PERLUNICODE(1) Perl Programmers Reference Guide PERLUNICODE(1)
NAME
perlunicode - Unicode support in Perl
DESCRIPTION
Important Caveats
Unicode support is an extensive requirement. While Perl does not
implement the Unicode standard or the accompanying technical reports
from cover to cover, Perl does support many Unicode features.
People who want to learn to use Unicode in Perl, should probably read
the Perl Unicode tutorial, perlunitut and perluniintro, 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>;.
Safest if you "use feature 'unicode_strings'"
In order to preserve backward compatibility, Perl does not turn on
full internal Unicode support unless the pragma "use feature
'unicode_strings'" is specified. (This is automatically selected
if you use "use 5.012" or higher.) Failure to do this can trigger
unexpected surprises. See "The "Unicode Bug"" below.
This pragma doesn't affect I/O, and there are still several places
where Unicode isn't fully supported, such as in filenames.
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 ":encoding(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;".
"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.
Starting in Perl 5.14, Perl-level operations work with characters
rather than bytes within the scope of a "use feature 'unicode_strings'"
(or equivalently "use 5.012" or higher). (This is not true if bytes
have been explicitly requested by "use bytes", nor necessarily true for
interactions with the platform's operating system.)
For earlier Perls, and when "unicode_strings" is not in effect, Perl
provides a fairly safe environment that can handle both types of
semantics in 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.
When "use locale" (but not "use locale ':not_characters'") is in
effect, Perl uses the semantics associated with the current locale.
("use locale" overrides "use feature 'unicode_strings'" in the same
scope; while "use locale ':not_characters'" effectively also selects
"use feature 'unicode_strings'" in its scope; see perllocale.)
Otherwise, Perl uses the platform's native byte semantics for
characters whose code points are less than 256, and Unicode semantics
for those greater than 255. On EBCDIC platforms, this is almost
seamless, as the EBCDIC code pages that Perl handles are equivalent to
Unicode's first 256 code points. (The exception is that EBCDIC regular
expression case-insensitive matching rules are not as as robust as
Unicode's.) But on ASCII platforms, Perl 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 "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.
If strings operating under byte semantics and strings with Unicode
character data are concatenated, the new string will have character
semantics. This can cause surprises: See "BUGS", below. You can
choose to be warned when this happens. See encoding::warnings.
Under character semantics, many operations that formerly operated on
bytes now operate on characters. A character in Perl is logically just
a number ranging from 0 to 2**31 or so. Larger characters may encode
into longer sequences of bytes internally, but this internal detail is
mostly hidden for Perl code. See perluniintro for more.
Effects of Character Semantics
Character semantics have the following effects:
o Strings--including hash keys--and regular expression patterns may
contain characters that have an ordinal value larger than 255.
If you use a Unicode editor to edit your program, Unicode
characters may occur directly within the literal strings in UTF-8
encoding, or UTF-16. (The former requires a BOM or "use utf8", the
latter requires a BOM.)
Unicode characters can also be added to a string by using the
"\N{U+...}" notation. The Unicode code for the desired character,
in hexadecimal, should be placed in the braces, after the "U". For
instance, a smiley face is "\N{U+263A}".
Alternatively, you can use the "\x{...}" notation for characters
0x100 and above. For characters below 0x100 you may get byte
semantics instead of character semantics; see "The "Unicode Bug"".
On EBCDIC machines there is the additional problem that the value
for such characters gives the EBCDIC character rather than the
Unicode one, thus it is more portable to use "\N{U+...}" instead.
Additionally, you can use the "\N{...}" notation and put the
official Unicode character name within the braces, such as
"\N{WHITE SMILING FACE}". This automatically loads the charnames
module with the ":full" and ":short" options. If you prefer
different options for this module, you can instead, before the
"\N{...}", explicitly load it with your desired options; for
example,
use charnames ':loose';
o If an appropriate encoding is specified, identifiers within the
Perl script may contain Unicode alphanumeric characters, including
ideographs. Perl does not currently attempt to canonicalize
variable names.
o Regular expressions match characters instead of bytes. "." matches
a character instead of a byte.
o Bracketed character classes in regular expressions match characters
instead of bytes and match against the character properties
specified in the Unicode properties database. "\w" can be used to
match a Japanese ideograph, for instance.
o Named Unicode properties, scripts, and block ranges may be used
(like bracketed character classes) by using the "\p{}" "matches
property" construct and the "\P{}" negation, "doesn't match
property". See "Unicode Character Properties" for more details.
