



perlunicode - Unicode support in Perl


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

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:


Also, the use of Unicode may present security issues that aren't obvious, see "Security Implications of Unicode".

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.


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).

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:

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:

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:

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

Unicode Character Properties



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.








See "Beyond Unicode code points" for special considerations when matching Unicode properties against non-Unicode code points.



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
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


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 describes how to use them.



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.)



and only the first of these match:


And only the last two of these match:


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.



The explanation above has omitted some detail; refer to UAX#24 "Unicode Script Property":

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}.





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





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).




This matches any of the 1_114_112 Unicode code points. It is a synonym for \p{Unicode}.








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, including one called "compat", which means some miscellaneous type of decomposition that doesn't fit into the other decomposition categories that Unicode has chosen.








This is a synonym for \p{Present_In=*}



Mnemonic: Perl's (original) space



Mnemonic: Perl's (original) word.


There are several of these, which are equivalents, using the \p{} notation, for Posix classes and are described in "POSIX Character Classes" in perlrecharclass.











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}



This matches any of the 1_114_112 Unicode code points. \p{Any}.







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

package Lang;  # property package name not required


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.



sub InKana {
    return <<END;


You could also have used the existing block property names:

sub InKana {
    return <<'END';


sub InKana {
    return <<'END';


sub InNotKana {
    return <<'END';


sub InNotKana {
    return <<'END';



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)


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", version 13, November 2013.

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     - Experimental  [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 and all other Unicode character properties, as R2.7 asks (see "Unicode Character Properties" above).
[3] Perl has \d \D \s \S \w \W \X [:prop:] [:^prop:], plus all the properties specified by These are described above in "Other Properties"

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 lookahead



    in Perl can be written as:


    But in this particular example, you probably really want



  • CPAN module Unicode::Regex::Set


  • "User-Defined Character Properties"

    "+" for union, "-" for removal (set-difference), "&" for intersection

[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.

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.


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". 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      - Partial       [10]
RL2.3   Default Word Boundaries         - Done          [11]
RL2.4   Default Case Conversion         - Done
RL2.5   Name Properties                 - Done
RL2.6   Wildcard Properties             - Missing
RL2.7   Full Properties                 - Done
[9] Unicode has rewritten this portion of UTS#18 to say that getting canonical equivalence (see UAX#15 "Unicode Normalization Forms") 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} but we don't have a "Grapheme Cluster Mode".
[11] see UAX#29 "Unicode Text Segmentation",

Level 3 - Tailored Support

RL3.1   Tailored Punctuation            - Missing
RL3.2   Tailored Grapheme Clusters      - Missing       [12]
RL3.3   Tailored Word Boundaries        - Missing
RL3.4   Tailored Loose Matches          - Retracted by Unicode
RL3.5   Tailored Ranges                 - Retracted by Unicode
RL3.6   Context Matching                - Missing       [13]
RL3.7   Incremental Matches             - Missing
RL3.8   Unicode Set Sharing             - Unicode is proposing
                                          to retract this
RL3.9   Possible Match Sets             - Missing
RL3.10  Folded Matching                 - Retracted by Unicode
RL3.11  Submatchers                     - Missing
[12] Perl has Unicode::Collate, but it isn't integrated with regular expressions. See UTS#10 "Unicode Collation Algorithms".
[13] Perl has (?<=x) and (?=x), but lookaheads or lookbehinds should see outside of the target substring

Unicode Encodings


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:


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:

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 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.

Also, note the following:

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.)


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 "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}";
        print /\w/ || 0;

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:

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]).



"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.


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.


Porting code from perl-5.6.X



See also "The "Unicode Bug"" above.

Interaction with Extensions





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:



sub param {
  if (defined $value) {
  } else {
    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.



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]).

