Provided by: perl-doc_5.18.2-2ubuntu1.7_all 
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. Nor does it change the internal representation of
strings, only their interpretation. 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
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. That means that non-ASCII
characters 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:
• 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';
• 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.
• 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.)
• 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]" and starting in
Perl v5.18, experimentally, a vertical tab.
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 experimental feature "(?[ ])" in perlre provides an
alternative which allows more complex definitions.) 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:
• 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 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.
• 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.
• 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.
• 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>
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.
• 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 - experimental [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] The experimental feature in v5.18 "(?[...])" accomplishes this. See "(?[ ])" in
perlre. If you don't want to use an experimental feature, you can use one of the
following:
• Regular expression look-ahead
You can mimic class subtraction using lookahead. For example, what UTS#18
might write as
[{Block=Greek}-[{UNASSIGNED}]]
in Perl can be written as:
(?!\p{Unassigned})\p{Block=Greek}
(?=\p{Assigned})\p{Block=Greek}
But in this particular example, you probably really want
\p{Greek}
which will match assigned characters known to be part of the Greek script.
• CPAN module Unicode::Regex::Set
It does implement the full UTS#18 grouping, intersection, union, and removal
(subtraction) syntax.
• "User-Defined Character Properties"
'+' for union, '-' for removal (set-difference), '&' for intersection
[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 (i.e. there is no empty line between
\r and \n). For CRLF, try the ":crlf" layer (see PerlIO).
[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
• 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)
• Level 3 - Tailored Support
RL3.1 Tailored Punctuation - MISSING
RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
RL3.3 Tailored Word Boundaries - MISSING
RL3.4 Tailored Loose Matches - MISSING
RL3.5 Tailored Ranges - MISSING
RL3.6 Context Matching - MISSING [19]
RL3.7 Incremental Matches - MISSING
( RL3.8 Unicode Set Sharing )
RL3.9 Possible Match Sets - MISSING
RL3.10 Folded Matching - MISSING [20]
RL3.11 Submatchers - MISSING
[17] see UAX#10 "Unicode Collation Algorithms"
[18] have Unicode::Collate but not integrated to regexes
[19] have (?<=x) and (?=x), but look-aheads or look-behinds
should see outside of the target substring
[20] need insensitive matching for linguistic features other
than case; for example, hiragana to katakana, wide and
narrow, simplified Han to traditional Han (see UTR#30
"Character Foldings")
Unicode Encodings
Unicode characters are assigned to code points, which are abstract numbers. To use these
numbers, various encodings are needed.
• UTF-8
UTF-8 is a variable-length (1 to 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.
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:
• 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.
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 v5.16.0) 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 "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:
• 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.
• 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.
• 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.
• 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.
• "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(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.
• "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_string(buf, len)" returns true if "len" bytes of the buffer are valid UTF-8.
• "is_utf8_char_buf(buf, buf_end)" returns true if the pointer points to a valid UTF-8
character.
• "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.
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@perl.org.
In earlier versions, when byte and character data were concatenated, the new string was
sometimes created by decoding the byte strings as ISO 8859-1 (Latin-1), even if the old
Unicode string used EBCDIC.
If you find any of these, please report them as bugs.
Porting code from perl-5.6.X
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer was required to use
the "utf8" pragma to declare that a given scope expected to deal with Unicode data and had
to make sure that only Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to your code. The
examples are written such that the code will continue to work under 5.6, so you should be
safe to try them out.
• A filehandle that should read or write UTF-8
if ($] > 5.008) {
binmode $fh, ":encoding(utf8)";
}
• A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension that has no mention of Unicode
in the manpage, you need to make sure that the UTF8 flag is stripped off. Note that at
the time of this writing (January 2012) the mentioned modules are not UTF-8-aware.
Please check the documentation to verify if this is still true.
if ($] > 5.008) {
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 ($] > 5.008) {
require Encode;
$val = Encode::decode_utf8($val);
}
• Same thing, if you are really sure it is UTF-8
if ($] > 5.008) {
require Encode;
Encode::_utf8_on($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 (January 2012), 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, $what) = @_;
if ($] < 5.008) {
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 $] > 5.008;
SEE ALSO
perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes, perlretut,
"${^UNICODE}" in perlvar <http://www.unicode.org/reports/tr44>).