Provided by: perl-doc_5.22.1-9ubuntu0.9_all 
NAME
perlunicode - Unicode support in Perl
DESCRIPTION
If you haven't already, before reading this document, you should become familiar with both perlunitut and
perluniintro.
Unicode aims to UNI-fy the en-CODE-ings of all the world's character sets into a single Standard. For
quite a few of the various coding standards that existed when Unicode was first created, converting from
each to Unicode essentially meant adding a constant to each code point in the original standard, and
converting back meant just subtracting that same constant. For ASCII and ISO-8859-1, the constant is 0.
For ISO-8859-5, (Cyrillic) the constant is 864; for Hebrew (ISO-8859-8), it's 1488; Thai (ISO-8859-11),
3424; and so forth. This made it easy to do the conversions, and facilitated the adoption of Unicode.
And it worked; nowadays, those legacy standards are rarely used. Most everyone uses Unicode.
Unicode is a comprehensive standard. It specifies many things outside the scope of Perl, such as how to
display sequences of characters. For a full discussion of all aspects of Unicode, see
<http://www.unicode.org>.
Important Caveats
Even though some of this section may not be understandable to you on first reading, we think it's
important enough to highlight some of the gotchas before delving further, so here goes:
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.
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 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
Use the ":encoding(...)" layer to read from and write to filehandles using the specified encoding.
(See open.)
You should convert your non-ASCII, non-UTF-8 Perl scripts to be UTF-8.
See encoding.
"use utf8" still needed to enable UTF-8 in scripts
If your Perl script is itself encoded in UTF-8, the "use utf8" pragma must be explicitly included to
enable recognition of that (in string or regular expression literals, or in identifier names). This
is the only time when an explicit "use utf8" is needed. (See utf8).
"BOM"-marked scripts and UTF-16 scripts autodetected
However, if a Perl script begins 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 the appropriate Unicode encoding. ("BOM"-less UTF-8 cannot be effectively recognized or
differentiated from ISO 8859-1 or other eight-bit encodings.)
Byte and Character Semantics
Before Unicode, most encodings used 8 bits (a single byte) to encode each character. Thus a character
was a byte, and a byte was a character, and there could be only 256 or fewer possible characters. "Byte
Semantics" in the title of this section refers to this behavior. There was no need to distinguish
between "Byte" and "Character".
Then along comes Unicode which has room for over a million characters (and Perl allows for even more).
This means that a character may require more than a single byte to represent it, and so the two terms are
no longer equivalent. What matter are the characters as whole entities, and not usually the bytes that
comprise them. That's what the term "Character Semantics" in the title of this section refers to.
Perl had to change internally to decouple "bytes" from "characters". It is important that you too change
your ideas, if you haven't already, so that "byte" and "character" no longer mean the same thing in your
mind.
The basic building block of Perl strings has always been a "character". The changes basically come down
to that the implementation no longer thinks that a character is always just a single byte.
There are various things to note:
• String handling functions, for the most part, continue to operate in terms of characters.
"length()", for example, returns the number of characters in a string, just as before. But that
number no longer is necessarily the same as the number of bytes in the string (there may be more
bytes than characters). The other such functions include "chop()", "chomp()", "substr()", "pos()",
"index()", "rindex()", "sort()", "sprintf()", and "write()".
The exceptions are:
• the bit-oriented "vec"
• the byte-oriented "pack"/"unpack" "C" format
However, the "W" specifier does operate on whole characters, as does the "U" specifier.
• some operators that interact with the platform's operating system
Operators dealing with filenames are examples.
• when the functions are called from within the scope of the "use bytes" pragma
Likely, you should use this only for debugging anyway.
• Strings--including hash keys--and regular expression patterns may contain characters that have
ordinal values 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".)
"Creating Unicode" in perluniintro gives other ways to place non-ASCII characters in your strings.
• The "chr()" and "ord()" functions work on whole characters.
• Regular expressions match whole characters. For example, "." matches a whole character instead of
only a single byte.
• The "tr///" operator translates whole characters. (Note that the "tr///CU" functionality has been
removed. For similar functionality to that, see "pack('U0', ...)" and "pack('C0', ...)").
• "scalar reverse()" reverses by character rather than by byte.
