \&rewrite,
          suppress => \@charList,
          table => $filename,
          undefName => qr/$undefName/,
          undefChar => qr/$undefChar/,
          upper_before_lower => $bool,
          variable => $variable,
       );

    UCA_Version
        If the revision (previously "tracking version") number of UCA is
        given, behavior of that revision is emulated on collating. If
        omitted, the return value of "UCA_Version()" is used.

        The following revisions are supported. The default is 43.

             UCA       Unicode Standard         DUCET (@version)
           -------------------------------------------------------
              8              3.1                3.0.1 (3.0.1d9)
              9     3.1 with Corrigendum 3      3.1.1
             11             4.0.0
             14             4.1.0
             16             5.0.0
             18             5.1.0
             20             5.2.0
             22             6.0.0
             24             6.1.0
             26             6.2.0
             28             6.3.0
             30             7.0.0
             32             8.0.0
             34             9.0.0
             36            10.0.0
             38            11.0.0
             40            12.0.0
             41            12.1.0
             43            13.0.0

        * See below for "long_contraction" with "UCA_Version" 22 and 24.

        * Noncharacters (e.g. U+FFFF) are not ignored, and can be overridden
        since "UCA_Version" 22.

        * Out-of-range codepoints (greater than U+10FFFF) are not ignored,
        and can be overridden since "UCA_Version" 22.

        * Fully ignorable characters were ignored, and would not interrupt
        contractions with "UCA_Version" 9 and 11.

        * Treatment of ignorables after variables and some behaviors were
        changed at "UCA_Version" 9.

        * Characters regarded as CJK unified ideographs (cf. "overrideCJK")
        depend on "UCA_Version".

        * Many hangul jamo are assigned at "UCA_Version" 20, that will
        affect "hangul_terminator".

    alternate
        -- see 3.2.2 Alternate Weighting, version 8 of UTS #10

        For backward compatibility, "alternate" (old name) can be used as an
        alias for "variable".

    backwards
        -- see 3.4 Backward Accents, UTS #10.

             backwards => $levelNumber or \@levelNumbers

        Weights in reverse order; ex. level 2 (diacritic ordering) in
        French. If omitted (or $levelNumber is "undef" or "\@levelNumbers"
        is "[]"), forwards at all the levels.

    entry
        -- see 5 Tailoring; 9.1 Allkeys File Format, UTS #10.

        If the same character (or a sequence of characters) exists in the
        collation element table through "table", mapping to collation
        elements is overridden. If it does not exist, the mapping is defined
        additionally.

            entry => <<'ENTRY', # for DUCET v4.0.0 (allkeys-4.0.0.txt)
        0063 0068 ; [.0E6A.0020.0002.0063] # ch
        0043 0068 ; [.0E6A.0020.0007.0043] # Ch
        0043 0048 ; [.0E6A.0020.0008.0043] # CH
        006C 006C ; [.0F4C.0020.0002.006C] # ll
        004C 006C ; [.0F4C.0020.0007.004C] # Ll
        004C 004C ; [.0F4C.0020.0008.004C] # LL
        00F1      ; [.0F7B.0020.0002.00F1] # n-tilde
        006E 0303 ; [.0F7B.0020.0002.00F1] # n-tilde
        00D1      ; [.0F7B.0020.0008.00D1] # N-tilde
        004E 0303 ; [.0F7B.0020.0008.00D1] # N-tilde
        ENTRY

            entry => <<'ENTRY', # for DUCET v4.0.0 (allkeys-4.0.0.txt)
        00E6 ; [.0E33.0020.0002.00E6][.0E8B.0020.0002.00E6] # ae ligature as <a><e>
        00C6 ; [.0E33.0020.0008.00C6][.0E8B.0020.0008.00C6] # AE ligature as <A><E>
        ENTRY

        NOTE: The code point in the UCA file format (before ';') must be a
        Unicode code point (defined as hexadecimal), but not a native code
        point. So 0063 must always denote "U+0063", but not a character of
        "\x63".

        Weighting may vary depending on collation element table. So ensure
        the weights defined in "entry" will be consistent with those in the
        collation element table loaded via "table".

        In DUCET v4.0.0, primary weight of "C" is 0E60 and that of "D" is
        "0E6D". So setting primary weight of "CH" to "0E6A" (as a value
        between 0E60 and "0E6D") makes ordering as "C < CH < D". Exactly
        speaking DUCET already has some characters between "C" and "D":
        "small capital C" ("U+1D04") with primary weight 0E64,
        "c-hook/C-hook" ("U+0188/U+0187") with 0E65, and "c-curl" ("U+0255")
        with 0E69. Then primary weight "0E6A" for "CH" makes "CH" ordered
        between "c-curl" and "D".

    hangul_terminator
        -- see 7.1.4 Trailing Weights, UTS #10.

