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perlunicode (1)


perlunicode - Unicode support in Perl


Please see following description for synopsis


Perl Programmers Reference Guide                   PERLUNICODE(1)

     perlunicode - Unicode support in Perl

  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, before
     reading this reference document.

     Also, the use of Unicode may present security issues that
     aren't obvious.  Read Unicode Security Considerations

     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 ":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;".

     Regular Expressions
         The regular expression compiler produces polymorphic
         opcodes.  That is, the pattern adapts to the data and
         automatically switches to the Unicode character scheme
         when presented with data that is internally encoded in
         UTF-8, or instead uses a traditional byte scheme when
         presented with byte data.

     "use utf8" still needed to enable UTF-8/UTF-EBCDIC in
         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

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         UTF-8 cannot be effectively recognized or differentiated
         from ISO 8859-1 or other eight-bit encodings.)

     "use encoding" needed to upgrade non-Latin-1 byte strings
         By default, there is a fundamental asymmetry in Perl's
         Unicode model: implicit upgrading from byte strings to
         Unicode strings assumes that they were encoded in ISO
         8859-1 (Latin-1), but Unicode strings are downgraded
         with UTF-8 encoding.  This happens because the first 256
         codepoints in Unicode happens to agree with Latin-1.

         See "Byte and Character Semantics" for more details.

  Byte and Character Semantics
     Beginning with version 5.6, Perl uses logically-wide
     characters to represent strings internally.

     In future, Perl-level operations will be expected to work
     with characters rather than bytes.

     However, as an interim compatibility measure, Perl aims to
     provide a safe migration path from byte semantics to
     character semantics for 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.

     Under byte semantics, when "use locale" is in effect, Perl
     uses the semantics associated with the current locale.
     Absent a "use locale", and absent a "use feature
     'unicode_strings'" pragma, Perl currently 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 "bytes" pragma will always, regardless of platform,
     force byte semantics in a particular lexical scope.  See

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     The "use feature 'unicode_strings'" pragma is intended to
     always, regardless of platform, force character (Unicode)
     semantics in a particular lexical scope.  In release 5.12,
     it is partially implemented, applying only to case changes.
     See "The "Unicode Bug"" below.

     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.

     Unless explicitly stated, Perl operators use character
     semantics for Unicode data and byte semantics for non-
     Unicode data.  The decision to use character semantics is
     made transparently.  If input data comes from a Unicode
     source--for example, if a character encoding layer is added
     to a filehandle or a literal Unicode string constant appears
     in a program--character semantics apply.  Otherwise, byte
     semantics are in effect.  The "bytes" pragma should be used
     to force byte semantics on Unicode data, and the "use
     feature 'unicode_strings'" pragma to force Unicode semantics
     on byte data (though in 5.12 it isn't fully implemented).

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

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

  Effects of Character Semantics
     Character semantics have the following effects:

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

         If you use a Unicode editor to edit your program,
         Unicode characters may occur directly within the literal
         strings in UTF-8 encoding, or UTF-16.  (The former
         requires a BOM or "use utf8", the latter requires a

         Unicode characters can also be added to a string by

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         using the "\N{U+...}" notation.  The Unicode code for
         the desired character, in hexadecimal, should be placed
         in the braces, after the "U". For instance, a smiley
         face is "\N{U+263A}".

         Alternatively, you can use the "\x{...}" notation for
         characters 0x100 and above.  For characters below 0x100
         you may get byte semantics instead of character
         semantics;  see "The "Unicode Bug"".  On EBCDIC machines
         there is the additional problem that the value for such
         characters gives the EBCDIC character rather than the
         Unicode one.

