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PERLUNICODE(1) Perl Programmers Reference Guide PERLUNICODE(1)
NAME
perlunicode - Unicode support in Perl
DESCRIPTION
Important Caveats
Unicode support is an extensive requirement. While Perl does not implement the Unicode standard or the accompanying
technical reports from cover to cover, Perl does support many Unicode features.
People who want to learn to use Unicode in Perl, should probably read the Perl Unicode tutorial, perlunitut, before
reading this reference document.
Also, the use of Unicode may present security issues that aren't obvious. Read Unicode Security Considerations
<http://www.unicode.org/reports/tr36>;.
Input and Output Layers
Perl knows when a filehandle uses Perl's internal Unicode encodings (UTF-8, or UTF-EBCDIC if in EBCDIC) if the
filehandle is opened with the ":utf8" layer. Other encodings can be converted to Perl's encoding on input or from
Perl's encoding on output by use of the ":encoding(...)" layer. See open.
To indicate that Perl source itself is in UTF-8, use "use utf8;".
Regular Expressions
The regular expression compiler produces polymorphic opcodes. That is, the pattern adapts to the data and
automatically switches to the Unicode character scheme when presented with data that is internally encoded in UTF-8,
or instead uses a traditional byte scheme when presented with byte data.
"use utf8" still needed to enable UTF-8/UTF-EBCDIC in scripts
As a compatibility measure, the "use utf8" pragma must be explicitly included to enable recognition of UTF-8 in the
Perl scripts themselves (in string or regular expression literals, or in identifier names) on ASCII-based machines or
to recognize UTF-EBCDIC on EBCDIC-based machines. These are the only times when an explicit "use utf8" is needed.
See utf8.
BOM-marked scripts and UTF-16 scripts autodetected
If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE, or UTF-8), or if the script looks like non-
BOM-marked UTF-16 of either endianness, Perl will correctly read in the script as Unicode. (BOMless UTF-8 cannot be
effectively recognized or differentiated from ISO 8859-1 or other eight-bit encodings.)
"use encoding" needed to upgrade non-Latin-1 byte strings
By default, there is a fundamental asymmetry in Perl's Unicode model: implicit upgrading from byte strings to Unicode
strings assumes that they were encoded in ISO 8859-1 (Latin-1), but Unicode strings are downgraded with UTF-8
encoding. This happens because the first 256 codepoints in Unicode happens to agree with Latin-1.
See "Byte and Character Semantics" for more details.
Byte and Character Semantics
Beginning with version 5.6, Perl uses logically-wide characters to represent strings internally.
In future, Perl-level operations will be expected to work with characters rather than bytes.
However, as an interim compatibility measure, Perl aims to provide a safe migration path from byte semantics to character
semantics for programs. For operations where Perl can unambiguously decide that the input data are characters, Perl
switches to character semantics. For operations where this determination cannot be made without additional information
from the user, Perl decides in favor of compatibility and chooses to use byte semantics.
Under byte semantics, when "use locale" is in effect, Perl uses the semantics associated with the current locale. Absent
a "use locale", and absent a "use feature 'unicode_strings'" pragma, Perl currently uses US-ASCII (or Basic Latin in
Unicode terminology) byte semantics, meaning that characters whose ordinal numbers are in the range 128 - 255 are
undefined except for their ordinal numbers. This means that none have case (upper and lower), nor are any a member of
character classes, like "[:alpha:]" or "\w". (But all do belong to the "\W" class or the Perl regular expression
extension "[:^alpha:]".)
This behavior preserves compatibility with earlier versions of Perl, which allowed byte semantics in Perl operations only
if none of the program's inputs were marked as being a source of Unicode character data. Such data may come from
filehandles, from calls to external programs, from information provided by the system (such as %ENV), or from literals
and constants in the source text.
The "bytes" pragma will always, regardless of platform, force byte semantics in a particular lexical scope. See bytes.
The "use feature 'unicode_strings'" pragma is intended to always, regardless of platform, force character (Unicode)
semantics in a particular lexical scope. In release 5.12, it is partially implemented, applying only to case changes.
See "The "Unicode Bug"" below.
The "utf8" pragma is primarily a compatibility device that enables recognition of UTF-(8|EBCDIC) in literals encountered
by the parser. Note that this pragma is only required while Perl defaults to byte semantics; when character semantics
become the default, this pragma may become a no-op. See utf8.
Unless explicitly stated, Perl operators use character semantics for Unicode data and byte semantics for non-Unicode
data. The decision to use character semantics is made transparently. If input data comes from a Unicode source--for
example, if a character encoding layer is added to a filehandle or a literal Unicode string constant appears in a
program--character semantics apply. Otherwise, byte semantics are in effect. The "bytes" pragma should be used to force
byte semantics on Unicode data, and the "use feature 'unicode_strings'" pragma to force Unicode semantics on byte data
(though in 5.12 it isn't fully implemented).
If strings operating under byte semantics and strings with Unicode character data are concatenated, the new string will
have character semantics. This can cause surprises: See "BUGS", below. You can choose to be warned when this happens.
See encoding::warnings.
Under character semantics, many operations that formerly operated on bytes now operate on characters. A character in Perl
is logically just a number ranging from 0 to 2**31 or so. Larger characters may encode into longer sequences of bytes
internally, but this internal detail is mostly hidden for Perl code. See perluniintro for more.
Effects of Character Semantics
Character semantics have the following effects:
o Strings--including hash keys--and regular expression patterns may contain characters that have an ordinal value
larger than 255.
If you use a Unicode editor to edit your program, Unicode characters may occur directly within the literal strings in
UTF-8 encoding, or UTF-16. (The former requires a BOM or "use utf8", the latter requires a BOM.)
Unicode characters can also be added to a string by using the "\N{U+...}" notation. The Unicode code for the desired
character, in hexadecimal, should be placed in the braces, after the "U". For instance, a smiley face is
"\N{U+263A}".
Alternatively, you can use the "\x{...}" notation for characters 0x100 and above. For characters below 0x100 you may
get byte semantics instead of character semantics; see "The "Unicode Bug"". On EBCDIC machines there is the
additional problem that the value for such characters gives the EBCDIC character rather than the Unicode one.
Additionally, if you
use charnames ':full';
you can use the "\N{...}" notation and put the official Unicode character name within the braces, such as "\N{WHITE
SMILING FACE}". See charnames.
o If an appropriate encoding is specified, identifiers within the Perl script may contain Unicode alphanumeric
characters, including ideographs. Perl does not currently attempt to canonicalize variable names.
o Regular expressions match characters instead of bytes. "." matches a character instead of a byte.
o Bracketed character classes in regular expressions match characters instead of bytes and match against the character
properties specified in the Unicode properties database. "\w" can be used to match a Japanese ideograph, for
instance.
o Named Unicode properties, scripts, and block ranges may be used (like bracketed character classes) by using the
"\p{}" "matches property" construct and the "\P{}" negation, "doesn't match property". See "Unicode Character
Properties" for more details.