        If a true value is given (non-zero but should be positive), it will
        be added as a terminator primary weight to the end of every standard
        Hangul syllable. Secondary and any higher weights for terminator are
        set to zero. If the value is false or "hangul_terminator" key does
        not exist, insertion of terminator weights will not be performed.

        Boundaries of Hangul syllables are determined according to
        conjoining Jamo behavior in the Unicode Standard and
        HangulSyllableType.txt.

        Implementation Note: (1) For expansion mapping (Unicode character
        mapped to a sequence of collation elements), a terminator will not
        be added between collation elements, even if Hangul syllable
        boundary exists there. Addition of terminator is restricted to the
        next position to the last collation element.

        (2) Non-conjoining Hangul letters (Compatibility Jamo, halfwidth
        Jamo, and enclosed letters) are not automatically terminated with a
        terminator primary weight. These characters may need terminator
        included in a collation element table beforehand.

    highestFFFF
        -- see 2.4 Tailored noncharacter weights, UTS #35 (LDML) Part 5:
        Collation.

        If the parameter is made true, "U+FFFF" has a highest primary
        weight. When a boolean of "$coll->ge($str, "abc")" and
        "$coll->le($str, "abc\x{FFFF}")" is true, it is expected that $str
        begins with "abc", or another primary equivalent. $str may be
        "abcd", "abc012", but should not include "U+FFFF" such as
        "abc\x{FFFF}xyz".

        "$coll->le($str, "abc\x{FFFF}")" works like "$coll->lt($str, "abd")"
        almost, but the latter has a problem that you should know which
        letter is next to "c". For a certain language where "ch" as the next
        letter, "abch" is greater than "abc\x{FFFF}", but less than "abd".

        Note: This is equivalent to "(entry => 'FFFF ;
        [.FFFE.0020.0005.FFFF]')". Any other character than "U+FFFF" can be
        tailored by "entry".

    identical
        -- see A.3 Deterministic Comparison, UTS #10.

        By default, strings whose weights are equal should be equal, even
        though their code points are not equal. Completely ignorable
        characters are ignored.

        If the parameter is made true, a final, tie-breaking level is used.
        If no difference of weights is found after the comparison through
        all the level specified by "level", the comparison with code points
        will be performed. For the tie-breaking comparison, the sort key has
        code points of the original string appended. Completely ignorable
        characters are not ignored.

        If "preprocess" and/or "normalization" is applied, the code points
        of the string after them (in NFD by default) are used.

    ignoreChar
    ignoreName
        -- see 3.6 Variable Weighting, UTS #10.

        Makes the entry in the table completely ignorable; i.e. as if the
        weights were zero at all level.

        Through "ignoreChar", any character matching "qr/$ignoreChar/" will
        be ignored. Through "ignoreName", any character whose name (given in
        the "table" file as a comment) matches "qr/$ignoreName/" will be
        ignored.

        E.g. when 'a' and 'e' are ignorable, 'element' is equal to 'lament'
        (or 'lmnt').

    ignore_level2
        -- see 5.1 Parametric Tailoring, UTS #10.

        By default, case-sensitive comparison (that is level 3 difference)
        won't ignore accents (that is level 2 difference).

        If the parameter is made true, accents (and other primary ignorable
        characters) are ignored, even though cases are taken into account.

        NOTE: "level" should be 3 or greater.

    katakana_before_hiragana
        -- see 7.2 Tertiary Weight Table, UTS #10.

        By default, hiragana is before katakana. If the parameter is made
        true, this is reversed.

        NOTE: This parameter simplemindedly assumes that any
        hiragana/katakana distinctions must occur in level 3, and their
        weights at level 3 must be same as those mentioned in 7.3.1, UTS
        #10. If you define your collation elements which violate this
        requirement, this parameter does not work validly.

    level
        -- see 4.3 Form Sort Key, UTS #10.

        Set the maximum level. Any higher levels than the specified one are
        ignored.

          Level 1: alphabetic ordering
          Level 2: diacritic ordering
          Level 3: case ordering
          Level 4: tie-breaking (e.g. in the case when variable is 'shifted')

          ex.level => 2,

        If omitted, the maximum is the 4th.

        NOTE: The DUCET includes weights over 0xFFFF at the 4th level. But
        this module only uses weights within 0xFFFF. When "variable" is
        'blanked' or 'non-ignorable' (other than 'shifted' and
        'shift-trimmed'), the level 4 may be unreliable.

        See also "identical".

    long_contraction
        -- see 3.8.2 Well-Formedness of the DUCET, 4.2 Produce Array, UTS
        #10.