         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

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     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).

     o   Most operators that deal with positions or lengths in a
         string will automatically switch to using character
         positions, including "chop()", "chomp()", "substr()",
         "pos()", "index()", "rindex()", "sprintf()", "write()",
         and "length()".  An operator that specifically does not
         switch is "vec()".  Operators that really don't care
         include operators that treat strings as a bucket of bits
         such as "sort()", and operators dealing with filenames.

     o   The "pack()"/"unpack()" letter "C" does not change,
         since it is often used for byte-oriented formats.
         Again, think "char" in the C language.

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

     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

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         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
     Most Unicode character properties are accessible by using
     regular expressions.  They are used (like bracketed
     character classes) by using the "\p{}" "matches property"
     construct and the "\P{}" negation, "doesn't match property".

     Note that 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

     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}",

     This formality is needed when properties are not binary,
     that is if they can take on more values than just True and
     False.  For example, the Bidi_Class (see "Bidirectional
     Character Types" below), can take on a number of different
     values, such as Left, Right, Whitespace, and others.  To
     match these, one needs to specify the property name
     (Bidi_Class), and the value being matched against (Left,
     Right, etc.).  This is done, as in the examples above, by
     having the two components separated by an equal sign (or
     interchangeably, a colon), like "\p{Bidi_Class: Left}".

     All Unicode-defined character properties may be written in
     these compound forms of "\p{property=value}" or
     "\p{property:value}", but Perl provides some additional
     properties that are written only in the single form, as well
     as single-form short-cuts for all binary properties and

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     certain others described below, in which you may omit the
     property name and the equals or colon separator.

     Most Unicode character properties have at least two synonyms
     (or aliases if you prefer), a short one that is easier to
     type, and a longer one which is more descriptive and hence
     it is easier to understand what it means.  Thus the "L" and
     "Letter" 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 the 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, in most cases, white space and
     even hyphens can 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 employed is in the middle of numbers, and the
     Perl extension properties that begin or end with an
     underscore.  Stricter matching cares about white space
     (except adjacent to the non-word characters) and 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}".


     Every Unicode character is assigned a general category,
     which is the "most usual categorization of a character"
     (from <>).

     The compound way of writing these is like
     "\p{General_Category=Number}" (short, "\p{gc:n}").  But Perl
     furnishes shortcuts in which everything up through the equal
     or colon separator is omitted.  So you can instead just

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     write "\pN".

     Here are the short and long forms of the General Category

         Short       Long

         L           Letter
         LC, L&      Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
         Lu          Uppercase_Letter
         Ll          Lowercase_Letter
         Lt          Titlecase_Letter
         Lm          Modifier_Letter
         Lo          Other_Letter

         M           Mark
         Mn          Nonspacing_Mark
         Mc          Spacing_Mark
         Me          Enclosing_Mark

         N           Number
         Nd          Decimal_Number (also Digit)
         Nl          Letter_Number
         No          Other_Number

         P           Punctuation (also Punct)
         Pc          Connector_Punctuation
         Pd          Dash_Punctuation
         Ps          Open_Punctuation
         Pe          Close_Punctuation
         Pi          Initial_Punctuation
                     (may behave like Ps or Pe depending on usage)
         Pf          Final_Punctuation
                     (may behave like Ps or Pe depending on usage)
         Po          Other_Punctuation

         S           Symbol
         Sm          Math_Symbol
         Sc          Currency_Symbol
         Sk          Modifier_Symbol
         So          Other_Symbol

         Z           Separator
         Zs          Space_Separator
         Zl          Line_Separator
         Zp          Paragraph_Separator

         C           Other
         Cc          Control (also Cntrl)
         Cf          Format
         Cs          Surrogate   (not usable)
         Co          Private_Use

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         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 cases, which are both aliases for
     the set consisting of everything matched by "Ll", "Lu", and

     Because Perl hides the need for the user to understand the
     internal representation of Unicode characters, there is no
     need to implement the somewhat messy concept of surrogates.
     "Cs" is therefore not supported.

