1PERLUNICODE(1)         Perl Programmers Reference Guide         PERLUNICODE(1)
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NAME

6       perlunicode - Unicode support in Perl
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DESCRIPTION

9       If you haven't already, before reading this document, you should become
10       familiar with both perlunitut and perluniintro.
11
12       Unicode aims to UNI-fy the en-CODE-ings of all the world's character
13       sets into a single Standard.   For quite a few of the various coding
14       standards that existed when Unicode was first created, converting from
15       each to Unicode essentially meant adding a constant to each code point
16       in the original standard, and converting back meant just subtracting
17       that same constant.  For ASCII and ISO-8859-1, the constant is 0.  For
18       ISO-8859-5, (Cyrillic) the constant is 864; for Hebrew (ISO-8859-8),
19       it's 1488; Thai (ISO-8859-11), 3424; and so forth.  This made it easy
20       to do the conversions, and facilitated the adoption of Unicode.
21
22       And it worked; nowadays, those legacy standards are rarely used.  Most
23       everyone uses Unicode.
24
25       Unicode is a comprehensive standard.  It specifies many things outside
26       the scope of Perl, such as how to display sequences of characters.  For
27       a full discussion of all aspects of Unicode, see
28       <http://www.unicode.org>.
29
30   Important Caveats
31       Even though some of this section may not be understandable to you on
32       first reading, we think it's important enough to highlight some of the
33       gotchas before delving further, so here goes:
34
35       Unicode support is an extensive requirement. While Perl does not
36       implement the Unicode standard or the accompanying technical reports
37       from cover to cover, Perl does support many Unicode features.
38
39       Also, the use of Unicode may present security issues that aren't
40       obvious, see "Security Implications of Unicode".
41
42       Safest if you "use feature 'unicode_strings'"
43           In order to preserve backward compatibility, Perl does not turn on
44           full internal Unicode support unless the pragma
45           "use feature 'unicode_strings'" is specified.  (This is
46           automatically selected if you "use 5.012" or higher.)  Failure to
47           do this can trigger unexpected surprises.  See "The "Unicode Bug""
48           below.
49
50           This pragma doesn't affect I/O.  Nor does it change the internal
51           representation of strings, only their interpretation.  There are
52           still several places where Unicode isn't fully supported, such as
53           in filenames.
54
55       Input and Output Layers
56           Use the ":encoding(...)" layer  to read from and write to
57           filehandles using the specified encoding.  (See open.)
58
59       You must convert your non-ASCII, non-UTF-8 Perl scripts to be UTF-8.
60           The encoding module has been deprecated since perl 5.18 and the
61           perl internals it requires have been removed with perl 5.26.
62
63       "use utf8" still needed to enable UTF-8 in scripts
64           If your Perl script is itself encoded in UTF-8, the "use utf8"
65           pragma must be explicitly included to enable recognition of that
66           (in string or regular expression literals, or in identifier names).
67           This is the only time when an explicit "use utf8" is needed.  (See
68           utf8).
69
70           If a Perl script begins with the bytes that form the UTF-8 encoding
71           of the Unicode BYTE ORDER MARK ("BOM", see "Unicode Encodings"),
72           those bytes are completely ignored.
73
74       UTF-16 scripts autodetected
75           If a Perl script begins with the Unicode "BOM" (UTF-16LE,
76           UTF16-BE), or if the script looks like non-"BOM"-marked UTF-16 of
77           either endianness, Perl will correctly read in the script as the
78           appropriate Unicode encoding.
79
80   Byte and Character Semantics
81       Before Unicode, most encodings used 8 bits (a single byte) to encode
82       each character.  Thus a character was a byte, and a byte was a
83       character, and there could be only 256 or fewer possible characters.
84       "Byte Semantics" in the title of this section refers to this behavior.
85       There was no need to distinguish between "Byte" and "Character".
86
87       Then along comes Unicode which has room for over a million characters
88       (and Perl allows for even more).  This means that a character may
89       require more than a single byte to represent it, and so the two terms
90       are no longer equivalent.  What matter are the characters as whole
91       entities, and not usually the bytes that comprise them.  That's what
92       the term "Character Semantics" in the title of this section refers to.
93
94       Perl had to change internally to decouple "bytes" from "characters".
95       It is important that you too change your ideas, if you haven't already,
96       so that "byte" and "character" no longer mean the same thing in your
97       mind.
98
99       The basic building block of Perl strings has always been a "character".
100       The changes basically come down to that the implementation no longer
101       thinks that a character is always just a single byte.
102
103       There are various things to note:
104
105       ·   String handling functions, for the most part, continue to operate
106           in terms of characters.  "length()", for example, returns the
107           number of characters in a string, just as before.  But that number
108           no longer is necessarily the same as the number of bytes in the
109           string (there may be more bytes than characters).  The other such
110           functions include "chop()", "chomp()", "substr()", "pos()",
111           "index()", "rindex()", "sort()", "sprintf()", and "write()".
112
113           The exceptions are:
114
115           ·   the bit-oriented "vec"
116
117
118
119           ·   the byte-oriented "pack"/"unpack" "C" format
120
121               However, the "W" specifier does operate on whole characters, as
122               does the "U" specifier.
123
124           ·   some operators that interact with the platform's operating
125               system
126
127               Operators dealing with filenames are examples.
128
129           ·   when the functions are called from within the scope of the
130               "use bytes" pragma
131
132               Likely, you should use this only for debugging anyway.
133
134       ·   Strings--including hash keys--and regular expression patterns may
135           contain characters that have ordinal values larger than 255.
136
137           If you use a Unicode editor to edit your program, Unicode
138           characters may occur directly within the literal strings in UTF-8
139           encoding, or UTF-16.  (The former requires a "use utf8", the latter
140           may require a "BOM".)
141
142           "Creating Unicode" in perluniintro gives other ways to place non-
143           ASCII characters in your strings.
144
145       ·   The "chr()" and "ord()" functions work on whole characters.
146
147       ·   Regular expressions match whole characters.  For example, "."
148           matches a whole character instead of only a single byte.
149
150       ·   The "tr///" operator translates whole characters.  (Note that the
151           "tr///CU" functionality has been removed.  For similar
152           functionality to that, see "pack('U0', ...)" and "pack('C0',
153           ...)").
154
155       ·   "scalar reverse()" reverses by character rather than by byte.
156
157       ·   The bit string operators, "& | ^ ~" and (starting in v5.22) "&. |.
158           ^.  ~." can operate on bit strings encoded in UTF-8, but this can
159           give unexpected results if any of the strings contain code points
160           above 0xFF.  Starting in v5.28, it is a fatal error to have such an
161           operand.  Otherwise, the operation is performed on a non-UTF-8 copy
162           of the operand.  If you're not sure about the encoding of a string,
163           downgrade it before using any of these operators; you can use
164           "utf8::utf8_downgrade()".
165
166       The bottom line is that Perl has always practiced "Character
167       Semantics", but with the advent of Unicode, that is now different than
168       "Byte Semantics".
169
170   ASCII Rules versus Unicode Rules
171       Before Unicode, when a character was a byte was a character, Perl knew
172       only about the 128 characters defined by ASCII, code points 0 through
173       127 (except for under "use locale").  That left the code points 128 to
174       255 as unassigned, and available for whatever use a program might want.
175       The only semantics they have is their ordinal numbers, and that they
176       are members of none of the non-negative character classes.  None are
177       considered to match "\w" for example, but all match "\W".
178
179       Unicode, of course, assigns each of those code points a particular
180       meaning (along with ones above 255).  To preserve backward
181       compatibility, Perl only uses the Unicode meanings when there is some
182       indication that Unicode is what is intended; otherwise the non-ASCII
183       code points remain treated as if they are unassigned.
184
185       Here are the ways that Perl knows that a string should be treated as
186       Unicode:
187
188       ·   Within the scope of "use utf8"
189
190           If the whole program is Unicode (signified by using 8-bit Unicode
191           Transformation Format), then all literal strings within it must be
192           Unicode.
193
194       ·   Within the scope of "use feature 'unicode_strings'"
195
196           This pragma was created so you can explicitly tell Perl that
197           operations executed within its scope are to use Unicode rules.
198           More operations are affected with newer perls.  See "The "Unicode
199           Bug"".
200
201       ·   Within the scope of "use 5.012" or higher
202
203           This implicitly turns on "use feature 'unicode_strings'".
204
205       ·   Within the scope of "use locale 'not_characters'", or "use locale"
206           and the current locale is a UTF-8 locale.
207
208           The former is defined to imply Unicode handling; and the latter
209           indicates a Unicode locale, hence a Unicode interpretation of all
210           strings within it.
211
212       ·   When the string contains a Unicode-only code point
213
214           Perl has never accepted code points above 255 without them being
215           Unicode, so their use implies Unicode for the whole string.
216
217       ·   When the string contains a Unicode named code point "\N{...}"
218
219           The "\N{...}" construct explicitly refers to a Unicode code point,
220           even if it is one that is also in ASCII.  Therefore the string
221           containing it must be Unicode.
222
223       ·   When the string has come from an external source marked as Unicode
224
225           The "-C" command line option can specify that certain inputs to the
226           program are Unicode, and the values of this can be read by your
227           Perl code, see "${^UNICODE}" in perlvar.
228
229       ·   When the string has been upgraded to UTF-8
230
231           The function "utf8::utf8_upgrade()" can be explicitly used to
232           permanently (unless a subsequent "utf8::utf8_downgrade()" is
233           called) cause a string to be treated as Unicode.
234
235       ·   There are additional methods for regular expression patterns
236
237           A pattern that is compiled with the "/u" or "/a" modifiers is
238           treated as Unicode (though there are some restrictions with "/a").
239           Under the "/d" and "/l" modifiers, there are several other
240           indications for Unicode; see "Character set modifiers" in perlre.
241
242       Note that all of the above are overridden within the scope of "use
243       bytes"; but you should be using this pragma only for debugging.
244
245       Note also that some interactions with the platform's operating system
246       never use Unicode rules.
247
248       When Unicode rules are in effect:
249
250       ·   Case translation operators use the Unicode case translation tables.
