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diff --git a/Master/tlpkg/tlperl/lib/pods/perlunicode.pod b/Master/tlpkg/tlperl/lib/pods/perlunicode.pod new file mode 100644 index 00000000000..140d1340b20 --- /dev/null +++ b/Master/tlpkg/tlperl/lib/pods/perlunicode.pod @@ -0,0 +1,1762 @@ +=head1 NAME + +perlunicode - Unicode support in Perl + +=head1 DESCRIPTION + +=head2 Important Caveats + +Unicode support is an extensive requirement. While Perl does not +implement the Unicode standard or the accompanying technical reports +from cover to cover, Perl does support many Unicode features. + +People who want to learn to use Unicode in Perl, should probably read +the 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 Input and Output Layers + +Perl knows when a filehandle uses Perl's internal Unicode encodings +(UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with +the ":utf8" layer. Other encodings can be converted to Perl's +encoding on input or from Perl's encoding on output by use of the +":encoding(...)" layer. See L<open>. + +To indicate that Perl source itself is in UTF-8, use C<use utf8;>. + +=item Regular Expressions + +The regular expression compiler produces polymorphic opcodes. That is, +the pattern adapts to the data and automatically switches to the Unicode +character scheme when presented with data that is internally encoded in +UTF-8, or instead uses a traditional byte scheme when presented with +byte data. + +=item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts + +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 BOM-marked scripts and UTF-16 scripts autodetected + +If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE, +or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either +endianness, Perl will correctly read in the script as Unicode. +(BOMless UTF-8 cannot be effectively recognized or differentiated from +ISO 8859-1 or other eight-bit encodings.) + +=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. + +=back + +=head2 Byte and Character Semantics + +Beginning with version 5.6, Perl uses logically-wide characters to +represent strings internally. + +In future, Perl-level operations will be expected to work with +characters rather than bytes. + +However, as an interim compatibility measure, Perl aims to +provide a safe migration path from byte semantics to character +semantics for programs. For operations where Perl can unambiguously +decide that the input data are characters, Perl switches to +character semantics. For operations where this determination cannot +be made without additional information from the user, Perl decides in +favor of compatibility and chooses to use byte semantics. + +Under byte semantics, when C<use locale> is in effect, Perl uses the +semantics associated with the current locale. Absent a C<use locale>, and +absent a C<use feature 'unicode_strings'> pragma, Perl currently uses US-ASCII +(or Basic Latin in Unicode terminology) byte semantics, meaning that characters +whose ordinal numbers are in the range 128 - 255 are undefined except for their +ordinal numbers. This means that none have case (upper and lower), nor are any +a member of character classes, like 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<bytes> pragma will always, regardless of platform, force byte +semantics in a particular lexical scope. See L<bytes>. + +The C<use feature 'unicode_strings'> pragma is intended to always, regardless +of platform, force character (Unicode) semantics in a particular lexical scope. +In release 5.12, it is partially implemented, applying only to case changes. +See L</The "Unicode Bug"> below. + +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>. + +Unless explicitly stated, Perl operators use character semantics +for Unicode data and byte semantics for non-Unicode data. +The decision to use character semantics is made transparently. If +input data comes from a Unicode source--for example, if a character +encoding layer is added to a filehandle or a literal Unicode +string constant appears in a program--character semantics apply. +Otherwise, byte semantics are in effect. The C<bytes> pragma should +be used to force byte semantics on Unicode data, and the C<use feature +'unicode_strings'> pragma to force Unicode semantics on byte data (though in +5.12 it isn't fully implemented). + +If strings operating under byte semantics and strings with Unicode +character data are concatenated, the new string will have +character semantics. This can cause surprises: See 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: + +=over 4 + +=item * + +Strings--including hash keys--and regular expression patterns may +contain characters that have an ordinal value larger than 255. + +If you use a Unicode editor to edit your program, Unicode characters may +occur directly within the literal strings in UTF-8 encoding, or UTF-16. +(The former requires a BOM or C<use utf8>, the latter requires a 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}>. + +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. + +Additionally, if you + + use charnames ':full'; + +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>. + +=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. + +=item * + +Regular expressions match characters instead of bytes. "." matches +a character instead of a 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. + +=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. + +=item * + +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. + +=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', ...). + +=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). + +=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. + +=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. + +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. + +=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. + +=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. + +=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"> for