PERLUNICODE(1) Perl Programmers Reference Guide PERLUNICODE(1)NAMEperlunicode - 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.
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 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:
· 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.
· 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.
· Regular expressions match characters instead of bytes. "." matches
a character instead of a byte.
· 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.
· Named Unicode properties, scripts, and block ranges may be used
like character classes via 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.
· 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.
· 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', ...).
· 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).
· 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.
· 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.
· 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.
· 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.
· You can define your own mappings to be used in lc(), lcfirst(),
uc(), and ucfirst() (or their string-inlined versions). See "User-
Defined Case Mappings" for more details.
· 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 character classes via the "\p{}"
"matches property" construct and the "\P{}" negation, "doesn't match
property".
For instance, "\p{Uppercase}" matches any 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 properties, so "\p{L}" is equivalent to "\pL".
More formally, "\p{Uppercase}" matches any 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 aliases for the set of "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 can be matched 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:
· A single hexadecimal number denoting a Unicode code point to
include.
· Two hexadecimal numbers separated by horizontal whitespace (space
or tabular characters) denoting a range of Unicode code points to
include.
· 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.
· 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.
· 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.
· 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.
· 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
· 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).
· 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.
· 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.
· UTF-EBCDIC
Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
· 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.)
· 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.
· 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.
· 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:
· 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.
· 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:
· 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.
· 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?
· chdir, chmod, chown, chroot, exec, link, lstat, mkdir, rename,
rmdir, stat, symlink, truncate, unlink, utime, -X
· %ENV
· glob (aka the <*>)
· open, opendir, sysopen
· qx (aka the backtick operator), system
· 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:
· Changing the case of a scalar, that is, using "uc()", "ucfirst()",
"lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in regular
expression substitutions.
· Using caseless ("/i") regular expression matching
· Matching a number of properties in regular expressions, such as
"\w"
· 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.
· "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.
· "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.
· "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.
· "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.
· "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).
· is_utf8_char(s) returns true if the pointer points to a valid UTF-8
character.
· "is_utf8_string(buf, len)" returns true if "len" bytes of the
buffer are valid UTF-8.
· "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.
· "utf8_distance(a, b)" will tell the distance in characters between
the two pointers pointing to the same UTF-8 encoded buffer.
· "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.
· "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.
· "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 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 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.
· A filehandle that should read or write UTF-8
if ($] > 5.007) {
binmode $fh, ":encoding(utf8)";
}
· A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension that has no
mention of Unicode in the manpage, you need to make sure that the
UTF8 flag is stripped off. Note that at the time of this writing
(October 2002) the mentioned modules are not UTF-8-aware. Please
check the documentation to verify if this is still true.
if ($] > 5.007) {
require Encode;
$val = Encode::encode_utf8($val); # make octets
}
· 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);
}
· Same thing, if you are really sure it is UTF-8
if ($] > 5.007) {
require Encode;
Encode::_utf8_on($val);
}
· 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 $ret;
}
}
}
}
· A large scalar that you know can only contain ASCII
Scalars that contain only ASCII and are marked as UTF-8 are
sometimes a drag to your program. If you recognize such a
situation, just remove the UTF8 flag:
utf8::downgrade($val) if $] > 5.007;
SEE ALSO
perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes,
perlretut, "${^UNICODE}" in perlvar
<http://www.unicode.org/reports/tr44>).
perl v5.12.2 2010-09-06 PERLUNICODE(1)
