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Perl Unicode introduction

perluniintro - Perl Unicode introduction


perluniintro - Perl Unicode introduction


This document gives a general idea of Unicode and how to use Unicode in Perl.


Unicode is a character set standard with plans to cover all of the writing systems of the world, plus many other symbols.

Unicode and ISO/IEC 10646 are coordinated standards that provide code points for the characters in almost all modern character set standards, covering more than 30 writing systems and hundreds of languages, including all commercially important modern languages. All characters in the largest Chinese, Japanese, and Korean dictionaries are also encoded. The standards will eventually cover almost all characters in more than 250 writing systems and thousands of languages.

A Unicode character is an abstract entity. It is not bound to any particular integer width, and especially not to the C language char. Unicode is language neutral and display neutral: it doesn't encode the language of the text, and it doesn't define fonts or other graphical layout details. Unicode operates on characters and on text built from those characters.

Unicode defines characters like LATIN CAPITAL LETTER A or GREEK SMALL LETTER ALPHA, and then unique numbers for those, hexadecimal 0x0041 or 0x03B1 for those particular characters. Such unique numbers are called code points.

The Unicode standard prefers using hexadecimal notation for the code points. (In case this notation, numbers like 0x0041, is unfamiliar to you, take a peek at a later section, Hexadecimal Notation.) The Unicode standard uses the notation U+0041 LATIN CAPITAL LETTER A, which gives the hexadecimal code point, and the normative name of the character.

Unicode also defines various properties for the characters, like ``uppercase'' or ``lowercase'', ``decimal digit'', or ``punctuation'': these properties are independent of the names of the characters. Furthermore, various operations on the characters like uppercasing, lowercasing, and collating (sorting), are defined.

A Unicode character consists either of a single code point, or a base character (like LATIN CAPITAL LETTER A), followed by one or more modifiers (like COMBINING ACUTE ACCENT). This sequence of a base character and modifiers is called a combining character sequence.

Whether to call these combining character sequences, as a whole, ``characters'' depends on your point of view. If you are a programmer, you probably would tend towards seeing each element in the sequences as one unit, one ``character'', but from the user viewpoint, the sequence as a whole is probably considered one ``character'', since that's probably what it looks like in the context of the user's language.

With this ``as a whole'' view of characters, the number of characters is open-ended. But in the programmer's ``one unit is one character'' point of view, the concept of ``characters'' is more deterministic, and so we take that point of view in this document: one ``character'' is one Unicode code point, be it a base character or a combining character.

For some of the combinations there are precomposed characters, for example LATIN CAPITAL LETTER A WITH ACUTE is defined as a single code point. These precomposed characters are, however, often available only for some combinations, and mainly they are meant to support round-trip conversions between Unicode and legacy standards (like the ISO 8859), and in general case the composing method is more extensible. To support conversion between the different compositions of the characters, various normalization forms are also defined.

Because of backward compatibility with legacy encodings, the ``a unique number for every character'' breaks down a bit: ``at least one number for every character'' is closer to truth. (This happens when the same character has been encoded in several legacy encodings.) The converse is also not true: not every code point has an assigned character. Firstly, there are unallocated code points within otherwise used blocks. Secondly, there are special Unicode control characters that do not represent true characters.

A common myth about Unicode is that it would be ``16-bit'', that is, 0x10000 (or 65536) characters from 0x0000 to 0xFFFF. This is untrue. Since Unicode 2.0 Unicode has been defined all the way up to 21 bits (0x10FFFF), and since 3.1 characters have been defined beyond 0xFFFF. The first 0x10000 characters are called the Plane 0, or the Basic Multilingual Plane (BMP). With the Unicode 3.1, 17 planes in all are defined (but nowhere near full of defined characters yet).

Another myth is that the 256-character blocks have something to do with languages: a block per language. Also this is untrue. The division into the blocks exists but it is almost completely accidental, an artifact of how the characters have been historically allocated. Instead, there is a concept called scripts, which may be more useful: there is Latin script, Greek script, and so on. Scripts usually span several parts of several blocks. For further information see the Unicode::UCD manpage.