more details. + +=back + +=over 4 + +=item * + +And finally, C<scalar reverse()> reverses by character rather than by byte. + +=back + +=head2 Unicode Character Properties + +Most Unicode character properties are accessible by using regular expressions. +They are used (like bracketed character classes) by using the C<\p{}> "matches +property" construct and the C<\P{}> negation, "doesn't match property". + +Note that the only time that Perl considers a sequence of individual code +points as a single logical character is in the C<\X> construct, already +mentioned above. Therefore "character" in this discussion means a single +Unicode code point. + +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 +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<\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 a number of different +values, such as Left, Right, Whitespace, and others. To match these, one needs +to specify the property name (Bidi_Class), and the value being matched against +(Left, Right, etc.). This is done, as in the examples above, by having the +two components separated by an equal sign (or interchangeably, a colon), like +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 +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 +separator. + +Most Unicode character properties have at least two synonyms (or aliases if you +prefer), a short one that is easier to type, and a longer one which is more +descriptive and hence it is easier to understand what it means. Thus the "L" +and "Letter" above are equivalent and can be used interchangeably. Likewise, +"Upper" is a synonym for "Uppercase", and we could have written +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>. + +Upper/lower case differences in the property names and values are irrelevant, +thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>. +Similarly, you can add or subtract underscores anywhere in the middle of a +word, so that these are also equivalent to C<\p{U_p_p_e_r}>. And white space +is irrelevant adjacent to non-word characters, such as the braces and the equals +or colon separators so C<\p{ Upper }> and C<\p{ Upper_case : Y }> are +equivalent to these as well. In fact, in most cases, white space and even +hyphens can be added or deleted anywhere. So even C<\p{ Up-per case = Yes}> is +equivalent. All this is called "loose-matching" by Unicode. The few places +where stricter matching is employed is in the middle of numbers, and the Perl +extension properties that begin or end with an underscore. Stricter matching +cares about white space (except adjacent to the non-word characters) and +hyphens, and non-interior underscores. + +You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret +(^) between the first brace and the property name: C<\p{^Tamil}> is +equal to C<\P{Tamil}>. + +=head3 B<General_Category> + +Every Unicode character is assigned a general category, which is the "most +usual categorization of a character" (from +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 +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: + + Short Long + + L Letter + LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}]) + Lu Uppercase_Letter + Ll Lowercase_Letter + Lt Titlecase_Letter + Lm Modifier_Letter + Lo Other_Letter + + M Mark + Mn Nonspacing_Mark + Mc Spacing_Mark + Me Enclosing_Mark + + N Number + Nd Decimal_Number (also Digit) + Nl Letter_Number + No Other_Number + + P Punctuation (also Punct) + Pc Connector_Punctuation + Pd Dash_Punctuation + Ps Open_Punctuation + Pe Close_Punctuation + Pi Initial_Punctuation + (may behave like Ps or Pe depending on usage) + Pf Final_Punctuation + (may behave like Ps or Pe depending on usage) + Po Other_Punctuation + + S Symbol + Sm Math_Symbol + Sc Currency_Symbol + Sk Modifier_Symbol + So Other_Symbol + + Z Separator + Zs Space_Separator + Zl Line_Separator + Zp Paragraph_Separator + + C Other + Cc Control (also Cntrl) + Cf Format + Cs Surrogate (not usable) + Co Private_Use + Cn Unassigned + +Single-letter properties match all characters in any of the +two-letter sub-properties starting with the same letter. +C<LC> and C<L&> are special cases, which are both aliases for the set consisting of everything matched by C<Ll>, C<Lu>, and C<Lt>. + +Because Perl hides the need for the user to understand the internal +representation of Unicode characters, there is no need to implement +the somewhat messy concept of surrogates. C<Cs> is therefore not +supported. + +=head3 B<Bidirectional Character Types> + +Because scripts differ in their directionality (Hebrew is +written right to left, for example) Unicode supplies these properties in +the Bidi_Class class: + + Property Meaning + + L Left-to-Right + LRE Left-to-Right Embedding + LRO Left-to-Right Override + R Right-to-Left + AL Arabic Letter + RLE Right-to-Left Embedding + RLO Right-to-Left Override + PDF Pop Directional Format + EN European Number + ES European Separator + ET European Terminator + AN Arabic Number + CS Common Separator + NSM Non-Spacing Mark + BN Boundary Neutral + B Paragraph Separator + S Segment Separator + WS Whitespace + ON Other Neutrals + +This property is always written in the compound form. +For example, C<\p{Bidi_Class:R}> matches characters that are normally +written right to left. + +=head3 B<Scripts> + +The world's languages are written in a number of scripts. This sentence +(unless you're reading it in translation) is written in Latin, while Russian is +written in Cyrllic, and Greek is written in, well, Greek; Japanese mainly in +Hiragana or Katakana. There