You can define your own character properties and use them in the regular expression with the "\p{}" or "\P{}"
construct. See "User-Defined Character Properties" for more details.
o The special pattern "\X" matches a logical character, an "extended grapheme cluster" in Standardese. In Unicode what
appears to the user to be a single character, for example an accented "G", may in fact be composed of a sequence of
characters, in this case a "G" followed by an accent character. "\X" will match the entire sequence.
o The "tr///" operator translates characters instead of bytes. Note that the "tr///CU" functionality has been removed.
For similar functionality see pack('U0', ...) and pack('C0', ...).
o Case translation operators use the Unicode case translation tables when character input is provided. Note that
"uc()", or "\U" in interpolated strings, translates to uppercase, while "ucfirst", or "\u" in interpolated strings,
translates to titlecase in languages that make the distinction (which is equivalent to uppercase in languages without
the distinction).
o Most operators that deal with positions or lengths in a string will automatically switch to using character
positions, including "chop()", "chomp()", "substr()", "pos()", "index()", "rindex()", "sprintf()", "write()", and
"length()". An operator that specifically does not switch is "vec()". Operators that really don't care include
operators that treat strings as a bucket of bits such as "sort()", and operators dealing with filenames.
o The "pack()"/"unpack()" letter "C" does not change, since it is often used for byte-oriented formats. Again, think
"char" in the C language.
There is a new "U" specifier that converts between Unicode characters and code points. There is also a "W" specifier
that is the equivalent of "chr"/"ord" and properly handles character values even if they are above 255.
o The "chr()" and "ord()" functions work on characters, similar to "pack("W")" and "unpack("W")", not "pack("C")" and
"unpack("C")". "pack("C")" and "unpack("C")" are methods for emulating byte-oriented "chr()" and "ord()" on Unicode
strings. While these methods reveal the internal encoding of Unicode strings, that is not something one normally
needs to care about at all.
o The bit string operators, "& | ^ ~", can operate on character data. However, for backward compatibility, such as
when using bit string operations when characters are all less than 256 in ordinal value, one should not use "~" (the
bit complement) with characters of both values less than 256 and values greater than 256. Most importantly,
DeMorgan's laws ("~($x|$y) eq ~$x&~$y" and "~($x&$y) eq ~$x|~$y") will not hold. The reason for this mathematical
faux pas is that the complement cannot return both the 8-bit (byte-wide) bit complement and the full character-wide
bit complement.
o You can define your own mappings to be used in "lc()", "lcfirst()", "uc()", and "ucfirst()" (or their double-quoted
string inlined versions such as "\U"). See "User-Defined Case Mappings" for more details.
o And finally, "scalar reverse()" reverses by character rather than by byte.
Unicode Character Properties
Most Unicode character properties are accessible by using regular expressions. They are used (like bracketed character
classes) by using the "\p{}" "matches property" construct and the "\P{}" negation, "doesn't match property".
Note that the only time that Perl considers a sequence of individual code points as a single logical character is in the
"\X" construct, already mentioned above. Therefore "character" in this discussion means a single Unicode code point.
For instance, "\p{Uppercase}" matches any single character with the Unicode "Uppercase" property, while "\p{L}" matches
any character with a General_Category of "L" (letter) property. Brackets are not required for single letter property
names, so "\p{L}" is equivalent to "\pL".
More formally, "\p{Uppercase}" matches any single character whose Unicode Uppercase property value is True, and
"\P{Uppercase}" matches any character whose Uppercase property value is False, and they could have been written as
"\p{Uppercase=True}" and "\p{Uppercase=False}", respectively.
This formality is needed when properties are not binary, that is if they can take on more values than just True and
False. For example, the Bidi_Class (see "Bidirectional Character Types" below), can take on a number of different
values, such as Left, Right, Whitespace, and others. To match these, one needs to specify the property name
(Bidi_Class), and the value being matched against (Left, Right, etc.). This is done, as in the examples above, by having
the two components separated by an equal sign (or interchangeably, a colon), like "\p{Bidi_Class: Left}".
All Unicode-defined character properties may be written in these compound forms of "\p{property=value}" or
"\p{property:value}", but Perl provides some additional properties that are written only in the single form, as well as
single-form short-cuts for all binary properties and certain others described below, in which you may omit the property
name and the equals or colon separator.
Most Unicode character properties have at least two synonyms (or aliases if you prefer), a short one that is easier to
type, and a longer one which is more descriptive and hence it is easier to understand what it means. Thus the "L" and
"Letter" above are equivalent and can be used interchangeably. Likewise, "Upper" is a synonym for "Uppercase", and we
could have written "\p{Uppercase}" equivalently as "\p{Upper}". Also, there are typically various synonyms for the
values the property can be. For binary properties, "True" has 3 synonyms: "T", "Yes", and "Y"; and "False has
correspondingly "F", "No", and "N". But be careful. A short form of a value for one property may not mean the same
thing as the same short form for another. Thus, for the General_Category property, "L" means "Letter", but for the
Bidi_Class property, "L" means "Left". A complete list of properties and synonyms is in perluniprops.
Upper/lower case differences in the property names and values are irrelevant, thus "\p{Upper}" means the same thing as
"\p{upper}" or even "\p{UpPeR}". Similarly, you can add or subtract underscores anywhere in the middle of a word, so
that these are also equivalent to "\p{U_p_p_e_r}". And white space is irrelevant adjacent to non-word characters, such
as the braces and the equals or colon separators so "\p{ Upper }" and "\p{ Upper_case : Y }" are equivalent to these
as well. In fact, in most cases, white space and even hyphens can be added or deleted anywhere. So even "\p{ Up-per
case = Yes}" is equivalent. All this is called "loose-matching" by Unicode. The few places where stricter matching is
employed is in the middle of numbers, and the Perl extension properties that begin or end with an underscore. Stricter
matching cares about white space (except adjacent to the non-word characters) and hyphens, and non-interior underscores.
You can also use negation in both "\p{}" and "\P{}" by introducing a caret (^) between the first brace and the property
name: "\p{^Tamil}" is equal to "\P{Tamil}".
General_Category
Every Unicode character is assigned a general category, which is the "most usual categorization of a character" (from
<http://www.unicode.org/reports/tr44>;).
The compound way of writing these is like "\p{General_Category=Number}" (short, "\p{gc:n}"). But Perl furnishes
shortcuts in which everything up through the equal or colon separator is omitted. So you can instead just write "\pN".
Here are the short and long forms of the General Category properties:
Short Long
L Letter
LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
Lu Uppercase_Letter
Ll Lowercase_Letter
Lt Titlecase_Letter
Lm Modifier_Letter
Lo Other_Letter
M Mark
Mn Nonspacing_Mark
Mc Spacing_Mark
Me Enclosing_Mark
N Number
Nd Decimal_Number (also Digit)
Nl Letter_Number
No Other_Number
P Punctuation (also Punct)
Pc Connector_Punctuation
Pd Dash_Punctuation
Ps Open_Punctuation
Pe Close_Punctuation
Pi Initial_Punctuation
(may behave like Ps or Pe depending on usage)
Pf Final_Punctuation
(may behave like Ps or Pe depending on usage)
Po Other_Punctuation
S Symbol
Sm Math_Symbol
Sc Currency_Symbol
Sk Modifier_Symbol
So Other_Symbol
Z Separator
Zs Space_Separator
Zl Line_Separator
Zp Paragraph_Separator
C Other
Cc Control (also Cntrl)
Cf Format
Cs Surrogate (not usable)
Co Private_Use
Cn Unassigned
Single-letter properties match all characters in any of the two-letter sub-properties starting with the same letter.
