perlunicode
(1)
Name
perlunicode - Unicode support in Perl
Synopsis
Please see following description for synopsis
Description
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
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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.
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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
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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
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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
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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
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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
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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
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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.
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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
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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}".
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"\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{Dt=NonCanon}")
"\p{Decomposition_Type: Non_Canonical}" (Short:
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
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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]".
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"\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
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"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}"
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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
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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.
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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 "lc()",
"lcfirst()", "uc()", and "ucfirst()" (or their string-
inlined versions, "\L", "\l", "\U", and "\u"). The
principle is similar to that of user-defined character
properties: to define subroutines with names "ToLower" (for
"lc()" and "lcfirst()"); "ToTitle" (for "ucfirst()"); and
"ToUpper" (for "uc()").
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 a mapping for "uc()" (and "\U") 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. The "utf8::ToSpecFoo" hashes in those
files 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.
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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:
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(?!\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
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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
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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
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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
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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
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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
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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
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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($string), or to use the standard module
Encode. Also, a scalar that has any characters whose
ordinal is above 0x100, or which were specified using either
of the "\N{...}" notations will automatically have character
semantics.
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Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
Sometimes (see "When Unicode Does Not Happen" or "The
"Unicode Bug"") there are situations where you simply need
to force a byte string into UTF-8, or vice versa. The low-
level calls utf8::upgrade($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"
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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.
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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
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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:
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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
perlbug@perl.org.
In earlier versions, when byte and character data were
concatenated, the new string was sometimes created by
decoding the byte strings as 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
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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
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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 $] > 5.007;
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perlunitut, perluniintro, perluniprops, Encode, open, utf8,
bytes, perlretut, "${^UNICODE}" in perlvar
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<http://www.unicode.org/reports/tr44>).
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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