• The bit string operators, "& | ^ ~" and (starting in v5.22) "&. |. ^. ~." can operate on characters
that don't fit into a byte. However, the current behavior is likely to change. You should not use
these operators on strings that are encoded in UTF-8. If you're not sure about the encoding of a
string, downgrade it before using any of these operators; you can use "utf8::utf8_downgrade()".
The bottom line is that Perl has always practiced "Character Semantics", but with the advent of Unicode,
that is now different than "Byte Semantics".
ASCII Rules versus Unicode Rules
Before Unicode, when a character was a byte was a character, Perl knew only about the 128 characters
defined by ASCII, code points 0 through 127 (except for under "use locale"). That left the code points
128 to 255 as unassigned, and available for whatever use a program might want. The only semantics they
have is their ordinal numbers, and that they are members of none of the non-negative character classes.
None are considered to match "\w" for example, but all match "\W".
Unicode, of course, assigns each of those code points a particular meaning (along with ones above 255).
To preserve backward compatibility, Perl only uses the Unicode meanings when there is some indication
that Unicode is what is intended; otherwise the non-ASCII code points remain treated as if they are
unassigned.
Here are the ways that Perl knows that a string should be treated as Unicode:
• Within the scope of "use utf8"
If the whole program is Unicode (signified by using 8-bit Unicode Transformation Format), then all
strings within it must be Unicode.
• Within the scope of "use feature 'unicode_strings'"
This pragma was created so you can explicitly tell Perl that operations executed within its scope are
to use Unicode rules. More operations are affected with newer perls. See "The "Unicode Bug"".
• Within the scope of "use 5.012" or higher
This implicitly turns on "use feature 'unicode_strings'".
• Within the scope of "use locale 'not_characters'", or "use locale" and the current locale is a UTF-8
locale.
The former is defined to imply Unicode handling; and the latter indicates a Unicode locale, hence a
Unicode interpretation of all strings within it.
• When the string contains a Unicode-only code point
Perl has never accepted code points above 255 without them being Unicode, so their use implies
Unicode for the whole string.
• When the string contains a Unicode named code point "\N{...}"
The "\N{...}" construct explicitly refers to a Unicode code point, even if it is one that is also in
ASCII. Therefore the string containing it must be Unicode.
• When the string has come from an external source marked as Unicode
The "-C" command line option can specify that certain inputs to the program are Unicode, and the
values of this can be read by your Perl code, see "${^UNICODE}" in perlvar.
• When the string has been upgraded to UTF-8
The function "utf8::utf8_upgrade()" can be explicitly used to permanently (unless a subsequent
"utf8::utf8_downgrade()" is called) cause a string to be treated as Unicode.
• There are additional methods for regular expression patterns
A pattern that is compiled with the "/u" or "/a" modifiers is treated as Unicode (though there are
some restrictions with "/a"). Under the "/d" and "/l" modifiers, there are several other indications
for Unicode; see "Character set modifiers" in perlre.
Note that all of the above are overridden within the scope of "use bytes"; but you should be using this
pragma only for debugging.
Note also that some interactions with the platform's operating system never use Unicode rules.
When Unicode rules are in effect:
• Case translation operators use the Unicode case translation tables.
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).
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.)
• Character classes in regular expressions match based on the character properties specified in the
Unicode properties database.
"\w" can be used to match a Japanese ideograph, for instance; and "[[:digit:]]" a Bengali number.
• 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.
Extended Grapheme Clusters (Logical characters)
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. The Unicode standard calls these "extended grapheme clusters" (which is an
improved version of the no-longer much used "grapheme cluster"); Perl furnishes the "\X" regular
expression construct to match such sequences in their entirety.
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, like
ISO-8859-1, which has quite a few of them. For example, "LATIN CAPITAL LETTER E WITH ACUTE" was already
in this standard when Unicode came along. Unicode therefore added it to its repertoire as that single
character. 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
"E" and the "COMBINING ACCENT" 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. A string may be comprised as much as possible of precomposed characters, or it may be
comprised of entirely decomposed characters. Unicode calls these respectively, "Normalization Form
Composed" (NFC) and "Normalization Form Decomposed". The "Unicode::Normalize" module contains functions
that convert between the two. A string may also have both composed characters and decomposed characters;
this module can be used to make it all one or the other.
You may be presented with strings in any of these equivalent forms. There is currently nothing in Perl 5
that ignores the differences. So you'll have to specially hanlde it. The usual advice is to convert
your inputs to "NFD" before processing further.
