perlunicode - Unicode support in Perl
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.
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.
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.
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.
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
is in effect (which overrides
use feature 'unicode_strings'
in the same scope), Perl uses the
semantics associated
with the current locale. 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.
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.
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.
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.
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 double-quoted string inlined
versions such as \U
). See
User-Defined Case-Mappings
for more details.
(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 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.)
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
.
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.
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 property gives what script a given character is in,
and the property can be specified with the compound form like
\p{Script=Hebrew}
(short: \p{sc=hebr}
). Perl furnishes shortcuts for all
script names. You can omit everything up through the equals (or colon), and
simply write \p{Latin}
or \P{Cyrillic}
.
A complete list of scripts and their shortcuts is in perluniprops.
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}
.
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. The digits 0-9 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. (Note 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. Only sets are used across several languages 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 property is likely to be the one 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:
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?
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.
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 synonyms for compound-form Unicode properties (for those, you'll have to refer to the Unicode Standard.
\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{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.
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.
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:
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.
This featured is deprecated and is scheduled to be removed in Perl 5.16. The CPAN module Unicode::Casing provides better functionality without the drawbacks described below.
You can define your own mappings to be used in lc(),
lcfirst(), uc(), and ucfirst() (or their string-inlined versions,
\L
, \l
, \U
, and \u
). The mappings are currently only valid
on strings encoded in UTF-8, but see below for a partial workaround for
this restriction.
The principle is similar to that of user-defined character
properties: define subroutines that do the mappings.
ToLower
is used for lc(), \L
, lcfirst(), and \l
; ToTitle
for
ucfirst() and \u
; and ToUpper
for uc() and \U
.
ToUpper()
should look something like this:
- sub ToUpper {
- return <<END;
- 0061\t007A\t0041
- 0101\t\t0100
- END
- }
This sample ToUpper()
has the effect of mapping "a-z" to "A-Z", 0x101
to 0x100, and all other characters map to themselves. The first
returned line means to map the code point at 0x61 ("a") to 0x41 ("A"),
the code point at 0x62 ("b") to 0x42 ("B"), ..., 0x7A ("z") to 0x5A
("Z"). The second line maps just the code point 0x101 to 0x100. Since
there are no other mappings defined, all other code points map to
themselves.
This mechanism is not well behaved as far as affecting other packages
and scopes. All non-threaded programs have exactly one uppercasing
behavior, one lowercasing behavior, and one titlecasing behavior in
effect for utf8-encoded strings for the duration of the program. Each
of these behaviors is irrevocably determined the first time the
corresponding function is called to change a utf8-encoded string's case.
If a corresponding To-
function has been defined in the package that
makes that first call, the mapping defined by that function will be the
mapping used for the duration of the program's execution across all
packages and scopes. If no corresponding To-
function has been
defined in that package, the standard official mapping will be used for
all packages and scopes, and any corresponding To-
function anywhere
will be ignored. Threaded programs have similar behavior. If the
program's casing behavior has been decided at the time of a thread's
creation, the thread will inherit that behavior. But, if the behavior
hasn't been decided, the thread gets to decide for itself, and its
decision does not affect other threads nor its creator.
As shown by the example above, 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 official mappings in the directory
$Config{privlib}
/unicore/To/. The mapping data is returned as the
here-document. The utf8::ToSpecFoo hashes in those files are special
exception mappings derived from
$Config{privlib}
/unicore/SpecialCasing.txt. (The "Digit" and
"Fold" mappings that one can see in the directory are not directly
user-accessible, one can use either the Unicode::UCD module, or just match
case-insensitively, which is what uses the "Fold" mapping. Neither are user
overridable.)
If you have many mappings to change, you can take the official mapping data, change by hand the affected code points, and place the whole thing into your subroutine. But this will only be valid on Perls that use the same Unicode version. Another option would be to have your subroutine read the official mapping files and overwrite the affected code points.
If you have only a few mappings to change, starting in 5.14 you can use the following trick, here illustrated for Turkish.
This takes the official mappings and overrides just one, for "LATIN SMALL LETTER I". The keys to the hash must be the bytes that form the UTF-8 (on EBCDIC platforms, UTF-EBCDIC) of the character, as illustrated by the inverse function.
This example is for an ASCII platform, and \xc4\xb0
is the string of
bytes that together form the UTF-8 that represents \N{LATIN CAPITAL
LETTER I WITH DOT ABOVE}
, U+0130
. You can avoid having to figure out
these bytes, and at the same time make it work on all platforms by
instead writing:
This works because utf8::encode()
takes the single character and
converts it to the sequence of bytes that constitute it. Note that we took
advantage of the fact that "i"
is the same in UTF-8 or UTF_EBCIDIC as not;
otherwise we would have had to write
- $utf8::ToSpecLower{$sequence} = "\N{LATIN SMALL LETTER I}";
in the ToLower example, and in the ToUpper example, use
- my $sequence = "\N{LATIN SMALL LETTER I}";
- utf8::encode($sequence);
A big caveat to the above trick and to this whole mechanism in general,
is that they work only on strings encoded in UTF-8. You can partially
get around this by using use subs
. (But better to just convert to
use Unicode::Casing.) For example:
(The trick illustrated here does work in earlier releases, but only if all the
characters you want to override have ordinal values of 256 or higher, or
if you use the other tricks given just below.)
The mappings are in effect only for the package they are defined in, and only
on scalars that have been marked as having Unicode characters, for example by
using utf8::upgrade()
. Although probably not advisable, you can
cause the mappings to be used globally by importing into CORE::GLOBAL
(see CORE).