     Bidirectional Character Types

     Because scripts differ in their directionality (Hebrew is
     written right to left, for example) Unicode supplies these
     properties in the Bidi_Class class:

         Property    Meaning

         L           Left-to-Right
         LRE         Left-to-Right Embedding
         LRO         Left-to-Right Override
         R           Right-to-Left
         AL          Arabic Letter
         RLE         Right-to-Left Embedding
         RLO         Right-to-Left Override
         PDF         Pop Directional Format
         EN          European Number
         ES          European Separator
         ET          European Terminator
         AN          Arabic Number
         CS          Common Separator
         NSM         Non-Spacing Mark
         BN          Boundary Neutral
         B           Paragraph Separator
         S           Segment Separator
         WS          Whitespace
         ON          Other Neutrals

     This property is always written in the compound form.  For
     example, "\p{Bidi_Class:R}" matches characters that are
     normally written right to left.


     The world's languages are written in a number of scripts.
     This sentence (unless you're reading it in translation) is
     written in Latin, while Russian is written in Cyrllic, and
     Greek is written in, well, Greek; Japanese mainly in
     Hiragana or Katakana.  There are many more.

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     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

     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}".


     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 block as well
     as several more, 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 digits,
     because digits are shared across many scripts. Digits 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.

     For more about scripts versus blocks, see UAX#24 "Unicode
     Script Property": <>

     The Script property is likely to be the one you want to use
     when processing natural language; the Block property may be
     useful in working with the nuts and bolts of Unicode.

     Block names are matched in the compound form, like
     "\p{Block: Arrows}" or "\p{Blk=Hebrew}".  Unlike most other
     properties only a few block names have a Unicode-defined
     short name.  But Perl does provide a (slight) shortcut:  You
     can say, for example "\p{In_Arrows}" or "\p{In_Hebrew}".
     For backwards compatibility, the "In" prefix may be omitted
     if there is no naming conflict with a script or any other
     property, and you can even use an "Is" prefix instead in

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     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 just prefer to always use "\p{Block: foo}" and
     "\p{Script: bar}" instead of the shortcuts, for clarity, and
     because they can't remember the difference between 'In' and
     'Is' anyway (or aren't confident that those who eventually
     will read their code will know).

     A complete list of blocks and their shortcuts is in

     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.  A number of
     these are just synonyms for the Unicode ones, but some are
     genunine extensions, including a couple that are in the
     compound form.  And quite a few of these are actually
     recommended by Unicode (in

     This section gives some details on all the extensions that
     aren't synonyms for compound-form Unicode properties (for
     those, you'll have to refer to the Unicode Standard

         This matches any of the 1_114_112 Unicode code points.
         It is a synonym for "\p{Any}".

         This matches any "\p{Alphabetic}" or
         "\p{Decimal_Number}" character.

         This matches any of the 1_114_112 Unicode code points.
         It is a synonym for "\p{All}".

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         This matches any assigned code point; that is, any code
         point whose general category is not Unassigned (or
         equivalently, not Cn).

         This is the same as "\h" and "\p{HorizSpace}":  A
         character that changes the spacing horizontally.

     "\p{Decomposition_Type: Non_Canonical}"    (Short:
         Matches a character that has a non-canonical

         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 and/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 they
         can be combined variously 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" construct
         to match such sequences.)

         But Unicode's intent is to unify the existing character
         set standards and practices, and a number of 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 first the
         character "LATIN CAPITAL LETTER E", then the character

         "LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-
         composed" character, and the 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

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         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 kind of
         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
         <>), 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".

         Perl has added the "Non_Canonical" type, for your
         convenience, to mean any of the compatibility

         Matches any character that is graphic.  Theoretically,
         this means a character that on a printer would cause ink
         to be used.

         This is the same as "\h" and "\p{Blank}":  A character
         that changes the spacing horizontally.

         This is a synonym for "\p{Present_In=*}"

         This is the same as "\s", restricted to ASCII, namely
         "[ \f\n\r\t]".