For more detailed information, see <http://unicode.org/reports/tr15/>.
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 (see "General_Category"
below). 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" property (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" 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.)
See "Beyond Unicode code points" for special considerations when matching Unicode properties against non-
Unicode code points.
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 values the "General Category" property can have:
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 a "Bidi_Class" property. Some of the values this property can have are:
Value 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. Unlike the "General_Category" property, this property can have
more values added in a future Unicode release. Those listed above comprised the complete set for many
Unicode releases, but others were added in Unicode 6.3; you can always find what the current ones are in
in perluniprops. And <http://www.unicode.org/reports/tr9/> describes how to use them.
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. New code should probably be
using "Script_Extensions" and not plain "Script".
(Actually, besides "Common", the "Inherited" script, contains characters that are used in multiple
scripts. These are modifier characters which 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 the "Is" Prefix
For backward compatibility (with Perl 5.6), all properties writable without using the compound form
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 the 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, no longer recommended) 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 don't do this for
new code because your code could break in new releases, and this has already happened: There was a time
in very early Unicode releases when "\p{Hebrew}" would have matched the block Hebrew; now it doesn't.
Using the "In" prefix avoids this ambiguity, so far. But new versions of Unicode continue to add new
properties whose names begin with "In". There is a possibility that one of them someday will conflict
with your usage. Since this is just a Perl extension, Unicode's name will take precedence and your code
will become broken. Also, Unicode is free to add a script whose name begins with "In"; that would cause
problems.
So it's clearer and best to use the compound form when specifying blocks. And be sure that is what you
really really want to do. In most cases scripts are what you want instead.
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 every possible code point. It is equivalent to "qr/./s". Unlike all the other non-
user-defined "\p{}" property matches, no warning is ever generated if this is property is matched
against a non-Unicode code point (see "Beyond Unicode code points" below).
"\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{Unicode}".
"\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.
The "Extended Grapheme Clusters (Logical characters)" section above talked about canonical
decompositions. 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 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 other
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, 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 until v5.18, 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{Unicode}"
This matches any of the 1_114_112 Unicode code points. "\p{Any}".
"\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
when 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 code point to include.
• Two hexadecimal numbers separated by horizontal whitespace (space or tabular characters) denoting a
range of 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 unassigned 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.
Unlike non-user-defined "\p{}" property matches, no warning is ever generated if these properties are
matched against a non-Unicode code point (see "Beyond Unicode code points" below).
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] "\N{U+...}" and "\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 starting 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, 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] Perl treats "\n" as the start- and end-line delimiter. Unicode specifies more characters that
should be so-interpreted.
These are:
VT U+000B (\v in C)
FF U+000C (\f)
CR U+000D (\r)
NEL U+0085
LS U+2028
PS U+2029
"^" and "$" in regular expression patterns are supposed to match all these, but don't. These
characters also don't, but should, affect "<>" $., and script line numbers.
Also, lines should not be split within "CRLF" (i.e. there is no empty line between "\r" and
"\n"). For "CRLF", try the ":crlf" layer (see PerlIO).
[9] But "Unicode::LineBreak" is available.
This module supplies line breaking conformant with UAX#14 "Unicode Line Breaking Algorithm"
<http://www.unicode.org/reports/tr14>.
[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 - DONE [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 and \b{gcb} 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. In most of Perl's
documentation, including elsewhere in this document, the term "UTF-8" means also "UTF-EBCDIC". But
in this section, "UTF-8" refers only to the encoding used on ASCII platforms. It is a superset of
7-bit US-ASCII, so anything encoded in ASCII has the identical representation when encoded in UTF-8.
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.
• UTF-EBCDIC
Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe. This means that all the basic
characters (which includes all those that have ASCII equivalents (like "A", "0", "%", etc.) are the
same in both EBCDIC and UTF-EBCDIC.)
UTF-EBCDIC is used on EBCDIC platforms. The largest Unicode code points take 5 bytes to represent
(instead of 4 in UTF-8), and Perl extends it to a maximum of 7 bytes to encode pode points up to what
can fit in a 32-bit word (instead of 13 bytes and a 64-bit word in UTF-8).
• UTF-16, UTF-16BE, UTF-16LE, Surrogates, and "BOM"'s (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 "BOM"'s, 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 ASCII
platform 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, except 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.
