=head1 DESCRIPTION
+If you haven't already, before reading this document, you should become
+familiar with both L<perlunitut> and L<perluniintro>.
+
+Unicode aims to B<UNI>-fy the en-B<CODE>-ings of all the world's
+character sets into a single Standard. For quite a few of the various
+coding standards that existed when Unicode was first created, converting
+from each to Unicode essentially meant adding a constant to each code
+point in the original standard, and converting back meant just
+subtracting that same constant. For ASCII and ISO-8859-1, the constant
+is 0. For ISO-8859-5, (Cyrillic) the constant is 864; for Hebrew
+(ISO-8859-8), it's 1488; Thai (ISO-8859-11), 3424; and so forth. This
+made it easy to do the conversions, and facilitated the adoption of
+Unicode.
+
+And it worked; nowadays, those legacy standards are rarely used. Most
+everyone uses Unicode.
+
+Unicode is a comprehensive standard. It specifies many things outside
+the scope of Perl, such as how to display sequences of characters. For
+a full discussion of all aspects of Unicode, see
+L<http://www.unicode.org>.
+
=head2 Important Caveats
+Even though some of this section may not be understandable to you on
+first reading, we think it's important enough to highlight some of the
+gotchas before delving further, so here goes:
+
Unicode support is an extensive requirement. While Perl does not
implement the Unicode standard or the accompanying technical reports
from cover to cover, Perl does support many Unicode features.
-People who want to learn to use Unicode in Perl, should probably read
-the L<Perl Unicode tutorial, perlunitut|perlunitut>, before reading
-this reference document.
-
Also, the use of Unicode may present security issues that aren't obvious.
Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
=over 4
-=item Safest if you "use feature 'unicode_strings'"
+=item Safest if you C<use feature 'unicode_strings'>
In order to preserve backward compatibility, Perl does not turn
on full internal Unicode support unless the pragma
-C<use feature 'unicode_strings'> is specified. (This is automatically
-selected if you use C<use 5.012> or higher.) Failure to do this can
+L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
+is specified. (This is automatically
+selected if you S<C<use 5.012>> or higher.) Failure to do this can
trigger unexpected surprises. See L</The "Unicode Bug"> below.
-This pragma doesn't affect I/O, and there are still several places
-where Unicode isn't fully supported, such as in filenames.
+This pragma doesn't affect I/O. Nor does it change the internal
+representation of strings, only their interpretation. There are still
+several places where Unicode isn't fully supported, such as in
+filenames.
=item Input and Output Layers
-Perl knows when a filehandle uses Perl's internal Unicode encodings
-(UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with
-the ":encoding(utf8)" layer. Other encodings can be converted to Perl's
-encoding on input or from Perl's encoding on output by use of the
-":encoding(...)" layer. See L<open>.
-
-To indicate that Perl source itself is in UTF-8, use C<use utf8;>.
+Use the C<:encoding(...)> layer to read from and write to
+filehandles using the specified encoding. (See L<open>.)
-=item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts
+=item You should convert your non-ASCII, non-UTF-8 Perl scripts to be
+UTF-8.
-As a compatibility measure, the C<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. B<These are the only times when an explicit C<use utf8>
-is needed.> See L<utf8>.
+See L<encoding>.
-=item BOM-marked scripts and UTF-16 scripts autodetected
+=item C<use utf8> still needed to enable L<UTF-8|/Unicode Encodings> in scripts
-If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE,
-or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either
-endianness, Perl will correctly read in the script as Unicode.
-(BOMless UTF-8 cannot be effectively recognized or differentiated from
-ISO 8859-1 or other eight-bit encodings.)
+If your Perl script is itself encoded in L<UTF-8|/Unicode Encodings>,
+the S<C<use utf8>> pragma must be explicitly included to enable
+recognition of that (in string or regular expression literals, or in
+identifier names). B<This is the only time when an explicit S<C<use
+utf8>> is needed.> (See L<utf8>).
-=item C<use encoding> needed to upgrade non-Latin-1 byte strings
+=item C<BOM>-marked scripts and L<UTF-16|/Unicode Encodings> scripts autodetected
-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 I<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 L</"Byte and Character Semantics"> for more details.
+However, if a Perl script begins with the Unicode C<BOM> (UTF-16LE,
+UTF16-BE, or UTF-8), or if the script looks like non-C<BOM>-marked
+UTF-16 of either endianness, Perl will correctly read in the script as
+the appropriate Unicode encoding. (C<BOM>-less UTF-8 cannot be
+effectively recognized or differentiated from ISO 8859-1 or other
+eight-bit encodings.)
=back
=head2 Byte and Character Semantics
-Beginning with version 5.6, Perl uses logically-wide characters to
-represent strings internally.
-
-Starting in Perl 5.14, Perl-level operations work with
-characters rather than bytes within the scope of a
-C<L<use feature 'unicode_strings'|feature>> (or equivalently
-C<use 5.012> or higher). (This is not true if bytes have been
-explicitly requested by C<L<use bytes|bytes>>, nor necessarily true
-for interactions with the platform's operating system.)
-
-For earlier Perls, and when C<unicode_strings> is not in effect, Perl
-provides a fairly safe environment that can handle both types of
-semantics in programs. For operations where Perl can unambiguously
-decide that the input data are characters, Perl switches to character
-semantics. For operations where this determination cannot be made
-without additional information from the user, Perl decides in favor of
-compatibility and chooses to use byte semantics.
-
-When C<use locale> is in effect (which overrides
-C<use feature 'unicode_strings'>), Perl uses the semantics associated
-with the current locale. Otherwise, Perl uses the platform's native
-byte semantics for characters whose code points are less than 256, and
-Unicode semantics for those greater than 255. On EBCDIC platforms, this
-is almost seamless, as the EBCDIC code pages that Perl handles are
-equivalent to Unicode's first 256 code points. (The exception is that
-EBCDIC regular expression case-insensitive matching rules are not as
-as robust as Unicode's.) But on ASCII platforms, Perl uses US-ASCII
-(or Basic Latin in Unicode terminology) byte semantics, meaning that characters
-whose ordinal numbers are in the range 128 - 255 are undefined except for their
-ordinal numbers. This means that none have case (upper and lower), nor are any
-a member of character classes, like C<[:alpha:]> or C<\w>. (But all do belong
-to the C<\W> class or the Perl regular expression extension C<[:^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 C<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 L<utf8>.
-
-If strings operating under byte semantics and strings with Unicode
-character data are concatenated, the new string will have
-character semantics. This can cause surprises: See L</BUGS>, below.
-You can choose to be warned when this happens. See L<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 L<perluniintro> for more.
-
-=head2 Effects of Character Semantics
-
-Character semantics have the following effects:
+Before Unicode, most encodings used 8 bits (a single byte) to encode
+each character. Thus a character was a byte, and a byte was a
+character, and there could be only 256 or fewer possible characters.
+"Byte Semantics" in the title of this section refers to
+this behavior. There was no need to distinguish between "Byte" and
+"Character".
+
+Then along comes Unicode which has room for over a million characters
+(and Perl allows for even more). This means that a character may
+require more than a single byte to represent it, and so the two terms
+are no longer equivalent. What matter are the characters as whole
+entities, and not usually the bytes that comprise them. That's what the
+term "Character Semantics" in the title of this section refers to.
+
+Perl had to change internally to decouple "bytes" from "characters".
+It is important that you too change your ideas, if you haven't already,
+so that "byte" and "character" no longer mean the same thing in your
+mind.
+
+The basic building block of Perl strings has always been a "character".
+The changes basically come down to that the implementation no longer
+thinks that a character is always just a single byte.
+
+There are various things to note:
=over 4
=item *
+String handling functions, for the most part, continue to operate in
+terms of characters. C<length()>, for example, returns the number of
+characters in a string, just as before. But that number no longer is
+necessarily the same as the number of bytes in the string (there may be
+more bytes than characters). The other such functions include
+C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
+C<sort()>, C<sprintf()>, and C<write()>.
+
+The exceptions are:
+
+=over 4
+
+=item *
+
+the bit-oriented C<vec>
+
+E<nbsp>
+
+=item *
+
+the byte-oriented C<pack>/C<unpack> C<"C"> format
+
+However, the C<W> specifier does operate on whole characters, as does the
+C<U> specifier.
+
+=item *
+
+some operators that interact with the platform's operating system
+
+Operators dealing with filenames are examples.
+
+=item *
+
+when the functions are called from within the scope of the
+S<C<L<use bytes|bytes>>> pragma
+
+Likely, you should use this only for debugging anyway.
+
+=back
+
+=item *
+
Strings--including hash keys--and regular expression patterns may
-contain characters that have an ordinal value larger than 255.
+contain characters that have ordinal values larger than 255.
If you use a Unicode editor to edit your program, Unicode characters may
occur directly within the literal strings in UTF-8 encoding, or UTF-16.
-(The former requires a BOM or C<use utf8>, the latter requires a BOM.)
+(The former requires a C<BOM> or C<use utf8>, the latter requires a C<BOM>.)
-Unicode characters can also be added to a string by using the C<\N{U+...}>
-notation. The Unicode code for the desired character, in hexadecimal,
-should be placed in the braces, after the C<U>. For instance, a smiley face is
-C<\N{U+263A}>.
+L<perluniintro/Creating Unicode> gives other ways to place non-ASCII
+characters in your strings.
-Alternatively, you can use the C<\x{...}> notation for characters 0x100 and
-above. For characters below 0x100 you may get byte semantics instead of
-character semantics; see L</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.
+=item *
-Additionally, if you
+The C<chr()> and C<ord()> functions work on whole characters.
- use charnames ':full';
+=item *
-you can use the C<\N{...}> notation and put the official Unicode
-character name within the braces, such as C<\N{WHITE SMILING FACE}>.
-See L<charnames>.
+Regular expressions match whole characters. For example, C<"."> matches
+a whole character instead of only a single byte.
=item *
-If an appropriate L<encoding> is specified, identifiers within the
-Perl script may contain Unicode alphanumeric characters, including
-ideographs. Perl does not currently attempt to canonicalize variable
-names.
+The C<tr///> operator translates whole characters. (Note that the
+C<tr///CU> functionality has been removed. For similar functionality to
+that, see C<pack('U0', ...)> and C<pack('C0', ...)>).
=item *
-Regular expressions match characters instead of bytes. "." matches
-a character instead of a byte.
+C<scalar reverse()> reverses by character rather than by byte.
=item *
-Bracketed character classes in regular expressions match characters instead of
-bytes and match against the character properties specified in the
-Unicode properties database. C<\w> can be used to match a Japanese
-ideograph, for instance.
+The bit string operators, C<& | ^ ~> and (starting in v5.22)
+C<&. |. ^. ~.> can operate on characters that don't fit into a byte.
+However, the current behavior is likely to change. You should not use
+these operators on strings that are encoded in UTF-8. If you're not
+sure about the encoding of a string, downgrade it before using any of
+these operators; you can use
+L<C<utf8::utf8_downgrade()>|utf8/Utility functions>.
-=item *
+=back
-Named Unicode properties, scripts, and block ranges may be used (like bracketed
-character classes) by using the C<\p{}> "matches property" construct and
-the C<\P{}> negation, "doesn't match property".
-See L</"Unicode Character Properties"> for more details.
+The bottom line is that Perl has always practiced "Character Semantics",
+but with the advent of Unicode, that is now different than "Byte
+Semantics".
-You can define your own character properties and use them
-in the regular expression with the C<\p{}> or C<\P{}> construct.
-See L</"User-Defined Character Properties"> for more details.