The Unicode code points are just abstract numbers. To input and output these abstract numbers, the numbers must be encoded somehow. Unicode defines several character encoding forms, of which UTF-8 is perhaps the most popular. UTF-8 is a variable length encoding that encodes Unicode characters as 1 to 6 bytes (only 4 with the currently defined characters). Other encodings include UTF-16 and UTF-32 and their big and little endian variants (UTF-8 is byteorder independent). The ISO/IEC 10646 defines the UCS-2 and UCS-4 encoding forms.

For more information about encodings, for example to learn what surrogates and byte order marks (BOMs) are, see perlunicode.

Perl's Unicode Support

Starting from Perl 5.6.0, Perl has had the capability of handling Unicode natively. The first recommended release for serious Unicode work is Perl 5.8.0, however. The maintenance release 5.6.1 fixed many of the problems of the initial implementation of Unicode, but for example regular expressions didn't really work with Unicode.

Starting from Perl 5.8.0, the use of use utf8 is no longer necessary. In earlier releases the utf8 pragma was used to declare that operations in the current block or file would be Unicode-aware. This model was found to be wrong, or at least clumsy: the Unicodeness is now carried with the data, not attached to the operations. (There is one remaining case where an explicit use utf8 is needed: if your Perl script itself is encoded in UTF-8, you can use UTF-8 in your variable and subroutine names, and in your string and regular expression literals, by saying use utf8. This is not the default because that would break existing scripts having legacy 8-bit data in them.)

Perl's Unicode Model

Perl supports both the old, pre-5.6, model of strings of eight-bit native bytes, and strings of Unicode characters. The principle is that Perl tries to keep its data as eight-bit bytes for as long as possible, but as soon as Unicodeness cannot be avoided, the data is transparently upgraded to Unicode.

Internally, Perl currently uses either whatever the native eight-bit character set of the platform (for example Latin-1) or UTF-8 to encode Unicode strings. Specifically, if all code points in the string are 0xFF or less, Perl uses the native eight-bit character set. Otherwise, it uses UTF-8.

A user of Perl does not normally need to know nor care how Perl happens to encode its internal strings, but it becomes relevant when outputting Unicode strings to a stream without a discipline (one with the ``default default''). In such a case, the raw bytes used internally (the native character set or UTF-8, as appropriate for each string) will be used, and a ``Wide character'' warning will be issued if those strings contain a character beyond 0x00FF.

For example,

      perl -e 'print "\x{DF}\n", "\x{0100}\x{DF}\n"'

produces a fairly useless mixture of native bytes and UTF-8, as well as a warning.

To output UTF-8 always, use the ``:utf8'' output discipline. Prepending

      binmode(STDOUT, ":utf8");

to this sample program ensures the output is completely UTF-8, and of course, removes the warning.

Perl 5.8.0 also supports Unicode on EBCDIC platforms. There, the support is somewhat harder to implement since additional conversions are needed at every step. Because of these difficulties, the Unicode support isn't quite as full as in other, mainly ASCII-based, platforms (the Unicode support is better than in the 5.6 series, which didn't work much at all for EBCDIC platform). On EBCDIC platforms, the internal Unicode encoding form is UTF-EBCDIC instead of UTF-8 (the difference is that as UTF-8 is ``ASCII-safe'' in that ASCII characters encode to UTF-8 as-is, UTF-EBCDIC is ``EBCDIC-safe'').

Creating Unicode

To create Unicode characters in literals for code points above 0xFF, use the \x{...} notation in doublequoted strings:

    my $smiley = "\x{263a}";

Similarly in regular expression literals

    $smiley =~ /\x{263a}/;

At run-time you can use chr():

    my $hebrew_alef = chr(0x05d0);

(See Further Resources for how to find all these numeric codes.)

Naturally, ord() will do the reverse: turn a character to a code point.

Note that \x.. (no {} and only two hexadecimal digits), \x{...}, and chr(...) for arguments less than 0x100 (decimal 256) generate an eight-bit character for backward compatibility with older Perls. For arguments of 0x100 or more, Unicode characters are always produced. If you want to force the production of Unicode characters regardless of the numeric value, use pack("U", ...) instead of \x.., \x{...}, or chr().

