Byte and Character Semantics

In future, Perl-level operations will be expected to work with characters rather than bytes.

However, as an interim compatibility measure, Perl aims to provide a safe migration path from byte semantics to character semantics for programs. For operations where Perl can unambiguously decide that the input data are characters, Perl switches to character semantics. For operations where this determination cannot be made without additional information from the user, Perl decides in favor of compatibility and chooses to use byte semantics.

This behavior preserves compatibility with earlier versions of Perl, which allowed byte semantics in Perl operations only if none of the program's inputs were marked as being as source of Unicode character data. Such data may come from filehandles, from calls to external programs, from information provided by the system (such as %ENV ), or from literals and constants in the source text.

On Windows platforms, if the -C command line switch is used or the ${^WIDE_SYSTEM_CALLS} global flag is set to 1, all system calls will use the corresponding wide-character APIs. This feature is available only on Windows to conform to the API standard already established for that platform--and there are very few non-Windows platforms that have Unicode-aware APIs.

The bytes pragma will always, regardless of platform, force byte semantics in a particular lexical scope. See the bytes manpage .

The utf8 pragma is primarily a compatibility device that enables recognition of UTF-(8|EBCDIC) in literals encountered by the parser. Note that this pragma is only required while Perl defaults to byte semantics; when character semantics become the default, this pragma may become a no-op. See utf8.

Unless explicitly stated, Perl operators use character semantics for Unicode data and byte semantics for non-Unicode data. The decision to use character semantics is made transparently. If input data comes from a Unicode source--for example, if a character encoding layer is added to a filehandle or a literal Unicode string constant appears in a program--character semantics apply. Otherwise, byte semantics are in effect. The bytes pragma should be used to force byte semantics on Unicode data.

If strings operating under byte semantics and strings with Unicode character data are concatenated, the new string will be upgraded to ISO 8859-1 (Latin-1), even if the old Unicode string used EBCDIC. This translation is done without regard to the system's native 8-bit encoding, so to change this for systems with non-Latin-1 and non-EBCDIC native encodings use the encoding pragma. See the encoding manpage .

Under character semantics, many operations that formerly operated on bytes now operate on characters. A character in Perl is logically just a number ranging from 0 to 2**31 or so. Larger characters may encode into longer sequences of bytes internally, but this internal detail is mostly hidden for Perl code. See the perluniintro manpage for more.

Effects of Character Semantics

• Strings--including hash keys--and regular expression patterns may contain characters that have an ordinal value larger than 255.

  If you use a Unicode editor to edit your program, Unicode characters may occur directly within the literal strings in one of the various Unicode encodings (UTF-8, UTF-EBCDIC, UCS-2, etc.), but will be recognized as such and converted to Perl's internal representation only if the appropriate the encoding manpage is specified.

  Unicode characters can also be added to a string by using the \x{...} notation. The Unicode code for the desired character, in hexadecimal, should be placed in the braces. For instance, a smiley face is \x{263A}. This encoding scheme only works for characters with a code of 0x100 or above.

  Additionally, if you

  use charnames ':full';

  you can use the \N{...} notation and put the official Unicode character name within the braces, such as \N{WHITE SMILING FACE}.

• If an appropriate the encoding manpage is specified, identifiers within the Perl script may contain Unicode alphanumeric characters, including ideographs. Perl does not currently attempt to canonicalize variable names.

• Regular expressions match characters instead of bytes. ``.'' matches a character instead of a byte. The \C pattern is provided to force a match a single byte--a char in C, hence \C.

• Character classes in regular expressions match characters instead of bytes and match against the character properties specified in the Unicode properties database. \w can be used to match a Japanese ideograph, for instance.

• Named Unicode properties, scripts, and block ranges may be used like character classes via the \p{} ``matches property'' construct and the \P{} negation, ``doesn't match property''.

  For instance, \p{Lu} matches any character with the Unicode ``Lu'' (Letter, uppercase) property, while \p{M} matches any character with an ``M'' (mark--accents and such) property. Brackets are not required for single letter properties, so \p{M} is equivalent to \pM. Many predefined properties are available, such as \p{Mirrored} and \p{Tibetan}.