are many more. + +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}>. + +A complete list of scripts and their shortcuts is in L<perluniprops>. + +=head3 B<Use of "Is" Prefix> + +For backward compatibility (with Perl 5.6), all properties 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}>. + +=head3 B<Blocks> + +In addition to B<scripts>, Unicode also defines B<blocks> of +characters. The difference between scripts and blocks is that the +concept of scripts is closer to natural languages, while the concept +of blocks is more of an artificial grouping based on groups of Unicode +characters with consecutive ordinal values. For example, the "Basic Latin" +block is all characters whose ordinals are between 0 and 127, inclusive, in +other words, the ASCII characters. The "Latin" script contains some letters +from this block as well as several more, like "Latin-1 Supplement", +"Latin Extended-A", etc., but it does not contain all the characters from +those blocks. It does not, for example, contain digits, because digits are +shared across many scripts. Digits and similar groups, like punctuation, are in +the script called 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. + +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 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 just prefer to always use C<\p{Block: foo}> and C<\p{Script: bar}> +instead of the shortcuts, for clarity, and because they can't remember the +difference between 'In' and 'Is' anyway (or aren't confident that those who +eventually will read their code will know). + +A complete list of blocks and their shortcuts is in L<perluniprops>. + +=head3 B<Other Properties> + +There are many more properties than the very basic ones described here. +A complete list is in L<perluniprops>. + +Unicode defines all its properties in the compound form, so all single-form +properties are Perl extensions. A number of these are just synonyms for the +Unicode ones, but some are genunine extensions, including a couple that are in +the compound form. And quite a few of these are actually recommended by Unicode +(in L<http://www.unicode.org/reports/tr18>). + +This section gives some details on all the extensions that aren't synonyms for +compound-form Unicode properties (for those, you'll have to refer to the +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}>. + +=item B<C<\p{Alnum}>> + +This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character. + +=item B<C<\p{Any}>> + +This matches any of the 1_114_112 Unicode code points. It is a synonym for +C<\p{All}>. + +=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). + +=item B<C<\p{Blank}>> + +This is the same as C<\h> and C<\p{HorizSpace}>: A character that changes the +spacing horizontally. + +=item B<C<\p{Decomposition_Type: Non_Canonical}>> (Short: C<\p{Dt=NonCanon}>) + +Matches a character that has a non-canonical decomposition. + +To understand the use of this rarely used property=value combination, it is +necessary to know some basics about decomposition. +Consider a character, say H. It could appear with various marks around it, +such as an acute accent, or a circumflex, or various hooks, circles, arrows, +I<etc.>, above, below, to one side and/or the other, etc. There are many +possibilities among the world's languages. The number of combinations is +astronomical, and if there were a character for each combination, it would +soon exhaust Unicode's more than a million possible characters. So Unicode +took a different approach: there is a character for the base H, and a +character for each of the possible marks, and they can be combined variously +to get a final logical character. So a logical character--what appears to be a +single character--can be a sequence of more than one individual characters. +This is called an "extended grapheme cluster". (Perl furnishes the C<\X> +construct to match such sequences.) + +But Unicode's intent is to unify the existing character set standards and +practices, and a number of pre-existing standards have single characters that +mean the same thing as some of these combinations. An example is ISO-8859-1, +which has quite a few of these in the Latin-1 range, an example being "LATIN +CAPITAL LETTER E WITH ACUTE". Because this character was in this pre-existing +standard, Unicode added it to its repertoire. But this character is considered +by Unicode to be equivalent to the sequence consisting of first the character +"LATIN CAPITAL LETTER E", then the character "COMBINING ACUTE ACCENT". + +"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and +the equivalence with the sequence is called canonical equivalence. All +pre-composed characters are said to have a decomposition (into the equivalent +sequence) and the decomposition type is also called canonical. + +However, many more characters have a different type of decomposition, a +"compatible" or "non-canonical" decomposition. The sequences that form these +decompositions are not considered canonically equivalent to the pre-composed +character. An example, again in the Latin-1 range, is the "SUPERSCRIPT ONE". +It is kind of like a regular digit 1, but not exactly; its decomposition +into the digit 1 is called a "compatible" decomposition, specifically a +"super" decomposition. There are several such compatibility +decompositions (see 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. + +Note that most Unicode characters don't have a decomposition, so their +decomposition type is "None". + +Perl has added the C<Non_Canonical> type, for your convenience, to mean any of +the compatibility decompositions. + +=item B<C<\p{Graph}>> + +Matches any character that is graphic. Theoretically, this means a character +that on a printer would cause ink to be used. + +=item