Noncharacter code points
66 code points are set aside in Unicode as "noncharacter code points". These all have the "Unassigned"
("Cn") "General_Category", and no character will ever be assigned to any of them. They are the 32 code
points between "U+FDD0" and "U+FDEF" inclusive, and the 34 code points:
U+FFFE U+FFFF
U+1FFFE U+1FFFF
U+2FFFE U+2FFFF
...
U+EFFFE U+EFFFF
U+FFFFE U+FFFFF
U+10FFFE U+10FFFF
Until Unicode 7.0, the noncharacters were "forbidden for use in open interchange of Unicode text data",
so that code that processed those streams could use these code points as sentinels that could be mixed in
with character data, and would always be distinguishable from that data. (Emphasis above and in the next
paragraph are added in this document.)
Unicode 7.0 changed the wording so that they are "not recommended for use in open interchange of Unicode
text data". The 7.0 Standard goes on to say:
"If a noncharacter is received in open interchange, an application is not required to interpret it in
any way. It is good practice, however, to recognize it as a noncharacter and to take appropriate
action, such as replacing it with "U+FFFD" replacement character, to indicate the problem in the
text. It is not recommended to simply delete noncharacter code points from such text, because of the
potential security issues caused by deleting uninterpreted characters. (See conformance clause C7 in
Section 3.2, Conformance Requirements, and Unicode Technical Report #36, "Unicode Security
Considerations" <http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
This change was made because it was found that various commercial tools like editors, or for things like
source code control, had been written so that they would not handle program files that used these code
points, effectively precluding their use almost entirely! And that was never the intent. They've always
been meant to be usable within an application, or cooperating set of applications, at will.
If you're writing code, such as an editor, that is supposed to be able to handle any Unicode text data,
then you shouldn't be using these code points yourself, and instead allow them in the input. If you need
sentinels, they should instead be something that isn't legal Unicode. For UTF-8 data, you can use the
bytes 0xC1 and 0xC2 as sentinels, as they never appear in well-formed UTF-8. (There are equivalents for
UTF-EBCDIC). You can also store your Unicode code points in integer variables and use negative values as
sentinels.
If you're not writing such a tool, then whether you accept noncharacters as input is up to you (though
the Standard recommends that you not). If you do strict input stream checking with Perl, these code
points continue to be forbidden. This is to maintain backward compatibility (otherwise potential
security holes could open up, as an unsuspecting application that was written assuming the noncharacters
would be filtered out before getting to it, could now, without warning, start getting them). To do
strict checking, you can use the layer ":encoding('UTF-8')".
Perl continues to warn (using the warning category "nonchar", which is a sub-category of "utf8") if an
attempt is made to output noncharacters.
Beyond Unicode code points
The maximum Unicode code point is "U+10FFFF", and Unicode only defines operations on code points up
through that. But Perl works on 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 any are
output.
Since Unicode rules are not defined on these code points, if a Unicode-defined operation is done on them,
Perl uses what we believe are sensible rules, while generally warning, using the "non_unicode" category.
For example, "uc("\x{11_0000}")" will generate such a warning, returning the input parameter as its
result, since Perl defines the uppercase of every non-Unicode code point to be the code point itself.
(All the case changing operations, not just uppercasing, work this way.)
The situation with matching Unicode properties in regular expressions, the "\p{}" and "\P{}" constructs,
against these code points is not as clear cut, and how these are handled has changed as we've gained
experience.
One possibility is to treat any match against these code points as undefined. But since Perl doesn't
have the concept of a match being undefined, it converts this to failing or "FALSE". This is almost, but
not quite, what Perl did from v5.14 (when use of these code points became generally reliable) through
v5.18. The difference is that Perl treated all "\p{}" matches as failing, but all "\P{}" matches as
succeeding.
One problem with this is that it leads to unexpected, and confusting results in some cases:
chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
That is, it treated both matches as undefined, and converted that to false (raising a warning on each).
The first case is the expected result, but the second is likely counterintuitive: "How could both be
false when they are complements?" Another problem was that the implementation optimized many Unicode
property matches down to already existing simpler, faster operations, which don't raise the warning. We
chose to not forgo those optimizations, which help the vast majority of matches, just to generate a
warning for the unlikely event that an above-Unicode code point is being matched against.