"LC" and "L&" are special cases, which are both aliases for the set consisting of everything matched by "Ll", "Lu", and
"Lt".
Because Perl hides the need for the user to understand the internal representation of Unicode characters, there is no
need to implement the somewhat messy concept of surrogates. "Cs" is therefore not supported.
Bidirectional Character Types
Because scripts differ in their directionality (Hebrew is written right to left, for example) Unicode supplies these
properties in the Bidi_Class class:
Property Meaning
L Left-to-Right
LRE Left-to-Right Embedding
LRO Left-to-Right Override
R Right-to-Left
AL Arabic Letter
RLE Right-to-Left Embedding
RLO Right-to-Left Override
PDF Pop Directional Format
EN European Number
ES European Separator
ET European Terminator
AN Arabic Number
CS Common Separator
NSM Non-Spacing Mark
BN Boundary Neutral
B Paragraph Separator
S Segment Separator
WS Whitespace
ON Other Neutrals
This property is always written in the compound form. For example, "\p{Bidi_Class:R}" matches characters that are
normally written right to left.
Scripts
The world's languages are written in a number of scripts. This sentence (unless you're reading it in translation) is
written in Latin, while Russian is written in Cyrllic, and Greek is written in, well, Greek; Japanese mainly in Hiragana
or Katakana. There are many more.
The Unicode Script property gives what script a given character is in, and the property can be specified with the
compound form like "\p{Script=Hebrew}" (short: "\p{sc=hebr}"). Perl furnishes shortcuts for all script names. You can
omit everything up through the equals (or colon), and simply write "\p{Latin}" or "\P{Cyrillic}".
A complete list of scripts and their shortcuts is in perluniprops.
Use of "Is" Prefix
For backward compatibility (with Perl 5.6), all properties mentioned so far may have "Is" or "Is_" prepended to their
name, so "\P{Is_Lu}", for example, is equal to "\P{Lu}", and "\p{IsScript:Arabic}" is equal to "\p{Arabic}".
Blocks
In addition to scripts, Unicode also defines blocks of characters. The difference between scripts and blocks is that the
concept of scripts is closer to natural languages, while the concept of blocks is more of an artificial grouping based on
groups of Unicode characters with consecutive ordinal values. For example, the "Basic Latin" block is all characters
whose ordinals are between 0 and 127, inclusive, in other words, the ASCII characters. The "Latin" script contains some
letters from this block as well as several more, like "Latin-1 Supplement", "Latin Extended-A", etc., but it does not
contain all the characters from those blocks. It does not, for example, contain digits, because digits are shared across
many scripts. Digits and similar groups, like punctuation, are in the script called "Common". There is also a script
called "Inherited" for characters that modify other characters, and inherit the script value of the controlling
character.
For more about scripts versus blocks, see UAX#24 "Unicode Script Property": <http://www.unicode.org/reports/tr24>;
The Script property is likely to be the one you want to use when processing natural language; the Block property may be
useful in working with the nuts and bolts of Unicode.
Block names are matched in the compound form, like "\p{Block: Arrows}" or "\p{Blk=Hebrew}". Unlike most other properties
only a few block names have a Unicode-defined short name. But Perl does provide a (slight) shortcut: You can say, for
example "\p{In_Arrows}" or "\p{In_Hebrew}". For backwards compatibility, the "In" prefix may be omitted if there is no
naming conflict with a script or any other property, and you can even use an "Is" prefix instead in those cases. But it
is not a good idea to do this, for a couple reasons:
1. It is confusing. There are many naming conflicts, and you may forget some. For example, "\p{Hebrew}" means the
script Hebrew, and NOT the block Hebrew. But would you remember that 6 months from now?
2. It is unstable. A new version of Unicode may pre-empt the current meaning by creating a property with the same name.
There was a time in very early Unicode releases when "\p{Hebrew}" would have matched the block Hebrew; now it
doesn't.
Some people just prefer to always use "\p{Block: foo}" and "\p{Script: bar}" instead of the shortcuts, for clarity, and
because they can't remember the difference between 'In' and 'Is' anyway (or aren't confident that those who eventually
will read their code will know).
A complete list of blocks and their shortcuts is in perluniprops.
Other Properties
There are many more properties than the very basic ones described here. A complete list is in perluniprops.
Unicode defines all its properties in the compound form, so all single-form properties are Perl extensions. A number of
these are just synonyms for the Unicode ones, but some are genunine extensions, including a couple that are in the
compound form. And quite a few of these are actually recommended by Unicode (in <http://www.unicode.org/reports/tr18>;).
This section gives some details on all the extensions that aren't synonyms for compound-form Unicode properties (for
those, you'll have to refer to the Unicode Standard <http://www.unicode.org/reports/tr44>;.
"\p{All}"
This matches any of the 1_114_112 Unicode code points. It is a synonym for "\p{Any}".
"\p{Alnum}"
This matches any "\p{Alphabetic}" or "\p{Decimal_Number}" character.
"\p{Any}"
This matches any of the 1_114_112 Unicode code points. It is a synonym for "\p{All}".
"\p{Assigned}"
This matches any assigned code point; that is, any code point whose general category is not Unassigned (or
equivalently, not Cn).
"\p{Blank}"
This is the same as "\h" and "\p{HorizSpace}": A character that changes the spacing horizontally.
"\p{Decomposition_Type: Non_Canonical}" (Short: "\p{Dt=NonCanon}")
Matches a character that has a non-canonical decomposition.
To understand the use of this rarely used property=value combination, it is necessary to know some basics about
decomposition. Consider a character, say H. It could appear with various marks around it, such as an acute accent,
or a circumflex, or various hooks, circles, arrows, etc., above, below, to one side and/or the other, etc. There are
many possibilities among the world's languages. The number of combinations is astronomical, and if there were a
character for each combination, it would soon exhaust Unicode's more than a million possible characters. So Unicode
took a different approach: there is a character for the base H, and a character for each of the possible marks, and
they can be combined variously to get a final logical character. So a logical character--what appears to be a single
character--can be a sequence of more than one individual characters. This is called an "extended grapheme cluster".
(Perl furnishes the "\X" construct to match such sequences.)
But Unicode's intent is to unify the existing character set standards and practices, and a number of pre-existing
standards have single characters that mean the same thing as some of these combinations. An example is ISO-8859-1,
which has quite a few of these in the Latin-1 range, an example being "LATIN CAPITAL LETTER E WITH ACUTE". Because
this character was in this pre-existing standard, Unicode added it to its repertoire. But this character is
considered by Unicode to be equivalent to the sequence consisting of first the character "LATIN CAPITAL LETTER E",
then the character "COMBINING ACUTE ACCENT".