251
252           Note that "uc()", or "\U" in interpolated strings, translates to
253           uppercase, while "ucfirst", or "\u" in interpolated strings,
254           translates to titlecase in languages that make the distinction
255           (which is equivalent to uppercase in languages without the
256           distinction).
257
258           There is a CPAN module, "Unicode::Casing", which allows you to
259           define your own mappings to be used in "lc()", "lcfirst()", "uc()",
260           "ucfirst()", and "fc" (or their double-quoted string inlined
261           versions such as "\U").  (Prior to Perl 5.16, this functionality
262           was partially provided in the Perl core, but suffered from a number
263           of insurmountable drawbacks, so the CPAN module was written
264           instead.)
265
266       ·   Character classes in regular expressions match based on the
267           character properties specified in the Unicode properties database.
268
269           "\w" can be used to match a Japanese ideograph, for instance; and
270           "[[:digit:]]" a Bengali number.
271
272       ·   Named Unicode properties, scripts, and block ranges may be used
273           (like bracketed character classes) by using the "\p{}" "matches
274           property" construct and the "\P{}" negation, "doesn't match
275           property".
276
277           See "Unicode Character Properties" for more details.
278
279           You can define your own character properties and use them in the
280           regular expression with the "\p{}" or "\P{}" construct.  See "User-
281           Defined Character Properties" for more details.
282
283   Extended Grapheme Clusters (Logical characters)
284       Consider a character, say "H".  It could appear with various marks
285       around it, such as an acute accent, or a circumflex, or various hooks,
286       circles, arrows, etc., above, below, to one side or the other, etc.
287       There are many possibilities among the world's languages.  The number
288       of combinations is astronomical, and if there were a character for each
289       combination, it would soon exhaust Unicode's more than a million
290       possible characters.  So Unicode took a different approach: there is a
291       character for the base "H", and a character for each of the possible
292       marks, and these can be variously combined to get a final logical
293       character.  So a logical character--what appears to be a single
294       character--can be a sequence of more than one individual characters.
295       The Unicode standard calls these "extended grapheme clusters" (which is
296       an improved version of the no-longer much used "grapheme cluster");
297       Perl furnishes the "\X" regular expression construct to match such
298       sequences in their entirety.
299
300       But Unicode's intent is to unify the existing character set standards
301       and practices, and several pre-existing standards have single
302       characters that mean the same thing as some of these combinations, like
303       ISO-8859-1, which has quite a few of them. For example, "LATIN CAPITAL
304       LETTER E WITH ACUTE" was already in this standard when Unicode came
305       along.  Unicode therefore added it to its repertoire as that single
306       character.  But this character is considered by Unicode to be
307       equivalent to the sequence consisting of the character "LATIN CAPITAL
308       LETTER E" followed by the character "COMBINING ACUTE ACCENT".
309
310       "LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed"
311       character, and its equivalence with the "E" and the "COMBINING ACCENT"
312       sequence is called canonical equivalence.  All pre-composed characters
313       are said to have a decomposition (into the equivalent sequence), and
314       the decomposition type is also called canonical.  A string may be
315       comprised as much as possible of precomposed characters, or it may be
316       comprised of entirely decomposed characters.  Unicode calls these
317       respectively, "Normalization Form Composed" (NFC) and "Normalization
318       Form Decomposed".  The "Unicode::Normalize" module contains functions
319       that convert between the two.  A string may also have both composed
320       characters and decomposed characters; this module can be used to make
321       it all one or the other.
322
323       You may be presented with strings in any of these equivalent forms.
324       There is currently nothing in Perl 5 that ignores the differences.  So
325       you'll have to specially handle it.  The usual advice is to convert
326       your inputs to "NFD" before processing further.
327
328       For more detailed information, see <http://unicode.org/reports/tr15/>.
329
330   Unicode Character Properties
331       (The only time that Perl considers a sequence of individual code points
332       as a single logical character is in the "\X" construct, already
333       mentioned above.   Therefore "character" in this discussion means a
334       single Unicode code point.)
335
336       Very nearly all Unicode character properties are accessible through
337       regular expressions by using the "\p{}" "matches property" construct
338       and the "\P{}" "doesn't match property" for its negation.
339
340       For instance, "\p{Uppercase}" matches any single character with the
341       Unicode "Uppercase" property, while "\p{L}" matches any character with
342       a "General_Category" of "L" (letter) property (see "General_Category"
343       below).  Brackets are not required for single letter property names, so
344       "\p{L}" is equivalent to "\pL".
345
346       More formally, "\p{Uppercase}" matches any single character whose
347       Unicode "Uppercase" property value is "True", and "\P{Uppercase}"
348       matches any character whose "Uppercase" property value is "False", and
349       they could have been written as "\p{Uppercase=True}" and
350       "\p{Uppercase=False}", respectively.
351
352       This formality is needed when properties are not binary; that is, if
353       they can take on more values than just "True" and "False".  For
354       example, the "Bidi_Class" property (see "Bidirectional Character Types"
355       below), can take on several different values, such as "Left", "Right",
356       "Whitespace", and others.  To match these, one needs to specify both
357       the property name ("Bidi_Class"), AND the value being matched against
358       ("Left", "Right", etc.).  This is done, as in the examples above, by
359       having the two components separated by an equal sign (or
360       interchangeably, a colon), like "\p{Bidi_Class: Left}".
361
362       All Unicode-defined character properties may be written in these
363       compound forms of "\p{property=value}" or "\p{property:value}", but
364       Perl provides some additional properties that are written only in the
365       single form, as well as single-form short-cuts for all binary
366       properties and certain others described below, in which you may omit
367       the property name and the equals or colon separator.
368
369       Most Unicode character properties have at least two synonyms (or
370       aliases if you prefer): a short one that is easier to type and a longer
371       one that is more descriptive and hence easier to understand.  Thus the
372       "L" and "Letter" properties above are equivalent and can be used
373       interchangeably.  Likewise, "Upper" is a synonym for "Uppercase", and
374       we could have written "\p{Uppercase}" equivalently as "\p{Upper}".
375       Also, there are typically various synonyms for the values the property
376       can be.   For binary properties, "True" has 3 synonyms: "T", "Yes", and
377       "Y"; and "False" has correspondingly "F", "No", and "N".  But be
378       careful.  A short form of a value for one property may not mean the
379       same thing as the same short form for another.  Thus, for the
380       "General_Category" property, "L" means "Letter", but for the
381       "Bidi_Class" property, "L" means "Left".  A complete list of properties
382       and synonyms is in perluniprops.
383
384       Upper/lower case differences in property names and values are
385       irrelevant; thus "\p{Upper}" means the same thing as "\p{upper}" or
386       even "\p{UpPeR}".  Similarly, you can add or subtract underscores
387       anywhere in the middle of a word, so that these are also equivalent to
388       "\p{U_p_p_e_r}".  And white space is irrelevant adjacent to non-word
389       characters, such as the braces and the equals or colon separators, so
390       "\p{   Upper  }" and "\p{ Upper_case : Y }" are equivalent to these as
391       well.  In fact, white space and even hyphens can usually be added or
392       deleted anywhere.  So even "\p{ Up-per case = Yes}" is equivalent.  All
393       this is called "loose-matching" by Unicode.  The few places where
394       stricter matching is used is in the middle of numbers, and in the Perl
395       extension properties that begin or end with an underscore.  Stricter
396       matching cares about white space (except adjacent to non-word
397       characters), hyphens, and non-interior underscores.
398
399       You can also use negation in both "\p{}" and "\P{}" by introducing a
400       caret ("^") between the first brace and the property name: "\p{^Tamil}"
401       is equal to "\P{Tamil}".
402
403       Almost all properties are immune to case-insensitive matching.  That
404       is, adding a "/i" regular expression modifier does not change what they
405       match.  There are two sets that are affected.  The first set is
406       "Uppercase_Letter", "Lowercase_Letter", and "Titlecase_Letter", all of
407       which match "Cased_Letter" under "/i" matching.  And the second set is
408       "Uppercase", "Lowercase", and "Titlecase", all of which match "Cased"
409       under "/i" matching.  This set also includes its subsets "PosixUpper"
410       and "PosixLower" both of which under "/i" match "PosixAlpha".  (The
411       difference between these sets is that some things, such as Roman
412       numerals, come in both upper and lower case so they are "Cased", but
413       aren't considered letters, so they aren't "Cased_Letter"'s.)
414
415       See "Beyond Unicode code points" for special considerations when
416       matching Unicode properties against non-Unicode code points.
417
418       General_Category
419
420       Every Unicode character is assigned a general category, which is the
421       "most usual categorization of a character" (from
422       <http://www.unicode.org/reports/tr44>).
423
424       The compound way of writing these is like "\p{General_Category=Number}"
425       (short: "\p{gc:n}").  But Perl furnishes shortcuts in which everything
426       up through the equal or colon separator is omitted.  So you can instead
427       just write "\pN".
428
429       Here are the short and long forms of the values the "General Category"
430       property can have:
431
432           Short       Long
433
434           L           Letter
435           LC, L&      Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
436           Lu          Uppercase_Letter
437           Ll          Lowercase_Letter
438           Lt          Titlecase_Letter
439           Lm          Modifier_Letter
440           Lo          Other_Letter
441
442           M           Mark
443           Mn          Nonspacing_Mark
444           Mc          Spacing_Mark
445           Me          Enclosing_Mark
446
447           N           Number
448           Nd          Decimal_Number (also Digit)
449           Nl          Letter_Number
450           No          Other_Number
451
452           P           Punctuation (also Punct)
453           Pc          Connector_Punctuation
454           Pd          Dash_Punctuation
455           Ps          Open_Punctuation
456           Pe          Close_Punctuation
457           Pi          Initial_Punctuation
458                       (may behave like Ps or Pe depending on usage)
459           Pf          Final_Punctuation
460                       (may behave like Ps or Pe depending on usage)
461           Po          Other_Punctuation
462
463           S           Symbol
464           Sm          Math_Symbol
465           Sc          Currency_Symbol
466           Sk          Modifier_Symbol
467           So          Other_Symbol
468
469           Z           Separator
470           Zs          Space_Separator
471           Zl          Line_Separator
472           Zp          Paragraph_Separator
473
474           C           Other
475           Cc          Control (also Cntrl)
476           Cf          Format
477           Cs          Surrogate
478           Co          Private_Use
479           Cn          Unassigned
480
481       Single-letter properties match all characters in any of the two-letter
482       sub-properties starting with the same letter.  "LC" and "L&" are
483       special: both are aliases for the set consisting of everything matched
484       by "Ll", "Lu", and "Lt".