  The official Unicode script and block names have spaces and dashes as separators, but for convenience you can use dashes, spaces, or underbars, and case is unimportant. It is recommended, however, that for consistency you use the following naming: the official Unicode script, property, or block name (see below for the additional rules that apply to block names) with whitespace and dashes removed, and the words ``uppercase-first-lowercase-rest''. Latin-1 Supplement thus becomes Latin1Supplement.

  You can also use negation in both \p{} and \P{} by introducing a caret (^) between the first brace and the property name: \p{^Tamil} is equal to \P{Tamil}.

  Here are the basic Unicode General Category properties, followed by their long form. You can use either; \p{Lu} and \p{LowercaseLetter}, for instance, are identical.

  Short Long L Letter Lu UppercaseLetter Ll LowercaseLetter Lt TitlecaseLetter Lm ModifierLetter Lo OtherLetter M Mark Mn NonspacingMark Mc SpacingMark Me EnclosingMark N Number Nd DecimalNumber Nl LetterNumber No OtherNumber P Punctuation Pc ConnectorPunctuation Pd DashPunctuation Ps OpenPunctuation Pe ClosePunctuation Pi InitialPunctuation (may behave like Ps or Pe depending on usage) Pf FinalPunctuation (may behave like Ps or Pe depending on usage) Po OtherPunctuation S Symbol Sm MathSymbol Sc CurrencySymbol Sk ModifierSymbol So OtherSymbol Z Separator Zs SpaceSeparator Zl LineSeparator Zp ParagraphSeparator C Other Cc Control Cf Format Cs Surrogate (not usable) Co PrivateUse Cn Unassigned

  Single-letter properties match all characters in any of the two-letter sub-properties starting with the same letter. L& is a special case, which is an alias for Ll, Lu, and Lt.

  Because Perl hides the need for the user to understand the internal representation of Unicode characters, there is no need to implement the somewhat messy concept of surrogates. Cs is therefore not supported.

  Because scripts differ in their directionality--Hebrew is written right to left, for example--Unicode supplies these properties:

  Property Meaning BidiL Left-to-Right BidiLRE Left-to-Right Embedding BidiLRO Left-to-Right Override BidiR Right-to-Left BidiAL Right-to-Left Arabic BidiRLE Right-to-Left Embedding BidiRLO Right-to-Left Override BidiPDF Pop Directional Format BidiEN European Number BidiES European Number Separator BidiET European Number Terminator BidiAN Arabic Number BidiCS Common Number Separator BidiNSM Non-Spacing Mark BidiBN Boundary Neutral BidiB Paragraph Separator BidiS Segment Separator BidiWS Whitespace BidiON Other Neutrals

  For example, \p{BidiR} matches characters that are normally written right to left.

Scripts

Extended property classes can supplement the basic properties, defined by the PropList Unicode database:

and there are further derived properties:

For backward compatibility (with Perl 5.6), all properties mentioned so far may have Is prepended to their name, so \P{IsLu}, for example, is equal to \P{Lu}.

Blocks

For more about scripts, see the UTR #24:

For more about blocks, see:

Block names are given with the In prefix. For example, the Katakana block is referenced via \p{InKatakana}. The In prefix may be omitted if there is no naming conflict with a script or any other property, but it is recommended that In always be used for block tests to avoid confusion.

These block names are supported:

• The special pattern \X matches any extended Unicode sequence--``a combining character sequence'' in Standardese--where the first character is a base character and subsequent characters are mark characters that apply to the base character. \X is equivalent to (?:\PM\pM*).

• The tr/// operator translates characters instead of bytes. Note that the tr///CU functionality has been removed. For similar functionality see pack('U0', ...) and pack('C0', ...).

• Case translation operators use the Unicode case translation tables when character input is provided. Note that uc() , or \U in interpolated strings, translates to uppercase, while ucfirst , or \u in interpolated strings, translates to titlecase in languages that make the distinction.

• Most operators that deal with positions or lengths in a string will automatically switch to using character positions, including chop() , substr() , pos() , index() , rindex() , sprintf() , write() , and length() . Operators that specifically do not switch include vec() , pack() , and unpack() . Operators that really don't care include chomp() , operators that treats strings as a bucket of bits such as sort() , and operators dealing with filenames.

• The pack() / unpack() letters c and C do not change, since they are often used for byte-oriented formats. Again, think char in the C language.