"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and the equivalence with the sequence is
called canonical equivalence. All pre-composed characters are said to have a decomposition (into the equivalent
sequence) and the decomposition type is also called canonical.
However, many more characters have a different type of decomposition, a "compatible" or "non-canonical"
decomposition. The sequences that form these decompositions are not considered canonically equivalent to the pre-
composed character. An example, again in the Latin-1 range, is the "SUPERSCRIPT ONE". It is kind of like a regular
digit 1, but not exactly; its decomposition into the digit 1 is called a "compatible" decomposition, specifically a
"super" decomposition. There are several such compatibility decompositions (see
<http://www.unicode.org/reports/tr44>;), including one called "compat" which means some miscellaneous type of
decomposition that doesn't fit into the decomposition categories that Unicode has chosen.
Note that most Unicode characters don't have a decomposition, so their decomposition type is "None".
Perl has added the "Non_Canonical" type, for your convenience, to mean any of the compatibility decompositions.
"\p{Graph}"
Matches any character that is graphic. Theoretically, this means a character that on a printer would cause ink to be
used.
"\p{HorizSpace}"
This is the same as "\h" and "\p{Blank}": A character that changes the spacing horizontally.
"\p{In=*}"
This is a synonym for "\p{Present_In=*}"
"\p{PerlSpace}"
This is the same as "\s", restricted to ASCII, namely "[ \f\n\r\t]".
Mnemonic: Perl's (original) space
"\p{PerlWord}"
This is the same as "\w", restricted to ASCII, namely "[A-Za-z0-9_]"
Mnemonic: Perl's (original) word.
"\p{PosixAlnum}"
This matches any alphanumeric character in the ASCII range, namely "[A-Za-z0-9]".
"\p{PosixAlpha}"
This matches any alphabetic character in the ASCII range, namely "[A-Za-z]".
"\p{PosixBlank}"
This matches any blank character in the ASCII range, namely "[ \t]".
"\p{PosixCntrl}"
This matches any control character in the ASCII range, namely "[\x00-\x1F\x7F]"
"\p{PosixDigit}"
This matches any digit character in the ASCII range, namely "[0-9]".
"\p{PosixGraph}"
This matches any graphical character in the ASCII range, namely "[\x21-\x7E]".
"\p{PosixLower}"
This matches any lowercase character in the ASCII range, namely "[a-z]".
"\p{PosixPrint}"
This matches any printable character in the ASCII range, namely "[\x20-\x7E]". These are the graphical characters
plus SPACE.
"\p{PosixPunct}"
This matches any punctuation character in the ASCII range, namely "[\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]". These
are the graphical characters that aren't word characters. Note that the Posix standard includes in its definition of
punctuation, those characters that Unicode calls "symbols."
"\p{PosixSpace}"
This matches any space character in the ASCII range, namely "[ \f\n\r\t\x0B]" (the last being a vertical tab).
"\p{PosixUpper}"
This matches any uppercase character in the ASCII range, namely "[A-Z]".
"\p{Present_In: *}" (Short: "\p{In=*}")
This property is used when you need to know in what Unicode version(s) a character is.
The "*" above stands for some two digit Unicode version number, such as 1.1 or 4.0; or the "*" can also be
"Unassigned". This property will match the code points whose final disposition has been settled as of the Unicode
release given by the version number; "\p{Present_In: Unassigned}" will match those code points whose meaning has yet
to be assigned.
For example, "U+0041" "LATIN CAPITAL LETTER A" was present in the very first Unicode release available, which is 1.1,
so this property is true for all valid "*" versions. On the other hand, "U+1EFF" was not assigned until version 5.1
when it became "LATIN SMALL LETTER Y WITH LOOP", so the only "*" that would match it are 5.1, 5.2, and later.
Unicode furnishes the "Age" property from which this is derived. The problem with Age is that a strict
interpretation of it (which Perl takes) has it matching the precise release a code point's meaning is introduced in.
Thus "U+0041" would match only 1.1; and "U+1EFF" only 5.1. This is not usually what you want.
Some non-Perl implementations of the Age property may change its meaning to be the same as the Perl Present_In
property; just be aware of that.
Another confusion with both these properties is that the definition is not that the code point has been assigned, but
that the meaning of the code point has been determined. This is because 66 code points will always be unassigned,
and, so the Age for them is the Unicode version the decision to make them so was made in. For example, "U+FDD0" is
to be permanently unassigned to a character, and the decision to do that was made in version 3.1, so "\p{Age=3.1}"
matches this character and "\p{Present_In: 3.1}" and up matches as well.
"\p{Print}"
This matches any character that is graphical or blank, except controls.
"\p{SpacePerl}"
This is the same as "\s", including beyond ASCII.
Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab which both the Posix standard and Unicode
consider to be space.)
"\p{VertSpace}"
This is the same as "\v": A character that changes the spacing vertically.
"\p{Word}"
This is the same as "\w", including beyond ASCII.
User-Defined Character Properties
You can define your own binary character properties by defining subroutines whose names begin with "In" or "Is". The
subroutines can be defined in any package. The user-defined properties can be used in the regular expression "\p" and
"\P" constructs; if you are using a user-defined property from a package other than the one you are in, you must specify
its package in the "\p" or "\P" construct.
# assuming property Is_Foreign defined in Lang::
package main; # property package name required
if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
package Lang; # property package name not required
if ($txt =~ /\p{IsForeign}+/) { ... }
Note that the effect is compile-time and immutable once defined.
The subroutines must return a specially-formatted string, with one or more newline-separated lines. Each line must be
one of the following:
o A single hexadecimal number denoting a Unicode code point to include.
o Two hexadecimal numbers separated by horizontal whitespace (space or tabular characters) denoting a range of Unicode
code points to include.
o Something to include, prefixed by "+": a built-in character property (prefixed by "utf8::") or a user-defined
character property, to represent all the characters in that property; two hexadecimal code points for a range; or a
single hexadecimal code point.
o Something to exclude, prefixed by "-": an existing character property (prefixed by "utf8::") or a user-defined
character property, to represent all the characters in that property; two hexadecimal code points for a range; or a
single hexadecimal code point.
o Something to negate, prefixed "!": an existing character property (prefixed by "utf8::") or a user-defined character
property, to represent all the characters in that property; two hexadecimal code points for a range; or a single
hexadecimal code point.
o Something to intersect with, prefixed by "&": an existing character property (prefixed by "utf8::") or a user-defined
character property, for all the characters except the characters in the property; two hexadecimal code points for a
range; or a single hexadecimal code point.
For example, to define a property that covers both the Japanese syllabaries (hiragana and katakana), you can define
sub InKana {
return <<END;
3040\t309F
30A0\t30FF
END
}
Imagine that the here-doc end marker is at the beginning of the line. Now you can use "\p{InKana}" and "\P{InKana}".