You can also use the charnames pragma to invoke characters by name in doublequoted strings:

    use charnames ':full';

    my $arabic_alef = "\N{ARABIC LETTER ALEF}";

And, as mentioned above, you can also pack() numbers into Unicode characters:

   my $georgian_an  = pack("U", 0x10a0);

Note that both \x{...} and \N{...} are compile-time string constants: you cannot use variables in them. if you want similar run-time functionality, use chr() and charnames::vianame().

Also note that if all the code points for pack ``U'' are below 0x100, bytes will be generated, just like if you were using chr().

   my $bytes = pack("U*", 0x80, 0xFF);

If you want to force the result to Unicode characters, use the special "U0" prefix. It consumes no arguments but forces the result to be in Unicode characters, instead of bytes.

   my $chars = pack("U0U*", 0x80, 0xFF);

Handling Unicode

Handling Unicode is for the most part transparent: just use the strings as usual. Functions like index(), length(), and substr() will work on the Unicode characters; regular expressions will work on the Unicode characters (see perlunicode and perlretut).

Note that Perl does not consider combining character sequences to be characters, such for example

    use charnames ':full';


will print 2, not 1. The only exception is that regular expressions have \X for matching a combining character sequence.

When life is not quite so transparent is working with legacy encodings, and I/O, and certain special cases.

Legacy Encodings

When you combine legacy data and Unicode the legacy data needs to be upgraded to Unicode. Normally ISO 8859-1 (or EBCDIC, if applicable) is assumed. You can override this assumption by using the encoding pragma, for example

    use encoding 'latin2'; # ISO 8859-2

in which case literals (string or regular expression) and chr/ord in your whole script are assumed to produce Unicode characters from ISO 8859-2 code points. Note that the matching for the encoding names is forgiving: instead of latin2 you could have said Latin 2, or iso8859-2, and so forth. With just

    use encoding;

first the environment variable PERL_ENCODING will be consulted, and if that doesn't exist, ISO 8859-1 (Latin 1) will be assumed.

The Encode module knows about many encodings and it has interfaces for doing conversions between those encodings:

    use Encode 'from_to';

    from_to($data, "iso-8859-3", "utf-8"); # from legacy to utf-8

Unicode I/O

Normally, writing out Unicode data

    print FH $some_string_with_unicode, "\n";

produces raw bytes that Perl happens to use to internally encode the Unicode string (which depends on the system, as well as what characters happen to be in the string at the time). If any of the characters are at code points 0x100 or above, you will get a warning. To ensure that the output is explicitly rendered in the encoding you desire (and to avoid the warning), open the stream with the desired encoding. Some examples:

    open FH, ">:ucs2",      "file"

    open FH, ">:utf8",      "file";

    open FH, ">:Shift-JIS", "file";

and on already open streams use binmode():

    binmode(STDOUT, ":ucs2");

    binmode(STDOUT, ":utf8");

    binmode(STDOUT, ":Shift-JIS");

See documentation for the Encode module for many supported encodings.

Reading in a file that you know happens to be encoded in one of the Unicode encodings does not magically turn the data into Unicode in Perl's eyes. To do that, specify the appropriate discipline when opening files

    open(my $fh,'<:utf8', 'anything');

    my $line_of_unicode = <$fh>;

    open(my $fh,'<:Big5', 'anything');

    my $line_of_unicode = <$fh>;

The I/O disciplines can also be specified more flexibly with the open pragma; see open:

    use open ':utf8'; # input and output default discipline will be UTF-8

    open X, ">file";

    print X chr(0x100), "\n";

    close X;

    open Y, "<file";

    printf "%#x\n", ord(<Y>); # this should print 0x100

    close Y;

With the open pragma you can use the :locale discipline

    $ENV{LC_ALL} = $ENV{LANG} = 'ru_RU.KOI8-R';

    # the :locale will probe the locale environment variables like LC_ALL

    use open OUT => ':locale'; # russki parusski

    open(O, ">koi8");

    print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1

    close O;

    open(I, "<koi8");

    printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1

    close I;

or you can also use the ':encoding(...)' discipline

    open(my $epic,'<:encoding(iso-8859-7)','iliad.greek');

    my $line_of_unicode = <$epic>;

These methods install a transparent filter on the I/O stream that converts data from the specified encoding when it is read in from the stream. The result is always Unicode.