485
486       Bidirectional Character Types
487
488       Because scripts differ in their directionality (Hebrew and Arabic are
489       written right to left, for example) Unicode supplies a "Bidi_Class"
490       property.  Some of the values this property can have are:
491
492           Value       Meaning
493
494           L           Left-to-Right
495           LRE         Left-to-Right Embedding
496           LRO         Left-to-Right Override
497           R           Right-to-Left
498           AL          Arabic Letter
499           RLE         Right-to-Left Embedding
500           RLO         Right-to-Left Override
501           PDF         Pop Directional Format
502           EN          European Number
503           ES          European Separator
504           ET          European Terminator
505           AN          Arabic Number
506           CS          Common Separator
507           NSM         Non-Spacing Mark
508           BN          Boundary Neutral
509           B           Paragraph Separator
510           S           Segment Separator
511           WS          Whitespace
512           ON          Other Neutrals
513
514       This property is always written in the compound form.  For example,
515       "\p{Bidi_Class:R}" matches characters that are normally written right
516       to left.  Unlike the "General_Category" property, this property can
517       have more values added in a future Unicode release.  Those listed above
518       comprised the complete set for many Unicode releases, but others were
519       added in Unicode 6.3; you can always find what the current ones are in
520       perluniprops.  And <http://www.unicode.org/reports/tr9/> describes how
521       to use them.
522
523       Scripts
524
525       The world's languages are written in many different scripts.  This
526       sentence (unless you're reading it in translation) is written in Latin,
527       while Russian is written in Cyrillic, and Greek is written in, well,
528       Greek; Japanese mainly in Hiragana or Katakana.  There are many more.
529
530       The Unicode "Script" and "Script_Extensions" properties give what
531       script a given character is in.  The "Script_Extensions" property is an
532       improved version of "Script", as demonstrated below.  Either property
533       can be specified with the compound form like "\p{Script=Hebrew}"
534       (short: "\p{sc=hebr}"), or "\p{Script_Extensions=Javanese}" (short:
535       "\p{scx=java}").  In addition, Perl furnishes shortcuts for all
536       "Script_Extensions" property names.  You can omit everything up through
537       the equals (or colon), and simply write "\p{Latin}" or "\P{Cyrillic}".
538       (This is not true for "Script", which is required to be written in the
539       compound form.  Prior to Perl v5.26, the single form returned the plain
540       old "Script" version, but was changed because "Script_Extensions" gives
541       better results.)
542
543       The difference between these two properties involves characters that
544       are used in multiple scripts.  For example the digits '0' through '9'
545       are used in many parts of the world.  These are placed in a script
546       named "Common".  Other characters are used in just a few scripts.  For
547       example, the "KATAKANA-HIRAGANA DOUBLE HYPHEN" is used in both Japanese
548       scripts, Katakana and Hiragana, but nowhere else.  The "Script"
549       property places all characters that are used in multiple scripts in the
550       "Common" script, while the "Script_Extensions" property places those
551       that are used in only a few scripts into each of those scripts; while
552       still using "Common" for those used in many scripts.  Thus both these
553       match:
554
555        "0" =~ /\p{sc=Common}/     # Matches
556        "0" =~ /\p{scx=Common}/    # Matches
557
558       and only the first of these match:
559
560        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common}  # Matches
561        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
562
563       And only the last two of these match:
564
565        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana}  # No match
566        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana}  # No match
567        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
568        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
569
570       "Script_Extensions" is thus an improved "Script", in which there are
571       fewer characters in the "Common" script, and correspondingly more in
572       other scripts.  It is new in Unicode version 6.0, and its data are
573       likely to change significantly in later releases, as things get sorted
574       out.  New code should probably be using "Script_Extensions" and not
575       plain "Script".  If you compile perl with a Unicode release that
576       doesn't have "Script_Extensions", the single form Perl extensions will
577       instead refer to the plain "Script" property.  If you compile with a
578       version of Unicode that doesn't have the "Script" property, these
579       extensions will not be defined at all.
580
581       (Actually, besides "Common", the "Inherited" script, contains
582       characters that are used in multiple scripts.  These are modifier
583       characters which inherit the script value of the controlling character.
584       Some of these are used in many scripts, and so go into "Inherited" in
585       both "Script" and "Script_Extensions".  Others are used in just a few
586       scripts, so are in "Inherited" in "Script", but not in
587       "Script_Extensions".)
588
589       It is worth stressing that there are several different sets of digits
590       in Unicode that are equivalent to 0-9 and are matchable by "\d" in a
591       regular expression.  If they are used in a single language only, they
592       are in that language's "Script" and "Script_Extensions".  If they are
593       used in more than one script, they will be in "sc=Common", but only if
594       they are used in many scripts should they be in "scx=Common".
595
596       The explanation above has omitted some detail; refer to UAX#24 "Unicode
597       Script Property": <http://www.unicode.org/reports/tr24>.
598
599       A complete list of scripts and their shortcuts is in perluniprops.
600
601       Use of the "Is" Prefix
602
603       For backward compatibility (with ancient Perl 5.6), all properties
604       writable without using the compound form mentioned so far may have "Is"
605       or "Is_" prepended to their name, so "\P{Is_Lu}", for example, is equal
606       to "\P{Lu}", and "\p{IsScript:Arabic}" is equal to "\p{Arabic}".
607
608       Blocks
609
610       In addition to scripts, Unicode also defines blocks of characters.  The
611       difference between scripts and blocks is that the concept of scripts is
612       closer to natural languages, while the concept of blocks is more of an
613       artificial grouping based on groups of Unicode characters with
614       consecutive ordinal values. For example, the "Basic Latin" block is all
615       the characters whose ordinals are between 0 and 127, inclusive; in
616       other words, the ASCII characters.  The "Latin" script contains some
617       letters from this as well as several other blocks, like "Latin-1
618       Supplement", "Latin Extended-A", etc., but it does not contain all the
619       characters from those blocks. It does not, for example, contain the
620       digits 0-9, because those digits are shared across many scripts, and
621       hence are in the "Common" script.
622
623       For more about scripts versus blocks, see UAX#24 "Unicode Script
624       Property": <http://www.unicode.org/reports/tr24>
625
626       The "Script_Extensions" or "Script" properties are likely to be the
627       ones you want to use when processing natural language; the "Block"
628       property may occasionally be useful in working with the nuts and bolts
629       of Unicode.
630
631       Block names are matched in the compound form, like "\p{Block: Arrows}"
632       or "\p{Blk=Hebrew}".  Unlike most other properties, only a few block
633       names have a Unicode-defined short name.
634
635       Perl also defines single form synonyms for the block property in cases
636       where these do not conflict with something else.  But don't use any of
637       these, because they are unstable.  Since these are Perl extensions,
638       they are subordinate to official Unicode property names; Unicode
639       doesn't know nor care about Perl's extensions.  It may happen that a
640       name that currently means the Perl extension will later be changed
641       without warning to mean a different Unicode property in a future
642       version of the perl interpreter that uses a later Unicode release, and
643       your code would no longer work.  The extensions are mentioned here for
644       completeness:  Take the block name and prefix it with one of: "In" (for
645       example "\p{Blk=Arrows}" can currently be written as "\p{In_Arrows}");
646       or sometimes "Is" (like "\p{Is_Arrows}"); or sometimes no prefix at all
647       ("\p{Arrows}").  As of this writing (Unicode 9.0) there are no
648       conflicts with using the "In_" prefix, but there are plenty with the
649       other two forms.  For example, "\p{Is_Hebrew}" and "\p{Hebrew}" mean
650       "\p{Script_Extensions=Hebrew}" which is NOT the same thing as
651       "\p{Blk=Hebrew}".  Our advice used to be to use the "In_" prefix as a
652       single form way of specifying a block.  But Unicode 8.0 added
653       properties whose names begin with "In", and it's now clear that it's
654       only luck that's so far prevented a conflict.  Using "In" is only
655       marginally less typing than "Blk:", and the latter's meaning is clearer
656       anyway, and guaranteed to never conflict.  So don't take chances.  Use
657       "\p{Blk=foo}" for new code.  And be sure that block is what you really
658       really want to do.  In most cases scripts are what you want instead.
659
660       A complete list of blocks is in perluniprops.
661
662       Other Properties
663
664       There are many more properties than the very basic ones described here.
665       A complete list is in perluniprops.
666
667       Unicode defines all its properties in the compound form, so all single-
668       form properties are Perl extensions.  Most of these are just synonyms
669       for the Unicode ones, but some are genuine extensions, including
670       several that are in the compound form.  And quite a few of these are
671       actually recommended by Unicode (in
672       <http://www.unicode.org/reports/tr18>).
673
674       This section gives some details on all extensions that aren't just
675       synonyms for compound-form Unicode properties (for those properties,
676       you'll have to refer to the Unicode Standard
677       <http://www.unicode.org/reports/tr44>.
678
679       "\p{All}"
680           This matches every possible code point.  It is equivalent to
681           "qr/./s".  Unlike all the other non-user-defined "\p{}" property
682           matches, no warning is ever generated if this is property is
683           matched against a non-Unicode code point (see "Beyond Unicode code
684           points" below).
685
686       "\p{Alnum}"
687           This matches any "\p{Alphabetic}" or "\p{Decimal_Number}"
688           character.
689
690       "\p{Any}"
691           This matches any of the 1_114_112 Unicode code points.  It is a
692           synonym for "\p{Unicode}".
693
694       "\p{ASCII}"
695           This matches any of the 128 characters in the US-ASCII character
696           set, which is a subset of Unicode.
697
698       "\p{Assigned}"
699           This matches any assigned code point; that is, any code point whose
700           general category is not "Unassigned" (or equivalently, not "Cn").