You could also have used the existing block property names:
sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
END
}
Suppose you wanted to match only the allocated characters, not the raw block ranges: in other words, you want to remove
the non-characters:
sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
-utf8::IsCn
END
}
The negation is useful for defining (surprise!) negated classes.
sub InNotKana {
return <<'END';
!utf8::InHiragana
-utf8::InKatakana
+utf8::IsCn
END
}
Intersection is useful for getting the common characters matched by two (or more) classes.
sub InFooAndBar {
return <<'END';
+main::Foo
&main::Bar
END
}
It's important to remember not to use "&" for the first set; that would be intersecting with nothing (resulting in an
empty set).
User-Defined Case Mappings
You can also define your own mappings to be used in the lc(), lcfirst(), uc(), and ucfirst() (or their string-inlined
versions). The principle is similar to that of user-defined character properties: to define subroutines with names like
"ToLower" (for lc() and lcfirst()), "ToTitle" (for the first character in ucfirst()), and "ToUpper" (for uc(), and the
rest of the characters in ucfirst()).
The string returned by the subroutines needs to be two hexadecimal numbers separated by two tabulators: the two numbers
being, respectively, the source code point and the destination code point. For example:
sub ToUpper {
return <<END;
0061\t\t0041
END
}
defines an uc() mapping that causes only the character "a" to be mapped to "A"; all other characters will remain
unchanged.
(For serious hackers only) The above means you have to furnish a complete mapping; you can't just override a couple of
characters and leave the rest unchanged. You can find all the mappings in the directory $Config{privlib}/unicore/To/.
The mapping data is returned as the here-document, and the "utf8::ToSpecFoo" are special exception mappings derived from
<$Config{privlib}>/unicore/SpecialCasing.txt. The "Digit" and "Fold" mappings that one can see in the directory are not
directly user-accessible, one can use either the "Unicode::UCD" module, or just match case-insensitively (that's when the
"Fold" mapping is used).
The mappings will only take effect on scalars that have been marked as having Unicode characters, for example by using
"utf8::upgrade()". Old byte-style strings are not affected.
The mappings are in effect for the package they are defined in.
Character Encodings for Input and Output
See Encode.
Unicode Regular Expression Support Level
The following list of Unicode support for regular expressions describes all the features currently supported. The
references to "Level N" and the section numbers refer to the Unicode Technical Standard #18, "Unicode Regular
Expressions", version 11, in May 2005.
o Level 1 - Basic Unicode Support
RL1.1 Hex Notation - done [1]
RL1.2 Properties - done [2][3]
RL1.2a Compatibility Properties - done [4]
RL1.3 Subtraction and Intersection - MISSING [5]
RL1.4 Simple Word Boundaries - done [6]
RL1.5 Simple Loose Matches - done [7]
RL1.6 Line Boundaries - MISSING [8]
RL1.7 Supplementary Code Points - done [9]
[1] \x{...}
[2] \p{...} \P{...}
[3] supports not only minimal list, but all Unicode character
properties (see L</Unicode Character Properties>)
[4] \d \D \s \S \w \W \X [:prop:] [:^prop:]
[5] can use regular expression look-ahead [a] or
user-defined character properties [b] to emulate set operations
[6] \b \B
[7] note that Perl does Full case-folding in matching (but with bugs),
not Simple: for example U+1F88 is equivalent to U+1F00 U+03B9,
not with 1F80. This difference matters mainly for certain Greek
capital letters with certain modifiers: the Full case-folding
decomposes the letter, while the Simple case-folding would map
it to a single character.
[8] should do ^ and $ also on U+000B (\v in C), FF (\f), CR (\r),
CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS (U+2029);
should also affect <>, $., and script line numbers;
should not split lines within CRLF [c] (i.e. there is no empty
line between \r and \n)
[9] UTF-8/UTF-EBDDIC used in perl allows not only U+10000 to U+10FFFF
but also beyond U+10FFFF [d]
[a] You can mimic class subtraction using lookahead. For example, what UTS#18 might write as
[{Greek}-[{UNASSIGNED}]]
in Perl can be written as:
(?!\p{Unassigned})\p{InGreekAndCoptic}
(?=\p{Assigned})\p{InGreekAndCoptic}
But in this particular example, you probably really want
\p{GreekAndCoptic}
which will match assigned characters known to be part of the Greek script.
Also see the Unicode::Regex::Set module, it does implement the full UTS#18 grouping, intersection, union, and removal
(subtraction) syntax.
[b] '+' for union, '-' for removal (set-difference), '&' for intersection (see "User-Defined Character Properties")
[c] Try the ":crlf" layer (see PerlIO).
[d] U+FFFF will currently generate a warning message if 'utf8' warnings are
enabled
o Level 2 - Extended Unicode Support
RL2.1 Canonical Equivalents - MISSING [10][11]
RL2.2 Default Grapheme Clusters - MISSING [12]
RL2.3 Default Word Boundaries - MISSING [14]
RL2.4 Default Loose Matches - MISSING [15]
RL2.5 Name Properties - MISSING [16]
RL2.6 Wildcard Properties - MISSING
[10] see UAX#15 "Unicode Normalization Forms"
[11] have Unicode::Normalize but not integrated to regexes
[12] have \X but we don't have a "Grapheme Cluster Mode"
[14] see UAX#29, Word Boundaries
[15] see UAX#21 "Case Mappings"
[16] have \N{...} but neither compute names of CJK Ideographs
and Hangul Syllables nor use a loose match [e]
[e] "\N{...}" allows namespaces (see charnames).
o Level 3 - Tailored Support
RL3.1 Tailored Punctuation - MISSING
RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
RL3.3 Tailored Word Boundaries - MISSING
RL3.4 Tailored Loose Matches - MISSING
RL3.5 Tailored Ranges - MISSING
RL3.6 Context Matching - MISSING [19]
RL3.7 Incremental Matches - MISSING
( RL3.8 Unicode Set Sharing )
RL3.9 Possible Match Sets - MISSING
RL3.10 Folded Matching - MISSING [20]
RL3.11 Submatchers - MISSING
[17] see UAX#10 "Unicode Collation Algorithms"
[18] have Unicode::Collate but not integrated to regexes
[19] have (?<=x) and (?=x), but look-aheads or look-behinds should see
outside of the target substring
[20] need insensitive matching for linguistic features other than case;
for example, hiragana to katakana, wide and narrow, simplified Han
to traditional Han (see UTR#30 "Character Foldings")
Unicode Encodings
Unicode characters are assigned to code points, which are abstract numbers. To use these numbers, various encodings are
needed.
o UTF-8
UTF-8 is a variable-length (1 to 6 bytes, current character allocations require 4 bytes), byte-order independent
encoding. For ASCII (and we really do mean 7-bit ASCII, not another 8-bit encoding), UTF-8 is transparent.
The following table is from Unicode 3.2.
Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
U+0000..U+007F 00..7F
U+0080..U+07FF * C2..DF 80..BF
U+0800..U+0FFF E0 * A0..BF 80..BF
U+1000..U+CFFF E1..EC 80..BF 80..BF
U+D000..U+D7FF ED 80..9F 80..BF
U+D800..U+DFFF +++++++ utf16 surrogates, not legal utf8 +++++++
U+E000..U+FFFF EE..EF 80..BF 80..BF
U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
Note the gaps before several of the byte entries above marked by '*'. These are caused by legal UTF-8 avoiding non-
shortest encodings: it is technically possible to UTF-8-encode a single code point in different ways, but that is
explicitly forbidden, and the shortest possible encoding should always be used (and that is what Perl does).