The open pragma affects all the open() calls after the pragma by setting default disciplines. If you want to affect only certain streams, use explicit disciplines directly in the open() call.

You can switch encodings on an already opened stream by using binmode(); see perlfunc/binmode.

The :locale does not currently (as of Perl 5.8.0) work with open() and binmode(), only with the open pragma. The :utf8 and :encoding(...) methods do work with all of open(), binmode(), and the open pragma.

Similarly, you may use these I/O disciplines on output streams to automatically convert Unicode to the specified encoding when it is written to the stream. For example, the following snippet copies the contents of the file ``text.jis'' (encoded as ISO-2022-JP, aka JIS) to the file ``text.utf8'', encoded as UTF-8:

    open(my $nihongo, '<:encoding(iso2022-jp)', 'text.jis');

    open(my $unicode, '>:utf8',                 'text.utf8');

    while (<$nihongo>) { print $unicode }

The naming of encodings, both by the open() and by the open pragma, is similarly understanding as with the encoding pragma: koi8-r and KOI8R will both be understood.

Common encodings recognized by ISO, MIME, IANA, and various other standardisation organisations are recognised; for a more detailed list see Encode.

read() reads characters and returns the number of characters. seek() and tell() operate on byte counts, as do sysread() and sysseek().

Notice that because of the default behaviour of not doing any conversion upon input if there is no default discipline, it is easy to mistakenly write code that keeps on expanding a file by repeatedly encoding:


    open F, "file";

    local $/; ## read in the whole file of 8-bit characters

    $t = <F>;

    close F;

    open F, ">:utf8", "file";

    print F $t; ## convert to UTF-8 on output

    close F;

If you run this code twice, the contents of the file will be twice UTF-8 encoded. A use open ':utf8' would have avoided the bug, or explicitly opening also the file for input as UTF-8.

NOTE: the :utf8 and :encoding features work only if your Perl has been built with the new ``perlio'' feature. Almost all Perl 5.8 platforms do use ``perlio'', though: you can see whether yours is by running ``perl -V'' and looking for useperlio=define.

Displaying Unicode As Text

Sometimes you might want to display Perl scalars containing Unicode as simple ASCII (or EBCDIC) text. The following subroutine converts its argument so that Unicode characters with code points greater than 255 are displayed as ``\x{...}'', control characters (like ``\n'') are displayed as ``\x..'', and the rest of the characters as themselves:

   sub nice_string {


         map { $_ > 255 ?                  # if wide character...

               sprintf("\\x{%04X}", $_) :  # \x{...}

               chr($_) =~ /[[:cntrl:]]/ ?  # else if control character ...

               sprintf("\\x%02X", $_) :    # \x..

               chr($_)                     # else as themselves

         } unpack("U*", $_[0]));           # unpack Unicode characters


For example,




Special Cases

  • Bit Complement Operator ~ And vec()

    The bit complement operator ~ may produce surprising results if used on strings containing characters with ordinal values above 255. In such a case, the results are consistent with the internal encoding of the characters, but not with much else. So don't do that. Similarly for vec(): you will be operating on the internally encoded bit patterns of the Unicode characters, not on the code point values, which is very probably not what you want.

  • Peeking At Perl's Internal Encoding

    Normal users of Perl should never care how Perl encodes any particular Unicode string (because the normal ways to get at the contents of a string with Unicode -- via input and output -- should always be via explicitly-defined I/O disciplines). But if you must, there are two ways of looking behind the scenes.

    One way of peeking inside the internal encoding of Unicode characters is to use unpack("C*", ... to get the bytes, or unpack("H*", ...) to display the bytes:

        # this prints  c4 80  for the UTF-8 bytes 0xc4 0x80
        print join(" ", unpack("H*", pack("U", 0x100))), "\n";

    Yet another way would be to use the Devel::Peek module:

        perl -MDevel::Peek -e 'Dump(chr(0x100))'

    That shows the UTF8 flag in FLAGS and both the UTF-8 bytes and Unicode characters in PV. See also later in this document the discussion about the is_utf8 function of the Encode module.

Advanced Topics

  • String Equivalence

    The question of string equivalence turns somewhat complicated in Unicode: what do you mean by equal?