+=head2 ASCII Rules versus Unicode Rules
-=item *
+Before Unicode, when a character was a byte was a character,
+Perl knew only about the 128 characters defined by ASCII, code points 0
+through 127 (except for under S<C<use locale>>). That left the code
+points 128 to 255 as unassigned, and available for whatever use a
+program might want. The only semantics they have is their ordinal
+numbers, and that they are members of none of the non-negative character
+classes. None are considered to match C<\w> for example, but all match
+C<\W>.
+
+Unicode, of course, assigns each of those code points a particular
+meaning (along with ones above 255). To preserve backward
+compatibility, Perl only uses the Unicode meanings when there is some
+indication that Unicode is what is intended; otherwise the non-ASCII
+code points remain treated as if they are unassigned.
-The special pattern C<\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 C<G>, may in fact be composed of a sequence
-of characters, in this case a C<G> followed by an accent character. C<\X>
-will match the entire sequence.
+Here are the ways that Perl knows that a string should be treated as
+Unicode:
+
+=over
=item *
-The C<tr///> operator translates characters instead of bytes. Note
-that the C<tr///CU> functionality has been removed. For similar
-functionality see pack('U0', ...) and pack('C0', ...).
+Within the scope of S<C<use utf8>>
+
+If the whole program is Unicode (signified by using 8-bit B<U>nicode
+B<T>ransformation B<F>ormat), then all strings within it must be
+Unicode.
=item *
-Case translation operators use the Unicode case translation tables
-when character input is provided. Note that C<uc()>, or C<\U> in
-interpolated strings, translates to uppercase, while C<ucfirst>,
-or C<\u> in interpolated strings, translates to titlecase in languages
-that make the distinction (which is equivalent to uppercase in languages
-without the distinction).
+Within the scope of
+L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
+
+This pragma was created so you can explicitly tell Perl that operations
+executed within its scope are to use Unicode rules. More operations are
+affected with newer perls. See L</The "Unicode Bug">.
=item *
-Most operators that deal with positions or lengths in a string will
-automatically switch to using character positions, including
-C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
-C<sprintf()>, C<write()>, and C<length()>. An operator that
-specifically does not switch is C<vec()>. Operators that really don't
-care include operators that treat strings as a bucket of bits such as
-C<sort()>, and operators dealing with filenames.
+Within the scope of S<C<use 5.012>> or higher
+
+This implicitly turns on S<C<use feature 'unicode_strings'>>.
=item *
-The C<pack()>/C<unpack()> letter C<C> does I<not> change, since it is often
-used for byte-oriented formats. Again, think C<char> in the C language.
+Within the scope of
+L<S<C<use locale 'not_characters'>>|perllocale/Unicode and UTF-8>,
+or L<S<C<use locale>>|perllocale> and the current
+locale is a UTF-8 locale.
-There is a new C<U> specifier that converts between Unicode characters
-and code points. There is also a C<W> specifier that is the equivalent of
-C<chr>/C<ord> and properly handles character values even if they are above 255.
+The former is defined to imply Unicode handling; and the latter
+indicates a Unicode locale, hence a Unicode interpretation of all
+strings within it.
=item *
-The C<chr()> and C<ord()> functions work on characters, similar to
-C<pack("W")> and C<unpack("W")>, I<not> C<pack("C")> and
-C<unpack("C")>. C<pack("C")> and C<unpack("C")> are methods for
-emulating byte-oriented C<chr()> and C<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.
+When the string contains a Unicode-only code point
+
+Perl has never accepted code points above 255 without them being
+Unicode, so their use implies Unicode for the whole string.
=item *
-The bit string operators, C<& | ^ ~>, 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 C<~> (the bit complement) with characters of both
-values less than 256 and values greater than 256. Most importantly,
-DeMorgan's laws (C<~($x|$y) eq ~$x&~$y> and C<~($x&$y) eq ~$x|~$y>)
-will not hold. The reason for this mathematical I<faux pas> is that
-the complement cannot return B<both> the 8-bit (byte-wide) bit
-complement B<and> the full character-wide bit complement.
+When the string contains a Unicode named code point C<\N{...}>
+
+The C<\N{...}> construct explicitly refers to a Unicode code point,
+even if it is one that is also in ASCII. Therefore the string
+containing it must be Unicode.
=item *
-You can define your own mappings to be used in C<lc()>,
-C<lcfirst()>, C<uc()>, and C<ucfirst()> (or their double-quoted string inlined
-versions such as C<\U>). See
-L<User-Defined Case-Mappings|/"User-Defined Case Mappings (for serious hackers only)">
-for more details.
+When the string has come from an external source marked as
+Unicode
+
+The L<C<-C>|perlrun/-C [numberE<sol>list]> command line option can
+specify that certain inputs to the program are Unicode, and the values
+of this can be read by your Perl code, see L<perlvar/"${^UNICODE}">.
+
+=item * When the string has been upgraded to UTF-8
+
+The function L<C<utf8::utf8_upgrade()>|utf8/Utility functions>
+can be explicitly used to permanently (unless a subsequent
+C<utf8::utf8_downgrade()> is called) cause a string to be treated as
+Unicode.
+
+=item * There are additional methods for regular expression patterns
+
+A pattern that is compiled with the C<< /u >> or C<< /a >> modifiers is
+treated as Unicode (though there are some restrictions with C<< /a >>).
+Under the C<< /d >> and C<< /l >> modifiers, there are several other
+indications for Unicode; see L<perlre/Character set modifiers>.
=back
+Note that all of the above are overridden within the scope of
+C<L<use bytes|bytes>>; but you should be using this pragma only for
+debugging.
+
+Note also that some interactions with the platform's operating system
+never use Unicode rules.
+
+When Unicode rules are in effect:
+
=over 4
=item *
-And finally, C<scalar reverse()> reverses by character rather than by byte.
+Case translation operators use the Unicode case translation tables.
+
+Note that C<uc()>, or C<\U> in interpolated strings, translates to
+uppercase, while C<ucfirst>, or C<\u> in interpolated strings,
+translates to titlecase in languages that make the distinction (which is
+equivalent to uppercase in languages without the distinction).
+
+There is a CPAN module, C<L<Unicode::Casing>>, which allows you to
+define your own mappings to be used in C<lc()>, C<lcfirst()>, C<uc()>,
+C<ucfirst()>, and C<fc> (or their double-quoted string inlined versions
+such as C<\U>). (Prior to Perl 5.16, this functionality was partially
+provided in the Perl core, but suffered from a number of insurmountable
+drawbacks, so the CPAN module was written instead.)
+
+=item *
+
+Character classes in regular expressions match based on the character
+properties specified in the Unicode properties database.
+
+C<\w> can be used to match a Japanese ideograph, for instance; and
+C<[[:digit:]]> a Bengali number.
+
+=item *
+
+Named Unicode properties, scripts, and block ranges may be used (like
+bracketed character classes) by using the C<\p{}> "matches property"
+construct and the C<\P{}> negation, "doesn't match property".
+
+See L</"Unicode Character Properties"> for more details.
+
+You can define your own character properties and use them
+in the regular expression with the C<\p{}> or C<\P{}> construct.
+See L</"User-Defined Character Properties"> for more details.
=back
+=head2 Extended Grapheme Clusters (Logical characters)
+
+Consider a character, say C<H>. It could appear with various marks around it,
+such as an acute accent, or a circumflex, or various hooks, circles, arrows,
+I<etc.>, above, below, to one side or the other, I<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 C<H>, and a
+character for each of the possible marks, and these can be variously combined
+to get a final logical character. So a logical character--what appears to be a
+single character--can be a sequence of more than one individual characters.
+The Unicode standard calls these "extended grapheme clusters" (which
+is an improved version of the no-longer much used "grapheme cluster");
+Perl furnishes the C<\X> regular expression construct to match such
+sequences in their entirety.
+
+But Unicode's intent is to unify the existing character set standards and
+practices, and several pre-existing standards have single characters that
+mean the same thing as some of these combinations, like ISO-8859-1,
+which has quite a few of them. For example, C<"LATIN CAPITAL LETTER E
+WITH ACUTE"> was already in this standard when Unicode came along.
+Unicode therefore added it to its repertoire as that single character.
+But this character is considered by Unicode to be equivalent to the
+sequence consisting of the character C<"LATIN CAPITAL LETTER E">
+followed by the character C<"COMBINING ACUTE ACCENT">.
+
+C<"LATIN CAPITAL LETTER E WITH ACUTE"> is called a "pre-composed"
+character, and its equivalence with the "E" and the "COMBINING ACCENT"
+sequence is called canonical equivalence. All pre-composed characters
+are said to have a decomposition (into the equivalent sequence), and the
+decomposition type is also called canonical. A string may be comprised
+as much as possible of precomposed characters, or it may be comprised of
+entirely decomposed characters. Unicode calls these respectively,
+"Normalization Form Composed" (NFC) and "Normalization Form Decomposed".
+The C<L<Unicode::Normalize>> module contains functions that convert
+between the two. A string may also have both composed characters and
+decomposed characters; this module can be used to make it all one or the
+other.
+
+You may be presented with strings in any of these equivalent forms.
+There is currently nothing in Perl 5 that ignores the differences. So
+you'll have to specially hanlde it. The usual advice is to convert your
+inputs to C<NFD> before processing further.
+
+For more detailed information, see L<http://unicode.org/reports/tr15/>.
+
=head2 Unicode Character Properties
(The only time that Perl considers a sequence of individual code
and the C<\P{}> "doesn't match property" for its negation.
For instance, C<\p{Uppercase}> matches any single character with the Unicode
-"Uppercase" property, while C<\p{L}> matches any character with a
-General_Category of "L" (letter) property. Brackets are not
+C<"Uppercase"> property, while C<\p{L}> matches any character with a
+C<General_Category> of C<"L"> (letter) property (see
+L</General_Category> below). Brackets are not
required for single letter property names, so C<\p{L}> is equivalent to C<\pL>.
More formally, C<\p{Uppercase}> matches any single character whose Unicode
-Uppercase property value is True, and C<\P{Uppercase}> matches any character
-whose Uppercase property value is False, and they could have been written as
+C<Uppercase> property value is C<True>, and C<\P{Uppercase}> matches any character
+whose C<Uppercase> property value is C<False>, and they could have been written as
C<\p{Uppercase=True}> and C<\p{Uppercase=False}>, respectively.
This formality is needed when properties are not binary; that is, if they can
-take on more values than just True and False. For example, the Bidi_Class (see
-L</"Bidirectional Character Types"> below), can take on several 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
+take on more values than just C<True> and C<False>. For example, the
+C<Bidi_Class> property (see L</"Bidirectional Character Types"> below),
+can take on several different
+values, such as C<Left>, C<Right>, C<Whitespace>, and others. To match these, one needs
+to specify both the property name (C<Bidi_Class>), AND the value being
+matched against
+(C<Left>, C<Right>, I<etc.>). This is done, as in the examples above, by having the
two components separated by an equal sign (or interchangeably, a colon), like
C<\p{Bidi_Class: Left}>.
All Unicode-defined character properties may be written in these compound forms
-of C<\p{property=value}> or C<\p{property:value}>, but Perl provides some
+of C<\p{I<property>=I<value>}> or C<\p{I<property>:I<value>}>, but Perl provides some
additional properties that are written only in the single form, as well as
single-form short-cuts for all binary properties and certain others described
below, in which you may omit the property name and the equals or colon
Most Unicode character properties have at least two synonyms (or aliases if you
prefer): a short one that is easier to type and a longer one that is more
-descriptive and hence easier to understand. Thus the "L" and "Letter" properties
-above are equivalent and can be used interchangeably. Likewise,
-"Upper" is a synonym for "Uppercase", and we could have written
-C<\p{Uppercase}> equivalently as C<\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
-L<perluniprops>.
+descriptive and hence easier to understand. Thus the C<"L"> and
+C<"Letter"> properties above are equivalent and can be used
+interchangeably. Likewise, C<"Upper"> is a synonym for C<"Uppercase">,
+and we could have written C<\p{Uppercase}> equivalently as C<\p{Upper}>.