         Mnemonic: Perl's (original) space

         This is the same as "\w", restricted to ASCII, namely

         Mnemonic: Perl's (original) word.

         This matches any alphanumeric character in the ASCII
         range, namely "[A-Za-z0-9]".

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         This matches any alphabetic character in the ASCII
         range, namely "[A-Za-z]".

         This matches any blank character in the ASCII range,
         namely "[ \t]".

         This matches any control character in the ASCII range,
         namely "[\x00-\x1F\x7F]"

         This matches any digit character in the ASCII range,
         namely "[0-9]".

         This matches any graphical character in the ASCII range,
         namely "[\x21-\x7E]".

         This matches any lowercase character in the ASCII range,
         namely "[a-z]".

         This matches any printable character in the ASCII range,
         namely "[\x20-\x7E]".  These are the graphical
         characters plus SPACE.

         This matches any punctuation character in the ASCII
         range, namely "[\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]".
         These are the graphical characters that aren't word
         characters.  Note that the Posix standard includes in
         its definition of punctuation, those characters that
         Unicode calls "symbols."

         This matches any space character in the ASCII range,
         namely "[ \f\n\r\t\x0B]" (the last being a vertical

         This matches any uppercase character in the ASCII range,
         namely "[A-Z]".

     "\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

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         "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 the decision to make them so was made in.  For
         example, "U+FDD0" is to be permanently unassigned to a
         character, and the decision to do that was made in
         version 3.1, so "\p{Age=3.1}" matches this character and
         "\p{Present_In: 3.1}" and up matches as well.

         This matches any character that is graphical or blank,
         except controls.

         This is the same as "\s", including beyond ASCII.

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

         This is the same as "\v":  A character that changes the
         spacing vertically.


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         This is the same as "\w", including beyond ASCII.

  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

     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 include, prefixed by "+": a built-in
         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 exclude, prefixed by "-": 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 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

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         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

     For example, to define a property that covers both the
     Japanese syllabaries (hiragana and katakana), you can define

         sub InKana {
             return <<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';

     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';

     The negation is useful for defining (surprise!) negated

         sub InNotKana {
             return <<'END';

     Intersection is useful for getting the common characters
     matched by two (or more) classes.

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         sub InFooAndBar {
             return <<'END';

     It's important to remember not to use "&" for the first set;
     that would be intersecting with nothing (resulting in an
     empty set).

  User-Defined Case Mappings
     You can also define your own mappings to be used in "lc()",
     "lcfirst()", "uc()", and "ucfirst()" (or their string-
     inlined versions, "\L", "\l", "\U", and "\u").  The
     principle is similar to that of user-defined character
     properties: to define subroutines with names "ToLower" (for
     "lc()" and "lcfirst()"); "ToTitle" (for "ucfirst()"); and
     "ToUpper" (for "uc()").

     The string returned by the subroutines needs to be two
     hexadecimal numbers separated by two tabulators: the two
     numbers being, respectively, the source code point and the
     destination code point.  For example:

         sub ToUpper {
             return <<END;

     defines a mapping for "uc()" (and "\U") that causes only the
     character "a" to be mapped to "A"; all other characters will
     remain unchanged.

     (For serious hackers only)  The above means you have to
     furnish a complete mapping; you can't just override a couple
     of characters and leave the rest unchanged.  You can find
     all the mappings in the directory
     $Config{privlib}/unicore/To/.  The mapping data is returned
     as the here-document.  The "utf8::ToSpecFoo" hashes in those
     files are special exception mappings derived from
     $Config{privlib}/unicore/SpecialCasing.txt.  The "Digit" and
     "Fold" mappings that one can see in the directory are not
     directly user-accessible, one can use either the
     Unicode::UCD module, or just match case-insensitively
     (that's when the "Fold" mapping is used).

     The mappings will only take effect on scalars that have been
     marked as having Unicode characters, for example by using
     "utf8::upgrade()".  Old byte-style strings are not affected.