  There is a new U specifier that converts between Unicode characters and code points.

• The chr() and ord() functions work on characters, similar to pack(``U'') and unpack(``U'') , not pack(``C'') and unpack(``C'') . pack(``C'') and unpack(``C'') are methods for emulating byte-oriented chr() and ord() on Unicode strings. While these methods reveal the internal encoding of Unicode strings, that is not something one normally needs to care about at all.

• The bit string operators, & | ^ ~, can operate on character data. However, for backward compatibility, such as when using bit string operations when characters are all less than 256 in ordinal value, one should not use ~ (the bit complement) with characters of both values less than 256 and values greater than 256. Most importantly, DeMorgan's laws (~($x|$y) eq ~$x&~$y and ~($x&$y) eq ~$x|~$y) will not hold. The reason for this mathematical faux pas is that the complement cannot return both the 8-bit (byte-wide) bit complement and the full character-wide bit complement.

• lc() , uc() , lcfirst() , and ucfirst() work for the following cases:
  • the case mapping is from a single Unicode character to another single Unicode character, or

  • the case mapping is from a single Unicode character to more than one Unicode character.

  Things to do with locales (Lithuanian, Turkish, Azeri) do not work.since Perl does not understand the concept of Unicode locales.

  See the Unicode Technical Report #21, Case Mappings, for more details.

• And finally, scalar reverse() reverses by character rather than by byte.

User-Defined Character Properties

The subroutines must return a specially-formatted string, with one or more newline-separated lines. Each line must be one of the following:

• Two hexadecimal numbers separated by horizontal whitespace (space or tabular characters) denoting a range of Unicode code points to include.

• Something to include, prefixed by ``+'': a built-in character property (prefixed by ``utf8::''), to represent all the characters in that property; two hexadecimal code points for a range; or a single hexadecimal code point.

• Something to exclude, prefixed by ``-'': an existing character property (prefixed by ``utf8::''), for all the characters in that property; two hexadecimal code points for a range; or a single hexadecimal code point.

• Something to negate, prefixed ``!'': an existing character property (prefixed by ``utf8::'') for all the characters except the characters in the property; two hexadecimal code points for a range; or a single hexadecimal code point.

    sub InKana {
        return <<END;
    3040\t309F
    30A0\t30FF
    END
    }

Imagine that the here-doc end marker is at the beginning of the line. Now you can use \p{InKana} and \P{InKana}.

You could also have used the existing block property names:

    sub InKana {
        return <<'END';
    +utf8::InHiragana
    +utf8::InKatakana
    END
    }

Suppose you wanted to match only the allocated characters, not the raw block ranges: in other words, you want to remove the non-characters:

    sub InKana {
        return <<'END';
    +utf8::InHiragana
    +utf8::InKatakana
    -utf8::IsCn
    END
    }

The negation is useful for defining (surprise!) negated classes.

    sub InNotKana {
        return <<'END';
    !utf8::InHiragana
    -utf8::InKatakana
    +utf8::IsCn
    END
    }

You can also define your own mappings to be used in the lc() , lcfirst() , uc() , and ucfirst() (or their string-inlined versions). The principle is the same: define subroutines in the main package with names like ToLower (for lc() and lcfirst() ), ToTitle (for the first character in ucfirst() ), and ToUpper (for uc() , and the rest of the characters in ucfirst() ).

The string returned by the subroutines needs now to be three hexadecimal numbers separated by tabulators: start of the source range, end of the source range, and start of the destination range. For example:

    sub ToUpper {
        return <<END;
    0061\t0063\t0041
    END
    }

defines an uc() mapping that causes only the characters ``a'', ``b'', and ``c'' to be mapped to ``A'', ``B'', ``C'', all other characters will remain unchanged.

If there is no source range to speak of, that is, the mapping is from a single character to another single character, leave the end of the source range empty, but the two tabulator characters are still needed. For example:

    sub ToLower {
        return <<END;
    0041\t\t0061
    END
    }

defines a lc() mapping that causes only ``A'' to be mapped to ``a'', all other characters will remain unchanged.