Another way to look at it is via bits:
Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
0aaaaaaa 0aaaaaaa
00000bbbbbaaaaaa 110bbbbb 10aaaaaa
ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
As you can see, the continuation bytes all begin with "10", and the leading bits of the start byte tell how many
bytes there are in the encoded character.
o UTF-EBCDIC
Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
o UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)
The followings items are mostly for reference and general Unicode knowledge, Perl doesn't use these constructs
internally.
UTF-16 is a 2 or 4 byte encoding. The Unicode code points "U+0000..U+FFFF" are stored in a single 16-bit unit, and
the code points "U+10000..U+10FFFF" in two 16-bit units. The latter case is using surrogates, the first 16-bit unit
being the high surrogate, and the second being the low surrogate.
Surrogates are code points set aside to encode the "U+10000..U+10FFFF" range of Unicode code points in pairs of
16-bit units. The high surrogates are the range "U+D800..U+DBFF" and the low surrogates are the range
"U+DC00..U+DFFF". The surrogate encoding is
$hi = ($uni - 0x10000) / 0x400 + 0xD800;
$lo = ($uni - 0x10000) % 0x400 + 0xDC00;
and the decoding is
$uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
If you try to generate surrogates (for example by using chr()), you will get a warning, if warnings are turned on,
because those code points are not valid for a Unicode character.
Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16 itself can be used for in-memory computations, but
if storage or transfer is required either UTF-16BE (big-endian) or UTF-16LE (little-endian) encodings must be chosen.
This introduces another problem: what if you just know that your data is UTF-16, but you don't know which endianness?
Byte Order Marks, or BOMs, are a solution to this. A special character has been reserved in Unicode to function as a
byte order marker: the character with the code point "U+FEFF" is the BOM.
The trick is that if you read a BOM, you will know the byte order, since if it was written on a big-endian platform,
you will read the bytes "0xFE 0xFF", but if it was written on a little-endian platform, you will read the bytes "0xFF
0xFE". (And if the originating platform was writing in UTF-8, you will read the bytes "0xEF 0xBB 0xBF".)
The way this trick works is that the character with the code point "U+FFFE" is guaranteed not to be a valid Unicode
character, so the sequence of bytes "0xFF 0xFE" is unambiguously "BOM, represented in little-endian format" and
cannot be "U+FFFE", represented in big-endian format". (Actually, "U+FFFE" is legal for use by your program, even
for input/output, but better not use it if you need a BOM. But it is "illegal for interchange", so that an
unsuspecting program won't get confused.)
o UTF-32, UTF-32BE, UTF-32LE
The UTF-32 family is pretty much like the UTF-16 family, expect that the units are 32-bit, and therefore the
surrogate scheme is not needed. The BOM signatures will be "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00 0x00"
for LE.
o UCS-2, UCS-4
Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit encoding. Unlike UTF-16, UCS-2 is not extensible
beyond "U+FFFF", because it does not use surrogates. UCS-4 is a 32-bit encoding, functionally identical to UTF-32.
o UTF-7
A seven-bit safe (non-eight-bit) encoding, which is useful if the transport or storage is not eight-bit safe.
Defined by RFC 2152.
Security Implications of Unicode
Read Unicode Security Considerations <http://www.unicode.org/reports/tr36>;. Also, note the following:
o Malformed UTF-8
Unfortunately, the specification of UTF-8 leaves some room for interpretation of how many bytes of encoded output one
should generate from one input Unicode character. Strictly speaking, the shortest possible sequence of UTF-8 bytes
should be generated, because otherwise there is potential for an input buffer overflow at the receiving end of a
UTF-8 connection. Perl always generates the shortest length UTF-8, and with warnings on, Perl will warn about non-
shortest length UTF-8 along with other malformations, such as the surrogates, which are not real Unicode code points.
o Regular expressions behave slightly differently between byte data and character (Unicode) data. For example, the
"word character" character class "\w" will work differently depending on if data is eight-bit bytes or Unicode.
In the first case, the set of "\w" characters is either small--the default set of alphabetic characters, digits, and
the "_"--or, if you are using a locale (see perllocale), the "\w" might contain a few more letters according to your
language and country.
In the second case, the "\w" set of characters is much, much larger. Most importantly, even in the set of the first
256 characters, it will probably match different characters: unlike most locales, which are specific to a language
and country pair, Unicode classifies all the characters that are letters somewhere as "\w". For example, your locale
might not think that LATIN SMALL LETTER ETH is a letter (unless you happen to speak Icelandic), but Unicode does.
As discussed elsewhere, Perl has one foot (two hooves?) planted in each of two worlds: the old world of bytes and the
new world of characters, upgrading from bytes to characters when necessary. If your legacy code does not explicitly
use Unicode, no automatic switch-over to characters should happen. Characters shouldn't get downgraded to bytes,
either. It is possible to accidentally mix bytes and characters, however (see perluniintro), in which case "\w" in
regular expressions might start behaving differently. Review your code. Use warnings and the "strict" pragma.
Unicode in Perl on EBCDIC
The way Unicode is handled on EBCDIC platforms is still experimental. On such platforms, references to UTF-8 encoding in
this document and elsewhere should be read as meaning the UTF-EBCDIC specified in Unicode Technical Report 16, unless
ASCII vs. EBCDIC issues are specifically discussed. There is no "utfebcdic" pragma or ":utfebcdic" layer; rather, "utf8"
and ":utf8" are reused to mean the platform's "natural" 8-bit encoding of Unicode. See perlebcdic for more discussion of
the issues.
Locales
Usually locale settings and Unicode do not affect each other, but there are a couple of exceptions:
o You can enable automatic UTF-8-ification of your standard file handles, default "open()" layer, and @ARGV by using
either the "-C" command line switch or the "PERL_UNICODE" environment variable, see perlrun for the documentation of
the "-C" switch.
o Perl tries really hard to work both with Unicode and the old byte-oriented world. Most often this is nice, but
sometimes Perl's straddling of the proverbial fence causes problems.
When Unicode Does Not Happen
While Perl does have extensive ways to input and output in Unicode, and few other 'entry points' like the @ARGV which can
be interpreted as Unicode (UTF-8), there still are many places where Unicode (in some encoding or another) could be given
as arguments or received as results, or both, but it is not.
The following are such interfaces. Also, see "The "Unicode Bug"". For all of these interfaces Perl currently (as of
5.8.3) simply assumes byte strings both as arguments and results, or UTF-8 strings if the "encoding" pragma has been
used.
