PERLUNICODE(1) Perl Programmers Reference Guide PERLUNICODE(1)
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
<https://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, see "Security Implications of Unicode" below.
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 v5.12" 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 must convert your non-ASCII, non-UTF-8 Perl scripts to be UTF-8.
The encoding module has been deprecated since perl 5.18 and the perl
internals it requires have been removed with perl 5.26.
"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).
If a Perl script begins with the bytes that form the UTF-8 encoding
of the Unicode BYTE ORDER MARK ("BOM", see "Unicode Encodings"),
those bytes are completely ignored.
UTF-16 scripts autodetected
If a Perl script begins with the Unicode "BOM" (UTF-16LE, UTF16-BE),
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.
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 "use utf8", the latter may require 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 bit strings encoded in UTF-8, but this can
give unexpected results if any of the strings contain code points
above 0xFF. Starting in v5.28, it is a fatal error to have such an
operand. Otherwise, the operation is performed on a non-UTF-8 copy
of the operand. 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 literal 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 v5.12" 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 consist as
much as possible of precomposed characters, or it may consist 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 handle 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 short form spelled the same 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 generally 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
"name" property has some restrictions on this due to a few outlier
names. Full details are given in
<https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>.
The few places where stricter matching is used is in the middle of
numbers, the "name" property, 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
<https://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
perluniprops. And <https://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. The "Script_Extensions" property is an
improved version of "Script", as demonstrated below. 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_Extensions" 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", which is required to be written in the
compound form. Prior to Perl v5.26, the single form returned the plain
old "Script" version, but was changed because "Script_Extensions" gives
better results.)
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". If you compile perl with a Unicode release that doesn't
have "Script_Extensions", the single form Perl extensions will instead
refer to the plain "Script" property. If you compile with a version of
Unicode that doesn't have the "Script" property, these extensions will
not be defined at all.
(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_Extensions". 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".
The explanation above has omitted some detail; refer to UAX#24 "Unicode
Script Property": <https://www.unicode.org/reports/tr24>.
A complete list of scripts and their shortcuts is in perluniprops.
Use of the "Is" Prefix
For backward compatibility (with ancient 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": <https://www.unicode.org/reports/tr24>
The "Script_Extensions" or "Script" 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.
Perl also defines single form synonyms for the block property in cases
where these do not conflict with something else. But don't use any of
these, because they are unstable. Since these are Perl extensions, they
are subordinate to official Unicode property names; Unicode doesn't know
nor care about Perl's extensions. It may happen that a name that
currently means the Perl extension will later be changed without warning
to mean a different Unicode property in a future version of the perl
interpreter that uses a later Unicode release, and your code would no
longer work. The extensions are mentioned here for completeness: Take
the block name and prefix it with one of: "In" (for example
"\p{Blk=Arrows}" can currently be written as "\p{In_Arrows}"); or
sometimes "Is" (like "\p{Is_Arrows}"); or sometimes no prefix at all
("\p{Arrows}"). As of this writing (Unicode 9.0) there are no conflicts
with using the "In_" prefix, but there are plenty with the other two
forms. For example, "\p{Is_Hebrew}" and "\p{Hebrew}" mean
"\p{Script_Extensions=Hebrew}" which is NOT the same thing as
"\p{Blk=Hebrew}". Our advice used to be to use the "In_" prefix as a
single form way of specifying a block. But Unicode 8.0 added properties
whose names begin with "In", and it's now clear that it's only luck
that's so far prevented a conflict. Using "In" is only marginally less
typing than "Blk:", and the latter's meaning is clearer anyway, and
guaranteed to never conflict. So don't take chances. Use "\p{Blk=foo}"
for new code. And be sure that block is what you really really want to
do. In most cases scripts are what you want instead.
A complete list of blocks 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 <https://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
<https://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 any of the non-canonical decomposition
types. Canonical decompositions are introduced in the "Extended
Grapheme Clusters (Logical characters)" section above. However,
many more characters have a different type of decomposition,
generically called "compatible" decompositions, or "non-canonical".