You can define your own character properties and use them in the
regular expression with the "\p{}" or "\P{}" construct. See "User-
Defined Character Properties" for more details.
o The special pattern "\X" matches a logical character, an "extended
grapheme cluster" in Standardese. In Unicode what appears to the
user to be a single character, for example an accented "G", may in
fact be composed of a sequence of characters, in this case a "G"
followed by an accent character. "\X" will match the entire
sequence.
o The "tr///" operator translates characters instead of bytes. Note
that the "tr///CU" functionality has been removed. For similar
functionality see pack('U0', ...) and pack('C0', ...).
o Case translation operators use the Unicode case translation tables
when character input is provided. Note that "uc()", or "\U" in
interpolated strings, translates to uppercase, while "ucfirst", or
"\u" in interpolated strings, translates to titlecase in languages
that make the distinction (which is equivalent to uppercase in
languages without the distinction).
o Most operators that deal with positions or lengths in a string will
automatically switch to using character positions, including
"chop()", "chomp()", "substr()", "pos()", "index()", "rindex()",
"sprintf()", "write()", and "length()". An operator that
specifically does not switch is "vec()". Operators that really
don't care include operators that treat strings as a bucket of bits
such as "sort()", and operators dealing with filenames.
o The "pack()"/"unpack()" letter "C" does not change, since it is
often used for byte-oriented formats. Again, think "char" in the C
language.
There is a new "U" specifier that converts between Unicode
characters and code points. There is also a "W" specifier that is
the equivalent of "chr"/"ord" and properly handles character values
even if they are above 255.
o The "chr()" and "ord()" functions work on characters, similar to
"pack("W")" and "unpack("W")", not "pack("C")" and "unpack("C")".
"pack("C")" and "unpack("C")" are methods for emulating byte-
oriented "chr()" and "ord()" on Unicode strings. While these
methods reveal the internal encoding of Unicode strings, that is
not something one normally needs to care about at all.
o The bit string operators, "& | ^ ~", can operate on character data.
However, for backward compatibility, such as when using bit string
operations when characters are all less than 256 in ordinal value,
one should not use "~" (the bit complement) with characters of both
values less than 256 and values greater than 256. Most
importantly, DeMorgan's laws ("~($x|$y) eq ~$x&~$y" and "~($x&$y)
eq ~$x|~$y") will not hold. The reason for this mathematical faux
pas is that the complement cannot return both the 8-bit (byte-wide)
bit complement and the full character-wide bit complement.
o There is a CPAN module, Unicode::Casing, which allows you to define
your own mappings to be used in "lc()", "lcfirst()", "uc()",
"ucfirst()", and "fc" (or their double-quoted string inlined
versions such as "\U"). (Prior to Perl 5.16, this functionality
was partially provided in the Perl core, but suffered from a number
of insurmountable drawbacks, so the CPAN module was written
instead.)
o And finally, "scalar reverse()" reverses by character rather than
by byte.
Unicode Character Properties
(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.)
Very nearly all Unicode character properties are accessible through
regular expressions by using the "\p{}" "matches property" construct
and the "\P{}" "doesn't match property" for its negation.
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
several different values, such as Left, Right, Whitespace, and others.
To match these, one needs to specify both 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 that is more descriptive and hence easier to understand. Thus the
"L" and "Letter" properties 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 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, white space and even hyphens can usually 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 used is in the middle of numbers, and in the Perl
extension properties that begin or end with an underscore. Stricter
matching cares about white space (except adjacent to non-word
characters), 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}".
Almost all properties are immune to case-insensitive matching. That
is, adding a "/i" regular expression modifier does not change what they
match. There are two sets that are affected. The first set is
"Uppercase_Letter", "Lowercase_Letter", and "Titlecase_Letter", all of
which match "Cased_Letter" under "/i" matching. And the second set is
"Uppercase", "Lowercase", and "Titlecase", all of which match "Cased"
under "/i" matching. This set also includes its subsets "PosixUpper"
and "PosixLower" both of which under "/i" matching match "PosixAlpha".
(The difference between these sets is that some things, such as Roman
numerals, come in both upper and lower case so they are "Cased", but
aren't considered letters, so they aren't "Cased_Letter"s.)