    The short answer is that by default Perl compares equivalence (eq, ne) based only on code points of the characters. In the above case, the answer is no (because 0x00C1 != 0x0041). But sometimes any CAPITAL LETTER As being considered equal, or even any As of any case, would be desirable.

    The long answer is that you need to consider character normalization and casing issues: see the Unicode::Normalize manpage, and Unicode Technical Reports #15 and #21, Unicode Normalization Forms and Case Mappings,

    As of Perl 5.8.0, regular expression case-ignoring matching implements only 1:1 semantics: one character matches one character. In Case Mappings both 1:N and N:1 matches are defined.

  • String Collation

    People like to see their strings nicely sorted, or as Unicode parlance goes, collated. But again, what do you mean by collate?


    The short answer is that by default, Perl compares strings (lt, le, cmp, ge, gt) based only on the code points of the characters. In the above case, the answer is ``after'', since 0x00C1 > 0x00C0.

    The long answer is that ``it depends'', and a good answer cannot be given without knowing (at the very least) the language context. See the Unicode::Collate manpage, and Unicode Collation Algorithm


  • Character Ranges and Classes

    Character ranges in regular expression character classes (/[a-z]/) and in the tr/// (also known as y///) operator are not magically Unicode-aware. What this means that [A-Za-z] will not magically start to mean ``all alphabetic letters'' (not that it does mean that even for 8-bit characters, you should be using /[[:alpha:]]/ for that).

    For specifying things like that in regular expressions, you can use the various Unicode properties, \pL or perhaps \p{Alphabetic}, in this particular case. You can use Unicode code points as the end points of character ranges, but that means that particular code point range, nothing more. For further information (there are dozens of Unicode character classes), see perlunicode.

  • String-To-Number Conversions

    Unicode does define several other decimal (and numeric) characters than just the familiar 0 to 9, such as the Arabic and Indic digits. Perl does not support string-to-number conversion for digits other than ASCII 0 to 9 (and ASCII a to f for hexadecimal).

Questions With Answers

Will My Old Scripts Break?

Very probably not. Unless you are generating Unicode characters somehow, any old behaviour should be preserved. About the only behaviour that has changed and which could start generating Unicode is the old behaviour of chr() where supplying an argument more than 255 produced a character modulo 255 (for example, chr(300) was equal to chr(45)).

How Do I Make My Scripts Work With Unicode?

Very little work should be needed since nothing changes until you somehow generate Unicode data. The greatest trick will be getting input as Unicode, and for that see the earlier I/O discussion.

How Do I Know Whether My String Is In Unicode?

You shouldn't care. No, you really shouldn't. If you have to care (beyond the cases described above), it means that we didn't get the transparency of Unicode quite right.

Okay, if you insist:

    use Encode 'is_utf8';

    print is_utf8($string) ? 1 : 0, "\n";

But note that this doesn't mean that any of the characters in the string are necessary UTF-8 encoded, or that any of the characters have code points greater than 0xFF (255) or even 0x80 (128), or that the string has any characters at all. All the is_utf8() does is to return the value of the internal ``utf8ness'' flag attached to the $string. If the flag is on, characters added to that string will be automatically upgraded to UTF-8 (and even then only if they really need to be upgraded, that is, if their code point is greater than 0xFF).

Sometimes you might really need to know the byte length of a string instead of the character length. For that use the bytes pragma and its only defined function length():

    my $unicode = chr(0x100);

    print length($unicode), "\n"; # will print 1

    use bytes;

    print length($unicode), "\n"; # will print 2 (the 0xC4 0x80 of the UTF-8)

How Do I Detect Data That's Not Valid In a Particular Encoding?

Use the Encode package to try converting it. For example,

    use Encode 'encode_utf8';

    if (encode_utf8($string_of_bytes_that_I_think_is_utf8)) {

        # valid

    } else {

        # invalid


For UTF-8 only, you can use:

    use warnings;

    @chars = unpack("U0U*", $string_of_bytes_that_I_think_is_utf8);

If invalid, a Malformed UTF-8 character (byte 0x##) in unpack is produced. The ``U0'' means ``expect strictly UTF-8 encoded Unicode''. Without that the unpack("U*", ...) would accept also data like chr(0xFF), similarly to the pack as we saw earlier.