B<C<\p{HorizSpace}>> + +This is the same as C<\h> and C<\p{Blank}>: A character that changes the +spacing horizontally. + +=item B<C<\p{In=*}>> + +This is a synonym for C<\p{Present_In=*}> + +=item B<C<\p{PerlSpace}>> + +This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>. + +Mnemonic: Perl's (original) space + +=item B<C<\p{PerlWord}>> + +This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]> + +Mnemonic: Perl's (original) word. + +=item B<C<\p{PosixAlnum}>> + +This matches any alphanumeric character in the ASCII range, namely +C<[A-Za-z0-9]>. + +=item B<C<\p{PosixAlpha}>> + +This matches any alphabetic character in the ASCII range, namely C<[A-Za-z]>. + +=item B<C<\p{PosixBlank}>> + +This matches any blank character in the ASCII range, namely C<S<[ \t]>>. + +=item B<C<\p{PosixCntrl}>> + +This matches any control character in the ASCII range, namely C<[\x00-\x1F\x7F]> + +=item B<C<\p{PosixDigit}>> + +This matches any digit character in the ASCII range, namely C<[0-9]>. + +=item B<C<\p{PosixGraph}>> + +This matches any graphical character in the ASCII range, namely C<[\x21-\x7E]>. + +=item B<C<\p{PosixLower}>> + +This matches any lowercase character in the ASCII range, namely C<[a-z]>. + +=item B<C<\p{PosixPrint}>> + +This matches any printable character in the ASCII range, namely C<[\x20-\x7E]>. +These are the graphical characters plus SPACE. + +=item B<C<\p{PosixPunct}>> + +This matches any punctuation character in the ASCII range, namely +C<[\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]>. These are the +graphical characters that aren't word characters. Note that the Posix standard +includes in its definition of punctuation, those characters that Unicode calls +"symbols." + +=item B<C<\p{PosixSpace}>> + +This matches any space character in the ASCII range, namely +C<S<[ \f\n\r\t\x0B]>> (the last being a vertical tab). + +=item B<C<\p{PosixUpper}>> + +This matches any uppercase character in the ASCII range, namely C<[A-Z]>. + +=item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>) + +This property is used when you need to know in what Unicode version(s) a +character is. + +The "*" above stands for some two digit Unicode version number, such as +C<1.1> or C<4.0>; or the "*" can also be C<Unassigned>. This property will +match the code points whose final disposition has been settled as of the +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 +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 +would match it are 5.1, 5.2, and later. + +Unicode furnishes the C<Age> property from which this is derived. The problem +with Age is that a strict interpretation of it (which Perl takes) has it +matching the precise release a code point's meaning is introduced in. Thus +C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1. This is not usually what +you want. + +Some non-Perl implementations of the Age property may change its meaning to be +the same as the Perl Present_In property; just be aware of that. + +Another confusion with both these properties is that the definition is not +that the code point has been assigned, but that the meaning of the code point +has been determined. This is because 66 code points will always be +unassigned, and, so the Age for them is the Unicode version the decision to +make them so was made in. For example, 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 and C<\p{Present_In: 3.1}> and up +matches as well. + +=item B<C<\p{Print}>> + +This matches any character that is graphical or blank, except controls. + +=item B<C<\p{SpacePerl}>> + +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 to be space.) + +=item B<C<\p{VertSpace}>> + +This is the same as C<\v>: A character that changes the spacing vertically. + +=item B<C<\p{Word}>> + +This is the same as C<\w>, including beyond ASCII. + +=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 +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 +package other than the one you are in, you must specify its package in the +C<\p> or C<\P> construct. + + # assuming property Is_Foreign defined in Lang:: + package main; # property package name required + if ($txt =~ /\p{Lang::IsForeign}+/) { ... } + + package Lang; # property package name not required + if ($txt =~ /\p{IsForeign}+/) { ... } + + +Note that the effect is compile-time and immutable once defined. + +The subroutines must return a specially-formatted string, with one +or more newline-separated lines. Each line must be one of the following: + +=over 4 + +=item * + +A single hexadecimal number denoting a Unicode 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. + +=item * + +Something to include, prefixed by "+": a built-in character +property (prefixed by "utf8::") or a user-defined character property, +to represent all the characters in that property; two hexadecimal code +points for a range; or a single hexadecimal code point. + +=item * + +Something to exclude, prefixed by "-": an existing character +property (prefixed by "utf8::") or a user-defined character property, +to represent all the characters in that property; two hexadecimal code +points for a range; or a single hexadecimal code point. + +=item * + +Something to negate, prefixed "!": an existing character +property (prefixed by "utf8::") or a user-defined character property, +to represent all the characters in that property; two hexadecimal code +points for a range; or a single hexadecimal code point. + +=item * + +Something to intersect with, prefixed by "&": an existing character +property (prefixed by "utf8::") or a user-defined character property, +for all the characters except the characters in the property; two +hexadecimal code points for a range; or a single hexadecimal code point. + +=back + +For example, to define a property that covers both the