+Also, there are typically various synonyms for the values the property
+can be. For binary properties, C<"True"> has 3 synonyms: C<"T">,
+C<"Yes">, and C<"Y">; and C<"False"> has correspondingly C<"F">,
+C<"No">, and C<"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 C<L</General_Category>> property, C<"L"> means
+C<"Letter">, but for the L<C<Bidi_Class>|/Bidirectional Character Types>
+property, C<"L"> means C<"Left">. A complete list of properties and
+synonyms is in L<perluniprops>.
Upper/lower case differences in property names and values are irrelevant;
thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
hyphens, and non-interior underscores.
You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
-(^) between the first brace and the property name: C<\p{^Tamil}> is
+(C<^>) between the first brace and the property name: C<\p{^Tamil}> is
equal to C<\P{Tamil}>.
Almost all properties are immune to case-insensitive matching. That is,
and C<Titlecase>,
all of which match C<Cased> under C</i> matching.
This set also includes its subsets C<PosixUpper> and C<PosixLower> both
-of which under C</i> match
ing match C<PosixAlpha>.
+of which under C</i> match C<PosixAlpha>.
(The difference between these sets is that some things, such as Roman
-numerals, come in both upper and lower case so they are C<Cased>, but aren't considered
-letters, so they aren't C<Cased_Letter>s.)
+numerals, come in both upper and lower case so they are C<Cased>, but
+aren't considered letters, so they aren't C<Cased_Letter>'s.)
+
+See L</Beyond Unicode code points> for special considerations when
+matching Unicode properties against non-Unicode code points.
=head3 B<General_Category>
L<http://www.unicode.org/reports/tr44>).
The compound way of writing these is like C<\p{General_Category=Number}>
-(short, C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
+(short: C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
through the equal or colon separator is omitted. So you can instead just write
C<\pN>.
-Here are the short and long forms of the General Category properties:
+Here are the short and long forms of the values the C<General Category> property
+can have:
Short Long
=head3 B<Bidirectional Character Types>
Because scripts differ in their directionality (Hebrew and Arabic are
-written right to left, for example) Unicode supplies these properties in
-the Bidi_Class class:
+written right to left, for example) Unicode supplies a C<Bidi_Class> property.
+Some of the values this property can have are:
- Property Meaning
+ Value Meaning
L Left-to-Right
LRE Left-to-Right Embedding
This property is always written in the compound form.
For example, C<\p{Bidi_Class:R}> matches characters that are normally
-written right to left.
+written right to left. Unlike the
+C<L</General_Category>> property, this
+property can have more values added in a future Unicode release. Those
+listed above comprised the complete set for many Unicode releases, but
+others were added in Unicode 6.3; you can always find what the
+current ones are in L<perluniprops>. And
+L<http://www.unicode.org/reports/tr9/> describes how to use them.
=head3 B<Scripts>
written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
Hiragana or Katakana. There are many more.
-The Unicode Script property gives what script a given character is in,
-and the property can be specified with the compound form like
-C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>). Perl furnishes shortcuts for all
-script names. You can omit everything up through the equals (or colon), and
-simply write C<\p{Latin}> or C<\P{Cyrillic}>.
+The Unicode C<Script> and C<Script_Extensions> properties give what script a
+given character is in. Either property can be specified with the
+compound form like
+C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>), or
+C<\p{Script_Extensions=Javanese}> (short: C<\p{scx=java}>).
+In addition, Perl furnishes shortcuts for all
+C<Script> property names. You can omit everything up through the equals
+(or colon), and simply write C<\p{Latin}> or C<\P{Cyrillic}>.
+(This is not true for C<Script_Extensions>, which is required to be
+written in the compound form.)
+
+The difference between these two properties involves characters that are
+used in multiple scripts. For example the digits '0' through '9' are
+used in many parts of the world. These are placed in a script named
+C<Common>. Other characters are used in just a few scripts. For
+example, the C<"KATAKANA-HIRAGANA DOUBLE HYPHEN"> is used in both Japanese
+scripts, Katakana and Hiragana, but nowhere else. The C<Script>
+property places all characters that are used in multiple scripts in the
+C<Common> script, while the C<Script_Extensions> property places those
+that are used in only a few scripts into each of those scripts; while
+still using C<Common> for those used in many scripts. Thus both these
+match:
+
+ "0" =~ /\p{sc=Common}/ # Matches
+ "0" =~ /\p{scx=Common}/ # Matches
+
+and only the first of these match:
+
+ "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
+ "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
+
+And only the last two of these match:
+
+ "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
+ "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
+ "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
+ "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
+
+C<Script_Extensions> is thus an improved C<Script>, in which there are
+fewer characters in the C<Common> script, and correspondingly more in
+other scripts. It is new in Unicode version 6.0, and its data are likely
+to change significantly in later releases, as things get sorted out.
+New code should probably be using C<Script_Extensions> and not plain
+C<Script>.
+
+(Actually, besides C<Common>, the C<Inherited> script, contains
+characters that are used in multiple scripts. These are modifier
+characters which inherit the script value
+of the controlling character. Some of these are used in many scripts,
+and so go into C<Inherited> in both C<Script> and C<Script_Extensions>.
+Others are used in just a few scripts, so are in C<Inherited> in
+C<Script>, but not in C<Script_Extensions>.)
+
+It is worth stressing that there are several different sets of digits in
+Unicode that are equivalent to 0-9 and are matchable by C<\d> in a
+regular expression. If they are used in a single language only, they
+are in that language's C<Script> and C<Script_Extension>. If they are
+used in more than one script, they will be in C<sc=Common>, but only
+if they are used in many scripts should they be in C<scx=Common>.
A complete list of scripts and their shortcuts is in L<perluniprops>.
-=head3 B<Use of "Is" Prefix>
+=head3 B<Use of the C<"Is"> Prefix>
-For backward compatibility (with Perl 5.6), all properties mentioned
+For backward compatibility (with Perl 5.6), all properties writable
+without using the compound form mentioned
so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
C<\p{Arabic}>.
characters. The difference between scripts and blocks is that the
concept of scripts is closer to natural languages, while the concept
of blocks is more of an artificial grouping based on groups of Unicode
-characters with consecutive ordinal values. For example, the "Basic Latin"
-block is all characters whose ordinals are between 0 and 127, inclusive; in
-other words, the ASCII characters. The "Latin" script contains some letters
-from this as well as several other blocks, like "Latin-1 Supplement",
-"Latin Extended-A", etc., but it does not contain all the characters from
+characters with consecutive ordinal values. For example, the C<"Basic Latin">
+block is all the characters whose ordinals are between 0 and 127, inclusive; in
+other words, the ASCII characters. The C<"Latin"> script contains some letters
+from this as well as several other blocks, like C<"Latin-1 Supplement">,
+C<"Latin Extended-A">, I<etc.>, but it does not contain all the characters from
those blocks. It does not, for example, contain the digits 0-9, because
-those digits are shared across many scripts. The digits 0-9 and similar groups,
-like punctuation, are in the script called C<Common>. There is also a
-script called C<Inherited> for characters that modify other characters,
-and inherit the script value of the controlling character. (Note that
-there are several different sets of digits in Unicode that are
-equivalent to 0-9 and are matchable by C<\d> in a regular expression.
-If they are used in a single language only, they are in that language's
-script. Only sets are used across several languages are in the
-C<Common> script.)
+those digits are shared across many scripts, and hence are in the
+C<Common> script.
For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
L<http://www.unicode.org/reports/tr24>
-The Script property is likely to be the one you want to use when processing
-natural language; the Block property may occasionally be useful in working
+The C<Script> or C<Script_Extensions> properties are likely to be the
+ones you want to use when processing
+natural language; the C<Block> property may occasionally be useful in working
with the nuts and bolts of Unicode.
Block names are matched in the compound form, like C<\p{Block: Arrows}> or
C<\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 C<\p{In_Arrows}> or C<\p{In_Hebrew}>. For backwards
-compatibility, the C<In> prefix may be omitted if there is no naming conflict
-with a script or any other property, and you can even use an C<Is> prefix
-instead in those cases. But it is not a good idea to do this, for a couple
-reasons:
-
-=over 4
-
-=item 1
-
-It is confusing. There are many naming conflicts, and you may forget some.
-For example, C<\p{Hebrew}> means the I<script> Hebrew, and NOT the I<block>
-Hebrew. But would you remember that 6 months from now?
-
-=item 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 C<\p{Hebrew}> would have matched the I<block> Hebrew; now it
-doesn't.
-
-=back
-
-Some people prefer to always use C<\p{Block: foo}> and C<\p{Script: bar}>
-instead of the shortcuts, whether for clarity, because they can't remember the
-difference between 'In' and 'Is' anyway, or they aren't confident that those who
-eventually will read their code will know that difference.
-
-A complete list of blocks and their shortcuts is in L<perluniprops>.
+Unicode-defined short name.
+
+Perl also defines single form synonyms for the block property in cases
+where these do not conflict with something else. But don't use any of
+these, because they are unstable. Since these are Perl extensions, they
+are subordinate to official Unicode property names; Unicode doesn't know
+nor care about Perl's extensions. It may happen that a name that
+currently means the Perl extension will later be changed without warning
+to mean a different Unicode property in a future version of the perl
+interpreter that uses a later Unicode release, and your code would no
+longer work. The extensions are mentioned here for completeness: Take
+the block name and prefix it with one of: C<In> (for example
+C<\p{Blk=Arrows}> can currently be written as C<\p{In_Arrows}>); or
+sometimes C<Is> (like C<\p{Is_Arrows}>); or sometimes no prefix at all
+(C<\p{Arrows}>). As of this writing (Unicode 8.0) there are no
+conflicts with using the C<In_> prefix, but there are plenty with the
+other two forms. For example, C<\p{Is_Hebrew}> and C<\p{Hebrew}> mean
+C<\p{Script=Hebrew}> which is NOT the same thing as C<\p{Blk=Hebrew}>. Our
+advice used to be to use the C<In_> prefix as a single form way of
+specifying a block. But Unicode 8.0 added properties whose names begin
+with C<In>, and it's now clear that it's only luck that's so far
+prevented a conflict. Using C<In> is only marginally less typing than
+C<Blk:>, and the latter's meaning is clearer anyway, and guaranteed to
+never conflict. So don't take chances. Use C<\p{Blk=foo}> for new
+code. And be sure that block is what you really really want to do. In
+most cases scripts are what you want instead.
+
+A complete list of blocks is in L<perluniprops>.
=head3 B<Other Properties>
the compound form. And quite a few of these are actually recommended by Unicode
(in L<http://www.unicode.org/reports/tr18>).
-This section gives some details on all extensions that aren't synonyms for
-compound-form Unicode properties (for those, you'll have to refer to the
+This section gives some details on all extensions that aren't just
+synonyms for compound-form Unicode properties
+(for those properties, you'll have to refer to the
L<Unicode Standard|http://www.unicode.org/reports/tr44>.
=over
=item B<C<\p{All}>>
-This matches any of the 1_114_112 Unicode code points. It is a synonym for
-C<\p{Any}>.
+This matches every possible code point. It is equivalent to C<qr/./s>.
+Unlike all the other non-user-defined C<\p{}> property matches, no
+warning is ever generated if this is property is matched against a
+non-Unicode code point (see L</Beyond Unicode code points> below).
=item B<C<\p{Alnum}>>
=item B<C<\p{Any}>>
-This matches any of the 1_114_112 Unicode code points. It is a synonym for
-C<\p{All}>.
+This matches any of the 1_114_112 Unicode code points. It is a synonym
+for C<\p{Unicode}>.
=item B<C<\p{ASCII}>>
=item B<C<\p{Assigned}>>
-This matches any assigned code point; that is, any code point whose general
-category is not Unassigned (or equivalently, not Cn).