        If the parameter is made true, for a contraction with three or more
        characters (here nicknamed "long contraction"), initial substrings
        will be handled. For example, a contraction ABC, where A is a
        starter, and B and C are non-starters (character with non-zero
        combining character class), will be detected even if there is not AB
        as a contraction.

        Default: Usually false. If "UCA_Version" is 22 or 24, and the value
        of "long_contraction" is not specified in "new()", a true value is
        set implicitly. This is a workaround to pass Conformance Tests for
        Unicode 6.0.0 and 6.1.0.

        "change()" handles "long_contraction" explicitly only. If
        "long_contraction" is not specified in "change()", even though
        "UCA_Version" is changed, "long_contraction" will not be changed.

        Limitation: Scanning non-starters is one-way (no back tracking). If
        AB is found but not ABC is not found, other long contraction where
        the first character is A and the second is not B may not be found.

        Under "(normalization => undef)", detection step of discontiguous
        contractions will be skipped.

        Note: The following contractions in DUCET are not considered in
        steps S2.1.1 to S2.1.3, where they are discontiguous.

            0FB2 0F71 0F80 (TIBETAN VOWEL SIGN VOCALIC RR)
            0FB3 0F71 0F80 (TIBETAN VOWEL SIGN VOCALIC LL)

        For example "TIBETAN VOWEL SIGN VOCALIC RR" with "COMBINING TILDE
        OVERLAY" ("U+0344") is "0FB2 0344 0F71 0F80" in NFD. In this case
        "0FB2 0F80" ("TIBETAN VOWEL SIGN VOCALIC R") is detected, instead of
        "0FB2 0F71 0F80". Inserted 0344 makes "0FB2 0F71 0F80" discontiguous
        and lack of contraction "0FB2 0F71" prohibits "0FB2 0F71 0F80" from
        being detected.

    minimalFFFE
        -- see 1.1.1 U+FFFE, UTS #35 (LDML) Part 5: Collation.

        If the parameter is made true, "U+FFFE" has a minimal primary
        weight. The comparison between "$a1\x{FFFE}$a2" and "$b1\x{FFFE}$b2"
        first compares $a1 and $b1 at level 1, and then $a2 and $b2 at level
        1, as followed.

                "ab\x{FFFE}a"
                "Ab\x{FFFE}a"
                "ab\x{FFFE}c"
                "Ab\x{FFFE}c"
                "ab\x{FFFE}xyz"
                "abc\x{FFFE}def"
                "abc\x{FFFE}xYz"
                "aBc\x{FFFE}xyz"
                "abcX\x{FFFE}def"
                "abcx\x{FFFE}xyz"
                "b\x{FFFE}aaa"
                "bbb\x{FFFE}a"

        Note: This is equivalent to "(entry => 'FFFE ;
        [.0001.0020.0005.FFFE]')". Any other character than "U+FFFE" can be
        tailored by "entry".

    normalization
        -- see 4.1 Normalize, UTS #10.

        If specified, strings are normalized before preparation of sort keys
        (the normalization is executed after preprocess).

        A form name "Unicode::Normalize::normalize()" accepts will be
        applied as $normalization_form. Acceptable names include 'NFD',
        'NFC', 'NFKD', and 'NFKC'. See "Unicode::Normalize::normalize()" for
        detail. If omitted, 'NFD' is used.

        "normalization" is performed after "preprocess" (if defined).

        Furthermore, special values, "undef" and "prenormalized", can be
        used, though they are not concerned with
        "Unicode::Normalize::normalize()".

        If "undef" (not a string "undef") is passed explicitly as the value
        for this key, any normalization is not carried out (this may make
        tailoring easier if any normalization is not desired). Under
        "(normalization => undef)", only contiguous contractions are
        resolved; e.g. even if "A-ring" (and "A-ring-cedilla") is ordered
        after "Z", "A-cedilla-ring" would be primary equal to "A". In this
        point, "(normalization => undef, preprocess => sub { NFD(shift) })"
        is not equivalent to "(normalization => 'NFD')".

        In the case of "(normalization => "prenormalized")", any
        normalization is not performed, but discontiguous contractions with
        combining characters are performed. Therefore "(normalization =>
        'prenormalized', preprocess => sub { NFD(shift) })" is equivalent to
        "(normalization => 'NFD')". If source strings are finely
        prenormalized, "(normalization => 'prenormalized')" may save time
        for normalization.

        Except "(normalization => undef)", Unicode::Normalize is required
        (see also CAVEAT).

    overrideCJK
        -- see 7.1 Derived Collation Elements, UTS #10.

        By default, CJK unified ideographs are ordered in Unicode codepoint
        order, but those in the CJK Unified Ideographs block are less than
        those in the CJK Unified Ideographs Extension A etc.