Japanese +syllabaries (hiragana and katakana), you can define + + sub InKana { + return <<END; + 3040\t309F + 30A0\t30FF + END + } + +Imagine that the here-doc end marker is at the beginning of the line. +Now you can use C<\p{InKana}> and C<\P{InKana}>. + +You could also have used the existing block property names: + + sub InKana { + return <<'END'; + +utf8::InHiragana + +utf8::InKatakana + END + } + +Suppose you wanted to match only the allocated characters, +not the raw block ranges: in other words, you want to remove +the non-characters: + + sub InKana { + return <<'END'; + +utf8::InHiragana + +utf8::InKatakana + -utf8::IsCn + END + } + +The negation is useful for defining (surprise!) negated classes. + + sub InNotKana { + return <<'END'; + !utf8::InHiragana + -utf8::InKatakana + +utf8::IsCn + END + } + +Intersection is useful for getting the common characters matched by +two (or more) classes. + + sub InFooAndBar { + 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 + +You can also define your own mappings to be used in the lc(), +lcfirst(), uc(), and ucfirst() (or their string-inlined versions). +The principle is similar to that of user-defined character +properties: to define subroutines +with names like C<ToLower> (for lc() and lcfirst()), C<ToTitle> (for +the first character in ucfirst()), and C<ToUpper> (for uc(), and the +rest of the characters in ucfirst()). + +The string returned by the subroutines needs to be two hexadecimal numbers +separated by two tabulators: the two numbers being, respectively, the source +code point and the destination code point. For example: + + sub ToUpper { + return <<END; + 0061\t\t0041 + END + } + +defines an uc() mapping that causes only the character "a" +to be mapped to "A"; all other characters will remain unchanged. + +(For serious hackers only) The above means you have to furnish a complete +mapping; you can't just override a couple of characters and leave the rest +unchanged. You can find all the mappings in the directory +C<$Config{privlib}>/F<unicore/To/>. The mapping data is returned as the +here-document, and the C<utf8::ToSpecFoo> are special exception mappings +derived from <$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 C<Unicode::UCD> module, or just match +case-insensitively (that's when the "Fold" mapping is used). + +The mappings will only take effect on scalars that have been marked as having +Unicode characters, for example by using C<utf8::upgrade()>. +Old byte-style strings are not affected. + +The mappings are in effect for the package they are defined in. + +=head2 Character Encodings for Input and Output + +See L<Encode>. + +=head2 Unicode Regular Expression Support Level + +The following list of Unicode support for regular expressions describes +all the features currently supported. The references to "Level N" +and the section numbers refer to the Unicode Technical Standard #18, +"Unicode Regular Expressions", version 11, in May 2005. + +=over 4 + +=item * + +Level 1 - Basic Unicode Support + + RL1.1 Hex Notation - done [1] + RL1.2 Properties - done [2][3] + RL1.2a Compatibility Properties - done [4] + RL1.3 Subtraction and Intersection - MISSING [5] + RL1.4 Simple Word Boundaries - done [6] + RL1.5 Simple Loose Matches - done [7] + RL1.6 Line Boundaries - MISSING [8] + RL1.7 Supplementary Code Points - done [9] + + [1] \x{...} + [2] \p{...} \P{...} + [3] supports not only minimal list, but all Unicode character + properties (see L</Unicode Character Properties>) + [4] \d \D \s \S \w \W \X [:prop:] [:^prop:] + [5] can use regular expression look-ahead [a] or + user-defined character properties [b] to emulate set operations + [6] \b \B + [7] note that Perl does Full case-folding in matching (but with bugs), + not Simple: for example U+1F88 is equivalent to U+1F00 U+03B9, + not with 1F80. This difference matters mainly for certain Greek + capital letters with certain modifiers: the Full case-folding + decomposes the letter, while the Simple case-folding would map + it to a single character. + [8] should do ^ and $ also on U+000B (\v in C), FF (\f), CR (\r), + CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS (U+2029); + should also affect <>, $., and script line numbers; + should not split lines within CRLF [c] (i.e. there is no empty + line between \r and \n) + [9] UTF-8/UTF-EBDDIC used in perl allows not only U+10000 to U+10FFFF + but also beyond U+10FFFF [d] + +[a] You can mimic class subtraction using lookahead. +For example, what UTS#18 might write as + + [{Greek}-[{UNASSIGNED}]] + +in Perl can be written as: + + (?!\p{Unassigned})\p{InGreekAndCoptic} + (?=\p{Assigned})\p{InGreekAndCoptic} + +But in this particular example, you probably really want + + \p{GreekAndCoptic} + +which will match assigned characters known to be part of the Greek script. + +Also see the Unicode::Regex::Set module, it does implement the full +UTS#18 grouping, intersection, union, and removal (subtraction) syntax. + +[b] '+' for union, '-' for removal (set-difference), '&' for intersection +(see L</"User-Defined Character Properties">) + +[c] Try the C<:crlf> layer (see L<PerlIO>). + +[d] U+FFFF will currently generate a warning message if 'utf8' warnings are + enabled + +=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 - MISSING [14] + RL2.4 Default Loose Matches - MISSING [15] + RL2.5 Name Properties - MISSING [16] + RL2.6 Wildcard Properties - MISSING + + [10] see UAX#15 "Unicode Normalization Forms" + [11] have Unicode::Normalize but not integrated to regexes + [12] have \X but we don't have a "Grapheme Cluster Mode" + [14] see UAX#29, Word Boundaries + [15] see UAX#21 "Case Mappings" + [16] have \N{...} but neither compute names of CJK Ideographs + and Hangul Syllables nor use a loose match [e] + +[e] C<\N{...