701
702       "\p{Blank}"
703           This is the same as "\h" and "\p{HorizSpace}":  A character that
704           changes the spacing horizontally.
705
706       "\p{Decomposition_Type: Non_Canonical}"    (Short: "\p{Dt=NonCanon}")
707           Matches a character that has a non-canonical decomposition.
708
709           The "Extended Grapheme Clusters (Logical characters)" section above
710           talked about canonical decompositions.  However, many more
711           characters have a different type of decomposition, a "compatible"
712           or "non-canonical" decomposition.  The sequences that form these
713           decompositions are not considered canonically equivalent to the
714           pre-composed character.  An example is the "SUPERSCRIPT ONE".  It
715           is somewhat like a regular digit 1, but not exactly; its
716           decomposition into the digit 1 is called a "compatible"
717           decomposition, specifically a "super" decomposition.  There are
718           several such compatibility decompositions (see
719           <http://www.unicode.org/reports/tr44>), including one called
720           "compat", which means some miscellaneous type of decomposition that
721           doesn't fit into the other decomposition categories that Unicode
722           has chosen.
723
724           Note that most Unicode characters don't have a decomposition, so
725           their decomposition type is "None".
726
727           For your convenience, Perl has added the "Non_Canonical"
728           decomposition type to mean any of the several compatibility
729           decompositions.
730
731       "\p{Graph}"
732           Matches any character that is graphic.  Theoretically, this means a
733           character that on a printer would cause ink to be used.
734
735       "\p{HorizSpace}"
736           This is the same as "\h" and "\p{Blank}":  a character that changes
737           the spacing horizontally.
738
739       "\p{In=*}"
740           This is a synonym for "\p{Present_In=*}"
741
742       "\p{PerlSpace}"
743           This is the same as "\s", restricted to ASCII, namely "[ \f\n\r\t]"
744           and starting in Perl v5.18, a vertical tab.
745
746           Mnemonic: Perl's (original) space
747
748       "\p{PerlWord}"
749           This is the same as "\w", restricted to ASCII, namely
750           "[A-Za-z0-9_]"
751
752           Mnemonic: Perl's (original) word.
753
754       "\p{Posix...}"
755           There are several of these, which are equivalents, using the "\p{}"
756           notation, for Posix classes and are described in "POSIX Character
757           Classes" in perlrecharclass.
758
759       "\p{Present_In: *}"    (Short: "\p{In=*}")
760           This property is used when you need to know in what Unicode
761           version(s) a character is.
762
763           The "*" above stands for some two digit Unicode version number,
764           such as 1.1 or 4.0; or the "*" can also be "Unassigned".  This
765           property will match the code points whose final disposition has
766           been settled as of the Unicode release given by the version number;
767           "\p{Present_In: Unassigned}" will match those code points whose
768           meaning has yet to be assigned.
769
770           For example, "U+0041" "LATIN CAPITAL LETTER A" was present in the
771           very first Unicode release available, which is 1.1, so this
772           property is true for all valid "*" versions.  On the other hand,
773           "U+1EFF" was not assigned until version 5.1 when it became "LATIN
774           SMALL LETTER Y WITH LOOP", so the only "*" that would match it are
775           5.1, 5.2, and later.
776
777           Unicode furnishes the "Age" property from which this is derived.
778           The problem with Age is that a strict interpretation of it (which
779           Perl takes) has it matching the precise release a code point's
780           meaning is introduced in.  Thus "U+0041" would match only 1.1; and
781           "U+1EFF" only 5.1.  This is not usually what you want.
782
783           Some non-Perl implementations of the Age property may change its
784           meaning to be the same as the Perl "Present_In" property; just be
785           aware of that.
786
787           Another confusion with both these properties is that the definition
788           is not that the code point has been assigned, but that the meaning
789           of the code point has been determined.  This is because 66 code
790           points will always be unassigned, and so the "Age" for them is the
791           Unicode version in which the decision to make them so was made.
792           For example, "U+FDD0" is to be permanently unassigned to a
793           character, and the decision to do that was made in version 3.1, so
794           "\p{Age=3.1}" matches this character, as also does "\p{Present_In:
795           3.1}" and up.
796
797       "\p{Print}"
798           This matches any character that is graphical or blank, except
799           controls.
800
801       "\p{SpacePerl}"
802           This is the same as "\s", including beyond ASCII.
803
804           Mnemonic: Space, as modified by Perl.  (It doesn't include the
805           vertical tab until v5.18, which both the Posix standard and Unicode
806           consider white space.)
807
808       "\p{Title}" and  "\p{Titlecase}"
809           Under case-sensitive matching, these both match the same code
810           points as "\p{General Category=Titlecase_Letter}" ("\p{gc=lt}").
811           The difference is that under "/i" caseless matching, these match
812           the same as "\p{Cased}", whereas "\p{gc=lt}" matches
813           "\p{Cased_Letter").
814
815       "\p{Unicode}"
816           This matches any of the 1_114_112 Unicode code points.  "\p{Any}".
817
818       "\p{VertSpace}"
819           This is the same as "\v":  A character that changes the spacing
820           vertically.
821
822       "\p{Word}"
823           This is the same as "\w", including over 100_000 characters beyond
824           ASCII.
825
826       "\p{XPosix...}"
827           There are several of these, which are the standard Posix classes
828           extended to the full Unicode range.  They are described in "POSIX
829           Character Classes" in perlrecharclass.
830
831   User-Defined Character Properties
832       You can define your own binary character properties by defining
833       subroutines whose names begin with "In" or "Is".  (The experimental
834       feature "(?[ ])" in perlre provides an alternative which allows more
835       complex definitions.)  The subroutines can be defined in any package.
836       The user-defined properties can be used in the regular expression
837       "\p{}" and "\P{}" constructs; if you are using a user-defined property
838       from a package other than the one you are in, you must specify its
839       package in the "\p{}" or "\P{}" construct.
840
841           # assuming property Is_Foreign defined in Lang::
842           package main;  # property package name required
843           if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
844
845           package Lang;  # property package name not required
846           if ($txt =~ /\p{IsForeign}+/) { ... }
847
848       Note that the effect is compile-time and immutable once defined.
849       However, the subroutines are passed a single parameter, which is 0 if
850       case-sensitive matching is in effect and non-zero if caseless matching
851       is in effect.  The subroutine may return different values depending on
852       the value of the flag, and one set of values will immutably be in
853       effect for all case-sensitive matches, and the other set for all case-
854       insensitive matches.
855
856       Note that if the regular expression is tainted, then Perl will die
857       rather than calling the subroutine when the name of the subroutine is
858       determined by the tainted data.
859
860       The subroutines must return a specially-formatted string, with one or
861       more newline-separated lines.  Each line must be one of the following:
862
863       ·   A single hexadecimal number denoting a code point to include.
864
865       ·   Two hexadecimal numbers separated by horizontal whitespace (space
866           or tabular characters) denoting a range of code points to include.
867
868       ·   Something to include, prefixed by "+": a built-in character
869           property (prefixed by "utf8::") or a fully qualified (including
870           package name) user-defined character property, to represent all the
871           characters in that property; two hexadecimal code points for a
872           range; or a single hexadecimal code point.
873
874       ·   Something to exclude, prefixed by "-": an existing character
875           property (prefixed by "utf8::") or a fully qualified (including
876           package name) user-defined character property, to represent all the
877           characters in that property; two hexadecimal code points for a
878           range; or a single hexadecimal code point.
879
880       ·   Something to negate, prefixed "!": an existing character property
881           (prefixed by "utf8::") or a fully qualified (including package
882           name) user-defined character property, to represent all the
883           characters in that property; two hexadecimal code points for a
884           range; or a single hexadecimal code point.
885
886       ·   Something to intersect with, prefixed by "&": an existing character
887           property (prefixed by "utf8::") or a fully qualified (including
888           package name) user-defined character property, for all the
889           characters except the characters in the property; two hexadecimal
890           code points for a range; or a single hexadecimal code point.
891
892       For example, to define a property that covers both the Japanese
893       syllabaries (hiragana and katakana), you can define
894
895           sub InKana {
896               return <<END;
897           3040\t309F
898           30A0\t30FF
899           END
900           }
901
902       Imagine that the here-doc end marker is at the beginning of the line.
903       Now you can use "\p{InKana}" and "\P{InKana}".
904
905       You could also have used the existing block property names:
906
907           sub InKana {
908               return <<'END';
909           +utf8::InHiragana
910           +utf8::InKatakana
911           END
912           }
913
914       Suppose you wanted to match only the allocated characters, not the raw
915       block ranges: in other words, you want to remove the unassigned
916       characters:
917
918           sub InKana {
919               return <<'END';
920           +utf8::InHiragana
921           +utf8::InKatakana
922           -utf8::IsCn
923           END
924           }
925
926       The negation is useful for defining (surprise!) negated classes.
927
928           sub InNotKana {
929               return <<'END';
930           !utf8::InHiragana
931           -utf8::InKatakana
932           +utf8::IsCn
933           END
934           }
935
936       This will match all non-Unicode code points, since every one of them is
937       not in Kana.  You can use intersection to exclude these, if desired, as
938       this modified example shows:
939
940           sub InNotKana {
941               return <<'END';
942           !utf8::InHiragana
943           -utf8::InKatakana
944           +utf8::IsCn
945           &utf8::Any
946           END
947           }
948
949       &utf8::Any must be the last line in the definition.
950
951       Intersection is used generally for getting the common characters
952       matched by two (or more) classes.  It's important to remember not to
953       use "&" for the first set; that would be intersecting with nothing,
954       resulting in an empty set.
955
956       Unlike non-user-defined "\p{}" property matches, no warning is ever
957       generated if these properties are matched against a non-Unicode code
958       point (see "Beyond Unicode code points" below).
959
960   User-Defined Case Mappings (for serious hackers only)
961       This feature has been removed as of Perl 5.16.  The CPAN module
962       "Unicode::Casing" provides better functionality without the drawbacks
963       that this feature had.  If you are using a Perl earlier than 5.16, this
964       feature was most fully documented in the 5.14 version of this pod:
965       <http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
966
967   Character Encodings for Input and Output
968       See Encode.
969
970   Unicode Regular Expression Support Level
971       The following list of Unicode supported features for regular
972       expressions describes all features currently directly supported by core
973       Perl.  The references to "Level N" and the section numbers refer to
974       UTS#18 "Unicode Regular Expressions"
975       <http://www.unicode.org/reports/tr18>, version 13, November 2013.