+This matches any assigned code point; that is, any code point whose L<general
+category|/General_Category> is not C<Unassigned> (or equivalently, not C<Cn>).
=item B<C<\p{Blank}>>
Matches a character that has a non-canonical decomposition.
-To understand the use of this rarely used property=value combination, it is
-necessary to know some basics about decomposition.
-Consider a character, say H. It could appear with various marks around it,
-such as an acute accent, or a circumflex, or various hooks, circles, arrows,
-I<etc.>, above, below, to one side or the other, etc. There are many
-possibilities among the world's languages. The number of combinations is
-astronomical, and if there were a character for each combination, it would
-soon exhaust Unicode's more than a million possible characters. So Unicode
-took a different approach: there is a character for the base H, and a
-character for each of the possible marks, and these can be variously combined
-to get a final logical character. So a logical character--what appears to be a
-single character--can be a sequence of more than one individual characters.
-This is called an "extended grapheme cluster"; Perl furnishes the C<\X>
-regular expression construct to match such sequences.
-
-But Unicode's intent is to unify the existing character set standards and
-practices, and several pre-existing standards have single characters that
-mean the same thing as some of these combinations. An example is ISO-8859-1,
-which has quite a few of these in the Latin-1 range, an example being "LATIN
-CAPITAL LETTER E WITH ACUTE". Because this character was in this pre-existing
-standard, Unicode added it to its repertoire. But this character is considered
-by Unicode to be equivalent to the sequence consisting of the character
-"LATIN CAPITAL LETTER E" followed by the character "COMBINING ACUTE ACCENT".
-
-"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and
-its equivalence with the sequence is called canonical equivalence. All
-pre-composed characters are said to have a decomposition (into the equivalent
-sequence), and the decomposition type is also called canonical.
-
-However, many more characters have a different type of decomposition, a
-"compatible" or "non-canonical" decomposition. The sequences that form these
-decompositions are not considered canonically equivalent to the pre-composed
-character. An example, again in the Latin-1 range, is the "SUPERSCRIPT ONE".
-It is somewhat like a regular digit 1, but not exactly; its decomposition
-into the digit 1 is called a "compatible" decomposition, specifically a
+The L</Extended Grapheme Clusters (Logical characters)> section above
+talked about canonical decompositions. However, many more characters
+have a different type of decomposition, a "compatible" or
+"non-canonical" decomposition. The sequences that form these
+decompositions are not considered canonically equivalent to the
+pre-composed character. An example is the C<"SUPERSCRIPT ONE">. It is
+somewhat like a regular digit 1, but not exactly; its decomposition into
+the digit 1 is called a "compatible" decomposition, specifically a
"super" decomposition. There are several such compatibility
-decompositions (see L<http://www.unicode.org/reports/tr44>), including one
-called "compat", which means some miscellaneous type of decomposition
-that doesn't fit into the decomposition categories that Unicode has chosen.
+decompositions (see L<http://www.unicode.org/reports/tr44>), including
+one called "compat", which means some miscellaneous type of
+decomposition that doesn't fit into the other decomposition categories
+that Unicode has chosen.
Note that most Unicode characters don't have a decomposition, so their
-decomposition type is "None".
+decomposition type is C<"None">.
For your convenience, Perl has added the C<Non_Canonical> decomposition
type to mean any of the several compatibility decompositions.
=item B<C<\p{PerlSpace}>>
-This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>.
+This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>
+and starting in Perl v5.18, a vertical tab.
Mnemonic: Perl's (original) space
=item B<C<\p{Posix...}>>
-There are several of these, which are equivalents using the C<\p>
-notation for Posix classes and are described in
+There are several of these, which are equivalents, using the C<\p{}>
+notation, for Posix classes and are described in
L<perlrecharclass/POSIX Character Classes>.
=item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>)
Unicode release given by the version number; C<\p{Present_In: Unassigned}>
will match those code points whose meaning has yet to be assigned.
-For example, C<U+0041> "LATIN CAPITAL LETTER A" was present in the very first
+For example, C<U+0041> C<"LATIN CAPITAL LETTER A"> was present in the very first
Unicode release available, which is C<1.1>, so this property is true for all
valid "*" versions. On the other hand, C<U+1EFF> was not assigned until version
-5.1 when it became "LATIN SMALL LETTER Y WITH LOOP", so the only "*" that
+5.1 when it became C<"LATIN SMALL LETTER Y WITH LOOP">, so the only "*" that
would match it are 5.1, 5.2, and later.
Unicode furnishes the C<Age> property from which this is derived. The problem
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.
+the same as the Perl C<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 I<assigned>, but that the meaning of the code point
has been I<determined>. This is because 66 code points will always be
-unassigned, and so the Age for them is the Unicode version in which the decision
+unassigned, and so the C<Age> for them is the Unicode version in which the decision
to make them so was made. For example, C<U+FDD0> is to be permanently
unassigned to a character, and the decision to do that was made in version 3.1,
so C<\p{Age=3.1}> matches this character, as also does C<\p{Present_In: 3.1}> and up.
This is the same as C<\s>, including beyond ASCII.
Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab
-which both the Posix standard and Unicode consider white space.)
+until v5.18, which both the Posix standard and Unicode consider white space.)
+
+=item B<C<\p{Title}>> and B<C<\p{Titlecase}>>
+
+Under case-sensitive matching, these both match the same code points as
+C<\p{General Category=Titlecase_Letter}> (C<\p{gc=lt}>). The difference
+is that under C</i> caseless matching, these match the same as
+C<\p{Cased}>, whereas C<\p{gc=lt}> matches C<\p{Cased_Letter>).
+
+=item B<C<\p{Unicode}>>
+
+This matches any of the 1_114_112 Unicode code points.
+C<\p{Any}>.
=item B<C<\p{VertSpace}>>
=back
+
=head2 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
+whose names begin with C<"In"> or C<"Is">. (The experimental feature
+L<perlre/(?[ ])> provides an alternative which allows more complex
+definitions.) The subroutines can be defined in any
package. The user-defined properties can be used in the regular expression
-C<\p> and C<\P> constructs; if you are using a user-defined property from a
+C<\p{}> and C<\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
-C<\p> or C<\P> construct.
+C<\p{}> or C<\P{}> construct.
# assuming property Is_Foreign defined in Lang::
package main; # property package name required
matches.
Note that if the regular expression is tainted, then Perl will die rather
-than calling the subroutine, where the name of the subroutine is
+than calling the subroutine when the name of the subroutine is
determined by the tainted data.
The subroutines must return a specially-formatted string, with one
=item *
-A single hexadecimal number denoting a Unicode code point to include.
+A single hexadecimal number denoting a code point to include.
=item *
Two hexadecimal numbers separated by horizontal whitespace (space or
-tabular characters) denoting a range of Unicode code points to include.
+tabular characters) denoting a range of code points to include.
=item *
-Something to include, prefixed by "+": a built-in character
-property (prefixed by "utf8::") or a user-defined character property,
+Something to include, prefixed by C<"+">: a built-in character
+property (prefixed by C<"utf8::">) or a fully qualified (including package
+name) user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.
=item *
-Something to exclude, prefixed by "-": an existing character
-property (prefixed by "utf8::") or a user-defined character property,
+Something to exclude, prefixed by C<"-">: an existing character
+property (prefixed by C<"utf8::">) or a fully qualified (including package
+name) user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.
=item *
-Something to negate, prefixed "!": an existing character
-property (prefixed by "utf8::") or a user-defined character property,
+Something to negate, prefixed C<"!">: an existing character
+property (prefixed by C<"utf8::">) or a fully qualified (including package
+name) user-defined character property,
to represent all the characters in that property; two hexadecimal code
points for a range; or a single hexadecimal code point.
=item *
-Something to intersect with, prefixed by "&": an existing character
-property (prefixed by "utf8::") or a user-defined character property,
+Something to intersect with, prefixed by C<"&">: an existing character
+property (prefixed by C<"utf8::">) or a fully qualified (including package
+name) user-defined character property,
for all the characters except the characters in the property; two
hexadecimal code points for a range; or a single hexadecimal code point.
Suppose you wanted to match only the allocated characters,
not the raw block ranges: in other words, you want to remove
-the non-characters:
+the unassigned characters:
sub InKana {
return <<'END';
END
}
-Intersection is useful for getting the common characters matched by
-two (or more) classes.
+This will match all non-Unicode code points, since every one of them is
+not in Kana. You can use intersection to exclude these, if desired, as
+this modified example shows:
- sub InFooAndBar {
+ sub InNotKana {
return <<'END';
- +main::Foo
- &main::Bar
- END
- }
-
-It's important to remember not to use "&" for the first set; that
-would be intersecting with nothing, resulting in an empty set.
-
-=head2 User-Defined Case Mappings (for serious hackers only)
-
-B<This featured is deprecated and is scheduled to be removed in Perl
-5.16.>
-The CPAN module L<Unicode::Casing> provides better functionality
-without the drawbacks described below.
-
-You can define your own mappings to be used in C<lc()>,
-C<lcfirst()>, C<uc()>, and C<ucfirst()> (or their string-inlined versions,
-C<\L>, C<\l>, C<\U>, and C<\u>). The mappings are currently only valid
-on strings encoded in UTF-8, but see below for a partial workaround for
-this restriction.
-
-The principle is similar to that of user-defined character
-properties: define subroutines that do the mappings.
-C<ToLower> is used for C<lc()>, C<\L>, C<lcfirst()>, and C<\l>; C<ToTitle> for
-C<ucfirst()> and C<\u>; and C<ToUpper> for C<uc()> and C<\U>.
-
-C<ToUpper()> should look something like this:
-
- sub ToUpper {
- return <<END;
- 0061\t007A\t0041
- 0101\t\t0100
+ !utf8::InHiragana
+ -utf8::InKatakana
+ +utf8::IsCn
+ &utf8::Any
END
}
-This sample C<ToUpper()> has the effect of mapping "a-z" to "A-Z", 0x101
-to 0x100, and all other characters map to themselves. The first
-returned line means to map the code point at 0x61 ("a") to 0x41 ("A"),
-the code point at 0x62 ("b") to 0x42 ("B"), ..., 0x7A ("z") to 0x5A
-("Z"). The second line maps just the code point 0x101 to 0x100. Since
-there are no other mappings defined, all other code points map to
-themselves.
-
-This mechanism is not well behaved as far as affecting other packages
-and scopes. All non-threaded programs have exactly one uppercasing
-behavior, one lowercasing behavior, and one titlecasing behavior in
-effect for utf8-encoded strings for the duration of the program. Each
-of these behaviors is irrevocably determined the first time the
-corresponding function is called to change a utf8-encoded string's case.
-If a corresponding C<To-> function has been defined in the package that
-makes that first call, the mapping defined by that function will be the
-mapping used for the duration of the program's execution across all
-packages and scopes. If no corresponding C<To-> function has been
-defined in that package, the standard official mapping will be used for
-all packages and scopes, and any corresponding C<To-> function anywhere
-will be ignored. Threaded programs have similar behavior. If the
-program's casing behavior has been decided at the time of a thread's
-creation, the thread will inherit that behavior. But, if the behavior
-hasn't been decided, the thread gets to decide for itself, and its
-decision does not affect other threads nor its creator.
-
-As shown by the example above, you have to furnish a complete mapping;
-you can't just override a couple of characters and leave the rest
-unchanged. You can find all the official mappings in the directory
-C<$Config{privlib}>F</unicore/To/>. The mapping data is returned as the
-here-document. The C<utf8::ToSpecI<Foo>> hashes in those files are special
-exception mappings derived from
-C<$Config{privlib}>F</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 L<Unicode::UCD> module, or just match
-case-insensitively, which is what uses the "Fold" mapping. Neither are user
-overridable.)
