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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 http://www.unicode.org.

Important Caveats

Even though some of this section may not be understandable to you on first reading, we think it's important enough to highlight some of the gotchas before delving further, so here goes:


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 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 5.012 or higher

    This implicitly turns on use feature 'unicode_strings'.

  • Within the scope of use locale 'not_characters', or use locale and the current locale is a UTF-8 locale.

    The former is defined to imply Unicode handling; and the latter indicates a Unicode locale, hence a Unicode interpretation of all strings within it.

  • When the string contains a Unicode-only code point

    Perl has never accepted code points above 255 without them being Unicode, so their use implies Unicode for the whole string.

  • When the string contains a Unicode named code point \N{...}

    The \N{...} construct explicitly refers to a Unicode code point, even if it is one that is also in ASCII. Therefore the string containing it must be Unicode.

  • When the string has come from an external source marked as Unicode

    The -C command line option can specify that certain inputs to the program are Unicode, and the values of this can be read by your Perl code, see "${^UNICODE}" in perlvar.

  • When the string has been upgraded to UTF-8

    The function utf8::utf8_upgrade() can be explicitly used to permanently (unless a subsequent utf8::utf8_downgrade() is called) cause a string to be treated as Unicode.

  • There are additional methods for regular expression patterns

    A pattern that is compiled with the /u or /a modifiers is treated as Unicode (though there are some restrictions with /a). Under the /d and /l modifiers, there are several other indications for Unicode; see "Character set modifiers" in perlre.

Note that all of the above are overridden within the scope of use bytes; but you should be using this pragma only for debugging.

Note also that some interactions with the platform's operating system never use Unicode rules.

When Unicode rules are in effect:

  • Case translation operators use the Unicode case translation tables.



  • 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.


    See "Unicode Character Properties" for more details.


Extended Grapheme Clusters (Logical characters)

Consider a character, say H. It could appear with various marks around it, such as an acute accent, or a circumflex, or various hooks, circles, arrows, etc., above, below, to one side or the other, etc. There are many possibilities among the world's languages. The number of combinations is astronomical, and if there were a character for each combination, it would soon exhaust Unicode's more than a million possible characters. So Unicode took a different approach: there is a character for the base H, and a character for each of the possible marks, and these can be variously combined to get a final logical character. So a logical character--what appears to be a single character--can be a sequence of more than one individual characters. The Unicode standard calls these "extended grapheme clusters" (which is an improved version of the no-longer much used "grapheme cluster"); Perl furnishes the \X regular expression construct to match such sequences in their entirety.

But Unicode's intent is to unify the existing character set standards and practices, and several pre-existing standards have single characters that mean the same thing as some of these combinations, like ISO-8859-1, which has quite a few of them. For example, "LATIN CAPITAL LETTER E WITH ACUTE" was already in this standard when Unicode came along. Unicode therefore added it to its repertoire as that single character. But this character is considered by Unicode to be equivalent to the sequence consisting of the character "LATIN CAPITAL LETTER E" followed by the character "COMBINING ACUTE ACCENT".

"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and its equivalence with the "E" and the "COMBINING ACCENT" sequence is called canonical equivalence. All pre-composed characters are said to have a decomposition (into the equivalent sequence), and the decomposition type is also called canonical. A string may be comprised as much as possible of precomposed characters, or it may be comprised of entirely decomposed characters. Unicode calls these respectively, "Normalization Form Composed" (NFC) and "Normalization Form Decomposed". The Unicode::Normalize module contains functions that convert between the two. A string may also have both composed characters and decomposed characters; this module can be used to make it all one or the other.

You may be presented with strings in any of these equivalent forms. There is currently nothing in Perl 5 that ignores the differences. So you'll have to specially 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 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.


    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


    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 http://www.unicode.org/reports/tr9/ 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": http://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}.





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

















Matches a character that has a non-canonical decomposition.

The "Extended Grapheme Clusters (Logical characters)" section above talked about canonical decompositions. However, many more characters have a different type of decomposition, a "compatible" or "non-canonical" decomposition. The sequences that form these decompositions are not considered canonically equivalent to the pre-composed character. An example is the "SUPERSCRIPT ONE". It is somewhat like a regular digit 1, but not exactly; its decomposition into the digit 1 is called a "compatible" decomposition, specifically a "super" decomposition. There are several such compatibility decompositions (see http://www.unicode.org/reports/tr44), including one called "compat", which means some miscellaneous type of decomposition that doesn't fit into the other decomposition categories that Unicode has chosen.








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}










User-Defined Character Properties


    # assuming property Is_Foreign defined in Lang::
    package main;  # property package name required

    package Lang;  # property package name not required




  • A single hexadecimal number denoting a code point to include.







    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';




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 http://www.unicode.org/reports/tr18/#Compatibility_Properties. These are described above in "Other Properties"

The experimental feature "(?[...])" starting in v5.18 accomplishes this.


  • 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"


[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:



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).


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


  • 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


    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


    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 deprecated 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 going forward (much higher on 64-bit systems).


    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)





    and the decoding is



    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.)



  • 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


  • UTF-7


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:

    "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")."

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





One problem with this is that it leads to unexpected, and confusing results in some cases:










 use warnings FATAL => "non_unicode"




Security Implications of Unicode

First, read Unicode Security Considerations.

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").


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.)


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).




  • %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}";
         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:


    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:]].


  • 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 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 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


  • 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


      if ($] > 5.008) {
        require Encode;
        $val = Encode::encode("UTF-8", $val); # make octets
  • A scalar we got back from an extension


      if ($] > 5.008) {
        require Encode;
        $val = Encode::decode("UTF-8", $val);

      if ($] > 5.008) {
        require Encode;
  • A wrapper for DBI fetchrow_array and fetchrow_hashref


      sub fetchrow {
        # $what is one of fetchrow_{array,hashref}
        if ($] < 5.008) {
          return $sth->$what;
        } else {
          require Encode;
          if (wantarray) {
            my @arr = $sth->$what;
            for (@arr) {
            return @arr;
          } else {
            my $ret = $sth->$what;
            if (ref $ret) {
              for my $k (keys %$ret) {
                && /[^\000-\177]/
              return $ret;
            } else {
              return $ret;
  • A large scalar that you know can only contain ASCII




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]).


perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes, perlretut, "${^UNICODE}" in perlvar, http://www.unicode.org/reports/tr44).