}> allows namespaces (see L<charnames>). + +=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 + ( RL3.8 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") + +=back + +=head2 Unicode Encodings + +Unicode characters are assigned to I<code points>, which are abstract +numbers. To use these numbers, various encodings are needed. + +=over 4 + +=item * + +UTF-8 + +UTF-8 is a variable-length (1 to 6 bytes, current character allocations +require 4 bytes), byte-order independent encoding. For ASCII (and we +really do mean 7-bit ASCII, not another 8-bit encoding), UTF-8 is +transparent. + +The following table is from Unicode 3.2. + + Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte + + U+0000..U+007F 00..7F + U+0080..U+07FF * C2..DF 80..BF + U+0800..U+0FFF E0 * A0..BF 80..BF + U+1000..U+CFFF E1..EC 80..BF 80..BF + U+D000..U+D7FF ED 80..9F 80..BF + U+D800..U+DFFF +++++++ utf16 surrogates, not legal utf8 +++++++ + U+E000..U+FFFF EE..EF 80..BF 80..BF + U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF + U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF + U+100000..U+10FFFF F4 80..8F 80..BF 80..BF + +Note the gaps before several of the byte entries above marked by '*'. These are +caused by legal UTF-8 avoiding non-shortest encodings: it is technically +possible to UTF-8-encode a single code point in different ways, but that is +explicitly forbidden, and the shortest possible encoding should always be used +(and that is what Perl does). + +Another way to look at it is via bits: + + Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte + + 0aaaaaaa 0aaaaaaa + 00000bbbbbaaaaaa 110bbbbb 10aaaaaa + ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa + 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa + +As you can see, the continuation bytes all begin with "10", and the +leading bits of the start byte tell how many bytes there are in the +encoded character. + +=item * + +UTF-EBCDIC + +Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe. + +=item * + +UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks) + +The followings items are mostly for reference and general Unicode +knowledge, Perl doesn't use these constructs internally. + +UTF-16 is a 2 or 4 byte encoding. The Unicode code points +C<U+0000..U+FFFF> are stored in a single 16-bit unit, and the code +points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is +using I<surrogates>, the first 16-bit unit being the I<high +surrogate>, and the second being the I<low surrogate>. + +Surrogates are code points set aside to encode the C<U+10000..U+10FFFF> +range of Unicode code points in pairs of 16-bit units. The I<high +surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates> +are the range C<U+DC00..U+DFFF>. The surrogate encoding is + + $hi = ($uni - 0x10000) / 0x400 + 0xD800; + $lo = ($uni - 0x10000) % 0x400 + 0xDC00; + +and the decoding is + + $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00); + +If you try to generate surrogates (for example by using chr()), you +will get a warning, if warnings are turned on, because those code +points are not valid for a Unicode character. + +Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16 +itself can be used for in-memory computations, but if storage or +transfer is required either UTF-16BE (big-endian) or UTF-16LE +(little-endian) encodings must be chosen. + +This introduces another problem: what if you just know that your data +is UTF-16, but you don't know which endianness? Byte Order Marks, or +BOMs, are a solution to this. A special character has been reserved +in Unicode to function as a byte order marker: the character with the +code point C<U+FEFF> is the BOM. + +The trick is that if you read a BOM, you will know the byte order, +since if it was written on a big-endian platform, you will read the +bytes 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>.) + +The way this trick works is that the character with the code point +C<U+FFFE> is guaranteed not to be a valid Unicode character, so the +sequence of bytes C<0xFF 0xFE> is unambiguously "BOM, represented in +little-endian format" and cannot be C<U+FFFE>, represented in big-endian +format". (Actually, C<U+FFFE> is legal for use by your program, even for +input/output, but better not use it if you need a BOM. But it is "illegal for +interchange", so that an unsuspecting program won't get confused.) + +=item * + +UTF-32, UTF-32BE, UTF-32LE + +The UTF-32 family is pretty much like the UTF-16 family, expect that +the units are 32-bit, and therefore the surrogate scheme is not +needed. The BOM signatures will be C<0x00 0x00 0xFE 0xFF> for BE and +C<0xFF 0xFE 0x00 0x00> for LE. + +=item * + +UCS-2, UCS-4 + +Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit +encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>, +because it does not use surrogates. UCS-4 is a 32-bit encoding, +functionally identical to UTF-32. + +=item * + +UTF-7 + +A seven-bit safe (non-eight-bit) encoding, which is useful if the +transport or storage is not eight-bit safe. Defined by RFC 2152. + +=back + +=head2 Security Implications of Unicode + +Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>. +Also, note the following: + +=over 4 + +=item * + +Malformed UTF-8 + +Unfortunately, the specification of UTF-8 leaves some room for +interpretation of how many bytes of encoded output one should generate +from one input Unicode character. Strictly speaking, the shortest +possible sequence of UTF-8 bytes should be generated, +because otherwise there is potential for an input buffer overflow at +the receiving end of a UTF-8 connection. Perl always generates the +shortest length UTF-8, and with warnings on, Perl will warn about +non-shortest length UTF-8 along with other malformations, such as the +surrogates, which