One reason why Perl does not attempt to resolve the role of Unicode in these cases is that the answers are highly
dependent on the operating system and the file system(s). For example, whether filenames can be in Unicode, and in
exactly what kind of encoding, is not exactly a portable concept. Similarly for the qx and system: how well will the
'command line interface' (and which of them?) handle Unicode?
o chdir, chmod, chown, chroot, exec, link, lstat, mkdir, rename, rmdir, stat, symlink, truncate, unlink, utime, -X
o %ENV
o glob (aka the <*>)
o open, opendir, sysopen
o qx (aka the backtick operator), system
o readdir, readlink
The "Unicode Bug"
The term, the "Unicode bug" has been applied to an inconsistency with the Unicode characters whose ordinals are in the
Latin-1 Supplement block, that is, between 128 and 255. Without a locale specified, unlike all other characters or code
points, these characters have very different semantics in byte semantics versus character semantics.
In character semantics they are interpreted as Unicode code points, which means they have the same semantics as Latin-1
(ISO-8859-1).
In byte semantics, they are considered to be unassigned characters, meaning that the only semantics they have is their
ordinal numbers, and that they are not members of various character classes. None are considered to match "\w" for
example, but all match "\W". (On EBCDIC platforms, the behavior may be different from this, depending on the underlying
C language library functions.)
The behavior is known to have effects on these areas:
o Changing the case of a scalar, that is, using "uc()", "ucfirst()", "lc()", and "lcfirst()", or "\L", "\U", "\u" and
"\l" in regular expression substitutions.
o Using caseless ("/i") regular expression matching
o Matching a number of properties in regular expressions, such as "\w"
o User-defined case change mappings. You can create a "ToUpper()" function, for example, which overrides Perl's built-
in case mappings. The scalar must be encoded in utf8 for your function to actually be invoked.
This behavior can lead to unexpected results in which a string's semantics suddenly change if a code point above 255 is
appended to or removed from it, which changes the string's semantics from byte to character or vice versa. As an
example, consider the following program and its output:
$ perl -le'
$s1 = "\xC2";
$s2 = "\x{2660}";
for ($s1, $s2, $s1.$s2) {
print /\w/ || 0;
}
'
0
0
1
If there's no "\w" in "s1" or in "s2", why does their concatenation have one?
This anomaly stems from Perl's attempt to not disturb older programs that didn't use Unicode, and hence had no semantics
for characters outside of the ASCII range (except in a locale), along with Perl's desire to add Unicode support
seamlessly. The result wasn't seamless: these characters were orphaned.
Work is being done to correct this, but only some of it was complete in time for the 5.12 release. What has been
finished is the important part of the case changing component. Due to concerns, and some evidence, that older code might
have come to rely on the existing behavior, the new behavior must be explicitly enabled by the feature "unicode_strings"
in the feature pragma, even though no new syntax is involved.
See "lc" in perlfunc for details on how this pragma works in combination with various others for casing. Even though the
pragma only affects casing operations in the 5.12 release, it is planned to have it affect all the problematic behaviors
in later releases: you can't have one without them all.
In the meantime, a workaround is to always call utf8::upgrade($string), or to use the standard module Encode. Also, a
scalar that has any characters whose ordinal is above 0x100, or which were specified using either of the "\N{...}"
notations will automatically have character semantics.
Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
Sometimes (see "When Unicode Does Not Happen" or "The "Unicode Bug"") there are situations where you simply need to force
a byte string into UTF-8, or vice versa. The low-level calls utf8::upgrade($bytestring) and
utf8::downgrade($utf8string[, FAIL_OK]) are the answers.
Note that utf8::downgrade() can fail if the string contains characters that don't fit into a byte.
Calling either function on a string that already is in the desired state is a no-op.
Using Unicode in XS
If you want to handle Perl Unicode in XS extensions, you may find the following C APIs useful. See also "Unicode
Support" in perlguts for an explanation about Unicode at the XS level, and perlapi for the API details.
o "DO_UTF8(sv)" returns true if the "UTF8" flag is on and the bytes pragma is not in effect. "SvUTF8(sv)" returns true
if the "UTF8" flag is on; the bytes pragma is ignored. The "UTF8" flag being on does not mean that there are any
characters of code points greater than 255 (or 127) in the scalar or that there are even any characters in the
scalar. What the "UTF8" flag means is that the sequence of octets in the representation of the scalar is the
sequence of UTF-8 encoded code points of the characters of a string. The "UTF8" flag being off means that each octet
in this representation encodes a single character with code point 0..255 within the string. Perl's Unicode model is
not to use UTF-8 until it is absolutely necessary.
o "uvchr_to_utf8(buf, chr)" writes a Unicode character code point into a buffer encoding the code point as UTF-8, and
returns a pointer pointing after the UTF-8 bytes. It works appropriately on EBCDIC machines.
o "utf8_to_uvchr(buf, lenp)" reads UTF-8 encoded bytes from a buffer and returns the Unicode character code point and,
optionally, the length of the UTF-8 byte sequence. It works appropriately on EBCDIC machines.
o "utf8_length(start, end)" returns the length of the UTF-8 encoded buffer in characters. "sv_len_utf8(sv)" returns
the length of the UTF-8 encoded scalar.
o "sv_utf8_upgrade(sv)" converts the string of the scalar to its UTF-8 encoded form. "sv_utf8_downgrade(sv)" does the
opposite, if possible. "sv_utf8_encode(sv)" is like sv_utf8_upgrade except that it does not set the "UTF8" flag.
"sv_utf8_decode()" does the opposite of "sv_utf8_encode()". Note that none of these are to be used as general-
purpose encoding or decoding interfaces: "use Encode" for that. "sv_utf8_upgrade()" is affected by the encoding
pragma but "sv_utf8_downgrade()" is not (since the encoding pragma is designed to be a one-way street).
o is_utf8_char(s) returns true if the pointer points to a valid UTF-8 character.
o "is_utf8_string(buf, len)" returns true if "len" bytes of the buffer are valid UTF-8.
o "UTF8SKIP(buf)" will return the number of bytes in the UTF-8 encoded character in the buffer. "UNISKIP(chr)" will
return the number of bytes required to UTF-8-encode the Unicode character code point. "UTF8SKIP()" is useful for
example for iterating over the characters of a UTF-8 encoded buffer; "UNISKIP()" is useful, for example, in computing
the size required for a UTF-8 encoded buffer.
o "utf8_distance(a, b)" will tell the distance in characters between the two pointers pointing to the same UTF-8
encoded buffer.
o "utf8_hop(s, off)" will return a pointer to a UTF-8 encoded buffer that is "off" (positive or negative) Unicode
characters displaced from the UTF-8 buffer "s". Be careful not to overstep the buffer: "utf8_hop()" will merrily run
off the end or the beginning of the buffer if told to do so.
o "pv_uni_display(dsv, spv, len, pvlim, flags)" and "sv_uni_display(dsv, ssv, pvlim, flags)" are useful for debugging
the output of Unicode strings and scalars. By default they are useful only for debugging--they display all
characters as hexadecimal code points--but with the flags "UNI_DISPLAY_ISPRINT", "UNI_DISPLAY_BACKSLASH", and
"UNI_DISPLAY_QQ" you can make the output more readable.
o "ibcmp_utf8(s1, pe1, l1, u1, s2, pe2, l2, u2)" can be used to compare two strings case-insensitively in Unicode. For
case-sensitive comparisons you can just use "memEQ()" and "memNE()" as usual.