The result is undefined if you try to match a non-Unicode code point
(that is, one above 0x10FFFF) against a Unicode property. Currently, a
warning is raised, and the match will fail. In some cases, this is
counterintuitive, as both these fail:
chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails.
chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Fails!
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
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: both are aliases for the set consisting of everything matched
by "Ll", "Lu", and "Lt".
Bidirectional Character Types
Because scripts differ in their directionality (Hebrew and Arabic are
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 many different scripts. This
sentence (unless you're reading it in translation) is written in Latin,
while Russian is written in Cyrillic, and Greek is written in, well,
Greek; Japanese mainly in Hiragana or Katakana. There are many more.
The Unicode Script and Script_Extensions properties give what script a
given character is in. Either property can be specified with the
compound form like "\p{Script=Hebrew}" (short: "\p{sc=hebr}"), or
"\p{Script_Extensions=Javanese}" (short: "\p{scx=java}"). In addition,
Perl furnishes shortcuts for all "Script" property names. You can omit
everything up through the equals (or colon), and simply write
"\p{Latin}" or "\P{Cyrillic}". (This is not true for
"Script_Extensions", which is required to be written in the compound
form.)
The difference between these two properties involves characters that
are used in multiple scripts. For example the digits '0' through '9'
are used in many parts of the world. These are placed in a script
named "Common". Other characters are used in just a few scripts. For
example, the "KATAKANA-HIRAGANA DOUBLE HYPHEN" is used in both Japanese
scripts, Katakana and Hiragana, but nowhere else. The "Script"
property places all characters that are used in multiple scripts in the
"Common" script, while the "Script_Extensions" property places those
that are used in only a few scripts into each of those scripts; while
still using "Common" for those used in many scripts. Thus both these
match:
"0" =~ /\p{sc=Common}/ # Matches
"0" =~ /\p{scx=Common}/ # Matches
and only the first of these match:
"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
And only the last two of these match:
"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
"\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
"Script_Extensions" is thus an improved "Script", in which there are
fewer characters in the "Common" script, and correspondingly more in
other scripts. It is new in Unicode version 6.0, and its data are
likely to change significantly in later releases, as things get sorted
out.
(Actually, besides "Common", the "Inherited" script, contains
characters that are used in multiple scripts. These are modifier
characters which modify other characters, and inherit the script value
of the controlling character. Some of these are used in many scripts,
and so go into "Inherited" in both "Script" and "Script_Extensions".
Others are used in just a few scripts, so are in "Inherited" in
"Script", but not in "Script_Extensions".)
It is worth stressing that there are several different sets of digits
in Unicode that are equivalent to 0-9 and are matchable by "\d" in a
regular expression. If they are used in a single language only, they
are in that language's "Script" and "Script_Extension". If they are
used in more than one script, they will be in "sc=Common", but only if
they are used in many scripts should they be in "scx=Common".
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 as well as several other blocks, 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 the digits 0-9,
because those digits are shared across many scripts, and hence are in
the "Common" script.
For more about scripts versus blocks, see UAX#24 "Unicode Script
Property": <http://www.unicode.org/reports/tr24>;
The "Script" or "Script_Extensions" properties are likely to be the
ones you want to use when processing natural language; the Block
property may occasionally 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 prefer to always use "\p{Block: foo}" and "\p{Script: bar}"
instead of the shortcuts, whether for clarity, because they can't
remember the difference between 'In' and 'Is' anyway, or they aren't
confident that those who eventually will read their code will know that
difference.
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. Most of these are just synonyms
for the Unicode ones, but some are genuine extensions, including
several 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 extensions that aren't just
synonyms for compound-form Unicode properties (for those properties,
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{ASCII}"
This matches any of the 128 characters in the US-ASCII character
set, which is a subset of Unicode.
"\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
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 these can be
variously combined 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" regular
expression construct to match such sequences.
But Unicode's intent is to unify the existing character set
standards and practices, and several 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 the character "LATIN CAPITAL LETTER E" followed by
the character "COMBINING ACUTE ACCENT".
"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed"
character, and its 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
somewhat 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".
For your convenience, Perl has added the "Non_Canonical"
decomposition type to mean any of the several 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{Posix...}"
There are several of these, which are equivalents using the "\p"
notation for Posix classes and are described in "POSIX Character
Classes" in perlrecharclass.