-
-If you have many mappings to change, you can take the official mapping data,
-change by hand the affected code points, and place the whole thing into your
-subroutine. But this will only be valid on Perls that use the same Unicode
-version. Another option would be to have your subroutine read the official
-mapping files and overwrite the affected code points.
-
-If you have only a few mappings to change, starting in 5.14 you can use the
-following trick, here illustrated for Turkish.
-
- use Config;
- use charnames ":full";
-
- sub ToUpper {
- my $official = do "$Config{privlib}/unicore/To/Upper.pl";
- $utf8::ToSpecUpper{'i'} =
- "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
- return $official;
- }
+C<&utf8::Any> must be the last line in the definition.
-This takes the official mappings and overrides just one, for "LATIN SMALL
-LETTER I". The keys to the hash must be the bytes that form the UTF-8
-(on EBCDIC platforms, UTF-EBCDIC) of the character, as illustrated by
-the inverse function.
+Intersection is used generally for getting the common characters matched
+by two (or more) classes. It's important to remember not to use C<"&"> for
+the first set; that would be intersecting with nothing, resulting in an
+empty set.
- sub ToLower {
- my $official = do $lower;
- $utf8::ToSpecLower{"\xc4\xb0"} = "i";
- return $official;
- }
+Unlike non-user-defined C<\p{}> property matches, no warning is ever
+generated if these properties are matched against a non-Unicode code
+point (see L</Beyond Unicode code points> below).
-This example is for an ASCII platform, and C<\xc4\xb0> is the string of
-bytes that together form the UTF-8 that represents C<\N{LATIN CAPITAL
-LETTER I WITH DOT ABOVE}>, C<U+0130>. You can avoid having to figure out
-these bytes, and at the same time make it work on all platforms by
-instead writing:
-
- sub ToLower {
- my $official = do $lower;
- my $sequence = "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
- utf8::encode($sequence);
- $utf8::ToSpecLower{$sequence} = "i";
- return $official;
- }
+=head2 User-Defined Case Mappings (for serious hackers only)
-This works because C<utf8::encode()> takes the single character and
-converts it to the sequence of bytes that constitute it. Note that we took
-advantage of the fact that C<"i"> is the same in UTF-8 or UTF_EBCIDIC as not;
-otherwise we would have had to write
-
- $utf8::ToSpecLower{$sequence} = "\N{LATIN SMALL LETTER I}";
-
-in the ToLower example, and in the ToUpper example, use
-
- my $sequence = "\N{LATIN SMALL LETTER I}";
- utf8::encode($sequence);
-
-A big caveat to the above trick and to this whole mechanism in general,
-is that they work only on strings encoded in UTF-8. You can partially
-get around this by using C<use subs>. (But better to just convert to
-use L<Unicode::Casing>.) For example:
-(The trick illustrated here does work in earlier releases, but only if all the
-characters you want to override have ordinal values of 256 or higher, or
-if you use the other tricks given just below.)
-
-The mappings are in effect only for the package they are defined in, and only
-on scalars that have been marked as having Unicode characters, for example by
-using C<utf8::upgrade()>. Although probably not advisable, you can
-cause the mappings to be used globally by importing into C<CORE::GLOBAL>
-(see L<CORE>).
-
-You can partially get around the restriction that the source strings
-must be in utf8 by using C<use subs> (or by importing into C<CORE::GLOBAL>) by:
-
- use subs qw(uc ucfirst lc lcfirst);
-
- sub uc($) {
- my $string = shift;
- utf8::upgrade($string);
- return CORE::uc($string);
- }
-
- sub lc($) {
- my $string = shift;
- utf8::upgrade($string);
-
- # Unless an I is before a dot_above, it turns into a dotless i.
- # (The character class with the combining classes matches non-above
- # marks following the I. Any number of these may be between the 'I' and
- # the dot_above, and the dot_above will still apply to the 'I'.
- use charnames ":full";
- $string =~
- s/I
- (?! [^\p{ccc=0}\p{ccc=Above}]* \N{COMBINING DOT ABOVE} )
- /\N{LATIN SMALL LETTER DOTLESS I}/gx;
-
- # But when the I is followed by a dot_above, remove the
- # dot_above so the end result will be i.
- $string =~ s/I
- ([^\p{ccc=0}\p{ccc=Above}]* )
- \N{COMBINING DOT ABOVE}
- /i$1/gx;
- return CORE::lc($string);
- }
-
-These examples (also for Turkish) make sure the input is in UTF-8, and then
-call the corresponding official function, which will use the C<ToUpper()> and
-C<ToLower()> functions you have defined.
-(For Turkish, there are other required functions: C<ucfirst>, C<lcfirst>,
-and C<ToTitle>. These are very similar to the ones given above.)
-
-The reason this is only a partial fix is that it doesn't affect the C<\l>,
-C<\L>, C<\u>, and C<\U> case-change operations in regular expressions,
-which still require the source to be encoded in utf8 (see L</The "Unicode
-Bug">). (Again, use L<Unicode::Casing> instead.)
-
-The C<lc()> example shows how you can add context-dependent casing. Note
-that context-dependent casing suffers from the problem that the string
-passed to the casing function may not have sufficient context to make
-the proper choice. Also, it will not be called for C<\l>, C<\L>, C<\u>,
-and C<\U>.
+B<This feature has been removed as of Perl 5.16.>
+The CPAN module C<L<Unicode::Casing>> provides better functionality without
+the drawbacks that this feature had. If you are using a Perl earlier
+than 5.16, this feature was most fully documented in the 5.14 version of
+this pod:
+L<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
=head2 Character Encodings for Input and Output
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][9]
- RL1.7 Supplementary Code Points - done [10]
-
- [1] \x{...}
- [2] \p{...} \P{...}
- [3] supports not only minimal list, but all Unicode character
- properties (see L</Unicode Character Properties>)
- [4] \d \D \s \S \w \W \X [:prop:] [:^prop:]
- [5] can use regular expression look-ahead [a] or
- user-defined character properties [b] to emulate set
- operations
- [6] \b \B
- [7] note that Perl does Full case-folding in matching (but with
- bugs), not Simple: for example U+1F88 is equivalent to
- U+1F00 U+03B9, not with 1F80. This difference matters
- mainly for certain Greek capital letters with certain
- modifiers: the Full case-folding decomposes the letter,
- while the Simple case-folding would map it to a single
- character.
- [8] should do ^ and $ also on U+000B (\v in C), FF (\f), CR
- (\r), CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS
- (U+2029); should also affect <>, $., and script line
- numbers; should not split lines within CRLF [c] (i.e. there
- is no empty line between \r and \n)
- [9] Linebreaking conformant with UAX#14 "Unicode Line Breaking
- Algorithm" is available through the Unicode::LineBreaking
- module.
- [10] UTF-8/UTF-EBDDIC used in Perl allows not only U+10000 to
- U+10FFFF but also beyond U+10FFFF
-
-[a] You can mimic class subtraction using lookahead.
+ RL1.1 Hex Notation - done [1]
+ RL1.2 Properties - done [2][3]
+ RL1.2a Compatibility Properties - done [4]
+ RL1.3 Subtraction and Intersection - experimental [5]
+ RL1.4 Simple Word Boundaries - done [6]
+ RL1.5 Simple Loose Matches - done [7]
+ RL1.6 Line Boundaries - MISSING [8][9]
+ RL1.7 Supplementary Code Points - done [10]
+
+=over 4
+
+=item [1] C<\N{U+...}> and C<\x{...}>
+
+=item [2] C<\p{...}> C<\P{...}>
+
+=item [3] supports not only minimal list, but all Unicode character
+properties (see Unicode Character Properties above)
+
+=item [4] C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
+C<[:^I<prop>:]>
+
+=item [5] The experimental feature starting in v5.18 C<"(?[...])"> accomplishes
+this.
+
+See L<perlre/(?[ ])>. If you don't want to use an experimental
+feature, you can use one of the following:
+
+=over 4
+
+=item *
+
+Regular expression lookahead
+
+You can mimic class subtraction using lookahead.
For example, what UTS#18 might write as
- [{Greek}-[{UNASSIGNED}]]
+ [{Block=Greek}-[{UNASSIGNED}]]
in Perl can be written as:
- (?!\p{Unassigned})\p{InGreekAndCoptic}
- (?=\p{Assigned})\p{InGreekAndCoptic}
+ (?!\p{Unassigned})\p{Block=Greek}
+ (?=\p{Assigned})\p{Block=Greek}
But in this particular example, you probably really want
- \p{GreekAndCoptic}
+ \p{Greek}
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.
+=item *
-[b] '+' for union, '-' for removal (set-difference), '&' for intersection
-(see L</"User-Defined Character Properties">)
+CPAN module C<L<Unicode::Regex::Set>>
-[c] Try the C<:crlf> layer (see L<PerlIO>).
+It does implement the full UTS#18 grouping, intersection, union, and
+removal (subtraction) syntax.
=item *
-Level 2 - Extended Unicode Support
+L</"User-Defined Character Properties">
+
+C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
+
+=back
+
+=item [6] C<\b> C<\B>
+
+=item [7]
+Note that Perl does Full case-folding in matching, not Simple:
+
+For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
+C<U+1F80>. This difference matters mainly for certain Greek capital
+letters with certain modifiers: the Full case-folding decomposes the
+letter, while the Simple case-folding would map it to a single
+character.
+
+=item [8]
+Perl treats C<\n> as the start- and end-line delimiter. Unicode
+specifies more characters that should be so-interpreted.
+
+These are:
+
+ VT U+000B (\v in C)
+ FF U+000C (\f)
+ CR U+000D (\r)
+ NEL U+0085
+ LS U+2028
+ PS U+2029
+
+C<^> and C<$> in regular expression patterns are supposed to match all
+these, but don't.
+These characters also don't, but should, affect C<< <> >> C<$.>, and
+script line numbers.
+
+Also, lines should not be split within C<CRLF> (i.e. there is no
+empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
+layer (see L<PerlIO>).
- 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
+=item [9] But C<qr/\b{lb}/> and C<L<Unicode::LineBreak>> are available.
- [10] see UAX#15 "Unicode Normalization Forms"
- [11] have Unicode::Normalize but not integrated to regexes
- [12] have \X but we don't have a "Grapheme Cluster Mode"
- [14] see UAX#29, Word Boundaries
- [15] see UAX#21 "Case Mappings"
- [16] missing loose match [e]
+L<C<qrE<sol>\b{lb}E<sol>>|perlrebackslash/\b{lb}> supplies default line
+breaking conformant with
+L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
-[e] C<\N{...}> allows namespaces (see L<charnames>).
+And, the module C<L<Unicode::LineBreak>> also conformant with UAX#14,
+provides customizable line breaking.
+
+=item [10]
+UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
+C<U+10FFFF> but also beyond C<U+10FFFF>
+
+=back
+
+=item *
+
+Level 2 - Extended Unicode Support
+
+ RL2.1 Canonical Equivalents - MISSING [10][11]
+ RL2.2 Default Grapheme Clusters - MISSING [12]
+ RL2.3 Default Word Boundaries - DONE [14]
+ RL2.4 Default Loose Matches - MISSING [15]
+ RL2.5 Name Properties - DONE
+ RL2.6 Wildcard Properties - MISSING
+
+ [10] see UAX#15 "Unicode Normalization Forms"
+ [11] have Unicode::Normalize but not integrated to regexes
+ [12] have \X and \b{gcb} but we don't have a "Grapheme Cluster
+ Mode"
+ [14] see UAX#29, Word Boundaries
+ [15] This is covered in Chapter 3.13 (in Unicode 6.0)
=item *
Level 3 - Tailored Support
- RL3.1 Tailored Punctuation - MISSING
- RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
- RL3.3 Tailored Word Boundaries - MISSING
- RL3.4 Tailored Loose Matches - MISSING
- RL3.5 Tailored Ranges - MISSING
- RL3.6 Context Matching - MISSING [19]
- RL3.7 Incremental Matches - MISSING
+ RL3.1 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")
+ 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 lookaheads or lookbehinds
+ 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")
=back
UTF-8
UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
-encoding. For ASCII (and we really do mean 7-bit ASCII, not another
-8-bit encoding), UTF-8 is transparent.