How Do I Convert Binary Data Into a Particular Encoding, Or Vice Versa?

This probably isn't as useful as you might think. Normally, you shouldn't need to.

In one sense, what you are asking doesn't make much sense: Encodings are for characters, and binary data is not ``characters'', so converting ``data'' into some encoding isn't meaningful unless you know in what character set and encoding the binary data is in, in which case it's not binary data, now is it?

If you have a raw sequence of bytes that you know should be interpreted via a particular encoding, you can use Encode:

    use Encode 'from_to';

    from_to($data, "iso-8859-1", "utf-8"); # from latin-1 to utf-8

The call to from_to() changes the bytes in $data, but nothing material about the nature of the string has changed as far as Perl is concerned. Both before and after the call, the string $data contains just a bunch of 8-bit bytes. As far as Perl is concerned, the encoding of the string (as Perl sees it) remains as ``system-native 8-bit bytes''.

You might relate this to a fictional 'Translate' module:

   use Translate;

   my $phrase = "Yes";

   Translate::from_to($phrase, 'english', 'deutsch');

   ## phrase now contains "Ja"

The contents of the string changes, but not the nature of the string. Perl doesn't know any more after the call than before that the contents of the string indicates the affirmative.

Back to converting data, if you have (or want) data in your system's native 8-bit encoding (e.g. Latin-1, EBCDIC, etc.), you can use pack/unpack to convert to/from Unicode.

    $native_string  = pack("C*", unpack("U*", $Unicode_string));

    $Unicode_string = pack("U*", unpack("C*", $native_string));

If you have a sequence of bytes you know is valid UTF-8, but Perl doesn't know it yet, you can make Perl a believer, too:

    use Encode 'decode_utf8';

    $Unicode = decode_utf8($bytes);

You can convert well-formed UTF-8 to a sequence of bytes, but if you just want to convert random binary data into UTF-8, you can't. Any random collection of bytes isn't well-formed UTF-8. You can use unpack("C*", $string) for the former, and you can create well-formed Unicode data by pack("U*", 0xff, ...).

How Do I Display Unicode? How Do I Input Unicode?

See and

How Does Unicode Work With Traditional Locales?

In Perl, not very well. Avoid using locales through the locale pragma. Use only one or the other.

Hexadecimal Notation

The Unicode standard prefers using hexadecimal notation because that shows better the division of Unicode into blocks of 256 characters. Hexadecimal is also simply shorter than decimal. You can use decimal notation, too, but learning to use hexadecimal just makes life easier with the Unicode standard.

The 0x prefix means a hexadecimal number, the digits are 0-9 and a-f (or A-F, case doesn't matter). Each hexadecimal digit represents four bits, or half a byte. print 0x..., "\n" will show a hexadecimal number in decimal, and printf "%x\n", $decimal will show a decimal number in hexadecimal. If you have just the ``hexdigits'' of a hexadecimal number, you can use the hex() function.

    print 0x0009, "\n";    # 9

    print 0x000a, "\n";    # 10

    print 0x000f, "\n";    # 15

    print 0x0010, "\n";    # 16

    print 0x0011, "\n";    # 17

    print 0x0100, "\n";    # 256

    print 0x0041, "\n";    # 65

    printf "%x\n",  65;    # 41

    printf "%#x\n", 65;    # 0x41

    print hex("41"), "\n"; # 65

Further Resources


If you cannot upgrade your Perl to 5.8.0 or later, you can still do some Unicode processing by using the modules Unicode::String, Unicode::Map8, and Unicode::Map, available from CPAN. If you have the GNU recode installed, you can also use the Perl frontend Convert::Recode for character conversions.

The following are fast conversions from ISO 8859-1 (Latin-1) bytes to UTF-8 bytes, the code works even with older Perl 5 versions.

    # ISO 8859-1 to UTF-8


    # UTF-8 to ISO 8859-1



perlunicode, Encode, encoding, open, utf8, bytes, perlretut, the Unicode::Collate manpage, the Unicode::Normalize manpage, the Unicode::UCD manpage


Thanks to the kind readers of the,,, and mailing lists for their valuable feedback.


Copyright 2001-2002 Jarkko Hietaniemi <>

This document may be distributed under the same terms as Perl itself.