            In the CJK Unified Ideographs block:
            U+4E00..U+9FA5 if UCA_Version is 8, 9 or 11.
            U+4E00..U+9FBB if UCA_Version is 14 or 16.
            U+4E00..U+9FC3 if UCA_Version is 18.
            U+4E00..U+9FCB if UCA_Version is 20 or 22.
            U+4E00..U+9FCC if UCA_Version is 24 to 30.
            U+4E00..U+9FD5 if UCA_Version is 32 or 34.
            U+4E00..U+9FEA if UCA_Version is 36.
            U+4E00..U+9FEF if UCA_Version is 38, 40 or 41.
            U+4E00..U+9FFC if UCA_Version is 43.

            In the CJK Unified Ideographs Extension blocks:
            Ext.A (U+3400..U+4DB5)   if UCA_Version is  8 to 41.
            Ext.A (U+3400..U+4DBF)   if UCA_Version is 43.
            Ext.B (U+20000..U+2A6D6) if UCA_Version is  8 to 41.
            Ext.B (U+20000..U+2A6DD) if UCA_Version is 43.
            Ext.C (U+2A700..U+2B734) if UCA_Version is 20 or later.
            Ext.D (U+2B740..U+2B81D) if UCA_Version is 22 or later.
            Ext.E (U+2B820..U+2CEA1) if UCA_Version is 32 or later.
            Ext.F (U+2CEB0..U+2EBE0) if UCA_Version is 36 or later.
            Ext.G (U+30000..U+3134A) if UCA_Version is 43.

        Through "overrideCJK", ordering of CJK unified ideographs (including
        extensions) can be overridden.

        ex. CJK unified ideographs in the JIS code point order.

          overrideCJK => sub {
              my $u = shift;             # get a Unicode codepoint
              my $b = pack('n', $u);     # to UTF-16BE
              my $s = your_unicode_to_sjis_converter($b); # convert
              my $n = unpack('n', $s);   # convert sjis to short
              [ $n, 0x20, 0x2, $u ];     # return the collation element
          },

        The return value may be an arrayref of 1st to 4th weights as shown
        above. The return value may be an integer as the primary weight as
        shown below. If "undef" is returned, the default derived collation
        element will be used.

          overrideCJK => sub {
              my $u = shift;             # get a Unicode codepoint
              my $b = pack('n', $u);     # to UTF-16BE
              my $s = your_unicode_to_sjis_converter($b); # convert
              my $n = unpack('n', $s);   # convert sjis to short
              return $n;                 # return the primary weight
          },

        The return value may be a list containing zero or more of an
        arrayref, an integer, or "undef".

        ex. ignores all CJK unified ideographs.

          overrideCJK => sub {()}, # CODEREF returning empty list

           # where ->eq("Pe\x{4E00}rl", "Perl") is true
           # as U+4E00 is a CJK unified ideograph and to be ignorable.

        If a false value (including "undef") is passed, "overrideCJK" has no
        effect. "$Collator->change(overrideCJK => 0)" resets the old one.

        But assignment of weight for CJK unified ideographs in "table" or
        "entry" is still valid. If "undef" is passed explicitly as the value
        for this key, weights for CJK unified ideographs are treated as
        undefined. However when "UCA_Version" > 8, "(overrideCJK => undef)"
        has no special meaning.

        Note: In addition to them, 12 CJK compatibility ideographs
        ("U+FA0E", "U+FA0F", "U+FA11", "U+FA13", "U+FA14", "U+FA1F",
        "U+FA21", "U+FA23", "U+FA24", "U+FA27", "U+FA28", "U+FA29") are also
        treated as CJK unified ideographs. But they can't be overridden via
        "overrideCJK" when you use DUCET, as the table includes weights for
        them. "table" or "entry" has priority over "overrideCJK".

    overrideHangul
        -- see 7.1 Derived Collation Elements, UTS #10.

        By default, Hangul syllables are decomposed into Hangul Jamo, even
        if "(normalization => undef)". But the mapping of Hangul syllables
        may be overridden.

        This parameter works like "overrideCJK", so see there for examples.

        If you want to override the mapping of Hangul syllables, NFD and
        NFKD are not appropriate, since NFD and NFKD will decompose Hangul
        syllables before overriding. FCD may decompose Hangul syllables as
        the case may be.

        If a false value (but not "undef") is passed, "overrideHangul" has
        no effect. "$Collator->change(overrideHangul => 0)" resets the old
        one.

        If "undef" is passed explicitly as the value for this key, weight
        for Hangul syllables is treated as undefined without decomposition
        into Hangul Jamo. But definition of weight for Hangul syllables in
        "table" or "entry" is still valid.

    overrideOut
        -- see 7.1.1 Handling Ill-Formed Code Unit Sequences, UTS #10.