+encoding. In most of Perl's documentation, including elsewhere in this
+document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
+"UTF-8" refers only to the encoding used on ASCII platforms. It is a
+superset of 7-bit US-ASCII, so anything encoded in ASCII has the
+identical representation when encoded in UTF-8.
The following table is from Unicode 3.2.
- Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
+ 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+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
Another way to look at it is via bits:
- Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
+ Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
- 0aaaaaaa 0aaaaaaa
- 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
- ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
- 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
+ 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
+As you can see, the continuation bytes all begin with C<"10">, and the
leading bits of the start byte tell how many bytes there are in the
encoded character.
The original UTF-8 specification allowed up to 6 bytes, to allow
-encoding of numbers up to 0x7FFF_FFFF. Perl continues to allow those,
+encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
and has extended that up to 13 bytes to encode code points up to what
can fit in a 64-bit word. However, Perl will warn if you output any of
these as being non-portable; and under strict UTF-8 input protocols,
-they are forbidden.
-
-The Unicode non-character code points are also disallowed in UTF-8 in
-"open interchange". See L</Non-character code points>.
+they are forbidden. In addition, it is deprecated to use a code point
+larger than what a signed integer variable on your system can hold. On
+32-bit ASCII systems, this means C<0x7FFF_FFFF> is the legal maximum
+going forward (much higher on 64-bit systems).
=item *
UTF-EBCDIC
-Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
+Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
+This means that all the basic characters (which includes all
+those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
+are the same in both EBCDIC and UTF-EBCDIC.)
+
+UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
+bytes to represent a given code point than UTF-8 does; the largest
+Unicode code points take 5 bytes to represent (instead of 4 in UTF-8),
+and, extended for 64-bit words, it uses 14 bytes instead of 13 bytes in
+UTF-8.
=item *
-UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)
+UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
The followings items are mostly for reference and general Unicode
knowledge, Perl doesn't use these constructs internally.
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
+C<BOM>'s, are a solution to this. A special character has been reserved
in Unicode to function as a byte order marker: the character with the
-code point C<U+FEFF> is the BOM.
+code point C<U+FEFF> is the C<BOM>.
-The trick is that if you read a BOM, you will know the byte order,
+The trick is that if you read a C<BOM>, you will know the byte order,
since if it was written on a big-endian platform, you will read the
bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
you will read the bytes C<0xFF 0xFE>. (And if the originating platform
-was writing in UTF-8, you will read the bytes C<0xEF 0xBB 0xBF>.)
+was writing in ASCII platform UTF-8, you will read the bytes
+C<0xEF 0xBB 0xBF>.)
The way this trick works is that the character with the code point
C<U+FFFE> is not supposed to be in input streams, so the
-sequence of bytes C<0xFF 0xFE> is unambiguously "BOM, represented in
+sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
little-endian format" and cannot be C<U+FFFE>, represented in big-endian
format".
Surrogates have no meaning in Unicode outside their use in pairs to
represent other code points. However, Perl allows them to be
represented individually internally, for example by saying
-C<chr(0xD801)>, so that the all code points, not just those valid for open interchange, are
+C<chr(0xD801)>, so that all code points, not just those valid for open
+interchange, are
representable. Unicode does define semantics for them, such as their
-General Category is "Cs". But because their use is somewhat dangerous,
-Perl will warn (using the warning category "surrogate", which is a
-sub-category of "utf8") if an attempt is made
+C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
+Perl will warn (using the warning category C<"surrogate">, which is a
+sub-category of C<"utf8">) if an attempt is made
to do things like take the lower case of one, or match
case-insensitively, or to output them. (But don't try this on Perls
before 5.14.)
UTF-32, UTF-32BE, UTF-32LE
-The UTF-32 family is pretty much like the UTF-16 family, expect that
+The UTF-32 family is pretty much like the UTF-16 family, except that
the units are 32-bit, and therefore the surrogate scheme is not
-needed. UTF-32 is a fixed-width encoding The BOM signatures are
+needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
=item *
encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
because it does not use surrogates. UCS-4 is a 32-bit encoding,
functionally identical to UTF-32 (the difference being that
-UCS-4 forbids neither surrogates nor code points larger than 0x10_FFFF).
+UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
=item *
=back
-=head2 Non-character code points
-
-66 code points are set aside in Unicode as "non-character code points".
-These all have the Unassigned (Cn) General Category, and they never will
-be assigned. These are never supposed to be in legal Unicode input
-streams, so that code can use them as sentinels that can be mixed in
-with character data, and they always will be distinguishable from that data.
-To keep them out of Perl input streams, strict UTF-8 should be
-specified, such as by using the layer C<:encoding('UTF-8')>. The
-non-character code points are the 32 between U+FDD0 and U+FDEF, and the
-34 code points U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE, U+10FFFF.
-Some people are under the mistaken impression that these are "illegal",
-but that is not true. An application or cooperating set of applications
-can legally use them at will internally; but these code points are
-"illegal for open interchange". Therefore, Perl will not accept these
-from input streams unless lax rules are being used, and will warn
-(using the warning category "nonchar", which is a sub-category of "utf8") if
-an attempt is made to output them.
+=head2 Noncharacter code points
+
+66 code points are set aside in Unicode as "noncharacter code points".
+These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
+no character will ever be assigned to any of them. They are the 32 code
+points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
+points:
+
+ U+FFFE U+FFFF
+ U+1FFFE U+1FFFF
+ U+2FFFE U+2FFFF
+ ...
+ U+EFFFE U+EFFFF
+ U+FFFFE U+FFFFF
+ U+10FFFE U+10FFFF
+
+Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
+interchange of Unicode text data", so that code that processed those
+streams could use these code points as sentinels that could be mixed in
+with character data, and would always be distinguishable from that data.
+(Emphasis above and in the next paragraph are added in this document.)
+
+Unicode 7.0 changed the wording so that they are "B<not recommended> for
+use in open interchange of Unicode text data". The 7.0 Standard goes on
+to say:
+
+=over 4
+
+"If a noncharacter is received in open interchange, an application is
+not required to interpret it in any way. It is good practice, however,
+to recognize it as a noncharacter and to take appropriate action, such
+as replacing it with C<U+FFFD> replacement character, to indicate the
+problem in the text. It is not recommended to simply delete
+noncharacter code points from such text, because of the potential
+security issues caused by deleting uninterpreted characters. (See
+conformance clause C7 in Section 3.2, Conformance Requirements, and
+L<Unicode Technical Report #36, "Unicode Security
+Considerations"|http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
+
+=back
+
+This change was made because it was found that various commercial tools
+like editors, or for things like source code control, had been written
+so that they would not handle program files that used these code points,
+effectively precluding their use almost entirely! And that was never
+the intent. They've always been meant to be usable within an
+application, or cooperating set of applications, at will.
+
+If you're writing code, such as an editor, that is supposed to be able
+to handle any Unicode text data, then you shouldn't be using these code
+points yourself, and instead allow them in the input. If you need
+sentinels, they should instead be something that isn't legal Unicode.
+For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
+they never appear in well-formed UTF-8. (There are equivalents for
+UTF-EBCDIC). You can also store your Unicode code points in integer
+variables and use negative values as sentinels.
+
+If you're not writing such a tool, then whether you accept noncharacters
+as input is up to you (though the Standard recommends that you not). If
+you do strict input stream checking with Perl, these code points
+continue to be forbidden. This is to maintain backward compatibility
+(otherwise potential security holes could open up, as an unsuspecting
+application that was written assuming the noncharacters would be
+filtered out before getting to it, could now, without warning, start
+getting them). To do strict checking, you can use the layer
+C<:encoding('UTF-8')>.
+
+Perl continues to warn (using the warning category C<"nonchar">, which
+is a sub-category of C<"utf8">) if an attempt is made to output
+noncharacters.
=head2 Beyond Unicode code points
-The maximum Unicode code point is U+10FFFF. But Perl accepts code
+The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
+operations on code points up through that. But Perl works on code
points up to the maximum permissible unsigned number available on the
platform. However, Perl will not accept these from input streams unless
lax rules are being used, and will warn (using the warning category
-"non_unicode", which is a sub-category of "utf8") if an attempt is made to
-operate on or output them. For example, C<uc(0x11_0000)> will generate
-this warning, returning the input parameter as its result, as the upper
-case of every non-Unicode code point is the code point itself.
+C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
+
+Since Unicode rules are not defined on these code points, if a
+Unicode-defined operation is done on them, Perl uses what we believe are
+sensible rules, while generally warning, using the C<"non_unicode">
+category. For example, C<uc("\x{11_0000}")> will generate such a
+warning, returning the input parameter as its result, since Perl defines
+the uppercase of every non-Unicode code point to be the code point
+itself. (All the case changing operations, not just uppercasing, work
+this way.)
+
+The situation with matching Unicode properties in regular expressions,
+the C<\p{}> and C<\P{}> constructs, against these code points is not as
+clear cut, and how these are handled has changed as we've gained
+experience.
+
+One possibility is to treat any match against these code points as
+undefined. But since Perl doesn't have the concept of a match being
+undefined, it converts this to failing or C<FALSE>. This is almost, but
+not quite, what Perl did from v5.14 (when use of these code points
+became generally reliable) through v5.18. The difference is that Perl
+treated all C<\p{}> matches as failing, but all C<\P{}> matches as
+succeeding.
+
+One problem with this is that it leads to unexpected, and confusting
+results in some cases:
+
+ chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
+ chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
+
+That is, it treated both matches as undefined, and converted that to
+false (raising a warning on each). The first case is the expected
+result, but the second is likely counterintuitive: "How could both be
+false when they are complements?" Another problem was that the
+implementation optimized many Unicode property matches down to already
+existing simpler, faster operations, which don't raise the warning. We
+chose to not forgo those optimizations, which help the vast majority of
+matches, just to generate a warning for the unlikely event that an
+above-Unicode code point is being matched against.
+
+As a result of these problems, starting in v5.20, what Perl does is
+to treat non-Unicode code points as just typical unassigned Unicode
+characters, and matches accordingly. (Note: Unicode has atypical
+unassigned code points. For example, it has noncharacter code points,
+and ones that, when they do get assigned, are destined to be written
+Right-to-left, as Arabic and Hebrew are. Perl assumes that no
+non-Unicode code point has any atypical properties.)
+
+Perl, in most cases, will raise a warning when matching an above-Unicode
+code point against a Unicode property when the result is C<TRUE> for
+C<\p{}>, and C<FALSE> for C<\P{}>. For example:
+
+ chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
+ chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
+
+In both these examples, the character being matched is non-Unicode, so
+Unicode doesn't define how it should match. It clearly isn't an ASCII
+hex digit, so the first example clearly should fail, and so it does,
+with no warning. But it is arguable that the second example should have
+an undefined, hence C<FALSE>, result. So a warning is raised for it.
+
+Thus the warning is raised for many fewer cases than in earlier Perls,
+and only when what the result is could be arguable. It turns out that
+none of the optimizations made by Perl (or are ever likely to be made)
+cause the warning to be skipped, so it solves both problems of Perl's
+earlier approach. The most commonly used property that is affected by
+this change is C<\p{Unassigned}> which is a short form for
+C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
+code points are considered C<Unassigned>. In earlier releases the
+matches failed because the result was considered undefined.