976
977       Level 1 - Basic Unicode Support
978
979        RL1.1   Hex Notation                     - Done          [1]
980        RL1.2   Properties                       - Done          [2]
981        RL1.2a  Compatibility Properties         - Done          [3]
982        RL1.3   Subtraction and Intersection     - Experimental  [4]
983        RL1.4   Simple Word Boundaries           - Done          [5]
984        RL1.5   Simple Loose Matches             - Done          [6]
985        RL1.6   Line Boundaries                  - Partial       [7]
986        RL1.7   Supplementary Code Points        - Done          [8]
987
988       [1] "\N{U+...}" and "\x{...}"
989       [2] "\p{...}" "\P{...}".  This requirement is for a minimal list of
990       properties.  Perl supports these and all other Unicode character
991       properties, as R2.7 asks (see "Unicode Character Properties" above).
992       [3] Perl has "\d" "\D" "\s" "\S" "\w" "\W" "\X" "[:prop:]" "[:^prop:]",
993       plus all the properties specified by
994       <http://www.unicode.org/reports/tr18/#Compatibility_Properties>.  These
995       are described above in "Other Properties"
996       [4] The experimental feature "(?[...])" starting in v5.18 accomplishes
997           this.
998
999           See "(?[ ])" in perlre.  If you don't want to use an experimental
1000           feature, you can use one of the following:
1001
1002           ·   Regular expression lookahead
1003
1004               You can mimic class subtraction using lookahead.  For example,
1005               what UTS#18 might write as
1006
1007                   [{Block=Greek}-[{UNASSIGNED}]]
1008
1009               in Perl can be written as:
1010
1011                   (?!\p{Unassigned})\p{Block=Greek}
1012                   (?=\p{Assigned})\p{Block=Greek}
1013
1014               But in this particular example, you probably really want
1015
1016                   \p{Greek}
1017
1018               which will match assigned characters known to be part of the
1019               Greek script.
1020
1021           ·   CPAN module "Unicode::Regex::Set"
1022
1023               It does implement the full UTS#18 grouping, intersection,
1024               union, and removal (subtraction) syntax.
1025
1026           ·   "User-Defined Character Properties"
1027
1028               "+" for union, "-" for removal (set-difference), "&" for
1029               intersection
1030
1031       [5] "\b" "\B" meet most, but not all, the details of this requirement,
1032       but "\b{wb}" and "\B{wb}" do, as well as the stricter R2.3.
1033       [6] Note that Perl does Full case-folding in matching, not Simple:
1034
1035           For example "U+1F88" is equivalent to "U+1F00 U+03B9", instead of
1036           just "U+1F80".  This difference matters mainly for certain Greek
1037           capital letters with certain modifiers: the Full case-folding
1038           decomposes the letter, while the Simple case-folding would map it
1039           to a single character.
1040
1041       [7] The reason this is considered to be only partially implemented is
1042           that Perl has "qr/\b{lb}/" and "Unicode::LineBreak" that are
1043           conformant with UAX#14 "Unicode Line Breaking Algorithm"
1044           <http://www.unicode.org/reports/tr14>.  The regular expression
1045           construct provides default behavior, while the heavier-weight
1046           module provides customizable line breaking.
1047
1048           But Perl treats "\n" as the start- and end-line delimiter, whereas
1049           Unicode specifies more characters that should be so-interpreted.
1050
1051           These are:
1052
1053            VT   U+000B  (\v in C)
1054            FF   U+000C  (\f)
1055            CR   U+000D  (\r)
1056            NEL  U+0085
1057            LS   U+2028
1058            PS   U+2029
1059
1060           "^" and "$" in regular expression patterns are supposed to match
1061           all these, but don't.  These characters also don't, but should,
1062           affect "<>" $., and script line numbers.
1063
1064           Also, lines should not be split within "CRLF" (i.e. there is no
1065           empty line between "\r" and "\n").  For "CRLF", try the ":crlf"
1066           layer (see PerlIO).
1067
1068       [8] UTF-8/UTF-EBDDIC used in Perl allows not only "U+10000" to
1069       "U+10FFFF" but also beyond "U+10FFFF"
1070
1071       Level 2 - Extended Unicode Support
1072
1073        RL2.1   Canonical Equivalents           - Retracted     [9]
1074                                                  by Unicode
1075        RL2.2   Extended Grapheme Clusters      - Partial       [10]
1076        RL2.3   Default Word Boundaries         - Done          [11]
1077        RL2.4   Default Case Conversion         - Done
1078        RL2.5   Name Properties                 - Done
1079        RL2.6   Wildcard Properties             - Missing
1080        RL2.7   Full Properties                 - Done
1081
1082       [9] Unicode has rewritten this portion of UTS#18 to say that getting
1083       canonical equivalence (see UAX#15 "Unicode Normalization Forms"
1084       <http://www.unicode.org/reports/tr15>) is basically to be done at the
1085       programmer level.  Use NFD to write both your regular expressions and
1086       text to match them against (you can use Unicode::Normalize).
1087       [10] Perl has "\X" and "\b{gcb}" but we don't have a "Grapheme Cluster
1088       Mode".
1089       [11] see UAX#29 "Unicode Text Segmentation"
1090       <http://www.unicode.org/reports/tr29>,
1091
1092       Level 3 - Tailored Support
1093
1094        RL3.1   Tailored Punctuation            - Missing
1095        RL3.2   Tailored Grapheme Clusters      - Missing       [12]
1096        RL3.3   Tailored Word Boundaries        - Missing
1097        RL3.4   Tailored Loose Matches          - Retracted by Unicode
1098        RL3.5   Tailored Ranges                 - Retracted by Unicode
1099        RL3.6   Context Matching                - Missing       [13]
1100        RL3.7   Incremental Matches             - Missing
1101        RL3.8   Unicode Set Sharing             - Unicode is proposing
1102                                                  to retract this
1103        RL3.9   Possible Match Sets             - Missing
1104        RL3.10  Folded Matching                 - Retracted by Unicode
1105        RL3.11  Submatchers                     - Missing
1106
1107       [12] Perl has Unicode::Collate, but it isn't integrated with regular
1108       expressions.  See UTS#10 "Unicode Collation Algorithms"
1109       <http://www.unicode.org/reports/tr10>.
1110       [13] Perl has "(?<=x)" and "(?=x)", but lookaheads or lookbehinds
1111       should see outside of the target substring
1112
1113   Unicode Encodings
1114       Unicode characters are assigned to code points, which are abstract
1115       numbers.  To use these numbers, various encodings are needed.
1116
1117       ·   UTF-8
1118
1119           UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
1120           encoding.  In most of Perl's documentation, including elsewhere in
1121           this document, the term "UTF-8" means also "UTF-EBCDIC".  But in
1122           this section, "UTF-8" refers only to the encoding used on ASCII
1123           platforms.  It is a superset of 7-bit US-ASCII, so anything encoded
1124           in ASCII has the identical representation when encoded in UTF-8.
1125
1126           The following table is from Unicode 3.2.
1127
1128            Code Points            1st Byte  2nd Byte  3rd Byte 4th Byte
1129
1130              U+0000..U+007F       00..7F
1131              U+0080..U+07FF     * C2..DF    80..BF
1132              U+0800..U+0FFF       E0      * A0..BF    80..BF
1133              U+1000..U+CFFF       E1..EC    80..BF    80..BF
1134              U+D000..U+D7FF       ED        80..9F    80..BF
1135              U+D800..U+DFFF       +++++ utf16 surrogates, not legal utf8 +++++
1136              U+E000..U+FFFF       EE..EF    80..BF    80..BF
1137             U+10000..U+3FFFF      F0      * 90..BF    80..BF    80..BF
1138             U+40000..U+FFFFF      F1..F3    80..BF    80..BF    80..BF
1139            U+100000..U+10FFFF     F4        80..8F    80..BF    80..BF
1140
1141           Note the gaps marked by "*" before several of the byte entries
1142           above.  These are caused by legal UTF-8 avoiding non-shortest
1143           encodings: it is technically possible to UTF-8-encode a single code
1144           point in different ways, but that is explicitly forbidden, and the
1145           shortest possible encoding should always be used (and that is what
1146           Perl does).
1147
1148           Another way to look at it is via bits:
1149
1150                           Code Points  1st Byte  2nd Byte  3rd Byte  4th Byte
1151
1152                              0aaaaaaa  0aaaaaaa
1153                      00000bbbbbaaaaaa  110bbbbb  10aaaaaa
1154                      ccccbbbbbbaaaaaa  1110cccc  10bbbbbb  10aaaaaa
1155            00000dddccccccbbbbbbaaaaaa  11110ddd  10cccccc  10bbbbbb  10aaaaaa
1156
1157           As you can see, the continuation bytes all begin with "10", and the
1158           leading bits of the start byte tell how many bytes there are in the
1159           encoded character.
1160
1161           The original UTF-8 specification allowed up to 6 bytes, to allow
1162           encoding of numbers up to "0x7FFF_FFFF".  Perl continues to allow
1163           those, and has extended that up to 13 bytes to encode code points
1164           up to what can fit in a 64-bit word.  However, Perl will warn if
1165           you output any of these as being non-portable; and under strict
1166           UTF-8 input protocols, they are forbidden.  In addition, it is
1167           deprecated to use a code point larger than what a signed integer
1168           variable on your system can hold.  On 32-bit ASCII systems, this
1169           means "0x7FFF_FFFF" is the legal maximum going forward (much higher
1170           on 64-bit systems).
1171
1172       ·   UTF-EBCDIC
1173
1174           Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
1175           This means that all the basic characters (which includes all those
1176           that have ASCII equivalents (like "A", "0", "%", etc.)  are the
1177           same in both EBCDIC and UTF-EBCDIC.)
1178
1179           UTF-EBCDIC is used on EBCDIC platforms.  It generally requires more
1180           bytes to represent a given code point than UTF-8 does; the largest
1181           Unicode code points take 5 bytes to represent (instead of 4 in
1182           UTF-8), and, extended for 64-bit words, it uses 14 bytes instead of
1183           13 bytes in UTF-8.