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" (for
"superscript") decomposition. There are several such compatibility
decompositions (see <https://www.unicode.org/reports/tr44>).
"\p{Dt: Non_Canon}" is a Perl extension that uses just one name to
refer to the union of all of them.
Most Unicode characters don't have a decomposition, so their
decomposition type is "None". Hence, "Non_Canonical" is equivalent
to
qr/(?[ \P{DT=Canonical} - \p{DT=None} ])/
(Note that one of the non-canonical decompositions is named
"compat", which could perhaps have been better named
"miscellaneous". It includes just the things that Unicode couldn't
figure out a better generic name for.)
"\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 Unicode version number, such as 1.1 or
12.0; 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.
Comparison of "\N{...}" and "\p{name=...}"
Starting in Perl 5.32, you can specify a character by its name in
regular expression patterns using "\p{name=...}". This is in addition
to the longstanding method of using "\N{...}". The following summarizes
the differences between these two:
\N{...} \p{Name=...}
can interpolate only with eval yes [1]
custom names yes no [2]
name aliases yes yes [3]
named sequences yes yes [4]
name value parsing exact Unicode loose [5]
[1] The ability to interpolate means you can do something like
qr/\p{na=latin capital letter $which}/
and specify $which elsewhere.
[2] You can create your own names for characters, and override official
ones when using "\N{...}". See "CUSTOM ALIASES" in charnames.
[3] Some characters have multiple names (synonyms).
[4] Some particular sequences of characters are given a single name, in
addition to their individual ones.
[5] Exact name value matching means you have to specify case, hyphens,
underscores, and spaces precisely in the name you want. Loose
matching follows the Unicode rules
<https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>, where
these are mostly irrelevant. Except for a few outlier character
names, these are the same rules as are already used for any other
"\p{...}" property.
Wildcards in Property Values
Starting in Perl 5.30, it is possible to do something like this:
qr!\p{numeric_value=/\A[0-5]\z/}!
or, by abbreviating and adding "/x",
qr! \p{nv= /(?x) \A [0-5] \z / }!
This matches all code points whose numeric value is one of 0, 1, 2, 3,
4, or 5. This particular example could instead have been written as
qr! \A [ \p{nv=0}\p{nv=1}\p{nv=2}\p{nv=3}\p{nv=4}\p{nv=5} ] \z !xx
in earlier perls, so in this case this feature just makes things easier
and shorter to write. If we hadn't included the "\A" and "\z", these
would have matched things like "1/2" because that contains a 1 (as well
as a 2). As written, it matches things like subscripts that have these
numeric values. If we only wanted the decimal digits with those numeric
values, we could say,
qr! (?[ \d & \p{nv=/[0-5]/ ]) }!x
The "\d" gets rid of needing to anchor the pattern, since it forces the
result to only match "[0-9]", and the "[0-5]" further restricts it.
The text in the above examples enclosed between the "/" characters can
be just about any regular expression. It is independent of the main
pattern, so doesn't share any capturing groups, etc. The delimiters for
it must be ASCII punctuation, but it may NOT be delimited by "{", nor
"}" nor contain a literal "}", as that delimits the end of the enclosing
"\p{}". Like any pattern, certain other delimiters are terminated by
their mirror images. These are "(", ""["", and "<". If the delimiter
is any of "-", "_", "+", or "\", or is the same delimiter as is used for
the enclosing pattern, it must be preceded by a backslash escape, both
fore and aft.
Beware of using "$" to indicate to match the end of the string. It can
too easily be interpreted as being a punctuation variable, like $/.
No modifiers may follow the final delimiter. Instead, use
"(?adlupimnsx-imnsx)" in perlre and/or "(?adluimnsx-imnsx:pattern)" in
perlre to specify modifiers. However, certain modifiers are illegal in
your wildcard subpattern. The only character set modifier specifiable
is "/aa"; any other character set, and "-m", and "p", and "s" are all
illegal. Specifying modifiers like "qr/.../gc" that aren't legal in the
"(?...)" notation normally raise a warning, but with wildcard
subpatterns, their use is an error. The "m" modifier is ineffective;
everything that matches will be a single line.