        Perl seems to allow out-of-range values (greater than 0x10FFFF). By
        default, out-of-range values are replaced with "U+FFFD" (REPLACEMENT
        CHARACTER) when "UCA_Version" >= 22, or ignored when "UCA_Version"
        <= 20.

        When "UCA_Version" >= 22, the weights of out-of-range values can be
        overridden. Though "table" or "entry" are available for them,
        out-of-range values are too many.

        "overrideOut" can perform it algorithmically. This parameter works
        like "overrideCJK", so see there for examples.

        ex. ignores all out-of-range values.

          overrideOut => sub {()}, # CODEREF returning empty list

        If a false value (including "undef") is passed, "overrideOut" has no
        effect. "$Collator->change(overrideOut => 0)" resets the old one.

        NOTE ABOUT U+FFFD:

        UCA recommends that out-of-range values should not be ignored for
        security reasons. Say, "pe\x{110000}rl" should not be equal to
        "perl". However, "U+FFFD" is wrongly mapped to a variable collation
        element in DUCET for Unicode 6.0.0 to 6.2.0, that means out-of-range
        values will be ignored when "variable" isn't "Non-ignorable".

        The mapping of "U+FFFD" is corrected in Unicode 6.3.0. see
        <http://www.unicode.org/reports/tr10/tr10-28.html#Trailing_Weights>
        (7.1.4 Trailing Weights). Such a correction is reproduced by this.

          overrideOut => sub { 0xFFFD }, # CODEREF returning a very large integer

        This workaround is unnecessary since Unicode 6.3.0.

    preprocess
        -- see 5.4 Preprocessing, UTS #10.

        If specified, the coderef is used to preprocess each string before
        the formation of sort keys.

        ex. dropping English articles, such as "a" or "the". Then, "the pen"
        is before "a pencil".

             preprocess => sub {
                   my $str = shift;
                   $str =~ s/\b(?:an?|the)\s+//gi;
                   return $str;
                },

        "preprocess" is performed before "normalization" (if defined).

        ex. decoding strings in a legacy encoding such as shift-jis:

            $sjis_collator = Unicode::Collate->new(
                preprocess => \&your_shiftjis_to_unicode_decoder,
            );
            @result = $sjis_collator->sort(@shiftjis_strings);

        Note: Strings returned from the coderef will be interpreted
        according to Perl's Unicode support. See perlunicode, perluniintro,
        perlunitut, perlunifaq, utf8.

    rearrange
        -- see 3.5 Rearrangement, UTS #10.

        Characters that are not coded in logical order and to be rearranged.
        If "UCA_Version" is equal to or less than 11, default is:

            rearrange => [ 0x0E40..0x0E44, 0x0EC0..0x0EC4 ],

        If you want to disallow any rearrangement, pass "undef" or "[]" (a
        reference to empty list) as the value for this key.

        If "UCA_Version" is equal to or greater than 14, default is "[]"
        (i.e. no rearrangement).

        According to the version 9 of UCA, this parameter shall not be used;
        but it is not warned at present.

    rewrite
        If specified, the coderef is used to rewrite lines in "table" or
        "entry". The coderef will get each line, and then should return a
        rewritten line according to the UCA file format. If the coderef
        returns an empty line, the line will be skipped.

        e.g. any primary ignorable characters into tertiary ignorable:

            rewrite => sub {
                my $line = shift;
                $line =~ s/\[\.0000\..{4}\..{4}\./[.0000.0000.0000./g;
                return $line;
            },

        This example shows rewriting weights. "rewrite" is allowed to affect
        code points, weights, and the name.

        NOTE: "table" is available to use another table file; preparing a
        modified table once would be more efficient than rewriting lines on
        reading an unmodified table every time.

    suppress
        -- see 3.12 Special-Purpose Commands, UTS #35 (LDML) Part 5:
        Collation.

        Contractions beginning with the specified characters are suppressed,
        even if those contractions are defined in "table".

        An example for Russian and some languages using the Cyrillic script:

            suppress => [0x0400..0x0417, 0x041A..0x0437, 0x043A..0x045F],

        where 0x0400 stands for "U+0400", CYRILLIC CAPITAL LETTER IE WITH
        GRAVE.

        NOTE: Contractions via "entry" will not be suppressed.

    table
        -- see 3.8 Default Unicode Collation Element Table, UTS #10.

        You can use another collation element table if desired.

        The table file should locate in the Unicode/Collate directory on
        @INC. Say, if the filename is Foo.txt, the table file is searched as
        Unicode/Collate/Foo.txt in @INC.

        By default, allkeys.txt (as the filename of DUCET) is used. If you
        will prepare your own table file, any name other than allkeys.txt
        may be better to avoid namespace conflict.