1184
1185       ·   UTF-16, UTF-16BE, UTF-16LE, Surrogates, and "BOM"'s (Byte Order
1186           Marks)
1187
1188           The followings items are mostly for reference and general Unicode
1189           knowledge, Perl doesn't use these constructs internally.
1190
1191           Like UTF-8, UTF-16 is a variable-width encoding, but where UTF-8
1192           uses 8-bit code units, UTF-16 uses 16-bit code units.  All code
1193           points occupy either 2 or 4 bytes in UTF-16: code points
1194           "U+0000..U+FFFF" are stored in a single 16-bit unit, and code
1195           points "U+10000..U+10FFFF" in two 16-bit units.  The latter case is
1196           using surrogates, the first 16-bit unit being the high surrogate,
1197           and the second being the low surrogate.
1198
1199           Surrogates are code points set aside to encode the
1200           "U+10000..U+10FFFF" range of Unicode code points in pairs of 16-bit
1201           units.  The high surrogates are the range "U+D800..U+DBFF" and the
1202           low surrogates are the range "U+DC00..U+DFFF".  The surrogate
1203           encoding is
1204
1205               $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1206               $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
1207
1208           and the decoding is
1209
1210               $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
1211
1212           Because of the 16-bitness, UTF-16 is byte-order dependent.  UTF-16
1213           itself can be used for in-memory computations, but if storage or
1214           transfer is required either UTF-16BE (big-endian) or UTF-16LE
1215           (little-endian) encodings must be chosen.
1216
1217           This introduces another problem: what if you just know that your
1218           data is UTF-16, but you don't know which endianness?  Byte Order
1219           Marks, or "BOM"'s, are a solution to this.  A special character has
1220           been reserved in Unicode to function as a byte order marker: the
1221           character with the code point "U+FEFF" is the "BOM".
1222
1223           The trick is that if you read a "BOM", you will know the byte
1224           order, since if it was written on a big-endian platform, you will
1225           read the bytes "0xFE 0xFF", but if it was written on a little-
1226           endian platform, you will read the bytes "0xFF 0xFE".  (And if the
1227           originating platform was writing in ASCII platform UTF-8, you will
1228           read the bytes "0xEF 0xBB 0xBF".)
1229
1230           The way this trick works is that the character with the code point
1231           "U+FFFE" is not supposed to be in input streams, so the sequence of
1232           bytes "0xFF 0xFE" is unambiguously ""BOM", represented in little-
1233           endian format" and cannot be "U+FFFE", represented in big-endian
1234           format".
1235
1236           Surrogates have no meaning in Unicode outside their use in pairs to
1237           represent other code points.  However, Perl allows them to be
1238           represented individually internally, for example by saying
1239           "chr(0xD801)", so that all code points, not just those valid for
1240           open interchange, are representable.  Unicode does define semantics
1241           for them, such as their "General_Category" is "Cs".  But because
1242           their use is somewhat dangerous, Perl will warn (using the warning
1243           category "surrogate", which is a sub-category of "utf8") if an
1244           attempt is made to do things like take the lower case of one, or
1245           match case-insensitively, or to output them.  (But don't try this
1246           on Perls before 5.14.)
1247
1248       ·   UTF-32, UTF-32BE, UTF-32LE
1249
1250           The UTF-32 family is pretty much like the UTF-16 family, except
1251           that the units are 32-bit, and therefore the surrogate scheme is
1252           not needed.  UTF-32 is a fixed-width encoding.  The "BOM"
1253           signatures are "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00
1254           0x00" for LE.
1255
1256       ·   UCS-2, UCS-4
1257
1258           Legacy, fixed-width encodings defined by the ISO 10646 standard.
1259           UCS-2 is a 16-bit encoding.  Unlike UTF-16, UCS-2 is not extensible
1260           beyond "U+FFFF", because it does not use surrogates.  UCS-4 is a
1261           32-bit encoding, functionally identical to UTF-32 (the difference
1262           being that UCS-4 forbids neither surrogates nor code points larger
1263           than "0x10_FFFF").
1264
1265       ·   UTF-7
1266
1267           A seven-bit safe (non-eight-bit) encoding, which is useful if the
1268           transport or storage is not eight-bit safe.  Defined by RFC 2152.
1269
1270   Noncharacter code points
1271       66 code points are set aside in Unicode as "noncharacter code points".
1272       These all have the "Unassigned" ("Cn") "General_Category", and no
1273       character will ever be assigned to any of them.  They are the 32 code
1274       points between "U+FDD0" and "U+FDEF" inclusive, and the 34 code points:
1275
1276        U+FFFE   U+FFFF
1277        U+1FFFE  U+1FFFF
1278        U+2FFFE  U+2FFFF
1279        ...
1280        U+EFFFE  U+EFFFF
1281        U+FFFFE  U+FFFFF
1282        U+10FFFE U+10FFFF
1283
1284       Until Unicode 7.0, the noncharacters were "forbidden for use in open
1285       interchange of Unicode text data", so that code that processed those
1286       streams could use these code points as sentinels that could be mixed in
1287       with character data, and would always be distinguishable from that
1288       data.  (Emphasis above and in the next paragraph are added in this
1289       document.)
1290
1291       Unicode 7.0 changed the wording so that they are "not recommended for
1292       use in open interchange of Unicode text data".  The 7.0 Standard goes
1293       on to say:
1294
1295           "If a noncharacter is received in open interchange, an application
1296           is not required to interpret it in any way.  It is good practice,
1297           however, to recognize it as a noncharacter and to take appropriate
1298           action, such as replacing it with "U+FFFD" replacement character,
1299           to indicate the problem in the text.  It is not recommended to
1300           simply delete noncharacter code points from such text, because of
1301           the potential security issues caused by deleting uninterpreted
1302           characters.  (See conformance clause C7 in Section 3.2, Conformance
1303           Requirements, and Unicode Technical Report #36, "Unicode Security
1304           Considerations"
1305           <http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1306
1307       This change was made because it was found that various commercial tools
1308       like editors, or for things like source code control, had been written
1309       so that they would not handle program files that used these code
1310       points, effectively precluding their use almost entirely!  And that was
1311       never the intent.  They've always been meant to be usable within an
1312       application, or cooperating set of applications, at will.
1313
1314       If you're writing code, such as an editor, that is supposed to be able
1315       to handle any Unicode text data, then you shouldn't be using these code
1316       points yourself, and instead allow them in the input.  If you need
1317       sentinels, they should instead be something that isn't legal Unicode.
1318       For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1319       they never appear in well-formed UTF-8.  (There are equivalents for
1320       UTF-EBCDIC).  You can also store your Unicode code points in integer
1321       variables and use negative values as sentinels.
1322
1323       If you're not writing such a tool, then whether you accept
1324       noncharacters as input is up to you (though the Standard recommends
1325       that you not).  If you do strict input stream checking with Perl, these
1326       code points continue to be forbidden.  This is to maintain backward
1327       compatibility (otherwise potential security holes could open up, as an
1328       unsuspecting application that was written assuming the noncharacters
1329       would be filtered out before getting to it, could now, without warning,
1330       start getting them).  To do strict checking, you can use the layer
1331       ":encoding('UTF-8')".
1332
1333       Perl continues to warn (using the warning category "nonchar", which is
1334       a sub-category of "utf8") if an attempt is made to output
1335       noncharacters.
1336
1337   Beyond Unicode code points
1338       The maximum Unicode code point is "U+10FFFF", and Unicode only defines
1339       operations on code points up through that.  But Perl works on code
1340       points up to the maximum permissible unsigned number available on the
1341       platform.  However, Perl will not accept these from input streams
1342       unless lax rules are being used, and will warn (using the warning
1343       category "non_unicode", which is a sub-category of "utf8") if any are
1344       output.
1345
1346       Since Unicode rules are not defined on these code points, if a Unicode-
1347       defined operation is done on them, Perl uses what we believe are
1348       sensible rules, while generally warning, using the "non_unicode"
1349       category.  For example, "uc("\x{11_0000}")" will generate such a
1350       warning, returning the input parameter as its result, since Perl
1351       defines the uppercase of every non-Unicode code point to be the code
1352       point itself.  (All the case changing operations, not just uppercasing,
1353       work this way.)
1354
1355       The situation with matching Unicode properties in regular expressions,
1356       the "\p{}" and "\P{}" constructs, against these code points is not as
1357       clear cut, and how these are handled has changed as we've gained
1358       experience.
1359
1360       One possibility is to treat any match against these code points as
1361       undefined.  But since Perl doesn't have the concept of a match being
1362       undefined, it converts this to failing or "FALSE".  This is almost, but
1363       not quite, what Perl did from v5.14 (when use of these code points
1364       became generally reliable) through v5.18.  The difference is that Perl
1365       treated all "\p{}" matches as failing, but all "\P{}" matches as
1366       succeeding.
1367
1368       One problem with this is that it leads to unexpected, and confusing
1369       results in some cases:
1370
1371        chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Failed on <= v5.18
1372        chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Failed! on <= v5.18
1373
1374       That is, it treated both matches as undefined, and converted that to
1375       false (raising a warning on each).  The first case is the expected
1376       result, but the second is likely counterintuitive: "How could both be
1377       false when they are complements?"  Another problem was that the
1378       implementation optimized many Unicode property matches down to already
1379       existing simpler, faster operations, which don't raise the warning.  We
1380       chose to not forgo those optimizations, which help the vast majority of
1381       matches, just to generate a warning for the unlikely event that an
1382       above-Unicode code point is being matched against.