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     The mappings are in effect for the package they are defined

  Character Encodings for Input and Output
     See Encode.

  Unicode Regular Expression Support Level
     The following list of Unicode support for regular
     expressions describes all the features currently supported.
     The references to "Level N" and the section numbers refer to
     the Unicode Technical Standard #18, "Unicode Regular
     Expressions", version 11, in May 2005.

     o   Level 1 - Basic Unicode Support

                 RL1.1   Hex Notation                        - done          [1]
                 RL1.2   Properties                          - done          [2][3]
                 RL1.2a  Compatibility Properties            - done          [4]
                 RL1.3   Subtraction and Intersection        - MISSING       [5]
                 RL1.4   Simple Word Boundaries              - done          [6]
                 RL1.5   Simple Loose Matches                - done          [7]
                 RL1.6   Line Boundaries                     - MISSING       [8]
                 RL1.7   Supplementary Code Points           - done          [9]

                 [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]  UTF-8/UTF-EBDDIC used in perl allows not only U+10000 to U+10FFFF
                      but also beyond U+10FFFF [d]

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


         in Perl can be written as:

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         But in this particular example, you probably really want


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

         Also see the Unicode::Regex::Set module, it does
         implement the full UTS#18 grouping, intersection, union,
         and removal (subtraction) syntax.

         [b] '+' for union, '-' for removal (set-difference), '&'
         for intersection (see "User-Defined Character

         [c] Try the ":crlf" layer (see PerlIO).

         [d] U+FFFF will currently generate a warning message if
         'utf8' warnings are

     o   Level 2 - Extended Unicode Support

                 RL2.1   Canonical Equivalents           - MISSING       [10][11]
                 RL2.2   Default Grapheme Clusters       - MISSING       [12]
                 RL2.3   Default Word Boundaries         - MISSING       [14]
                 RL2.4   Default Loose Matches           - MISSING       [15]
                 RL2.5   Name Properties                 - 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] have \N{...} but neither compute names of CJK Ideographs
                      and Hangul Syllables nor use a loose match [e]

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

     o   Level 3 - Tailored Support

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                 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 6 bytes, current
         character allocations require 4 bytes), byte-order
         independent encoding. For ASCII (and we really do mean
         7-bit ASCII, not another 8-bit encoding), UTF-8 is

         The following table is from Unicode 3.2.

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

            U+0000..U+007F       00..7F
            U+0080..U+07FF     * C2..DF    80..BF
            U+0800..U+0FFF       E0      * A0..BF    80..BF
            U+1000..U+CFFF       E1..EC    80..BF    80..BF
            U+D000..U+D7FF       ED        80..9F    80..BF
            U+D800..U+DFFF       +++++++ utf16 surrogates, not legal utf8 +++++++
            U+E000..U+FFFF       EE..EF    80..BF    80..BF
           U+10000..U+3FFFF      F0      * 90..BF    80..BF    80..BF
           U+40000..U+FFFFF      F1..F3    80..BF    80..BF    80..BF
          U+100000..U+10FFFF     F4        80..8F    80..BF    80..BF

         Note the gaps before several of the byte entries above
         marked by '*'.  These are caused by legal UTF-8 avoiding
         non-shortest encodings: it is technically possible to
         UTF-8-encode a single code point in different ways, but

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         that is explicitly forbidden, and the shortest possible
         encoding should always be used (and that is what Perl

         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.

     o   UTF-EBCDIC

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

     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.

         UTF-16 is a 2 or 4 byte encoding.  The Unicode code
         points "U+0000..U+FFFF" are stored in a single 16-bit
         unit, and the code points "U+10000..U+10FFFF" in two
         16-bit units.  The latter case is using surrogates, the
         first 16-bit unit being the high surrogate, and the
         second being the low surrogate.