"\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 in which the decision to make them so was made.
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, as also does "\p{Present_In:
3.1}" and up.
"\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
white space.)
"\p{Title}" and "\p{Titlecase}"
Under case-sensitive matching, these both match the same code
points as "\p{General Category=Titlecase_Letter}" ("\p{gc=lt}").
The difference is that under "/i" caseless matching, these match
the same as "\p{Cased}", whereas "\p{gc=lt}" matches
"\p{Cased_Letter").
"\p{VertSpace}"
This is the same as "\v": A character that changes the spacing
vertically.
"\p{Word}"
This is the same as "\w", including over 100_000 characters beyond
ASCII.
"\p{XPosix...}"
There are several of these, which are the standard Posix classes
extended to the full Unicode range. They are described in "POSIX
Character Classes" in perlrecharclass.
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.
However, the subroutines are passed a single parameter, which is 0 if
case-sensitive matching is in effect and non-zero if caseless matching
is in effect. The subroutine may return different values depending on
the value of the flag, and one set of values will immutably be in
effect for all case-sensitive matches, and the other set for all case-
insensitive matches.
Note that if the regular expression is tainted, then Perl will die
rather than calling the subroutine, where the name of the subroutine is
determined by the tainted data.
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 fully qualified (including
package name) 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 fully qualified (including
package name) 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 fully qualified (including package
name) 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 fully qualified (including
package name) 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
}
This will match all non-Unicode code points, since every one of them is
not in Kana. You can use intersection to exclude these, if desired, as
this modified example shows:
sub InNotKana {
return <<'END';
!utf8::InHiragana
-utf8::InKatakana
+utf8::IsCn
&utf8::Any
END
}
&utf8::Any must be the last line in the definition.
Intersection is used generally for getting the common characters
matched by two (or more) classes. It's important to remember not to
use "&" for the first set; that would be intersecting with nothing,
resulting in an empty set.
(Note that official Unicode properties differ from these in that they
automatically exclude non-Unicode code points and a warning is raised
if a match is attempted on one of those.)
User-Defined Case Mappings (for serious hackers only)
This feature has been removed as of Perl 5.16. The CPAN module
Unicode::Casing provides better functionality without the drawbacks
that this feature had. If you are using a Perl earlier than 5.16, this
feature was most fully documented in the 5.14 version of this pod:
http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29
<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-
Mappings-%28for-serious-hackers-only%29>
Character Encodings for Input and Output
See Encode.
Unicode Regular Expression Support Level
The following list of Unicode supported features for regular
expressions describes all features currently directly supported by core
Perl. The references to "Level N" and the section numbers refer to the
Unicode Technical Standard #18, "Unicode Regular Expressions", version
13, from August 2008.
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][9]
RL1.7 Supplementary Code Points - done [10]
[1] \x{...}
[2] \p{...} \P{...}
[3] supports not only minimal list, but all Unicode character
properties (see Unicode Character Properties above)
[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, instead of just U+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] Linebreaking conformant with UAX#14 "Unicode Line Breaking
Algorithm" is available through the Unicode::LineBreaking
module.
[10] UTF-8/UTF-EBDDIC used in Perl allows not only U+10000 to
U+10FFFF but also beyond U+10FFFF
[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).
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 - DONE
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] This is covered in Chapter 3.13 (in Unicode 6.0)
o Level 3 - Tailored Support
RL3.1 Tailored Punctuation - MISSING
RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
RL3.3 Tailored Word Boundaries - MISSING
RL3.4 Tailored Loose Matches - MISSING
RL3.5 Tailored Ranges - MISSING
RL3.6 Context Matching - MISSING [19]
RL3.7 Incremental Matches - MISSING
( RL3.8 Unicode Set Sharing )
RL3.9 Possible Match Sets - MISSING
RL3.10 Folded Matching - MISSING [20]
RL3.11 Submatchers - MISSING
[17] see UAX#10 "Unicode Collation Algorithms"
[18] have Unicode::Collate but not integrated to regexes
[19] have (?<=x) and (?=x), but look-aheads or look-behinds
should see outside of the target substring
[20] need insensitive matching for linguistic features other
than case; for example, hiragana to katakana, wide and
narrow, simplified Han to traditional Han (see UTR#30
"Character Foldings")
Unicode Encodings
Unicode characters are assigned to code points, which are abstract
numbers. To use these numbers, various encodings are needed.
o UTF-8
UTF-8 is a variable-length (1 to 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 marked by "*" before several of the byte entries
above. 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.