1383
1384       As a result of these problems, starting in v5.20, what Perl does is to
1385       treat non-Unicode code points as just typical unassigned Unicode
1386       characters, and matches accordingly.  (Note: Unicode has atypical
1387       unassigned code points.  For example, it has noncharacter code points,
1388       and ones that, when they do get assigned, are destined to be written
1389       Right-to-left, as Arabic and Hebrew are.  Perl assumes that no non-
1390       Unicode code point has any atypical properties.)
1391
1392       Perl, in most cases, will raise a warning when matching an above-
1393       Unicode code point against a Unicode property when the result is "TRUE"
1394       for "\p{}", and "FALSE" for "\P{}".  For example:
1395
1396        chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails, no warning
1397        chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Succeeds, with warning
1398
1399       In both these examples, the character being matched is non-Unicode, so
1400       Unicode doesn't define how it should match.  It clearly isn't an ASCII
1401       hex digit, so the first example clearly should fail, and so it does,
1402       with no warning.  But it is arguable that the second example should
1403       have an undefined, hence "FALSE", result.  So a warning is raised for
1404       it.
1405
1406       Thus the warning is raised for many fewer cases than in earlier Perls,
1407       and only when what the result is could be arguable.  It turns out that
1408       none of the optimizations made by Perl (or are ever likely to be made)
1409       cause the warning to be skipped, so it solves both problems of Perl's
1410       earlier approach.  The most commonly used property that is affected by
1411       this change is "\p{Unassigned}" which is a short form for
1412       "\p{General_Category=Unassigned}".  Starting in v5.20, all non-Unicode
1413       code points are considered "Unassigned".  In earlier releases the
1414       matches failed because the result was considered undefined.
1415
1416       The only place where the warning is not raised when it might ought to
1417       have been is if optimizations cause the whole pattern match to not even
1418       be attempted.  For example, Perl may figure out that for a string to
1419       match a certain regular expression pattern, the string has to contain
1420       the substring "foobar".  Before attempting the match, Perl may look for
1421       that substring, and if not found, immediately fail the match without
1422       actually trying it; so no warning gets generated even if the string
1423       contains an above-Unicode code point.
1424
1425       This behavior is more "Do what I mean" than in earlier Perls for most
1426       applications.  But it catches fewer issues for code that needs to be
1427       strictly Unicode compliant.  Therefore there is an additional mode of
1428       operation available to accommodate such code.  This mode is enabled if
1429       a regular expression pattern is compiled within the lexical scope where
1430       the "non_unicode" warning class has been made fatal, say by:
1431
1432        use warnings FATAL => "non_unicode"
1433
1434       (see warnings).  In this mode of operation, Perl will raise the warning
1435       for all matches against a non-Unicode code point (not just the arguable
1436       ones), and it skips the optimizations that might cause the warning to
1437       not be output.  (It currently still won't warn if the match isn't even
1438       attempted, like in the "foobar" example above.)
1439
1440       In summary, Perl now normally treats non-Unicode code points as typical
1441       Unicode unassigned code points for regular expression matches, raising
1442       a warning only when it is arguable what the result should be.  However,
1443       if this warning has been made fatal, it isn't skipped.
1444
1445       There is one exception to all this.  "\p{All}" looks like a Unicode
1446       property, but it is a Perl extension that is defined to be true for all
1447       possible code points, Unicode or not, so no warning is ever generated
1448       when matching this against a non-Unicode code point.  (Prior to v5.20,
1449       it was an exact synonym for "\p{Any}", matching code points 0 through
1450       0x10FFFF.)
1451
1452   Security Implications of Unicode
1453       First, read Unicode Security Considerations
1454       <http://www.unicode.org/reports/tr36>.
1455
1456       Also, note the following:
1457
1458       ·   Malformed UTF-8
1459
1460           UTF-8 is very structured, so many combinations of bytes are
1461           invalid.  In the past, Perl tried to soldier on and make some sense
1462           of invalid combinations, but this can lead to security holes, so
1463           now, if the Perl core needs to process an invalid combination, it
1464           will either raise a fatal error, or will replace those bytes by the
1465           sequence that forms the Unicode REPLACEMENT CHARACTER, for which
1466           purpose Unicode created it.
1467
1468           Every code point can be represented by more than one possible
1469           syntactically valid UTF-8 sequence.  Early on, both Unicode and
1470           Perl considered any of these to be valid, but now, all sequences
1471           longer than the shortest possible one are considered to be
1472           malformed.
1473
1474           Unicode considers many code points to be illegal, or to be avoided.
1475           Perl generally accepts them, once they have passed through any
1476           input filters that may try to exclude them.  These have been
1477           discussed above (see "Surrogates" under UTF-16 in "Unicode
1478           Encodings", "Noncharacter code points", and "Beyond Unicode code
1479           points").
1480
1481       ·   Regular expression pattern matching may surprise you if you're not
1482           accustomed to Unicode.  Starting in Perl 5.14, several pattern
1483           modifiers are available to control this, called the character set
1484           modifiers.  Details are given in "Character set modifiers" in
1485           perlre.
1486
1487       As discussed elsewhere, Perl has one foot (two hooves?) planted in each
1488       of two worlds: the old world of ASCII and single-byte locales, and the
1489       new world of Unicode, upgrading when necessary.  If your legacy code
1490       does not explicitly use Unicode, no automatic switch-over to Unicode
1491       should happen.
1492
1493   Unicode in Perl on EBCDIC
1494       Unicode is supported on EBCDIC platforms.  See perlebcdic.
1495
1496       Unless ASCII vs. EBCDIC issues are specifically being discussed,
1497       references to UTF-8 encoding in this document and elsewhere should be
1498       read as meaning UTF-EBCDIC on EBCDIC platforms.  See "Unicode and UTF"
1499       in perlebcdic.
1500
1501       Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1502       hidden from you; "use utf8" (and NOT something like "use utfebcdic")
1503       declares the script is in the platform's "native" 8-bit encoding of
1504       Unicode.  (Similarly for the ":utf8" layer.)
1505
1506   Locales
1507       See "Unicode and UTF-8" in perllocale
1508
1509   When Unicode Does Not Happen
1510       There are still many places where Unicode (in some encoding or another)
1511       could be given as arguments or received as results, or both in Perl,
1512       but it is not, in spite of Perl having extensive ways to input and
1513       output in Unicode, and a few other "entry points" like the @ARGV array
1514       (which can sometimes be interpreted as UTF-8).
1515
1516       The following are such interfaces.  Also, see "The "Unicode Bug"".  For
1517       all of these interfaces Perl currently (as of v5.16.0) simply assumes
1518       byte strings both as arguments and results, or UTF-8 strings if the
1519       (deprecated) "encoding" pragma has been used.
1520
1521       One reason that Perl does not attempt to resolve the role of Unicode in
1522       these situations is that the answers are highly dependent on the
1523       operating system and the file system(s).  For example, whether
1524       filenames can be in Unicode and in exactly what kind of encoding, is
1525       not exactly a portable concept.  Similarly for "qx" and "system": how
1526       well will the "command-line interface" (and which of them?) handle
1527       Unicode?
1528
1529       ·   "chdir", "chmod", "chown", "chroot", "exec", "link", "lstat",
1530           "mkdir", "rename", "rmdir", "stat", "symlink", "truncate",
1531           "unlink", "utime", "-X"
1532
1533       ·   %ENV
1534
1535       ·   "glob" (aka the "<*>")
1536
1537       ·   "open", "opendir", "sysopen"
1538
1539       ·   "qx" (aka the backtick operator), "system"
1540
1541       ·   "readdir", "readlink"
1542
1543   The "Unicode Bug"
1544       The term, "Unicode bug" has been applied to an inconsistency with the
1545       code points in the "Latin-1 Supplement" block, that is, between 128 and
1546       255.  Without a locale specified, unlike all other characters or code
1547       points, these characters can have very different semantics depending on
1548       the rules in effect.  (Characters whose code points are above 255 force
1549       Unicode rules; whereas the rules for ASCII characters are the same
1550       under both ASCII and Unicode rules.)
1551
1552       Under Unicode rules, these upper-Latin1 characters are interpreted as
1553       Unicode code points, which means they have the same semantics as
1554       Latin-1 (ISO-8859-1) and C1 controls.
1555
1556       As explained in "ASCII Rules versus Unicode Rules", under ASCII rules,
1557       they are considered to be unassigned characters.
1558
1559       This can lead to unexpected results.  For example, a string's semantics
1560       can suddenly change if a code point above 255 is appended to it, which
1561       changes the rules from ASCII to Unicode.  As an example, consider the
1562       following program and its output:
1563
1564        $ perl -le'
1565            no feature "unicode_strings";
1566            $s1 = "\xC2";
1567            $s2 = "\x{2660}";
1568            for ($s1, $s2, $s1.$s2) {
1569                print /\w/ || 0;
1570            }
1571        '
1572        0
1573        0
1574        1
1575
1576       If there's no "\w" in "s1" nor in "s2", why does their concatenation
1577       have one?
1578
1579       This anomaly stems from Perl's attempt to not disturb older programs
1580       that didn't use Unicode, along with Perl's desire to add Unicode
1581       support seamlessly.  But the result turned out to not be seamless.  (By
1582       the way, you can choose to be warned when things like this happen.  See
1583       "encoding::warnings".)
1584
1585       "use feature 'unicode_strings'" was added, starting in Perl v5.12, to
1586       address this problem.  It affects these things:
1587
1588       ·   Changing the case of a scalar, that is, using "uc()", "ucfirst()",
1589           "lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in double-
1590           quotish contexts, such as regular expression substitutions.
1591
1592           Under "unicode_strings" starting in Perl 5.12.0, Unicode rules are
1593           generally used.  See "lc" in perlfunc for details on how this works
1594           in combination with various other pragmas.
1595
1596       ·   Using caseless ("/i") regular expression matching.
1597
1598           Starting in Perl 5.14.0, regular expressions compiled within the
1599           scope of "unicode_strings" use Unicode rules even when executed or
1600           compiled into larger regular expressions outside the scope.
1601
1602       ·   Matching any of several properties in regular expressions.
1603
1604           These properties are "\b" (without braces), "\B" (without braces),
1605           "\s", "\S", "\w", "\W", and all the Posix character classes except
1606           "[[:ascii:]]".
1607
1608           Starting in Perl 5.14.0, regular expressions compiled within the
1609           scope of "unicode_strings" use Unicode rules even when executed or
1610           compiled into larger regular expressions outside the scope.
1611
1612       ·   In "quotemeta" or its inline equivalent "\Q".