+
+The only place where the warning is not raised when it might ought to
+have been is if optimizations cause the whole pattern match to not even
+be attempted. For example, Perl may figure out that for a string to
+match a certain regular expression pattern, the string has to contain
+the substring C<"foobar">. Before attempting the match, Perl may look
+for that substring, and if not found, immediately fail the match without
+actually trying it; so no warning gets generated even if the string
+contains an above-Unicode code point.
+
+This behavior is more "Do what I mean" than in earlier Perls for most
+applications. But it catches fewer issues for code that needs to be
+strictly Unicode compliant. Therefore there is an additional mode of
+operation available to accommodate such code. This mode is enabled if a
+regular expression pattern is compiled within the lexical scope where
+the C<"non_unicode"> warning class has been made fatal, say by:
+
+ use warnings FATAL => "non_unicode"
+
+(see L<warnings>). In this mode of operation, Perl will raise the
+warning for all matches against a non-Unicode code point (not just the
+arguable ones), and it skips the optimizations that might cause the
+warning to not be output. (It currently still won't warn if the match
+isn't even attempted, like in the C<"foobar"> example above.)
+
+In summary, Perl now normally treats non-Unicode code points as typical
+Unicode unassigned code points for regular expression matches, raising a
+warning only when it is arguable what the result should be. However, if
+this warning has been made fatal, it isn't skipped.
+
+There is one exception to all this. C<\p{All}> looks like a Unicode
+property, but it is a Perl extension that is defined to be true for all
+possible code points, Unicode or not, so no warning is ever generated
+when matching this against a non-Unicode code point. (Prior to v5.20,
+it was an exact synonym for C<\p{Any}>, matching code points C<0>
+through C<0x10FFFF>.)
=head2 Security Implications of Unicode
-Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
+First, read
+L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
+
Also, note the following:
=over 4
=back
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.
+each of two worlds: the old world of ASCII and single-byte locales, and
+the new world of Unicode, upgrading when necessary.
If your legacy code does not explicitly use Unicode, no automatic
-switch-over to characters should happen. Characters shouldn't get
-downgraded to bytes, either. It is possible to accidentally mix bytes
-and characters, however (see L<perluniintro>), in which case C<\w> in
-regular expressions might start behaving differently (unless the C</a>
-modifier is in effect). Review your code. Use warnings and the C<strict> pragma.
+switch-over to Unicode should happen.
=head2 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 C<utfebcdic> pragma or
-":utfebcdic" layer; rather, "utf8" and ":utf8" are reused to mean
-the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
-for more discussion of the issues.
+Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
+
+Unless ASCII vs. EBCDIC issues are specifically being discussed,
+references to UTF-8 encoding in this document and elsewhere should be
+read as meaning UTF-EBCDIC on EBCDIC platforms.
+See L<perlebcdic/Unicode and UTF>.
+
+Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
+hidden from you; S<C<use utf8>> (and NOT something like
+S<C<use utfebcdic>>) declares the the script is in the platform's
+"native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
+layer.)
=head2 Locales
=head2 When Unicode Does Not Happen
-While Perl does have extensive ways to input and output in Unicode,
-and a few other "entry points" like the @ARGV array (which can sometimes be
-interpreted as UTF-8), there are still many places where Unicode
-(in some encoding or another) could be given as arguments or received as
-results, or both, but it is not.
+There are still many places where Unicode (in some encoding or
+another) could be given as arguments or received as results, or both in
+Perl, but it is not, in spite of Perl having extensive ways to input and
+output in Unicode, and a few other "entry points" like the C<@ARGV>
+array (which can sometimes be interpreted as UTF-8).
The following are such interfaces. Also, see L</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 (problematic) C<encoding> pragma has been used.
+currently (as of v5.16.0) simply assumes byte strings both as arguments
+and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
One reason that Perl does not attempt to resolve the role of Unicode in
these situations is that the answers are highly dependent on the operating
=item *
-chdir, chmod, chown, chroot, exec, link, lstat, mkdir,
-rename, rmdir, stat, symlink, truncate, unlink, utime, -X
+C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
+C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
=item *
-%ENV
+C<%ENV>
=item *
-glob (aka the <*>)
+C<glob> (aka the C<E<lt>*E<gt>>)
=item *
-open, opendir, sysopen
+C<open>, C<opendir>, C<sysopen>
=item *
-qx (aka the backtick operator), system
+C<qx> (aka the backtick operator), C<system>
=item *
-readdir, readlink
+C<readdir>, C<readlink>
=back
=head2 The "Unicode Bug"
-The term, the "Unicode bug" has been applied to an inconsistency
-on ASCII platforms with the
-Unicode code points in the Latin-1 Supplement block, that
-is, between 128 and 255. Without a locale specified, unlike all other
-characters or code points, these characters have very different semantics in
-byte semantics versus character semantics, unless
-C<use feature 'unicode_strings'> is specified.
-(The lesson here is to specify C<unicode_strings> to avoid the
-headaches.)
-
-In character semantics they are interpreted as Unicode code points, which means
-they have the same semantics as Latin-1 (ISO-8859-1).
-
-In byte semantics, they are considered to be unassigned characters, meaning
-that the only semantics they have is their ordinal numbers, and that they are
-not members of various character classes. None are considered to match C<\w>
-for example, but all match C<\W>.
-
-The behavior is known to have effects on these areas:
-
-=over 4
-
-=item *
-
-Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
-and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in regular expression
-substitutions.
-
-=item *
-
-Using caseless (C</i>) regular expression matching
+The term, "Unicode bug" has been applied to an inconsistency with the
+code points in the C<Latin-1 Supplement> block, that is, between
+128 and 255. Without a locale specified, unlike all other characters or
+code points, these characters can have very different semantics
+depending on the rules in effect. (Characters whose code points are
+above 255 force Unicode rules; whereas the rules for ASCII characters
+are the same under both ASCII and Unicode rules.)
-=item *
+Under Unicode rules, these upper-Latin1 characters are interpreted as
+Unicode code points, which means they have the same semantics as Latin-1
+(ISO-8859-1) and C1 controls.
-Matching any of several properties in regular expressions, namely C<\b>,
-C<\B>, C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
-I<except> C<[[:ascii:]]>.
+As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
+they are considered to be unassigned characters.
-=item *
-
-In C<quotemeta> or its inline equivalent C<\Q>, no characters
-code points above 127 are quoted in UTF-8 encoded strings, but in
-byte encoded strings, code points between 128-255 are always quoted.
-
-=item *
-
-User-defined case change mappings. You can create a C<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 invoked.
-
-=back
-
-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:
+This can lead to unexpected results. For example, a string's
+semantics can suddenly change if a code point above 255 is appended to
+it, which changes the rules from ASCII to Unicode. As an
+example, consider the following program and its output:
$ perl -le'
no feature 'unicode_strings';
0
1
-If there's no C<\w> in C<s1> or in C<s2>, why does their concatenation have one?
+If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
+have one?
This anomaly stems from Perl's attempt to not disturb older programs that
-didn't use Unicode, and hence had no semantics for characters outside of the
-ASCII range (except in a locale), along with Perl's desire to add Unicode
-support seamlessly. The result wasn't seamless: these characters were
-orphaned.
-
-Starting in Perl 5.14, C<use feature 'unicode_strings'> can be used to
-cause Perl to use Unicode semantics on all string operations within the
-scope of the feature subpragma. Regular expressions compiled in its
-scope retain that behavior even when executed or compiled into larger
-regular expressions outside the scope. (The pragma does not, however,
-affect the C<quotemeta> behavior. Nor does it affect the deprecated
-user-defined case changing operations--these still require a UTF-8
-encoded string to operate.)
-
-In Perl 5.12, the subpragma affected casing changes, but not regular
-expressions. See L<perlfunc/lc> for details on how this pragma works in
-combination with various others for casing.
-
-For earlier Perls, or when a string is passed to a function outside the
-subpragma's scope, a workaround is to always call C<utf8::upgrade($string)>,
-or to use the standard module L<Encode>. Also, a scalar that has any characters
-whose ordinal is above 0x100, or which were specified using either of the
-C<\N{...}> notations, will automatically have character semantics.
+didn't use Unicode, along with Perl's desire to add Unicode support
+seamlessly. But the result turned out to not be seamless. (By the way,
+you can choose to be warned when things like this happen. See
+C<L<encoding::warnings>>.)
-=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
-
-Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
-there are situations where you simply need to force a byte
-string into UTF-8, or vice versa. The low-level calls
-utf8::upgrade($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.
-
-=head2 Using Unicode in XS
-
-If you want to handle Perl Unicode in XS extensions, you may find the
-following C APIs useful. See also L<perlguts/"Unicode Support"> for an
-explanation about Unicode at the XS level, and L<perlapi> for the API
-details.
+L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
+was added, starting in Perl v5.12, to address this problem. It affects
+these things:
=over 4
=item *
-C<DO_UTF8(sv)> returns true if the C<UTF8> flag is on and the bytes
-pragma is not in effect. C<SvUTF8(sv)> returns true if the C<UTF8>
-flag is on; the bytes pragma is ignored. The C<UTF8> flag being on
-does B<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 C<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 C<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.
-
-=item *
+Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
+and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
+contexts, such as regular expression substitutions.
-C<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.
+Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
+generally used. See L<perlfunc/lc> for details on how this works
+in combination with various other pragmas.
=item *
-C<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.
-
-=item *
+Using caseless (C</i>) regular expression matching.
-C<utf8_length(start, end)> returns the length of the UTF-8 encoded buffer
-in characters. C<sv_len_utf8(sv)> returns the length of the UTF-8 encoded
-scalar.
+Starting in Perl 5.14.0, regular expressions compiled within
+the scope of C<unicode_strings> use Unicode rules
+even when executed or compiled into larger
+regular expressions outside the scope.
=item *
-C<sv_utf8_upgrade(sv)> converts the string of the scalar to its UTF-8
-encoded form. C<sv_utf8_downgrade(sv)> does the opposite, if
-possible. C<sv_utf8_encode(sv)> is like sv_utf8_upgrade except that
-it does not set the C<UTF8> flag. C<sv_utf8_decode()> does the
-opposite of C<sv_utf8_encode()>. Note that none of these are to be
-used as general-purpose encoding or decoding interfaces: C<use Encode>
-for that. C<sv_utf8_upgrade()> is affected by the encoding pragma
-but C<sv_utf8_downgrade()> is not (since the encoding pragma is
-designed to be a one-way street).
+Matching any of several properties in regular expressions.
-=item *
+These properties are C<\b> (without braces), C<\B> (without braces),
+C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
+I<except> C<[[:ascii:]]>.
-C<is_utf8_char(s)> returns true if the pointer points to a valid UTF-8
-character.
+Starting in Perl 5.14.0, regular expressions compiled within
+the scope of C<unicode_strings> use Unicode rules
+even when executed or compiled into larger
+regular expressions outside the scope.
=item *
-C<is_utf8_string(buf, len)> returns true if C<len> bytes of the buffer
-are valid UTF-8.
+In C<quotemeta> or its inline equivalent C<\Q>.
-=item *
+Starting in Perl 5.16.0, consistent quoting rules are used within the
+scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
+Prior to that, or outside its scope, no code points above 127 are quoted
+in UTF-8 encoded strings, but in byte encoded strings, code points
+between 128-255 are always quoted.
-C<UTF8SKIP(buf)> will return the number of bytes in the UTF-8 encoded
-character in the buffer. C<UNISKIP(chr)> will return the number of bytes
-required to UTF-8-encode the Unicode character code point. C<UTF8SKIP()>
-is useful for example for iterating over the characters of a UTF-8
-encoded buffer; C<UNISKIP()> is useful, for example, in computing
-the size required for a UTF-8 encoded buffer.
-
-=item *
+=back
-C<utf8_distance(a, b)> will tell the distance in characters between the
-two pointers pointing to the same UTF-8 encoded buffer.