By default, your pattern is matched case-insensitively, as if "/i" had
been specified. You can change this by saying "(?-i)" in your pattern.
There are also certain operations that are illegal. You can't nest
"\p{...}" and "\P{...}" calls within a wildcard subpattern, and "\G"
doesn't make sense, so is also prohibited.
And the "*" quantifier (or its equivalent "(0,}") is illegal.
This feature is not available when the left-hand side is prefixed by
"Is_", nor for any form that is marked as "Discouraged" in "Discouraged"
in perluniprops.
This experimental feature has been added to begin to implement
<https://www.unicode.org/reports/tr18/#Wildcard_Properties>. Using it
will raise a (default-on) warning in the
"experimental::uniprop_wildcards" category. We reserve the right to
change its operation as we gain experience.
Your subpattern can be just about anything, but for it to have some
utility, it should match when called with either or both of a) the full
name of the property value with underscores (and/or spaces in the Block
property) and some things uppercase; or b) the property value in all
lowercase with spaces and underscores squeezed out. For example,
qr!\p{Blk=/Old I.*/}!
qr!\p{Blk=/oldi.*/}!
would match the same things.
Another example that shows that within "\p{...}", "/x" isn't needed to
have spaces:
qr!\p{scx= /Hebrew|Greek/ }!
To be safe, we should have anchored the above example, to prevent
matches for something like "Hebrew_Braille", but there aren't any script
names like that, so far. A warning is issued if none of the legal
values for a property are matched by your pattern. It's likely that a
future release will raise a warning if your pattern ends up causing
every possible code point to match.
Starting in 5.32, the Name, Name Aliases, and Named Sequences properties
are allowed to be matched. They are considered to be a single
combination property, just as has long been the case for "\N{}". Loose
matching doesn't work in exactly the same way for these as it does for
the values of other properties. The rules are given in
<https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>. As a
result, Perl doesn't try loose matching for you, like it does in other
properties. All letters in names are uppercase, but you can add "(?i)"
to your subpattern to ignore case. If you're uncertain where a blank
is, you can use " ?" in your subpattern. No character name contains an
underscore, so don't bother trying to match one. The use of hyphens is
particularly problematic; refer to the above link. But note that, as of
Unicode 13.0, the only script in modern usage which has weirdnesses with
these is Tibetan; also the two Korean characters U+116C HANGUL JUNGSEONG
OE and U+1180 HANGUL JUNGSEONG O-E. Unicode makes no promises to not
add hyphen-problematic names in the future.
Using wildcards on these is resource intensive, given the hundreds of
thousands of legal names that must be checked against.
An example of using Name property wildcards is
qr!\p{name=/(SMILING|GRINNING) FACE/}!
Another is
qr/(?[ \p{name=\/CJK\/} - \p{ideographic} ])/
which is the 200-ish (as of Unicode 13.0) CJK characters that aren't
ideographs.
There are certain properties that wildcard subpatterns don't currently
work with. These are:
Bidi Mirroring Glyph
Bidi Paired Bracket
Case Folding
Decomposition Mapping
Equivalent Unified Ideograph
Lowercase Mapping
NFKC Case Fold
Titlecase Mapping
Uppercase Mapping
Nor is the "@unicode_property@" form implemented.