The original UTF-8 specification allowed up to 6 bytes, to allow
encoding of numbers up to 0x7FFF_FFFF. Perl continues to allow
those, and has extended that up to 13 bytes to encode code points
up to what can fit in a 64-bit word. However, Perl will warn if
you output any of these as being non-portable; and under strict
UTF-8 input protocols, they are forbidden.
The Unicode non-character code points are also disallowed in UTF-8
in "open interchange". See "Non-character code points".
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.
Like UTF-8, UTF-16 is a variable-width encoding, but where UTF-8
uses 8-bit code units, UTF-16 uses 16-bit code units. All code
points occupy either 2 or 4 bytes in UTF-16: code points
"U+0000..U+FFFF" are stored in a single 16-bit unit, and 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);
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 not supposed to be in input streams, 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".
Surrogates have no meaning in Unicode outside their use in pairs to
represent other code points. However, Perl allows them to be
represented individually internally, for example by saying
"chr(0xD801)", so that all code points, not just those valid for
open interchange, are representable. Unicode does define semantics
for them, such as their General Category is "Cs". But because
their use is somewhat dangerous, Perl will warn (using the warning
category "surrogate", which is a sub-category of "utf8") if an
attempt is made to do things like take the lower case of one, or
match case-insensitively, or to output them. (But don't try this
on Perls before 5.14.)
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. UTF-32 is a fixed-width encoding. The BOM signatures
are "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00 0x00" for LE.
o UCS-2, UCS-4
Legacy, fixed-width 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 (the difference
being that UCS-4 forbids neither surrogates nor code points larger
than 0x10_FFFF).
o UTF-7
A seven-bit safe (non-eight-bit) encoding, which is useful if the
transport or storage is not eight-bit safe. Defined by RFC 2152.
Non-character code points
66 code points are set aside in Unicode as "non-character code points".
These all have the Unassigned (Cn) General Category, and they never
will be assigned. These are never supposed to be in legal Unicode
input streams, so that code can use them as sentinels that can be mixed
in with character data, and they always will be distinguishable from
that data. To keep them out of Perl input streams, strict UTF-8 should
be specified, such as by using the layer ":encoding('UTF-8')". The
non-character code points are the 32 between U+FDD0 and U+FDEF, and the
34 code points U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE,
U+10FFFF. Some people are under the mistaken impression that these are
"illegal", but that is not true. An application or cooperating set of
applications can legally use them at will internally; but these code
points are "illegal for open interchange". Therefore, Perl will not
accept these from input streams unless lax rules are being used, and
will warn (using the warning category "nonchar", which is a sub-
category of "utf8") if an attempt is made to output them.
Beyond Unicode code points
The maximum Unicode code point is U+10FFFF. But Perl accepts code
points up to the maximum permissible unsigned number available on the
platform. However, Perl will not accept these from input streams
unless lax rules are being used, and will warn (using the warning
category "non_unicode", which is a sub-category of "utf8") if an
attempt is made to operate on or output them. For example,
"uc(0x11_0000)" will generate this warning, returning the input
parameter as its result, as the upper case of every non-Unicode code
point is the code point itself.
Security Implications of Unicode
Read Unicode Security Considerations
<http://www.unicode.org/reports/tr36>;. Also, note the following:
o Malformed UTF-8
Unfortunately, the original 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 Unicode code points valid for
interchange.
o Regular expression pattern matching may surprise you if you're not
accustomed to Unicode. Starting in Perl 5.14, several pattern
modifiers are available to control this, called the character set
modifiers. Details are given in "Character set modifiers" in
perlre.
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 (unless the "/a" modifier is in effect). 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
See "Unicode and UTF-8" in perllocale
When Unicode Does Not Happen
While Perl does have extensive ways to input and output in Unicode, and
a few other "entry points" like the @ARGV array (which can sometimes be
interpreted as UTF-8), there are still 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
(problematic) "encoding" pragma has been used.