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

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

         and the decoding is

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

         If you try to generate surrogates (for example by using
         chr()), you will get a warning, if warnings are turned
         on, because those code points are not valid for a

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         Unicode character.

         Because of the 16-bitness, UTF-16 is byte-order
         dependent.  UTF-16 itself can be used for in-memory
         computations, but if storage or transfer is required
         either UTF-16BE (big-endian) or UTF-16LE (little-endian)
         encodings must be chosen.

         This introduces another problem: what if you just know
         that your data is UTF-16, but you don't know which
         endianness?  Byte Order Marks, or BOMs, are a solution
         to this.  A special character has been reserved in
         Unicode to function as a byte order marker: the
         character with the code point "U+FEFF" is the BOM.

         The trick is that if you read a BOM, you will know the
         byte order, since if it was written on a big-endian
         platform, you will read the bytes "0xFE 0xFF", but if it
         was written on a little-endian platform, you will read
         the bytes "0xFF 0xFE".  (And if the originating platform
         was writing in UTF-8, you will read the bytes "0xEF 0xBB

         The way this trick works is that the character with the
         code point "U+FFFE" is guaranteed not to be a valid
         Unicode character, 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".  (Actually, "U+FFFE" is legal for use by
         your program, even for input/output, but better not use
         it if you need a BOM.  But it is "illegal for
         interchange", so that an unsuspecting program won't get

     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.  The BOM signatures will
         be "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00
         0x00" for LE.

     o   UCS-2, UCS-4

         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

     o   UTF-7

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         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.

  Security Implications of Unicode
     Read Unicode Security Considerations
     <>.  Also, note the

     o   Malformed UTF-8

         Unfortunately, the 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 real Unicode code points.

     o   Regular expressions behave slightly differently between
         byte data and character (Unicode) data.  For example,
         the "word character" character class "\w" will work
         differently depending on if data is eight-bit bytes or

         In the first case, the set of "\w" characters is either
         small--the default set of alphabetic characters, digits,
         and the "_"--or, if you are using a locale (see
         perllocale), the "\w" might contain a few more letters
         according to your language and country.

         In the second case, the "\w" set of characters is much,
         much larger.  Most importantly, even in the set of the
         first 256 characters, it will probably match different
         characters: unlike most locales, which are specific to a
         language and country pair, Unicode classifies all the
         characters that are letters somewhere as "\w".  For
         example, your locale might not think that LATIN SMALL
         LETTER ETH is a letter (unless you happen to speak
         Icelandic), but Unicode does.

         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

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         accidentally mix bytes and characters, however (see
         perluniintro), in which case "\w" in regular expressions
         might start behaving differently.  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.

     Usually locale settings and Unicode do not affect each
     other, but there are a couple of exceptions:

     o   You can enable automatic UTF-8-ification of your
         standard file handles, default "open()" layer, and @ARGV
         by using either the "-C" command line switch or the
         "PERL_UNICODE" environment variable, see perlrun for the
         documentation of the "-C" switch.

     o   Perl tries really hard to work both with Unicode and the
         old byte-oriented world. Most often this is nice, but
         sometimes Perl's straddling of the proverbial fence
         causes problems.

  When Unicode Does Not Happen
     While Perl does have extensive ways to input and output in
     Unicode, and few other 'entry points' like the @ARGV which
     can be interpreted as Unicode (UTF-8), there still are many
     places where Unicode (in some encoding or another) could be
     given as arguments or received as results, or both, but it
     is not.

     The following are such interfaces.  Also, see "The "Unicode
     Bug"".  For all of these interfaces Perl currently (as of
     5.8.3) simply assumes byte strings both as arguments and
     results, or UTF-8 strings if the "encoding" pragma has been

     One reason why Perl does not attempt to resolve the role of
     Unicode in these cases 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 the qx and system: how well will the
     'command line interface' (and which of them?) handle

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

     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 with the Unicode characters whose ordinals are
     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

     In character semantics they are interpreted as Unicode code
     points, which means they have the same semantics as Latin-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".  (On EBCDIC platforms,
     the behavior may be different from this, depending on the
     underlying C language library functions.)