        NOTE: When XSUB is used, the DUCET is compiled on building this
        module, and it may save time at the run time. Explicit saying
        "(table => 'allkeys.txt')", or using another table, or using
        "ignoreChar", "ignoreName", "undefChar", "undefName" or "rewrite"
        will prevent this module from using the compiled DUCET.

        If "undef" is passed explicitly as the value for this key, no file
        is read (but you can define collation elements via "entry").

        A typical way to define a collation element table without any file
        of table:

           $onlyABC = Unicode::Collate->new(
               table => undef,
               entry => << 'ENTRIES',
        0061 ; [.0101.0020.0002.0061] # LATIN SMALL LETTER A
        0041 ; [.0101.0020.0008.0041] # LATIN CAPITAL LETTER A
        0062 ; [.0102.0020.0002.0062] # LATIN SMALL LETTER B
        0042 ; [.0102.0020.0008.0042] # LATIN CAPITAL LETTER B
        0063 ; [.0103.0020.0002.0063] # LATIN SMALL LETTER C
        0043 ; [.0103.0020.0008.0043] # LATIN CAPITAL LETTER C
        ENTRIES
            );

        If "ignoreName" or "undefName" is used, character names should be
        specified as a comment (following "#") on each line.

    undefChar
    undefName
        -- see 6.3.3 Reducing the Repertoire, UTS #10.

        Undefines the collation element as if it were unassigned in the
        "table". This reduces the size of the table. If an unassigned
        character appears in the string to be collated, the sort key is made
        from its codepoint as a single-character collation element, as it is
        greater than any other assigned collation elements (in the codepoint
        order among the unassigned characters). But, it'd be better to
        ignore characters unfamiliar to you and maybe never used.

        Through "undefChar", any character matching "qr/$undefChar/" will be
        undefined. Through "undefName", any character whose name (given in
        the "table" file as a comment) matches "qr/$undefName/" will be
        undefined.

        ex. Collation weights for beyond-BMP characters are not stored in
        object:

            undefChar => qr/[^\0-\x{fffd}]/,

    upper_before_lower
        -- see 6.6 Case Comparisons, UTS #10.

        By default, lowercase is before uppercase. If the parameter is made
        true, this is reversed.

        NOTE: This parameter simplemindedly assumes that any
        lowercase/uppercase distinctions must occur in level 3, and their
        weights at level 3 must be same as those mentioned in 7.3.1, UTS
        #10. If you define your collation elements which differs from this
        requirement, this parameter doesn't work validly.

    variable
        -- see 3.6 Variable Weighting, UTS #10.

        This key allows for variable weighting of variable collation
        elements, which are marked with an ASTERISK in the table (NOTE: Many
        punctuation marks and symbols are variable in allkeys.txt).

           variable => 'blanked', 'non-ignorable', 'shifted', or 'shift-trimmed'.

        These names are case-insensitive. By default (if specification is
        omitted), 'shifted' is adopted.

           'Blanked'        Variable elements are made ignorable at levels 1 through 3;
                            considered at the 4th level.

           'Non-Ignorable'  Variable elements are not reset to ignorable.

           'Shifted'        Variable elements are made ignorable at levels 1 through 3
                            their level 4 weight is replaced by the old level 1 weight.
                            Level 4 weight for Non-Variable elements is 0xFFFF.

           'Shift-Trimmed'  Same as 'shifted', but all FFFF's at the 4th level
                            are trimmed.

  Methods for Collation
    "@sorted = $Collator->sort(@not_sorted)"
        Sorts a list of strings.

    "$result = $Collator->cmp($a, $b)"
        Returns 1 (when $a is greater than $b) or 0 (when $a is equal to $b)
        or -1 (when $a is less than $b).

    "$result = $Collator->eq($a, $b)"
    "$result = $Collator->ne($a, $b)"
    "$result = $Collator->lt($a, $b)"
    "$result = $Collator->le($a, $b)"
    "$result = $Collator->gt($a, $b)"
    "$result = $Collator->ge($a, $b)"
        They works like the same name operators as theirs.

           eq : whether $a is equal to $b.
           ne : whether $a is not equal to $b.
           lt : whether $a is less than $b.
           le : whether $a is less than $b or equal to $b.
           gt : whether $a is greater than $b.
           ge : whether $a is greater than $b or equal to $b.

    "$sortKey = $Collator->getSortKey($string)"
        -- see 4.3 Form Sort Key, UTS #10.

        Returns a sort key.

        You compare the sort keys using a binary comparison and get the
        result of the comparison of the strings using UCA.