Here's a complete example of matching IPV4 internet protocol addresses
in any (single) script
no warnings 'experimental::uniprop_wildcards';
# Can match a substring, so this intermediate regex needs to have
# context or anchoring in its final use. Using nt=de yields decimal
# digits. When specifying a subset of these, we must include \d to
# prevent things like U+00B2 SUPERSCRIPT TWO from matching
my $zero_through_255 =
qr/ \b (*sr: # All from same sript
(?[ \p{nv=0} & \d ])* # Optional leading zeros
( # Then one of:
\d{1,2} # 0 - 99
| (?[ \p{nv=1} & \d ]) \d{2} # 100 - 199
| (?[ \p{nv=2} & \d ])
( (?[ \p{nv=:[0-4]:} & \d ]) \d # 200 - 249
| (?[ \p{nv=5} & \d ])
(?[ \p{nv=:[0-5]:} & \d ]) # 250 - 255
)
)
)
\b
/x;
my $ipv4 = qr/ \A (*sr: $zero_through_255
(?: [.] $zero_through_255 ) {3}
)
\z
/x;
User-Defined Character Properties
You can define your own binary character properties by defining
subroutines whose names begin with "In" or "Is". (The regex sets
feature "(?[ ])" in perlre provides an alternative which allows more
complex definitions.) The subroutines can be defined in any package.
They override any Unicode properties expressed as the same names. The
user-defined properties can be used in the regular expression "\p{}" and
"\P{}" constructs; if you are using a user-defined property from a
package other than the one you are in, you must specify its package in
the "\p{}" or "\P{}" construct.
# assuming property IsForeign defined in Lang::
package main; # property package name required
if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
package Lang; # property package name not required
if ($txt =~ /\p{IsForeign}+/) { ... }
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. But the subroutine is never called more than once
for each flag value (zero vs non-zero). The return value is saved and
used instead of calling the sub ever again. If the sub is defined at
the time the pattern is compiled, it will be called then; if not, it
will be called the first time its value (for that flag) is needed during
execution.
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. The
second number must not be smaller than the first.
• 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. (Similarly using "-" for the first set does nothing).
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 UTS#18 "Unicode
Regular Expressions" <https://www.unicode.org/reports/tr18>, version 18,
October 2016.
Level 1 - Basic Unicode Support
RL1.1 Hex Notation - Done [1]
RL1.2 Properties - Done [2]
RL1.2a Compatibility Properties - Done [3]
RL1.3 Subtraction and Intersection - Done [4]
RL1.4 Simple Word Boundaries - Done [5]
RL1.5 Simple Loose Matches - Done [6]
RL1.6 Line Boundaries - Partial [7]
RL1.7 Supplementary Code Points - Done [8]
[1] "\N{U+...}" and "\x{...}"
[2] "\p{...}" "\P{...}". This requirement is for a minimal list of
properties. Perl supports these. See R2.7 for other properties.
[3] Perl has "\d" "\D" "\s" "\S" "\w" "\W" "\X" "[:prop:]" "[:^prop:]",
plus all the properties specified by
<https://www.unicode.org/reports/tr18/#Compatibility_Properties>.
These are described above in "Other Properties"
[4] The regex sets feature "(?[...])" starting in v5.18 accomplishes
this. See "(?[ ])" in perlre.
[5] "\b" "\B" meet most, but not all, the details of this requirement,
but "\b{wb}" and "\B{wb}" do, as well as the stricter R2.3.
[6] 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.
[7] The reason this is considered to be only partially implemented is
that Perl has "qr/\b{lb}/" and "Unicode::LineBreak" that are
conformant with UAX#14 "Unicode Line Breaking Algorithm"
<https://www.unicode.org/reports/tr14>. The regular expression
construct provides default behavior, while the heavier-weight module
provides customizable line breaking.
But Perl treats "\n" as the start- and end-line delimiter, whereas
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).
[8] 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 - Retracted [9]
by Unicode
RL2.2 Extended Grapheme Clusters and - Partial [10]
Character Classes with Strings
RL2.3 Default Word Boundaries - Done [11]
RL2.4 Default Case Conversion - Done
RL2.5 Name Properties - Done
RL2.6 Wildcards in Property Values - Partial [12]
RL2.7 Full Properties - Partial [13]
RL2.8 Optional Properties - Partial [14]
[9] Unicode has rewritten this portion of UTS#18 to say that getting
canonical equivalence (see UAX#15 "Unicode Normalization Forms"
<https://www.unicode.org/reports/tr15>) is basically to be done at the
programmer level. Use NFD to write both your regular expressions and
text to match them against (you can use Unicode::Normalize).