     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 a number of properties in regular expressions,
         such as "\w"

     o   User-defined case change mappings.  You can create a
         "ToUpper()" function, for example, which overrides
         Perl's built-in case mappings.  The scalar must be
         encoded in utf8 for your function to actually be

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     This behavior can lead to unexpected results in which a
     string's semantics suddenly change if a code point above 255
     is appended to or removed from it, which changes the
     string's semantics from byte to character or vice versa.  As
     an example, consider the following program and its output:

      $ perl -le'
          $s1 = "\xC2";
          $s2 = "\x{2660}";
          for ($s1, $s2, $s1.$s2) {
              print /\w/ || 0;

     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.

     Work is being done to correct this, but only some of it was
     complete in time for the 5.12 release.  What has been
     finished is the important part of the case changing
     component.  Due to concerns, and some evidence, that older
     code might have come to rely on the existing behavior, the
     new behavior must be explicitly enabled by the feature
     "unicode_strings" in the feature pragma, even though no new
     syntax is involved.

     See "lc" in perlfunc for details on how this pragma works in
     combination with various others for casing.  Even though the
     pragma only affects casing operations in the 5.12 release,
     it is planned to have it affect all the problematic
     behaviors in later releases: you can't have one without them

     In the meantime, 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

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  Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
     Sometimes (see "When Unicode Does Not Happen" or "The
     "Unicode Bug"") there are situations where you simply need
     to force a byte string into UTF-8, or vice versa.  The low-
     level calls utf8::upgrade($bytestring) and
     utf8::downgrade($utf8string[, FAIL_OK]) are the answers.

     Note that utf8::downgrade() can fail if the string contains
     characters that don't fit into a byte.

     Calling either function on a string that already is in the
     desired state is a no-op.

  Using Unicode in XS
     If you want to handle Perl Unicode in XS extensions, you may
     find the following C APIs useful.  See also "Unicode
     Support" in perlguts for an explanation about Unicode at the
     XS level, and perlapi for the API details.

     o   "DO_UTF8(sv)" returns true if the "UTF8" flag is on and
         the bytes pragma is not in effect.  "SvUTF8(sv)" returns
         true if the "UTF8" flag is on; the bytes pragma is
         ignored.  The "UTF8" flag being on does not mean that
         there are any characters of code points greater than 255
         (or 127) in the scalar or that there are even any
         characters in the scalar.  What the "UTF8" flag means is
         that the sequence of octets in the representation of the
         scalar is the sequence of UTF-8 encoded code points of
         the characters of a string.  The "UTF8" flag being off
         means that each octet in this representation encodes a
         single character with code point 0..255 within the
         string.  Perl's Unicode model is not to use UTF-8 until
         it is absolutely necessary.

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

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         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

     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.

     o   "pv_uni_display(dsv, spv, len, pvlim, flags)" and
         "sv_uni_display(dsv, ssv, pvlim, flags)" are useful for
         debugging the output of Unicode strings and scalars.  By
         default they are useful only for debugging--they display
         all characters as hexadecimal code points--but with the
         and "UNI_DISPLAY_QQ" you can make the output more

     o   "ibcmp_utf8(s1, pe1, l1, u1, s2, pe2, l2, u2)" can be
         used to compare two strings case-insensitively in
         Unicode.  For case-sensitive comparisons you can just
         use "memEQ()" and "memNE()" as usual.

     For more information, see perlapi, and utf8.c and utf8.h in
     the Perl source code distribution.

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  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 version of Unicode that you want
     from the Unicode web site <>).  These
     should replace the existing files in
     "\$Config{privlib}"/unicore.  ("\%Config" is available from
     the Config module.)  Follow the instructions in README.perl
     in that directory to change some of their names, and then
     run make.