For more information, see perlapi, and utf8.c and utf8.h in the Perl source code distribution.
Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
Perl by default comes with the latest supported Unicode version built in, but you can change to use any earlier one.
Download the files in the version of Unicode that you want from the Unicode web site <http://www.unicode.org>;). These
should replace the existing files in "\$Config{privlib}"/unicore. ("\%Config" is available from the Config module.)
Follow the instructions in README.perl in that directory to change some of their names, and then run make.
It is even possible to download them to a different directory, and then change utf8_heavy.pl in the directory
"\$Config{privlib}" to point to the new directory, or maybe make a copy of that directory before making the change, and
using @INC or the "-I" run-time flag to switch between versions at will (but because of caching, not in the middle of a
process), but all this is beyond the scope of these instructions.
BUGS
Interaction with Locales
Use of locales with Unicode data may lead to odd results. Currently, Perl attempts to attach 8-bit locale info to
characters in the range 0..255, but this technique is demonstrably incorrect for locales that use characters above that
range when mapped into Unicode. Perl's Unicode support will also tend to run slower. Use of locales with Unicode is
discouraged.
Problems with characters in the Latin-1 Supplement range
See "The "Unicode Bug""
Problems with case-insensitive regular expression matching
There are problems with case-insensitive matches, including those involving character classes (enclosed in [square
brackets]), characters whose fold is to multiple characters (such as the single character LATIN SMALL LIGATURE FFL
matches case-insensitively with the 3-character string "ffl"), and characters in the Latin-1 Supplement.
Interaction with Extensions
When Perl exchanges data with an extension, the extension should be able to understand the UTF8 flag and act accordingly.
If the extension doesn't know about the flag, it's likely that the extension will return incorrectly-flagged data.
So if you're working with Unicode data, consult the documentation of every module you're using if there are any issues
with Unicode data exchange. If the documentation does not talk about Unicode at all, suspect the worst and probably look
at the source to learn how the module is implemented. Modules written completely in Perl shouldn't cause problems.
Modules that directly or indirectly access code written in other programming languages are at risk.
For affected functions, the simple strategy to avoid data corruption is to always make the encoding of the exchanged data
explicit. Choose an encoding that you know the extension can handle. Convert arguments passed to the extensions to that
encoding and convert results back from that encoding. Write wrapper functions that do the conversions for you, so you can
later change the functions when the extension catches up.
To provide an example, let's say the popular Foo::Bar::escape_html function doesn't deal with Unicode data yet. The
wrapper function would convert the argument to raw UTF-8 and convert the result back to Perl's internal representation
like so:
sub my_escape_html ($) {
my($what) = shift;
return unless defined $what;
Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
}
Sometimes, when the extension does not convert data but just stores and retrieves them, you will be in a position to use
the otherwise dangerous Encode::_utf8_on() function. Let's say the popular "Foo::Bar" extension, written in C, provides a
"param" method that lets you store and retrieve data according to these prototypes:
$self->param($name, $value); # set a scalar
$value = $self->param($name); # retrieve a scalar
If it does not yet provide support for any encoding, one could write a derived class with such a "param" method:
sub param {
my($self,$name,$value) = @_;
utf8::upgrade($name); # make sure it is UTF-8 encoded
if (defined $value) {
utf8::upgrade($value); # make sure it is UTF-8 encoded
return $self->SUPER::param($name,$value);
} else {
my $ret = $self->SUPER::param($name);
Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
return $ret;
}
}
Some extensions provide filters on data entry/exit points, such as DB_File::filter_store_key and family. Look out for
such filters in the documentation of your extensions, they can make the transition to Unicode data much easier.
Speed
Some functions are slower when working on UTF-8 encoded strings than on byte encoded strings. All functions that need to
hop over characters such as length(), substr() or index(), or matching regular expressions can work much faster when the
underlying data are byte-encoded.
In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a caching scheme was introduced which will
hopefully make the slowness somewhat less spectacular, at least for some operations. In general, operations with UTF-8
encoded strings are still slower. As an example, the Unicode properties (character classes) like "\p{Nd}" are known to be
quite a bit slower (5-20 times) than their simpler counterparts like "\d" (then again, there 268 Unicode characters
matching "Nd" compared with the 10 ASCII characters matching "d").
Problems on EBCDIC platforms
There are a number of known problems with Perl on EBCDIC platforms. If you want to use Perl there, send email to
perlbugATperl.org.
In earlier versions, when byte and character data were concatenated, the new string was sometimes created by decoding the
byte strings as ISO 8859-1 (Latin-1), even if the old Unicode string used EBCDIC.
If you find any of these, please report them as bugs.
Porting code from perl-5.6.X
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer was required to use the "utf8" pragma to declare
that a given scope expected to deal with Unicode data and had to make sure that only Unicode data were reaching that
scope. If you have code that is working with 5.6, you will need some of the following adjustments to your code. The
examples are written such that the code will continue to work under 5.6, so you should be safe to try them out.
o A filehandle that should read or write UTF-8
if ($] > 5.007) {
binmode $fh, ":encoding(utf8)";
}
o A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension that has no mention of Unicode in the manpage, you need to
make sure that the UTF8 flag is stripped off. Note that at the time of this writing (October 2002) the mentioned
modules are not UTF-8-aware. Please check the documentation to verify if this is still true.
if ($] > 5.007) {
require Encode;
$val = Encode::encode_utf8($val); # make octets
}
o A scalar we got back from an extension
If you believe the scalar comes back as UTF-8, you will most likely want the UTF8 flag restored:
if ($] > 5.007) {
require Encode;
$val = Encode::decode_utf8($val);
}
o Same thing, if you are really sure it is UTF-8
if ($] > 5.007) {
require Encode;
Encode::_utf8_on($val);
}
o A wrapper for fetchrow_array and fetchrow_hashref
When the database contains only UTF-8, a wrapper function or method is a convenient way to replace all your
fetchrow_array and fetchrow_hashref calls. A wrapper function will also make it easier to adapt to future
enhancements in your database driver. Note that at the time of this writing (October 2002), the DBI has no
standardized way to deal with UTF-8 data. Please check the documentation to verify if that is still true.
sub fetchrow {
my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref}
if ($] < 5.007) {
return $sth->$what;
} else {
require Encode;
if (wantarray) {
my @arr = $sth->$what;
for (@arr) {
defined && /[^\000-\177]/ && Encode::_utf8_on($_);
}
return @arr;
} else {
my $ret = $sth->$what;
if (ref $ret) {
for my $k (keys %$ret) {
defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret->{$k};
}
return $ret;
} else {
defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
return $ret;
}
}
}
}
o A large scalar that you know can only contain ASCII
Scalars that contain only ASCII and are marked as UTF-8 are sometimes a drag to your program. If you recognize such a
situation, just remove the UTF8 flag:
utf8::downgrade($val) if $] > 5.007;
SEE ALSO
perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes, perlretut, "${^UNICODE}" in perlvar
<http://www.unicode.org/reports/tr44>;).
perl v5.12.4 2011-06-07 PERLUNICODE(1)