           $Collator->getSortKey($a) cmp $Collator->getSortKey($b)

              is equivalent to

           $Collator->cmp($a, $b)

    "$sortKeyForm = $Collator->viewSortKey($string)"
        Converts a sorting key into its representation form. If
        "UCA_Version" is 8, the output is slightly different.

           use Unicode::Collate;
           my $c = Unicode::Collate->new();
           print $c->viewSortKey("Perl"),"\n";

           # output:
           # [0B67 0A65 0B7F 0B03 | 0020 0020 0020 0020 | 0008 0002 0002 0002 | FFFF FFFF FFFF FFFF]
           #  Level 1               Level 2               Level 3               Level 4

  Methods for Searching
    The "match", "gmatch", "subst", "gsubst" methods work like "m//",
    "m//g", "s///", "s///g", respectively, but they are not aware of any
    pattern, but only a literal substring.

    DISCLAIMER: If "preprocess" or "normalization" parameter is true for
    $Collator, calling these methods ("index", "match", "gmatch", "subst",
    "gsubst") is croaked, as the position and the length might differ from
    those on the specified string.

    "rearrange" and "hangul_terminator" parameters are neglected.
    "katakana_before_hiragana" and "upper_before_lower" don't affect
    matching and searching, as it doesn't matter whether greater or less.

    "$position = $Collator->index($string, $substring[, $position])"
    "($position, $length) = $Collator->index($string, $substring[,
    $position])"
        If $substring matches a part of $string, returns the position of the
        first occurrence of the matching part in scalar context; in list
        context, returns a two-element list of the position and the length
        of the matching part.

        If $substring does not match any part of $string, returns -1 in
        scalar context and an empty list in list context.

        e.g. when the content of $str is ""Ich mu"ß" studieren Perl."", you
        say the following where $sub is ""M"ü"SS"",

          my $Collator = Unicode::Collate->new( normalization => undef, level => 1 );
                                             # (normalization => undef) is REQUIRED.
          my $match;
          if (my($pos,$len) = $Collator->index($str, $sub)) {
              $match = substr($str, $pos, $len);
          }

        and get ""mu"ß""" in $match, since ""mu"ß""" is primary equal to
        ""M"ü"SS"".

    "$match_ref = $Collator->match($string, $substring)"
    "($match) = $Collator->match($string, $substring)"
        If $substring matches a part of $string, in scalar context, returns
        a reference to the first occurrence of the matching part ($match_ref
        is always true if matches, since every reference is true); in list
        context, returns the first occurrence of the matching part.

        If $substring does not match any part of $string, returns "undef" in
        scalar context and an empty list in list context.

        e.g.

            if ($match_ref = $Collator->match($str, $sub)) { # scalar context
                print "matches [$$match_ref].\n";
            } else {
                print "doesn't match.\n";
            }

             or

            if (($match) = $Collator->match($str, $sub)) { # list context
                print "matches [$match].\n";
            } else {
                print "doesn't match.\n";
            }

    "@match = $Collator->gmatch($string, $substring)"
        If $substring matches a part of $string, returns all the matching
        parts (or matching count in scalar context).

        If $substring does not match any part of $string, returns an empty
        list.

    "$count = $Collator->subst($string, $substring, $replacement)"
        If $substring matches a part of $string, the first occurrence of the
        matching part is replaced by $replacement ($string is modified) and
        $count (always equals to 1) is returned.

        $replacement can be a "CODEREF", taking the matching part as an
        argument, and returning a string to replace the matching part (a bit
        similar to "s/(..)/$coderef->($1)/e").

    "$count = $Collator->gsubst($string, $substring, $replacement)"
        If $substring matches a part of $string, all the occurrences of the
        matching part are replaced by $replacement ($string is modified) and
        $count is returned.

        $replacement can be a "CODEREF", taking the matching part as an
        argument, and returning a string to replace the matching part (a bit
        similar to "s/(..)/$coderef->($1)/eg").

        e.g.

          my $Collator = Unicode::Collate->new( normalization => undef, level => 1 );
                                             # (normalization => undef) is REQUIRED.
          my $str = "Camel donkey zebra came\x{301}l CAMEL horse cam\0e\0l...";
          $Collator->gsubst($str, "camel", sub { "<b>$_[0]</b>" });

          # now $str is "<b>Camel</b> donkey zebra <b>came\x{301}l</b> <b>CAMEL</b> horse <b>cam\0e\0l</b>...";
          # i.e., all the camels are made bold-faced.

           Examples: levels and ignore_level2 - what does camel match?
          ---------------------------------------------------------------------------
           level  ignore_level2  |  camel  Camel  came\x{301}l  c-a-m-e-l  cam\0e\0l
          -----------------------|---------------------------------------------------
             1        false      |   yes    yes      yes          yes        yes
             2        false      |   yes    yes      no           yes        yes
             3        false      |   yes    no       no           yes        yes
             4        false      |   yes    no       no           no         yes
          -----------------------|---------------------------------------------------
             1        true       |   yes    yes      yes          yes        yes
             2        true       |   yes    yes      yes          yes        yes
             3        true       |   yes    no       yes          yes        yes
             4        true       |   yes    no       yes          no         yes
          ---------------------------------------------------------------------------
           note: if variable => non-ignorable, camel doesn't match c-a-m-e-l
                 at any level.