As a result of these problems, starting in v5.20, what Perl does is to treat non-Unicode code points as
just typical unassigned Unicode characters, and matches accordingly. (Note: Unicode has atypical
unassigned code points. For example, it has noncharacter code points, and ones that, when they do get
assigned, are destined to be written Right-to-left, as Arabic and Hebrew are. Perl assumes that no non-
Unicode code point has any atypical properties.)
Perl, in most cases, will raise a warning when matching an above-Unicode code point against a Unicode
property when the result is "TRUE" for "\p{}", and "FALSE" for "\P{}". For example:
chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
In both these examples, the character being matched is non-Unicode, so Unicode doesn't define how it
should match. It clearly isn't an ASCII hex digit, so the first example clearly should fail, and so it
does, with no warning. But it is arguable that the second example should have an undefined, hence
"FALSE", result. So a warning is raised for it.
Thus the warning is raised for many fewer cases than in earlier Perls, and only when what the result is
could be arguable. It turns out that none of the optimizations made by Perl (or are ever likely to be
made) cause the warning to be skipped, so it solves both problems of Perl's earlier approach. The most
commonly used property that is affected by this change is "\p{Unassigned}" which is a short form for
"\p{General_Category=Unassigned}". Starting in v5.20, all non-Unicode code points are considered
"Unassigned". In earlier releases the matches failed because the result was considered undefined.
The only place where the warning is not raised when it might ought to have been is if optimizations cause
the whole pattern match to not even be attempted. For example, Perl may figure out that for a string to
match a certain regular expression pattern, the string has to contain the substring "foobar". Before
attempting the match, Perl may look for that substring, and if not found, immediately fail the match
without actually trying it; so no warning gets generated even if the string contains an above-Unicode
code point.
This behavior is more "Do what I mean" than in earlier Perls for most applications. But it catches fewer
issues for code that needs to be strictly Unicode compliant. Therefore there is an additional mode of
operation available to accommodate such code. This mode is enabled if a regular expression pattern is
compiled within the lexical scope where the "non_unicode" warning class has been made fatal, say by:
use warnings FATAL => "non_unicode"
(see warnings). In this mode of operation, Perl will raise the warning for all matches against a non-
Unicode code point (not just the arguable ones), and it skips the optimizations that might cause the
warning to not be output. (It currently still won't warn if the match isn't even attempted, like in the
"foobar" example above.)
In summary, Perl now normally treats non-Unicode code points as typical Unicode unassigned code points
for regular expression matches, raising a warning only when it is arguable what the result should be.
However, if this warning has been made fatal, it isn't skipped.
There is one exception to all this. "\p{All}" looks like a Unicode property, but it is a Perl extension
that is defined to be true for all possible code points, Unicode or not, so no warning is ever generated
when matching this against a non-Unicode code point. (Prior to v5.20, it was an exact synonym for
"\p{Any}", matching code points 0 through 0x10FFFF.)
Security Implications of Unicode
First, 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
ASCII and single-byte locales, and the new world of Unicode, upgrading when necessary. If your legacy
code does not explicitly use Unicode, no automatic switch-over to Unicode should happen.
Unicode in Perl on EBCDIC
Unicode is supported on EBCDIC platforms. See perlebcdic.
Unless ASCII vs. EBCDIC issues are specifically being discussed, references to UTF-8 encoding in this
document and elsewhere should be read as meaning UTF-EBCDIC on EBCDIC platforms. See "Unicode and UTF"
in perlebcdic.
Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly hidden from you; "use utf8" (and
NOT something like "use utfebcdic") declares the the script is in the platform's "native" 8-bit encoding
of Unicode. (Similarly for the ":utf8" layer.)
Locales
See "Unicode and UTF-8" in perllocale
When Unicode Does Not Happen
There are still many places where Unicode (in some encoding or another) could be given as arguments or
received as results, or both in Perl, but it is not, in spite of Perl having 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).
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 (deprecated) "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 with the 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 can have very different semantics depending on the rules in effect.
(Characters whose code points are above 255 force Unicode rules; whereas the rules for ASCII characters
are the same under both ASCII and Unicode rules.)
Under Unicode rules, these upper-Latin1 characters are interpreted as Unicode code points, which means
they have the same semantics as Latin-1 (ISO-8859-1) and C1 controls.
As explained in "ASCII Rules versus Unicode Rules", under ASCII rules, they are considered to be
unassigned characters.
