perlunicode: Revamp

I've always had problems understanding the point of some of the
discussion of this pod, so I've finally rewritten parts to bring it
up-to-date with modern Unicode support and clarify things.

In particular the "byte" vs "character" semantics didn't make sense to
me.  Perl has always used character semantics (outside of a few places
noted in both pod versions); it's just that the advent of Unicode made
'byte' and 'character' no longer synonymous.  So I've split that section
of the old pod, with the added section entitled "ASCII rules vs Unicode
rules", which I think is more clear.

1 files changed, 558 insertions, 457 deletions

diff --git a/pod/perlunicode.pod b/pod/perlunicode.pod
index 71aa5df24b..8edbaace85 100644
--- a/pod/perlunicode.pod
+++ b/pod/perlunicode.pod
@@ -4,17 +4,38 @@ perlunicode - Unicode support in Perl
 
 =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> and
-L<perluniintro>, 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>.
 
@@ -36,108 +57,106 @@ 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 C<: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
-C<:encoding(...)>  layer.  See L<open>.
+Use the C<:encoding(...)> layer  to read from and write to
+filehandles using the specified encoding.  (See L<open>.)
 
-To indicate that Perl source itself is in UTF-8, use C<use utf8;>.
+=item You should convert your non-ASCII, non-UTF-8 Perl scripts to be
+UTF-8.
 
-=item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts
+See L<encoding>.
 
-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>.
+=item C<use utf8> still needed to enable L<UTF-8|/Unicode Encodings> in scripts
 
-=item C<BOM>-marked scripts and UTF-16 scripts autodetected
+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>).
 
-If a Perl script begins marked 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 Unicode.
-(C<BOM>less UTF-8 cannot be effectively recognized or differentiated from
-ISO 8859-1 or other eight-bit encodings.)
+=item C<BOM>-marked scripts and L<UTF-16|/Unicode Encodings> scripts autodetected
 
-=item C<use encoding> needed to upgrade non-Latin-1 byte strings
-
-By default, there is a fundamental asymmetry in Perl's Unicode model:
-implicit upgrading from byte strings to Unicode strings assumes that
-they were encoded in 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
 
-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> (but not C<use locale ':not_characters'>) is in
-effect, Perl uses the rules associated with the current locale.
-(C<use locale> overrides C<use feature 'unicode_strings'> in the same scope;
-while C<use locale ':not_characters'> effectively also selects
-C<use feature 'unicode_strings'> in its scope; see L<perllocale>.)
-Otherwise, Perl uses the platform's native
-byte semantics for characters whose code points are less than 256, and
-Unicode rules for those greater than 255.  That means that non-ASCII
-characters 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 C<%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 C<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 ordinal values larger than 255.
 
@@ -145,140 +164,235 @@ 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 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 C<0x100> and
-above.  For characters below C<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, thus it is more portable to use
-C<\N{U+...}> instead.
-
-Additionally, you can use the C<\N{...}> notation and put the official
-Unicode character name within the braces, such as
-C<\N{WHITE SMILING FACE}>.  This automatically loads the L<charnames>
-module with the C<:full> and C<:short> options.  If you prefer different
-options for this module, you can instead, before the C<\N{...}>,
-explicitly load it with your desired options; for example,
+=item *
 
-   use charnames ':loose';
+The C<chr()> and C<ord()> functions work on whole characters.
 
 =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.
+Regular expressions match whole characters.  For example, C<"."> matches
+a whole character instead of only a single byte.
 
 =item *
 
-Regular expressions match characters instead of bytes.  C<"."> matches
-a character instead of a byte.
+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 *
 
-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.
+C<scalar reverse()> reverses by character rather than by byte.
 
 =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.
+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>.
 
-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
 
-=item *
+The bottom line is that Perl has always practiced "Character Semantics",
+but with the advent of Unicode, that is now different than "Byte
+Semantics".
+
+=head2 ASCII Rules versus Unicode Rules
+
+Before Unicode, when a character was a byte was a character,
+Perl knew only about the 128 characters defined by ASCII, code points 0
+through 127 (except for under 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>.
 
-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.
+Unicode, of course, assigns each of those code points a particular
+meaning (along with ones above 255).  To preserve backward
+compatibility, Perl only uses the Unicode meanings when there is some
+indication that Unicode is what is intended; otherwise the non-ASCII
+code points remain treated as if they are unassigned.
+
+Here are the ways that Perl knows that a string should be treated as
+Unicode:
+
+=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 *
 
-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.)
+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 L<C<E<sol>u>|perlre/E<sol>u> modifier is
+treated as Unicode.  Under the C</d> modifier, there are several other
+indications of Unicode; see L<perlre/E<sol>d>.
 