are not real Unicode code points. + +=item * + +Regular expressions behave slightly differently between byte data and +character (Unicode) data. For example, the "word character" character +class C<\w> will work differently depending on if data is eight-bit bytes +or Unicode. + +In the first case, the set of C<\w> characters is either small--the +default set of alphabetic characters, digits, and the "_"--or, if you +are using a locale (see L<perllocale>), the C<\w> might contain a few +more letters according to your language and country. + +In the second case, the C<\w> set of characters is much, much larger. +Most importantly, even in the set of the first 256 characters, it will +probably match different characters: unlike most locales, which are +specific to a language and country pair, Unicode classifies all the +characters that are letters I<somewhere> as C<\w>. For example, your +locale might not think that LATIN SMALL LETTER ETH is a letter (unless +you happen to speak Icelandic), but Unicode does. + +As discussed elsewhere, Perl has one foot (two hooves?) planted in +each of two worlds: the old world of bytes and the new world of +characters, upgrading from bytes to characters when necessary. +If your legacy code does not explicitly use Unicode, no automatic +switch-over to characters should happen. Characters shouldn't get +downgraded to bytes, either. It is possible to accidentally mix bytes +and characters, however (see L<perluniintro>), in which case C<\w> in +regular expressions might start behaving differently. Review your +code. Use warnings and the C<strict> pragma. + +=back + +=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. + +=head2 Locales + +Usually locale settings and Unicode do not affect each other, but +there are a couple of exceptions: + +=over 4 + +=item * + +You can enable automatic UTF-8-ification of your standard file +handles, default C<open()> layer, and C<@ARGV> by using either +the C<-C> command line switch or the C<PERL_UNICODE> environment +variable, see L<perlrun> for the documentation of the C<-C> switch. + +=item * + +Perl tries really hard to work both with Unicode and the old +byte-oriented world. Most often this is nice, but sometimes Perl's +straddling of the proverbial fence causes problems. + +=back + +=head2 When Unicode Does Not Happen + +While Perl does have extensive ways to input and output in Unicode, +and few other 'entry points' like the @ARGV which can be interpreted +as Unicode (UTF-8), there still are many places where Unicode (in some +encoding or another) could be given as arguments or received as +results, or both, but it is not. + +The following are such interfaces. Also, see 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 C<encoding> pragma has been used. + +One reason why Perl does not attempt to resolve the role of Unicode in +these cases is that the answers are highly dependent on the operating +system and the file system(s). For example, whether filenames can be +in Unicode, and in exactly what kind of encoding, is not exactly a +portable concept. Similarly for the qx and system: how well will the +'command line interface' (and which of them?) handle Unicode? + +=over 4 + +=item * + +chdir, chmod, chown, chroot, exec, link, lstat, mkdir, +rename, rmdir, stat, symlink, truncate, unlink, utime, -X + +=item * + +%ENV + +=item * + +glob (aka the <*>) + +=item * + +open, opendir, sysopen + +=item * + +qx (aka the backtick operator), system + +=item * + +readdir, readlink + +=back + +=head2 The "Unicode Bug" + +The term, the "Unicode bug" has been applied to an inconsistency with the +Unicode characters whose ordinals are in the Latin-1 Supplement block, that +is, between 128 and 255. Without a locale specified, unlike all other +characters or code points, these characters have very different semantics in +byte semantics versus character semantics. + +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>. (On EBCDIC platforms, the behavior may +be different from this, depending on the underlying C language library +functions.) + +The behavior is known to have effects on these areas: + +=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 + +=item * + +Matching a number of properties in regular expressions, such as C<\w> + +=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: + + $ perl -le' + $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. + +Work is being done to correct this, but only some of it was complete in time +for the 5.12 release. What has been finished is the important part of the case +changing component. Due to concerns, and some evidence, that older code might +have come to rely on the existing behavior, the new behavior must be explicitly +enabled by the feature C<unicode_strings> in the L<feature> pragma, even though +no new syntax is involved. + +See L<perlfunc/lc> for details on how this pragma works in combination with +various others for casing. Even though the pragma only affects casing +operations in the 5.12 release, it is planned to have it affect all the +problematic behaviors in later releases: you can't have one without them all. + +In the meantime, a workaround is to always call utf8::upgrade($string), or to +use the standard module L<Encode>. Also, a scalar that has any characters +whose ordinal is above 0x100, or which were specified using either of the +C<\N{...