+You can see from the above that the effect of C<unicode_strings>
+increased over several Perl releases. (And Perl's support for Unicode
+continues to improve; it's best to use the latest available release in
+order to get the most complete and accurate results possible.) Note that
+C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
+higher.
-=item *
+For Perls earlier than those described above, or when a string is passed
+to a function outside the scope of C<unicode_strings>, see the next section.
-C<utf8_hop(s, off)> will return a pointer to a UTF-8 encoded buffer
-that is C<off> (positive or negative) Unicode characters displaced
-from the UTF-8 buffer C<s>. Be careful not to overstep the buffer:
-C<utf8_hop()> will merrily run off the end or the beginning of the
-buffer if told to do so.
+=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
-=item *
+Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
+there are situations where you simply need to force a byte
+string into UTF-8, or vice versa. The standard module L<Encode> can be
+used for this, or the low-level calls
+L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
+L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
-C<pv_uni_display(dsv, spv, len, pvlim, flags)> and
-C<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 B<all> characters as hexadecimal code
-points--but with the flags C<UNI_DISPLAY_ISPRINT>,
-C<UNI_DISPLAY_BACKSLASH>, and C<UNI_DISPLAY_QQ> you can make the
-output more readable.
+Note that C<utf8::downgrade()> can fail if the string contains characters
+that don't fit into a byte.
-=item *
+Calling either function on a string that already is in the desired state is a
+no-op.
-C<foldEQ_utf8(s1, pe1, l1, u1, s2, pe2, l2, u2)> can be used to
-compare two strings case-insensitively in Unicode. For case-sensitive
-comparisons you can just use C<memEQ()> and C<memNE()> as usual, except
-if one string is in utf8 and the other isn't.
+L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
+made to use Unicode rules.
-=back
+=head2 Using Unicode in XS
-For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
-in the Perl source code distribution.
+See L<perlguts/"Unicode Support"> for an introduction to Unicode at
+the XS level, and L<perlapi/Unicode Support> for the API details.
=head2 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.
+Perl by default comes with the latest supported Unicode version built-in, but
+the goal is to allow you to change to use any earlier one. In Perls
+v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
+Perl v5.18 and v5.24 are able to handle all earlier versions.
Download the files in the desired version of Unicode from the Unicode web
site L<http://www.unicode.org>). These should replace the existing files in
F<lib/unicore> in the Perl source tree. Follow the instructions in
F<README.perl> in that directory to change some of their names, and then build
-perl (see F<INSTALL>).
-
-It is even possible to copy the built files to a different directory, and then
-change F<utf8_heavy.pl> in the directory C<\$Config{privlib}> to point to the
-new directory, or maybe make a copy of that directory before making the change,
-and using C<@INC> or the C<-I> run-time flag to switch between versions at will
-(but because of caching, not in the middle of a process), but all this is
-beyond the scope of these instructions.
-
-=head1 BUGS
-
-=head2 Interaction with Locales
-
-See L<perllocale/Unicode and UTF-8>
-
-=head2 Problems with characters in the Latin-1 Supplement range
-
-See L</The "Unicode Bug">
-
-=head2 Interaction with Extensions
-
-When Perl exchanges data with an extension, the extension should be
-able to understand the UTF8 flag and act accordingly. If the
-extension doesn't recognize that flag, it's likely that the extension
-will return incorrectly-flagged data.
-
-So if you're working with Unicode data, consult the documentation of
-every module you're using if there are any issues with Unicode data
-exchange. If the documentation does not talk about Unicode at all,
-suspect the worst and probably look at the source to learn how the
-module is implemented. Modules written completely in Perl shouldn't
-cause problems. Modules that directly or indirectly access code written
-in other programming languages are at risk.
-
-For affected functions, the simple strategy to avoid data corruption is
-to always make the encoding of the exchanged data explicit. Choose an
-encoding that you know the extension can handle. Convert arguments passed
-to the extensions to that encoding and convert results back from that
-encoding. Write wrapper functions that do the conversions for you, so
-you can later change the functions when the extension catches up.
-
-To provide an example, let's say the popular Foo::Bar::escape_html
-function doesn't deal with Unicode data yet. The wrapper function
-would convert the argument to raw UTF-8 and convert the result back to
-Perl's internal representation like so:
-
- sub my_escape_html ($) {
- my($what) = shift;
- return unless defined $what;
- Encode::decode_utf8(Foo::Bar::escape_html(
- Encode::encode_utf8($what)));
- }
-
-Sometimes, when the extension does not convert data but just stores
-and retrieves them, you will be able to use the otherwise
-dangerous Encode::_utf8_on() function. Let's say the popular
-C<Foo::Bar> extension, written in C, provides a C<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 C<param> method:
-
- sub param {
- my($self,$name,$value) = @_;
- utf8::upgrade($name); # make sure it is UTF-8 encoded
- if (defined $value) {
- utf8::upgrade($value); # make sure it is UTF-8 encoded
- return $self->SUPER::param($name,$value);
- } else {
- my $ret = $self->SUPER::param($name);
- Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
- return $ret;
- }
- }
-
-Some extensions provide filters on data entry/exit points, such as
-DB_File::filter_store_key and family. Look out for such filters in
-the documentation of your extensions, they can make the transition to
-Unicode data much easier.
-
-=head2 Speed
-
-Some functions are slower when working on UTF-8 encoded strings than
-on byte encoded strings. All functions that need to hop over
-characters such as length(), substr() or index(), or matching regular
-expressions can work B<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 C<\p{Nd}> are known to
-be quite a bit slower (5-20 times) than their simpler counterparts
-like C<\d> (then again, there are hundreds of Unicode characters matching C<Nd>
-compared with the 10 ASCII characters matching C<d>).
-
-=head2 Problems on EBCDIC platforms
-
-There are a several known problems with Perl on EBCDIC platforms. If you
-want to use Perl there, send email to perlbug@perl.org.
-
-In earlier versions, when byte and character data were concatenated,
-the new string was sometimes created by
-decoding the byte strings as I<ISO 8859-1 (Latin-1)>, even if the
-old Unicode string used EBCDIC.
-
-If you find any of these, please report them as bugs.
+perl (see L<INSTALL>).
=head2 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 C<utf8> pragma to declare that a given scope
-expected to deal with Unicode data and had to make sure that only
-Unicode data were reaching that scope. If you have code that is
+Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
+programmer was required to use the C<utf8> pragma to declare that a
+given scope expected to deal with Unicode data and had to make sure that
+only Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to
-your code. The examples are written such that the code will continue
-to work under 5.6, so you should be safe to try them out.
+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.
-=over 4
+=over 3
=item *
A filehandle that should read or write UTF-8
- if ($] > 5.007) {
+ if ($] > 5.008) {
binmode $fh, ":encoding(utf8)";
}
A scalar that is going to be passed to some extension
-Be it Compress::Zlib, Apache::Request or any extension that has no
+Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
mention of Unicode in the manpage, you need to make sure that the
UTF8 flag is stripped off. Note that at the time of this writing
-(October 2002) the mentioned modules are not UTF-8-aware. Please
+(January 2012) the mentioned modules are not UTF-8-aware. Please
check the documentation to verify if this is still true.
- if ($] > 5.007) {
+ if ($] > 5.008) {
require Encode;
$val = Encode::encode_utf8($val); # make octets
}
If you believe the scalar comes back as UTF-8, you will most likely
want the UTF8 flag restored:
- if ($] > 5.007) {
+ if ($] > 5.008) {
require Encode;
$val = Encode::decode_utf8($val);
}
Same thing, if you are really sure it is UTF-8
- if ($] > 5.007) {
+ if ($] > 5.008) {
require Encode;
Encode::_utf8_on($val);
}
=item *
-A wrapper for fetchrow_array and fetchrow_hashref
+A wrapper for L<DBI> C<fetchrow_array> and C<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
+a convenient way to replace all your C<fetchrow_array> and
+C<fetchrow_hashref> calls. A wrapper function will also make it easier to
adapt to future enhancements in your database driver. Note that at the
-time of this writing (October 2002), the DBI has no standardized way
-to deal with UTF-8 data. Please check the documentation to verify if
+time of this writing (January 2012), the DBI has no standardized way
+to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
that is still true.
sub fetchrow {
# $what is one of fetchrow_{array,hashref}
my($self, $sth, $what) = @_;
- if ($] < 5.007) {
+ if ($] < 5.008) {
return $sth->$what;
} else {
require Encode;
a drag to your program. If you recognize such a situation, just remove
the UTF8 flag:
- utf8::downgrade($val) if $] > 5.007;
+ utf8::downgrade($val) if $] > 5.008;
=back
+=head1 BUGS
+
+See also L</The "Unicode Bug"> above.
+
+=head2 Interaction with Extensions
+
+When Perl exchanges data with an extension, the extension should be
+able to understand the UTF8 flag and act accordingly. If the
+extension doesn't recognize that flag, it's likely that the extension
+will return incorrectly-flagged data.
+
+So if you're working with Unicode data, consult the documentation of
+every module you're using if there are any issues with Unicode data
+exchange. If the documentation does not talk about Unicode at all,
+suspect the worst and probably look at the source to learn how the
+module is implemented. Modules written completely in Perl shouldn't
+cause problems. Modules that directly or indirectly access code written
+in other programming languages are at risk.
+
+For affected functions, the simple strategy to avoid data corruption is
+to always make the encoding of the exchanged data explicit. Choose an
+encoding that you know the extension can handle. Convert arguments passed
+to the extensions to that encoding and convert results back from that
+encoding. Write wrapper functions that do the conversions for you, so
+you can later change the functions when the extension catches up.
+
+To provide an example, let's say the popular C<Foo::Bar::escape_html>
+function doesn't deal with Unicode data yet. The wrapper function
+would convert the argument to raw UTF-8 and convert the result back to
+Perl's internal representation like so:
+
+ sub my_escape_html ($) {
+ my($what) = shift;
+ return unless defined $what;
+ Encode::decode_utf8(Foo::Bar::escape_html(
+ Encode::encode_utf8($what)));
+ }
+
+Sometimes, when the extension does not convert data but just stores
+and retrieves it, you will be able to use the otherwise
+dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
+the popular C<Foo::Bar> extension, written in C, provides a C<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 C<param> method:
+
+ sub param {
+ my($self,$name,$value) = @_;
+ utf8::upgrade($name); # make sure it is UTF-8 encoded
+ if (defined $value) {
+ utf8::upgrade($value); # make sure it is UTF-8 encoded
+ return $self->SUPER::param($name,$value);
+ } else {
+ my $ret = $self->SUPER::param($name);
+ Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
+ return $ret;
+ }
+ }
+
+Some extensions provide filters on data entry/exit points, such as
+C<DB_File::filter_store_key> and family. Look out for such filters in
+the documentation of your extensions; they can make the transition to
+Unicode data much easier.
+
+=head2 Speed
+
+Some functions are slower when working on UTF-8 encoded strings than
+on byte encoded strings. All functions that need to hop over
+characters such as C<length()>, C<substr()> or C<index()>, or matching
+regular expressions can work B<much> faster when the underlying data are
+byte-encoded.
+
+In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
+a caching scheme was introduced which improved the situation. In general,
+operations with UTF-8 encoded strings are still slower. As an example,
+the Unicode properties (character classes) like C<\p{Nd}> are known to
+be quite a bit slower (5-20 times) than their simpler counterparts
+like C<[0-9]> (then again, there are hundreds of Unicode characters matching
+C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
+
=head1 SEE ALSO
L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
-L<perlretut>, L<perlvar/"${^UNICODE}">
+L<perlretut>, L<perlvar/"${^UNICODE}">,
L<http://www.unicode.org/reports/tr44>).
=cut
https://perl5.git.perl.org / perl5.git / blobdiff