 =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
@@ -582,34 +696,28 @@ 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 preempt 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.
+Unicode-defined short name.  But Perl does provide a (slight, no longer
+recommended) 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 don't do this for new code because your code could break in new
+releases, and this has already happened: There was a time in very
+early Unicode releases when C<\p{Hebrew}> would have matched the
+I<block> Hebrew; now it doesn't.
+
+Using the C<In> prefix avoids this ambiguity, so far.  But new versions
+of Unicode continue to add new properties whose names begin with C<In>.
+There is a possibility that one of them someday will conflict with your
+usage.  Since this is just a Perl extension, Unicode's name will take
+precedence and your code will become broken.  Also, Unicode is free to
+add a script whose name begins with C<In>; that would cause problems.
+
+So it's clearer and best to use the compound form when specifying
+blocks.  And be sure that is what you really really want to do.  In most
+cases scripts are what you want instead.
 
 A complete list of blocks and their shortcuts is in L<perluniprops>.
 
@@ -666,41 +774,14 @@ spacing horizontally.
 
 Matches a character that has a non-canonical decomposition.
 
-To understand the use of this rarely used I<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 C<"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
-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 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 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
+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
@@ -1004,31 +1085,27 @@ Level 1 - Basic Unicode Support
 
 =over 4
 
-=item [1]
-
-C<\x{...}>
-
-=item [2]
-
-C<\p{...}> C<\P{...}>
+=item [1] C<\N{U+...}> and C<\x{...}>
 
-=item [3]
+=item [2] C<\p{...}> C<\P{...}>
 
-supports not only minimal list, but all Unicode character properties (see Unicode Character Properties above)
+=item [3] supports not only minimal list, but all Unicode character
+properties (see Unicode Character Properties above)
 
-=item [4]
+=item [4] C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
+C<[:^I<prop>:]>
 
-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.
 
-=item [5]
-
-The experimental feature 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:
+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 look-ahead
+=item *
+
+Regular expression look-ahead
 
 You can mimic class subtraction using lookahead.
 For example, what UTS#18 might write as
@@ -1046,47 +1123,60 @@ But in this particular example, you probably really want
 
 which will match assigned characters known to be part of the Greek script.
 
-=item * CPAN module C<L<Unicode::Regex::Set>>
+=item *
+
+CPAN module C<L<Unicode::Regex::Set>>
 
 It does implement the full UTS#18 grouping, intersection, union, and
 removal (subtraction) syntax.
 
-=item * L</"User-Defined Character Properties">
+=item *
+
+L</"User-Defined Character Properties">
 
 C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
 
 =back
 
-=item [6]
-
-C<\b> C<\B>
+=item [6] C<\b> C<\B>
 
 =item [7]
+Note that Perl does Full case-folding in matching, not Simple:
 
-Note that Perl does Full case-folding in matching (but with bugs), 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
+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.
 
-Should do C<^> and C<$> also on C<U+000B> (C<\v> in C), C<FF> (C<\f>),
-C<CR> (C<\r>), C<CRLF> (C<\r\n>), C<NEL> (C<U+0085>), C<LS> (C<U+2028>),
-and C<PS> (C<U+2029>); should also affect C<E<lt>E<gt>>, C<$.>, and
-script line numbers; should not split lines 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>).
+These are:
 
-=item [9]
+ VT   U+000B  (\v in C)
+ FF   U+000C  (\f)
+ CR   U+000D  (\r)
+ NEL  U+0085
+ LS   U+2028
+ PS   U+2029
 
-Linebreaking conformant with L<UAX#14 "Unicode Line Breaking
-Algorithm"|http://www.unicode.org/reports/tr14>
-is available through the C<L<Unicode::LineBreak>> module.
+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.
 
-=item [10]
+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>).
+
+=item [9] But C<L<Unicode::LineBreak>> is available.
 
+This module supplies line breaking conformant with
+L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
+
+=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>
 
@@ -1149,8 +1239,11 @@ numbers.  To use these numbers, various encodings are needed.
 UTF-8
 
 UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
-encoding. 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.
 
@@ -1198,6 +1291,14 @@ they are forbidden.
 UTF-EBCDIC
 
 Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
+This means that all the basic characters (which includes all
+those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
+are the same in both EBCDIC and UTF-EBCDIC.)
+
+UTF-EBCDIC is used on EBCDIC platforms.  The largest Unicode code points
+take 5 bytes to represent (instead of 4 in UTF-8), and Perl extends it
+to a maximum of 7 bytes to encode pode points up to what can fit in a
+32-bit word (instead of 13 bytes and a 64-bit word in UTF-8).
 