}> notations will automatically have character semantics. + +=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl) + +Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">) +there are situations where you simply need to force a byte +string into UTF-8, or vice versa. The low-level calls +utf8::upgrade($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. + +=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 * + +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. + +=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 * + +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. + +=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). + +=item * + +C<is_utf8_char(s)> returns true if the pointer points to a valid UTF-8 +character. + +=item * + +C<is_utf8_string(buf, len)> returns true if C<len> bytes of the buffer +are valid UTF-8. + +=item * + +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 * + +C<utf8_distance(a, b)> will tell the distance in characters between the +two pointers pointing to the same UTF-8 encoded buffer. + +=item * + +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. + +=item * + +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. + +=item * + +C<ibcmp_utf8(s1, pe1, l1, u1, s2, pe2, l2, u2)> can be used to +compare two strings case-insensitively in Unicode. For case-sensitive +comparisons you can just use C<memEQ()> and C<memNE()> as usual. + +=back + +For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h> +in the Perl source code distribution. + +=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. + +Download the files in the version of Unicode that you want from the Unicode web +site L<http://www.unicode.org>). These should replace the existing files in +C<\$Config{privlib}>/F<unicore>. (C<\%Config> is available from the Config +module.) Follow the instructions in F<README.perl> in that directory to change +some of their names, and then run F<make>. + +It is even possible to download them 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 + +Use of locales with Unicode data may lead to odd results. Currently, +Perl attempts to attach 8-bit locale info to characters in the range +0..255, but this technique is demonstrably incorrect for locales that +use characters above that range when mapped into Unicode. Perl's +Unicode support will also tend to run slower. Use of locales with +Unicode is discouraged. + +=head2 Problems with characters in the Latin-1 Supplement range + +See L</The "Unicode Bug"> + +=head2 Problems with case-insensitive regular expression matching + +There are problems with case-insensitive matches, including those involving +character classes (enclosed in [square brackets]), characters whose fold +is to multiple characters (such as the single character LATIN SMALL LIGATURE +FFL matches case-insensitively with the 3-character string C<ffl>), and +characters in the Latin-1 Supplement. + +=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 know about the flag, it's likely that the extension +will return incorrectly-flagged data. + +So if you're working with Unicode data, consult the documentation of +every module you're using if there are any 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 in a position 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 268 Unicode characters matching C<Nd> +compared with the 10 ASCII characters matching C<d>). + +=head2 Problems on EBCDIC platforms + +There are a number of known problems with Perl on EBCDIC platforms. If you +want to use Perl there, send email to 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. + +=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 +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. + +=over 4 + +=item * + +A filehandle that should read or write UTF-8 + + if ($] > 5.007) { + binmode $fh, ":encoding(utf8)"; + } + +=item * + +A scalar that is going to be passed to some extension + +Be it Compress::Zlib, Apache::Request or any extension that has no +mention of Unicode in the manpage, you need to make sure that the +UTF8 flag is stripped off. Note that at the time of this writing +(October 2002) the mentioned modules are not UTF-8-aware. Please +check the documentation to verify if this is still true. + + if ($] > 5.007) { + require Encode; + $val = Encode::encode_utf8($val); # make octets + } + +=item * + +A scalar we got back from an extension + +If you believe the scalar comes back as UTF-8, you will most likely +want the UTF8 flag restored: + + if ($] > 5.007) { + require Encode; + $val = Encode::decode_utf8($val); + } + +=item * + +Same thing, if you are really sure it is UTF-8 + + if ($] > 5.007) { + require Encode; + Encode::_utf8_on($val); + } + +=item * + +A wrapper for fetchrow_array and fetchrow_hashref + +When the database contains only UTF-8, a wrapper function or method is +a convenient way to replace all your fetchrow_array and +fetchrow_hashref calls. A wrapper function will also make it easier to +adapt to future enhancements in your database driver. Note that at the +time of this writing (October 2002), the DBI has no standardized way +to deal with UTF-8 data. Please check the documentation to verify if +that is still true. + + sub fetchrow { + my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref} + if ($] < 5.007) { + return $sth->$what; + } else { + require Encode; + if (wantarray) { + my @arr = $sth->$what; + for (@arr) { + defined && /[^\000-\177]/ && Encode::_utf8_on($_); + } + return @arr; + } else { + my $ret = $sth->$what; + if (ref $ret) { + for my $k (keys %$ret) { + defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret->{$k}; + } + return $ret; + } else { + defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret; + return $ret; + } + } + } + } + + +=item * + +A large scalar that you know can only contain ASCII + +Scalars that contain only ASCII and are marked as UTF-8 are sometimes +a drag to your program. If you recognize such a situation, just remove +the UTF8 flag: + + utf8::downgrade($val) if $] > 5.007; + +=back + +=head1 SEE ALSO + +L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>, +L<perlretut>, L<perlvar/"${^UNICODE}"> +L<http://www.unicode.org/reports/tr44>). + +=cut |