[10] Perl has "\X" and "\b{gcb}". Unicode has retracted their "Grapheme
Cluster Mode", and recently added string properties, which Perl does not
yet support.
[11] see UAX#29 "Unicode Text Segmentation"
<https://www.unicode.org/reports/tr29>,
[12] see "Wildcards in Property Values" above.
[13] Perl supports all the properties in the Unicode Character Database
(UCD). It does not yet support the listed properties that come from
other Unicode sources.
[14] The only optional property that Perl supports is Named Sequence.
None of these properties are in the UCD.
Level 3 - Tailored Support
This has been retracted by Unicode.
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. In addition, it is now illegal
to use a code point larger than what a signed integer variable on
your system can hold. On 32-bit ASCII systems, this means
"0x7FFF_FFFF" is the legal maximum (much higher on 64-bit systems).
• 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. It generally requires more
bytes to represent a given code point than UTF-8 does; the largest
Unicode code points take 5 bytes to represent (instead of 4 in
UTF-8), and, extended for 64-bit words, it uses 14 bytes instead of
13 bytes 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"
<https://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 0xC0 and 0xC1 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 signed 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 confusing
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
<https://www.unicode.org/reports/tr36>.
Also, note the following:
• Malformed UTF-8
UTF-8 is very structured, so many combinations of bytes are invalid.
In the past, Perl tried to soldier on and make some sense of invalid
combinations, but this can lead to security holes, so now, if the
Perl core needs to process an invalid combination, it will either
raise a fatal error, or will replace those bytes by the sequence
that forms the Unicode REPLACEMENT CHARACTER, for which purpose
Unicode created it.
Every code point can be represented by more than one possible
syntactically valid UTF-8 sequence. Early on, both Unicode and Perl
considered any of these to be valid, but now, all sequences longer
than the shortest possible one are considered to be malformed.
Unicode considers many code points to be illegal, or to be avoided.
Perl generally accepts them, once they have passed through any input
filters that may try to exclude them. These have been discussed
above (see "Surrogates" under UTF-16 in "Unicode Encodings",
"Noncharacter code points", and "Beyond Unicode code points").
• 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 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.
• In the ".." or range operator.
Starting in Perl 5.26.0, the range operator on strings treats their
lengths consistently within the scope of "unicode_strings". Prior to
that, or outside its scope, it could produce strings whose length in
characters exceeded that of the right-hand side, where the right-
hand side took up more bytes than the correct range endpoint.
• In "split"'s special-case whitespace splitting.
Starting in Perl 5.28.0, the "split" function with a pattern
specified as a string containing a single space handles whitespace
characters consistently within the scope of "unicode_strings". Prior
to that, or outside its scope, characters that are whitespace
according to Unicode rules but not according to ASCII rules were
treated as field contents rather than field separators when they
appear in byte-encoded strings.
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 v5.12" 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 and v5.24 are able to handle all earlier versions.
Download the files in the desired version of Unicode from the Unicode
web site <https://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 $fh, ":encoding(UTF-8)";
}
• A scalar that is going to be passed to some extension
Be it "Compress::Zlib", "Apache::Request" or any extension that has
no mention of Unicode in the manpage, you need to make sure that the
UTF8 flag is stripped off. Note that at the time of this writing
(January 2012) the mentioned modules are not UTF-8-aware. Please
check the documentation to verify if this is still true.
if ($] > 5.008) {
require Encode;
$val = Encode::encode("UTF-8", $val); # make octets
}
• A scalar we got back from an extension
If you believe the scalar comes back as UTF-8, you will most likely
want the UTF8 flag restored:
if ($] > 5.008) {
require Encode;
$val = Encode::decode("UTF-8", $val);
}
• Same thing, if you are really sure it is UTF-8
if ($] > 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("UTF-8", Foo::Bar::escape_html(
Encode::encode("UTF-8", $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,
<https://www.unicode.org/reports/tr44>).
perl v5.40.1 2026-08-30 PERLUNICODE(1)
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