This can lead to unexpected results. For example, a string's semantics can suddenly change if a code
point above 255 is appended to it, which changes the rules from ASCII to Unicode. 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" nor 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, along with
Perl's desire to add Unicode support seamlessly. But the result turned out to not be seamless. (By the
way, you can choose to be warned when things like this happen. See "encoding::warnings".)
"use feature 'unicode_strings'" was added, starting in Perl v5.12, to address this problem. It affects
these things:
• 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, Unicode rules 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
Unicode rules even when executed or compiled into larger regular expressions outside the scope.
• Matching any of several properties in regular expressions.
These properties are "\b" (without braces), "\B" (without braces), "\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
Unicode rules even when executed or compiled into larger regular expressions outside the scope.
• In "quotemeta" or its inline equivalent "\Q".
Starting in Perl 5.16.0, consistent quoting rules are used within the scope of "unicode_strings", as
described in "quotemeta" in perlfunc. Prior to that, or outside its scope, 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.
You can see from the above that the effect of "unicode_strings" increased over several Perl releases.
(And Perl's support for Unicode continues to improve; it's best to use the latest available release in
order to get the most complete and accurate results possible.) Note that "unicode_strings" is
automatically chosen if you "use 5.012" or higher.
For Perls earlier than those described above, or when a string is passed to a function outside the scope
of "unicode_strings", see the next section.
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 standard module Encode can be used for
this, or the low-level calls "utf8::upgrade($bytestring)" and "utf8::downgrade($utf8string[, FAIL_OK])".
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.
"ASCII Rules versus Unicode Rules" gives all the ways that a string is made to use Unicode rules.
Using Unicode in XS
See "Unicode Support" in perlguts for an introduction to Unicode at the XS level, and "Unicode Support"
in perlapi for the API details.
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 the goal is to allow you to
change to use any earlier one. In Perls v5.20 and v5.22, however, the earliest usable version is Unicode
5.1. Perl v5.18 is able to handle all earlier versions.
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).
Porting code from perl-5.6.X
Perls starting in 5.8 have 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 DBI "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 DBI 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 $] > 5.008;
BUGS
See also "The "Unicode Bug"" above.
Interaction with Extensions
When Perl exchanges data with an extension, the extension should be able to understand the UTF8 flag and
act accordingly. If the extension doesn't recognize that flag, it's likely that the extension will return
incorrectly-flagged data.
So if you're working with Unicode data, consult the documentation of every module you're using if there
are any issues with Unicode data exchange. If the documentation does not talk about Unicode at all,
suspect the worst and probably look at the source to learn how the module is implemented. Modules written
completely in Perl shouldn't cause problems. Modules that directly or indirectly access code written in
other programming languages are at risk.
For affected functions, the simple strategy to avoid data corruption is to always make the encoding of
the exchanged data explicit. Choose an encoding that you know the extension can handle. Convert arguments
passed to the extensions to that encoding and convert results back from that encoding. Write wrapper
functions that do the conversions for you, so you can later change the functions when the extension
catches up.
To provide an example, let's say the popular "Foo::Bar::escape_html" function doesn't deal with Unicode
data yet. The wrapper function would convert the argument to raw UTF-8 and convert the result back to
Perl's internal representation like so:
sub my_escape_html ($) {
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 it, 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 $ret;
}
}
Some extensions provide filters on data entry/exit points, such as "DB_File::filter_store_key" and
family. Look out for such filters in the documentation of your extensions; they can make the transition
to Unicode data much easier.
Speed
Some functions are slower when working on UTF-8 encoded strings than on byte encoded strings. All
functions that need to hop over characters such as "length()", "substr()" or "index()", or matching
regular expressions can work much faster when the underlying data are byte-encoded.
In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a caching scheme was introduced
which improved the situation. In general, operations with UTF-8 encoded strings are still slower. As an
example, the Unicode properties (character classes) like "\p{Nd}" are known to be quite a bit slower
(5-20 times) than their simpler counterparts like "[0-9]" (then again, there are hundreds of Unicode
characters matching "Nd" compared with the 10 ASCII characters matching "[0-9]").
SEE ALSO
perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes, perlretut, "${^UNICODE}" in perlvar,
<http://www.unicode.org/reports/tr44>).
perl v5.22.1 2020-10-19 PERLUNICODE(1)