 =item *
 
@@ -1509,25 +1610,25 @@ modifiers.  Details are given in L<perlre/Character set modifiers>.
 =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
-C<":utfebcdic"> layer; rather, C<"utf8"> and C<":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
 
@@ -1584,32 +1685,50 @@ C<readdir>, C<readlink>
 
 =head2 The "Unicode Bug"
 
-The term, "Unicode bug" has been applied to an inconsistency
-on ASCII platforms with the
-Unicode 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 have very different semantics in
-byte semantics versus character semantics, unless
-C<use feature 'unicode_strings'> is specified, directly or indirectly.
-(It is indirectly specified by a C<use v5.12> or higher.)
-
-In character semantics these upper-Latin1 characters are interpreted as
-Unicode code points, which means
-they have the same semantics as Latin-1 (ISO-8859-1).
-
-In byte semantics (without C<unicode_strings>), 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>.
-
-Perl 5.12.0 added C<unicode_strings> to force character semantics on
-these code points in some circumstances, which fixed portions of the
-bug; Perl 5.14.0 fixed almost all of it; and Perl 5.16.0 fixed the
-remainder (so far as we know, anyway).  The lesson here is to enable
-C<unicode_strings> to avoid the headaches described below.
-
-The old, problematic behavior affects these areas:
+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.)
+
+Under Unicode rules, these upper-Latin1 characters are interpreted as
+Unicode code points, which means they have the same semantics as Latin-1
+(ISO-8859-1) and C1 controls.
+
+As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
+they are considered to be unassigned characters.
+
+This can lead to unexpected results.  For example, a string's
+semantics can suddenly change if a code point above 255 is appended to
+it, which changes the rules from ASCII to Unicode.  As an
+example, consider the following program and its output:
+
+ $ perl -le'
+     no feature 'unicode_strings';
+     $s1 = "\xC2";
+     $s2 = "\x{2660}";
+     for ($s1, $s2, $s1.$s2) {
+         print /\w/ || 0;
+     }
+ '
+ 0
+ 0
+ 1
+
+If there's no 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, 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>>.)
+
+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
 
@@ -1618,79 +1737,63 @@ The old, problematic behavior affects these areas:
 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.
-Under C<unicode_strings> starting in Perl 5.12.0, character semantics are
+
+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 *
 
 Using caseless (C</i>) regular expression matching.
+
 Starting in Perl 5.14.0, regular expressions compiled within
-the scope of C<unicode_strings> use character semantics
+the scope of C<unicode_strings> use Unicode rules
 even when executed or compiled into larger
 regular expressions outside the scope.
 
 =item *
 
-Matching any of several properties in regular expressions, namely
-C<\b> (without braces), C<\B> (without braces), C<\s>, C<\S>, C<\w>,
-C<\W>, and all the Posix character classes
+Matching any of several properties in regular expressions.
+
+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:]]>.
+
 Starting in Perl 5.14.0, regular expressions compiled within
-the scope of C<unicode_strings> use character semantics
+the scope of C<unicode_strings> use Unicode rules
 even when executed or compiled into larger
 regular expressions outside the scope.
 
 =item *
 
-In C<quotemeta> or its inline equivalent C<\Q>, no code points above 127
-are quoted in UTF-8 encoded strings, but in byte encoded strings, code
-points between 128-255 are always quoted.
+In C<quotemeta> or its inline equivalent C<\Q>.
+
 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.
 
 =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:
-
- $ perl -le'
-     no feature 'unicode_strings';
-     $s1 = "\xC2";
-     $s2 = "\x{2660}";
-     for ($s1, $s2, $s1.$s2) {
-         print /\w/ || 0;
-     }
- '
- 0
- 0
- 1
-
-If there's no C<\w> in C<s1> or 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.
+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.
 
 For Perls earlier than those described above, or when a string is passed
-to a function outside the subpragma's scope, a workaround is to always
-call L<C<utf8::upgrade($string)>|utf8/Utility functions>,
-or to use the standard module L<Encode>.   Also, a scalar that has any characters
-whose ordinal is C<0x100> or above, or which were specified using either of the
-C<\N{...}> notations, will automatically have character semantics.
+to a function outside the scope of C<unicode_strings>, see the next section.
 
 =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
+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> are
-the answers.
+L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
 
 Note that C<utf8::downgrade()> can fail if the string contains characters
 that don't fit into a byte.
@@ -1698,6 +1801,8 @@ that don't fit into a byte.
 Calling either function on a string that already is in the desired state is a
 no-op.
 
+L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
+made to use Unicode rules.
 
 =head2 Using Unicode in XS
 
@@ -1706,8 +1811,10 @@ 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 is 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
@@ -1715,105 +1822,15 @@ 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 L<INSTALL>).
 
-=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 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 them, 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 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 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 3
 
@@ -1916,6 +1933,90 @@ the UTF8 flag:
 
 =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>,