     It is even possible to download them to a different
     directory, and then change 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

  Interaction with Locales
     Use of locales with Unicode data may lead to odd results.
     Currently, Perl attempts to attach 8-bit locale info to
     characters in the range 0..255, but this technique is
     demonstrably incorrect for locales that use characters above
     that range when mapped into Unicode.  Perl's Unicode support
     will also tend to run slower.  Use of locales with Unicode
     is discouraged.

  Problems with characters in the Latin-1 Supplement range
     See "The "Unicode Bug""

  Problems with case-insensitive regular expression matching
     There are problems with case-insensitive matches, including
     those involving character classes (enclosed in [square
     brackets]), characters whose fold is to multiple characters
     (such as the single character LATIN SMALL LIGATURE FFL
     matches case-insensitively with the 3-character string
     "ffl"), and characters in the Latin-1 Supplement.

  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 know about the 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

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     issues with Unicode data exchange. If the documentation does
     not talk about Unicode at all, suspect the worst and
     probably look at the source to learn how the module is
     implemented. Modules written completely in Perl shouldn't
     cause problems. Modules that directly or indirectly access
     code written in other programming languages are at risk.

     For affected functions, the simple strategy to avoid data
     corruption is to always make the encoding of the exchanged
     data explicit. Choose an encoding that you know the
     extension can handle. Convert arguments passed to the
     extensions to that encoding and convert results back from
     that encoding. Write wrapper functions that do the
     conversions for you, so you can later change the functions
     when the extension catches up.

     To provide an example, let's say the popular
     Foo::Bar::escape_html function doesn't deal with Unicode
     data yet. The wrapper function would convert the argument to
     raw UTF-8 and convert the result back to Perl's internal
     representation like so:

         sub my_escape_html ($) {
           my($what) = shift;
           return unless defined $what;

     Sometimes, when the extension does not convert data but just
     stores and retrieves them, you will be in a position to use
     the otherwise dangerous Encode::_utf8_on() function. Let's
     say the popular "Foo::Bar" extension, written in C, provides
     a "param" method that lets you store and retrieve data
     according to these prototypes:

         $self->param($name, $value);            # set a scalar
         $value = $self->param($name);           # retrieve a scalar

     If it does not yet provide support for any encoding, one
     could write a derived class with such a "param" method:

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         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.

     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 268
     Unicode characters matching "Nd" compared with the 10 ASCII
     characters matching "d").

  Problems on EBCDIC platforms
     There are a number of known problems with Perl on EBCDIC
     platforms.  If you want to use Perl there, send email to

     In earlier versions, when byte and character data were
     concatenated, the new string was sometimes created by
     decoding the byte strings as ISO 8859-1 (Latin-1), even if
     the old Unicode string used EBCDIC.

     If you find any of these, please report them as bugs.

  Porting code from perl-5.6.X
     Perl 5.8 has a different Unicode model from 5.6. In 5.6 the
     programmer was required to use the "utf8" pragma to declare
     that a given scope expected to deal with Unicode data and

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     had to make sure that only Unicode data were reaching that
     scope. If you have code that is working with 5.6, you will
     need some of the following adjustments to your code. The
     examples are written such that the code will continue to
     work under 5.6, so you should be safe to try them out.

     o   A filehandle that should read or write UTF-8

           if ($] > 5.007) {
             binmode $fh, ":encoding(utf8)";

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

         Be it Compress::Zlib, Apache::Request or any extension
         that has no mention of Unicode in the manpage, you need
         to make sure that the UTF8 flag is stripped off. Note
         that at the time of this writing (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;

     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

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         check the documentation to verify if that is still true.

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

     o   A large scalar that you know can only contain ASCII

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

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

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     perlunitut, perluniintro, perluniprops, Encode, open, utf8,
     bytes, perlretut, "${^UNICODE}" in perlvar

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     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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