1613
1614           Starting in Perl 5.16.0, consistent quoting rules are used within
1615           the scope of "unicode_strings", as described in "quotemeta" in
1616           perlfunc.  Prior to that, or outside its scope, no code points
1617           above 127 are quoted in UTF-8 encoded strings, but in byte encoded
1618           strings, code points between 128-255 are always quoted.
1619
1620       ·   In the ".." or range operator.
1621
1622           Starting in Perl 5.26.0, the range operator on strings treats their
1623           lengths consistently within the scope of "unicode_strings". Prior
1624           to that, or outside its scope, it could produce strings whose
1625           length in characters exceeded that of the right-hand side, where
1626           the right-hand side took up more bytes than the correct range
1627           endpoint.
1628
1629       ·   In "split"'s special-case whitespace splitting.
1630
1631           Starting in Perl 5.28.0, the "split" function with a pattern
1632           specified as a string containing a single space handles whitespace
1633           characters consistently within the scope of of "unicode_strings".
1634           Prior to that, or outside its scope, characters that are whitespace
1635           according to Unicode rules but not according to ASCII rules were
1636           treated as field contents rather than field separators when they
1637           appear in byte-encoded strings.
1638
1639       You can see from the above that the effect of "unicode_strings"
1640       increased over several Perl releases.  (And Perl's support for Unicode
1641       continues to improve; it's best to use the latest available release in
1642       order to get the most complete and accurate results possible.)  Note
1643       that "unicode_strings" is automatically chosen if you "use 5.012" or
1644       higher.
1645
1646       For Perls earlier than those described above, or when a string is
1647       passed to a function outside the scope of "unicode_strings", see the
1648       next section.
1649
1650   Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1651       Sometimes (see "When Unicode Does Not Happen" or "The "Unicode Bug"")
1652       there are situations where you simply need to force a byte string into
1653       UTF-8, or vice versa.  The standard module Encode can be used for this,
1654       or the low-level calls "utf8::upgrade($bytestring)" and
1655       "utf8::downgrade($utf8string[, FAIL_OK])".
1656
1657       Note that "utf8::downgrade()" can fail if the string contains
1658       characters that don't fit into a byte.
1659
1660       Calling either function on a string that already is in the desired
1661       state is a no-op.
1662
1663       "ASCII Rules versus Unicode Rules" gives all the ways that a string is
1664       made to use Unicode rules.
1665
1666   Using Unicode in XS
1667       See "Unicode Support" in perlguts for an introduction to Unicode at the
1668       XS level, and "Unicode Support" in perlapi for the API details.
1669
1670   Hacking Perl to work on earlier Unicode versions (for very serious hackers
1671       only)
1672       Perl by default comes with the latest supported Unicode version built-
1673       in, but the goal is to allow you to change to use any earlier one.  In
1674       Perls v5.20 and v5.22, however, the earliest usable version is Unicode
1675       5.1.  Perl v5.18 and v5.24 are able to handle all earlier versions.
1676
1677       Download the files in the desired version of Unicode from the Unicode
1678       web site <http://www.unicode.org>).  These should replace the existing
1679       files in lib/unicore in the Perl source tree.  Follow the instructions
1680       in README.perl in that directory to change some of their names, and
1681       then build perl (see INSTALL).
1682
1683   Porting code from perl-5.6.X
1684       Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6
1685       the programmer was required to use the "utf8" pragma to declare that a
1686       given scope expected to deal with Unicode data and had to make sure
1687       that only Unicode data were reaching that scope. If you have code that
1688       is working with 5.6, you will need some of the following adjustments to
1689       your code. The examples are written such that the code will continue to
1690       work under 5.6, so you should be safe to try them out.
1691
1692       ·  A filehandle that should read or write UTF-8
1693
1694            if ($] > 5.008) {
1695              binmode $fh, ":encoding(UTF-8)";
1696            }
1697
1698       ·  A scalar that is going to be passed to some extension
1699
1700          Be it "Compress::Zlib", "Apache::Request" or any extension that has
1701          no mention of Unicode in the manpage, you need to make sure that the
1702          UTF8 flag is stripped off. Note that at the time of this writing
1703          (January 2012) the mentioned modules are not UTF-8-aware. Please
1704          check the documentation to verify if this is still true.
1705
1706            if ($] > 5.008) {
1707              require Encode;
1708              $val = Encode::encode("UTF-8", $val); # make octets
1709            }
1710
1711       ·  A scalar we got back from an extension
1712
1713          If you believe the scalar comes back as UTF-8, you will most likely
1714          want the UTF8 flag restored:
1715
1716            if ($] > 5.008) {
1717              require Encode;
1718              $val = Encode::decode("UTF-8", $val);
1719            }
1720
1721       ·  Same thing, if you are really sure it is UTF-8
1722
1723            if ($] > 5.008) {
1724              require Encode;
1725              Encode::_utf8_on($val);
1726            }
1727
1728       ·  A wrapper for DBI "fetchrow_array" and "fetchrow_hashref"
1729
1730          When the database contains only UTF-8, a wrapper function or method
1731          is a convenient way to replace all your "fetchrow_array" and
1732          "fetchrow_hashref" calls. A wrapper function will also make it
1733          easier to adapt to future enhancements in your database driver. Note
1734          that at the time of this writing (January 2012), the DBI has no
1735          standardized way to deal with UTF-8 data. Please check the DBI
1736          documentation to verify if that is still true.
1737
1738            sub fetchrow {
1739              # $what is one of fetchrow_{array,hashref}
1740              my($self, $sth, $what) = @_;
1741              if ($] < 5.008) {
1742                return $sth->$what;
1743              } else {
1744                require Encode;
1745                if (wantarray) {
1746                  my @arr = $sth->$what;
1747                  for (@arr) {
1748                    defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1749                  }
1750                  return @arr;
1751                } else {
1752                  my $ret = $sth->$what;
1753                  if (ref $ret) {
1754                    for my $k (keys %$ret) {
1755                      defined
1756                      && /[^\000-\177]/
1757                      && Encode::_utf8_on($_) for $ret->{$k};
1758                    }
1759                    return $ret;
1760                  } else {
1761                    defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
1762                    return $ret;
1763                  }
1764                }
1765              }
1766            }
1767
1768       ·  A large scalar that you know can only contain ASCII
1769
1770          Scalars that contain only ASCII and are marked as UTF-8 are
1771          sometimes a drag to your program. If you recognize such a situation,
1772          just remove the UTF8 flag:
1773
1774            utf8::downgrade($val) if $] > 5.008;
1775

BUGS

1777       See also "The "Unicode Bug"" above.
1778
1779   Interaction with Extensions
1780       When Perl exchanges data with an extension, the extension should be
1781       able to understand the UTF8 flag and act accordingly. If the extension
1782       doesn't recognize that flag, it's likely that the extension will return
1783       incorrectly-flagged data.
1784
1785       So if you're working with Unicode data, consult the documentation of
1786       every module you're using if there are any issues with Unicode data
1787       exchange. If the documentation does not talk about Unicode at all,
1788       suspect the worst and probably look at the source to learn how the
1789       module is implemented. Modules written completely in Perl shouldn't
1790       cause problems. Modules that directly or indirectly access code written
1791       in other programming languages are at risk.
1792
1793       For affected functions, the simple strategy to avoid data corruption is
1794       to always make the encoding of the exchanged data explicit. Choose an
1795       encoding that you know the extension can handle. Convert arguments
1796       passed to the extensions to that encoding and convert results back from
1797       that encoding. Write wrapper functions that do the conversions for you,
1798       so you can later change the functions when the extension catches up.
1799
1800       To provide an example, let's say the popular "Foo::Bar::escape_html"
1801       function doesn't deal with Unicode data yet. The wrapper function would
1802       convert the argument to raw UTF-8 and convert the result back to Perl's
1803       internal representation like so:
1804
1805           sub my_escape_html ($) {
1806               my($what) = shift;
1807               return unless defined $what;
1808               Encode::decode("UTF-8", Foo::Bar::escape_html(
1809                                            Encode::encode("UTF-8", $what)));
1810           }
1811
1812       Sometimes, when the extension does not convert data but just stores and
1813       retrieves it, you will be able to use the otherwise dangerous
1814       "Encode::_utf8_on()" function. Let's say the popular "Foo::Bar"
1815       extension, written in C, provides a "param" method that lets you store
1816       and retrieve data according to these prototypes:
1817
1818           $self->param($name, $value);            # set a scalar
1819           $value = $self->param($name);           # retrieve a scalar
1820
1821       If it does not yet provide support for any encoding, one could write a
1822       derived class with such a "param" method:
1823
1824           sub param {
1825             my($self,$name,$value) = @_;
1826             utf8::upgrade($name);     # make sure it is UTF-8 encoded
1827             if (defined $value) {
1828               utf8::upgrade($value);  # make sure it is UTF-8 encoded
1829               return $self->SUPER::param($name,$value);
1830             } else {
1831               my $ret = $self->SUPER::param($name);
1832               Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
1833               return $ret;
1834             }
1835           }
1836
1837       Some extensions provide filters on data entry/exit points, such as
1838       "DB_File::filter_store_key" and family. Look out for such filters in
1839       the documentation of your extensions; they can make the transition to
1840       Unicode data much easier.
1841
1842   Speed
1843       Some functions are slower when working on UTF-8 encoded strings than on
1844       byte encoded strings.  All functions that need to hop over characters
1845       such as "length()", "substr()" or "index()", or matching regular
1846       expressions can work much faster when the underlying data are byte-
1847       encoded.
1848
1849       In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a
1850       caching scheme was introduced which improved the situation.  In
1851       general, operations with UTF-8 encoded strings are still slower. As an
1852       example, the Unicode properties (character classes) like "\p{Nd}" are
1853       known to be quite a bit slower (5-20 times) than their simpler
1854       counterparts like "[0-9]" (then again, there are hundreds of Unicode
1855       characters matching "Nd" compared with the 10 ASCII characters matching
1856       "[0-9]").
1857

SEE ALSO

1859       perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes,
1860       perlretut, "${^UNICODE}" in perlvar,
1861       <http://www.unicode.org/reports/tr44>).
1862
1863
1864
1865perl v5.28.2                      2018-11-01                    PERLUNICODE(1)
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