You can partially get around the restriction that the source strings
must be in utf8 by using use subs
(or by importing into CORE::GLOBAL
) by:
- use subs qw(uc ucfirst lc lcfirst);
- sub uc($) {
- my $string = shift;
- utf8::upgrade($string);
- return CORE::uc($string);
- }
- sub lc($) {
- my $string = shift;
- utf8::upgrade($string);
- # Unless an I is before a dot_above, it turns into a dotless i.
- # (The character class with the combining classes matches non-above
- # marks following the I. Any number of these may be between the 'I' and
- # the dot_above, and the dot_above will still apply to the 'I'.
- use charnames ":full";
- $string =~
- s/I
- (?! [^\p{ccc=0}\p{ccc=Above}]* \N{COMBINING DOT ABOVE} )
- /\N{LATIN SMALL LETTER DOTLESS I}/gx;
- # But when the I is followed by a dot_above, remove the
- # dot_above so the end result will be i.
- $string =~ s/I
- ([^\p{ccc=0}\p{ccc=Above}]* )
- \N{COMBINING DOT ABOVE}
- /i$1/gx;
- return CORE::lc($string);
- }
These examples (also for Turkish) make sure the input is in UTF-8, and then
call the corresponding official function, which will use the ToUpper()
and
ToLower()
functions you have defined.
(For Turkish, there are other required functions: ucfirst, lcfirst,
and ToTitle
. These are very similar to the ones given above.)
The reason this is only a partial fix is that it doesn't affect the \l
,
\L
, \u
, and \U
case-change operations in regular expressions,
which still require the source to be encoded in utf8 (see The Unicode Bug). (Again, use Unicode::Casing instead.)
The lc() example shows how you can add context-dependent casing. Note
that context-dependent casing suffers from the problem that the string
passed to the casing function may not have sufficient context to make
the proper choice. Also, it will not be called for \l
, \L
, \u
,
and \U
.
See Encode.
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.
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 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] 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).
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] missing 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 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 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.
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.
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.)
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.
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).
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.
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.
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.
Read Unicode Security Considerations. Also, note the following:
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.
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.
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.
See Unicode and UTF-8 in perllocale
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?
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 term, the "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.
(The lesson here is to specify unicode_strings
to avoid the
headaches.)
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
.
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 any of several properties in regular expressions, namely \b
,
\B
, \s, \S
, \w
, \W
, and all the Posix character classes
except [[:ascii:]]
.
In quotemeta or its inline equivalent \Q
, no characters
code points above 127 are quoted in UTF-8 encoded strings, but in
byte encoded strings, code points between 128-255 are always quoted.
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'
- 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.
Starting in Perl 5.14, use feature 'unicode_strings'
can be used to
cause Perl to use Unicode semantics on all string operations within the
scope of the feature subpragma. Regular expressions compiled in its
scope retain that behavior even when executed or compiled into larger
regular expressions outside the scope. (The pragma does not, however,
affect the quotemeta behavior. Nor does it affect the deprecated
user-defined case changing operations--these still require a UTF-8
encoded string to operate.)
In Perl 5.12, the subpragma affected casing changes, but not regular expressions. See lc for details on how this pragma works in combination with various others for casing.
For earlier Perls, or when a string is passed to a function outside the
subpragma's scope, a workaround is to always call utf8::upgrade($string)
,
or to use the standard module Encode. Also, a scalar that has any characters
whose ordinal is above 0x100, or which were specified using either of the
\N{...}
notations, will automatically have character semantics.
Sometimes (see When Unicode Does Not Happen or The Unicode Bug) there are situations where you simply need to force a byte string into UTF-8, or vice versa. The low-level calls utf8::upgrade($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.
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.
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.
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).
It is even possible to copy the built files 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.
See Unicode and UTF-8 in perllocale
See The Unicode Bug
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:
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 $ret;
- }
- }
Some extensions provide filters on data entry/exit points, such as DB_File::filter_store_key and family. Look out for such filters in the documentation of your extensions, they can make the transition to Unicode data much easier.
Some functions are slower when working on UTF-8 encoded strings than on byte encoded strings. All functions that need to hop over characters such as length(), substr() or index(), or matching regular expressions can work much faster when the underlying data are byte-encoded.
In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
a caching scheme was introduced which will hopefully make the slowness
somewhat less spectacular, at least for some operations. In general,
operations with UTF-8 encoded strings are still slower. As an example,
the Unicode properties (character classes) like \p{Nd}
are known to
be quite a bit slower (5-20 times) than their simpler counterparts
like \d
(then again, there are hundreds of Unicode characters matching Nd
compared with the 10 ASCII characters matching d
).
There are several known problems with Perl on EBCDIC platforms. If you want to use Perl there, send email to perlbug@perl.org.
In earlier versions, when byte and character data were concatenated, the new string was sometimes created by decoding the byte strings as ISO 8859-1 (Latin-1), even if the old Unicode string used EBCDIC.
If you find any of these, please report them as bugs.
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
was required to use the utf8
pragma to declare that a given scope
expected to deal with Unicode data and had to make sure that only
Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to
your code. The examples are written such that the code will continue
to work under 5.6, so you should be safe to try them out.
A filehandle that should read or write UTF-8
- if ($] > 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 {
- # $what is one of fetchrow_{array,hashref}
- my($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 $ret;
- }
- }
- }
- }
A large scalar that you know can only contain ASCII
Scalars that contain only ASCII and are marked as UTF-8 are sometimes a drag to your program. If you recognize such a situation, just remove the UTF8 flag:
- utf8::downgrade($val) if $] > 5.007;
perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes, perlretut, ${^UNICODE} in perlvar http://www.unicode.org/reports/tr44).