  Other Methods
    "%old_tailoring = $Collator->change(%new_tailoring)"
    "$modified_collator = $Collator->change(%new_tailoring)"
        Changes the value of specified keys and returns the changed part.

            $Collator = Unicode::Collate->new(level => 4);

            $Collator->eq("perl", "PERL"); # false

            %old = $Collator->change(level => 2); # returns (level => 4).

            $Collator->eq("perl", "PERL"); # true

            $Collator->change(%old); # returns (level => 2).

            $Collator->eq("perl", "PERL"); # false

        Not all "(key,value)"s are allowed to be changed. See also
        @Unicode::Collate::ChangeOK and @Unicode::Collate::ChangeNG.

        In the scalar context, returns the modified collator (but it is not
        a clone from the original).

            $Collator->change(level => 2)->eq("perl", "PERL"); # true

            $Collator->eq("perl", "PERL"); # true; now max level is 2nd.

            $Collator->change(level => 4)->eq("perl", "PERL"); # false

    "$version = $Collator->version()"
        Returns the version number (a string) of the Unicode Standard which
        the "table" file used by the collator object is based on. If the
        table does not include a version line (starting with @version),
        returns "unknown".

    "UCA_Version()"
        Returns the revision number of UTS #10 this module consults, that
        should correspond with the DUCET incorporated.

    "Base_Unicode_Version()"
        Returns the version number of UTS #10 this module consults, that
        should correspond with the DUCET incorporated.

EXPORT
    No method will be exported.

INSTALL
    Though this module can be used without any "table" file, to use this
    module easily, it is recommended to install a table file in the UCA
    format, by copying it under the directory <a place in
    @INC>/Unicode/Collate.

    The most preferable one is "The Default Unicode Collation Element Table"
    (aka DUCET), available from the Unicode Consortium's website:

       http://www.unicode.org/Public/UCA/

       http://www.unicode.org/Public/UCA/latest/allkeys.txt
       (latest version)

    If DUCET is not installed, it is recommended to copy the file from
    http://www.unicode.org/Public/UCA/latest/allkeys.txt to <a place in
    @INC>/Unicode/Collate/allkeys.txt manually.

CAVEATS
    Normalization
        Use of the "normalization" parameter requires the Unicode::Normalize
        module (see Unicode::Normalize).

        If you need not it (say, in the case when you need not handle any
        combining characters), assign "(normalization => undef)" explicitly.

        -- see 6.5 Avoiding Normalization, UTS #10.

    Conformance Test
        The Conformance Test for the UCA is available under
        <http://www.unicode.org/Public/UCA/>.

        For CollationTest_SHIFTED.txt, a collator via
        "Unicode::Collate->new( )" should be used; for
        CollationTest_NON_IGNORABLE.txt, a collator via
        "Unicode::Collate->new(variable => "non-ignorable", level => 3)".

        If "UCA_Version" is 26 or later, the "identical" level is preferred;
        "Unicode::Collate->new(identical => 1)" and
        "Unicode::Collate->new(identical => 1," "variable =>
        "non-ignorable", level => 3)" should be used.

        Unicode::Normalize is required to try The Conformance Test.

AUTHOR, COPYRIGHT AND LICENSE
    The Unicode::Collate module for perl was written by SADAHIRO Tomoyuki,
    <SADAHIRO AT cpan.org>. This module is Copyright(C) 2001-2020, SADAHIRO
    Tomoyuki. Japan. All rights reserved.

    This module is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

    The file Unicode/Collate/allkeys.txt was copied verbatim from
    <http://www.unicode.org/Public/UCA/13.0.0/allkeys.txt>. For this file,
    Copyright (c) 2020 Unicode, Inc.; distributed under the Terms of Use in
    <http://www.unicode.org/terms_of_use.html>

SEE ALSO
    Unicode Collation Algorithm - UTS #10
        <http://www.unicode.org/reports/tr10/>

    The Default Unicode Collation Element Table (DUCET)
        <http://www.unicode.org/Public/UCA/latest/allkeys.txt>

    The conformance test for the UCA
        <http://www.unicode.org/Public/UCA/latest/CollationTest.html>

        <http://www.unicode.org/Public/UCA/latest/CollationTest.zip>

    Hangul Syllable Type
        <http://www.unicode.org/Public/UNIDATA/HangulSyllableType.txt>

    Unicode Normalization Forms - UAX #15
        <http://www.unicode.org/reports/tr15/>

    Unicode Locale Data Markup Language (LDML) - UTS #35
        <http://www.unicode.org/reports/tr35/>


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