One reason that Perl does not attempt to resolve the role of Unicode in
these situations 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 "qx" and "system": how
well will the "command-line interface" (and which of them?) handle
Unicode?
o chdir, chmod, chown, chroot, exec, link, lstat, mkdir, rename,
rmdir, stat, symlink, truncate, unlink, utime, -X
o %ENV
o glob (aka the <*>)
o open, opendir, sysopen
o qx (aka the backtick operator), system
o readdir, readlink
The "Unicode Bug"
The term, "Unicode bug" has been applied to an inconsistency on ASCII
platforms with the Unicode code points 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, unless "use
feature 'unicode_strings'" is specified, directly or indirectly. (It
is indirectly specified by a "use v5.12" or higher.)
In character semantics these upper-Latin1 characters are interpreted as
Unicode code points, which means they have the same semantics as
Latin-1 (ISO-8859-1).
In byte semantics (without "unicode_strings"), 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".
Perl 5.12.0 added "unicode_strings" to force character semantics on
these code points in some circumstances, which fixed portions of the
bug; Perl 5.14.0 fixed almost all of it; and Perl 5.16.0 fixed the
remainder (so far as we know, anyway). The lesson here is to enable
"unicode_strings" to avoid the headaches described below.
The old, problematic behavior affects these areas:
o Changing the case of a scalar, that is, using "uc()", "ucfirst()",
"lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in double-
quotish contexts, such as regular expression substitutions. Under
"unicode_strings" starting in Perl 5.12.0, character semantics are
generally used. See "lc" in perlfunc for details on how this works
in combination with various other pragmas.
o Using caseless ("/i") regular expression matching. Starting in
Perl 5.14.0, regular expressions compiled within the scope of
"unicode_strings" use character semantics even when executed or
compiled into larger regular expressions outside the scope.
o Matching any of several properties in regular expressions, namely
"\b", "\B", "\s", "\S", "\w", "\W", and all the Posix character
classes except "[[:ascii:]]". Starting in Perl 5.14.0, regular
expressions compiled within the scope of "unicode_strings" use
character semantics even when executed or compiled into larger
regular expressions outside the scope.
o In "quotemeta" or its inline equivalent "\Q", no code points above
127 are quoted in UTF-8 encoded strings, but in byte encoded
strings, code points between 128-255 are always quoted. Starting
in Perl 5.16.0, consistent quoting rules are used within the scope
of "unicode_strings", as described in "quotemeta" in perlfunc.
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'
no feature 'unicode_strings';
$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.
For Perls earlier than those described above, or when a string is
passed to a function outside the subpragma's scope, a workaround is to
always call "utf8::upgrade($string)", or to use the standard module
Encode. Also, a scalar that has any characters whose ordinal is above
0x100, or which were specified using either of the "\N{...}" notations,
will automatically have character semantics.
Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
Sometimes (see "When Unicode Does Not Happen" or "The "Unicode Bug"")
there are situations where you simply need to force a byte string into
UTF-8, or vice versa. The low-level calls utf8::upgrade($bytestring)
and utf8::downgrade($utf8string[, FAIL_OK]) are the answers.
Note that utf8::downgrade() can fail if the string contains characters
that don't fit into a byte.
Calling either function on a string that already is in the desired
state is a no-op.
Using Unicode in XS
If you want to handle Perl Unicode in XS extensions, you may find the
following C APIs useful. See also "Unicode Support" in perlguts for an
explanation about Unicode at the XS level, and perlapi for the API
details.
o "DO_UTF8(sv)" returns true if the "UTF8" flag is on and the bytes
pragma is not in effect. "SvUTF8(sv)" returns true if the "UTF8"
flag is on; the bytes pragma is ignored. The "UTF8" flag being on
does not mean that there are any characters of code points greater
than 255 (or 127) in the scalar or that there are even any
characters in the scalar. What the "UTF8" flag means is that the
sequence of octets in the representation of the scalar is the
sequence of UTF-8 encoded code points of the characters of a
string. The "UTF8" flag being off means that each octet in this
representation encodes a single character with code point 0..255
within the string. Perl's Unicode model is not to use UTF-8 until
it is absolutely necessary.
o "uvchr_to_utf8(buf, chr)" writes a Unicode character code point
into a buffer encoding the code point as UTF-8, and returns a
pointer pointing after the UTF-8 bytes. It works appropriately on
EBCDIC machines.
o "utf8_to_uvchr_buf(buf, bufend, lenp)" reads UTF-8 encoded bytes
from a buffer and returns the Unicode character code point and,
optionally, the length of the UTF-8 byte sequence. It works
appropriately on EBCDIC machines.
o "utf8_length(start, end)" returns the length of the UTF-8 encoded
buffer in characters. "sv_len_utf8(sv)" returns the length of the
UTF-8 encoded scalar.
o "sv_utf8_upgrade(sv)" converts the string of the scalar to its
UTF-8 encoded form. "sv_utf8_downgrade(sv)" does the opposite, if
possible. "sv_utf8_encode(sv)" is like sv_utf8_upgrade except that
it does not set the "UTF8" flag. "sv_utf8_decode()" does the
opposite of "sv_utf8_encode()". Note that none of these are to be
used as general-purpose encoding or decoding interfaces: "use
Encode" for that. "sv_utf8_upgrade()" is affected by the encoding
pragma but "sv_utf8_downgrade()" is not (since the encoding pragma
is designed to be a one-way street).
o "is_utf8_string(buf, len)" returns true if "len" bytes of the
buffer are valid UTF-8.
o is_utf8_char(s) returns true if the pointer points to a valid UTF-8
character. However, this function should not be used because of
security concerns. Instead, use "is_utf8_string()".
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 "foldEQ_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, except if one string is in utf8 and the other isn't.
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 desired version of Unicode from the Unicode
web site <http://www.unicode.org>;). These should replace the existing
files in lib/unicore in the Perl source tree. Follow the instructions
in README.perl in that directory to change some of their names, and
then build perl (see INSTALL).
BUGS
Interaction with Locales
See "Unicode and UTF-8" in perllocale
Problems with characters in the Latin-1 Supplement range
See "The "Unicode Bug""
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 recognize that 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 able 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 are hundreds of Unicode characters matching
"Nd" compared with the 10 ASCII characters matching "d").
Problems on EBCDIC platforms
There are several known problems with Perl on EBCDIC platforms. If you
want to use Perl there, send email to perlbug AT 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.
o A filehandle that should read or write UTF-8
if ($] > 5.007) {
binmode $fh, ":encoding(utf8)";
}
o A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension that has no
mention of Unicode in the manpage, you need to make sure that the
UTF8 flag is stripped off. Note that at the time of this writing
(October 2002) the mentioned modules are not UTF-8-aware. Please
check the documentation to verify if this is still true.
if ($] > 5.007) {
require Encode;
$val = Encode::encode_utf8($val); # make octets
}
o A scalar we got back from an extension
If you believe the scalar comes back as UTF-8, you will most likely
want the UTF8 flag restored:
if ($] > 5.007) {
require Encode;
$val = Encode::decode_utf8($val);
}
o Same thing, if you are really sure it is UTF-8
if ($] > 5.007) {
require Encode;
Encode::_utf8_on($val);
}
o A wrapper for fetchrow_array and fetchrow_hashref
When the database contains only UTF-8, a wrapper function or method
is a convenient way to replace all your fetchrow_array and
fetchrow_hashref calls. A wrapper function will also make it easier
to adapt to future enhancements in your database driver. Note that
at the time of this writing (October 2002), the DBI has no
standardized way to deal with UTF-8 data. Please check the
documentation to verify if that is still true.
sub fetchrow {
# $what is one of fetchrow_{array,hashref}
my($self, $sth, $what) = @_;
if ($] < 5.007) {
return $sth->$what;
} else {
require Encode;
if (wantarray) {
my @arr = $sth->$what;
for (@arr) {
defined && /[^\000-\177]/ && Encode::_utf8_on($_);
}
return @arr;
} else {
my $ret = $sth->$what;
if (ref $ret) {
for my $k (keys %$ret) {
defined
&& /[^\000-\177]/
&& Encode::_utf8_on($_) for $ret->{$k};
}
return $ret;
} else {
defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
return $ret;
}
}
}
}
o A large scalar that you know can only contain ASCII
Scalars that contain only ASCII and are marked as UTF-8 are
sometimes a drag to your program. If you recognize such a situation,
just remove the UTF8 flag:
utf8::downgrade($val) if $] > 5.007;
SEE ALSO
perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes,
perlretut, "${^UNICODE}" in perlvar
<http://www.unicode.org/reports/tr44>;).
perl v5.16.3 2013-03-04 PERLUNICODE(1)
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