(For serious hackers only) If you want to introspect the default mappings, you can find the data in the directory $Config{privlib}/unicore/To/. The mapping data is returned as the here-document, and the utf8::ToSpecFoo are special exception mappings derived from <$Config{privlib}>/unicore/SpecialCasing.txt. The Digit and Fold mappings that one can see in the directory are not directly user-accessible, one can use either the Unicode::UCD module, or just match case-insensitively (that's when the Fold mapping is used).

A final note on the user-defined property tests and mappings: they will be used only if the scalar has been marked as having Unicode characters. Old byte-style strings will not be affected.

Character Encodings for Input and Output

Unicode Regular Expression Support Level

• Level 1 - Basic Unicode Support
  2.1 Hex Notation - done [1] Named Notation - done [2] 2.2 Categories - done [3][4] 2.3 Subtraction - MISSING [5][6] 2.4 Simple Word Boundaries - done [7] 2.5 Simple Loose Matches - done [8] 2.6 End of Line - MISSING [9][10] [ 1] \x{...} [ 2] \N{...} [ 3] . \p{...} \P{...} [ 4] now scripts (see UTR#24 Script Names) in addition to blocks [ 5] have negation [ 6] can use regular expression look-ahead [a] or user-defined character properties [b] to emulate subtraction [ 7] include Letters in word characters [ 8] note that Perl does Full case-folding in matching, not Simple: for example U+1F88 is equivalent with U+1F00 U+03B9, not with 1F80. This difference matters 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. [ 9] see UTR#13 Unicode Newline Guidelines [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029} (should also affect <>, $., and script line numbers) (the \x{85}, \x{2028} and \x{2029} do match \s)

  [a] You can mimic class subtraction using lookahead. For example, what TR18 might write as

  [{Greek}-[{UNASSIGNED}]]

  in Perl can be written as:

  (?!\p{Unassigned})\p{InGreekAndCoptic} (?=\p{Assigned})\p{InGreekAndCoptic}

  But in this particular example, you probably really want

  \p{GreekAndCoptic}

  which will match assigned characters known to be part of the Greek script.

  [b] See ``User-Defined Character Properties''.

• Level 2 - Extended Unicode Support
  3.1 Surrogates - MISSING [11] 3.2 Canonical Equivalents - MISSING [12][13] 3.3 Locale-Independent Graphemes - MISSING [14] 3.4 Locale-Independent Words - MISSING [15] 3.5 Locale-Independent Loose Matches - MISSING [16] [11] Surrogates are solely a UTF-16 concept and Perl's internal representation is UTF-8. The Encode module does UTF-16, though. [12] see UTR#15 Unicode Normalization [13] have Unicode::Normalize but not integrated to regexes [14] have \X but at this level . should equal that [15] need three classes, not just \w and \W [16] see UTR#21 Case Mappings

• Level 3 - Locale-Sensitive Support
  4.1 Locale-Dependent Categories - MISSING 4.2 Locale-Dependent Graphemes - MISSING [16][17] 4.3 Locale-Dependent Words - MISSING 4.4 Locale-Dependent Loose Matches - MISSING 4.5 Locale-Dependent Ranges - MISSING [16] see UTR#10 Unicode Collation Algorithms [17] have Unicode::Collate but not integrated to regexes

Unicode Encodings

• UTF-8

  UTF-8 is a variable-length (1 to 6 bytes, current character allocations require 4 bytes), byte-order independent encoding. For ASCII (and we really do mean 7-bit ASCII, not another 8-bit encoding), UTF-8 is transparent.

  The following table is from Unicode 3.2.

  Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte U+0000..U+007F 00..7F U+0080..U+07FF C2..DF 80..BF U+0800..U+0FFF E0 A0..BF 80..BF U+1000..U+CFFF E1..EC 80..BF 80..BF U+D000..U+D7FF ED 80..9F 80..BF U+D800..U+DFFF ******* ill-formed ******* U+E000..U+FFFF EE..EF 80..BF 80..BF U+10000..U+3FFFF F0 90..BF 80..BF 80..BF U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF U+100000..U+10FFFF F4 80..8F 80..BF 80..BF

  Note the A0..BF in U+0800..U+0FFF, the 80..9F in U+D000...U+D7FF, the 90..BF in U+10000..U+3FFFF, and the 80...8F in U+100000..U+10FFFF. The ``gaps'' are caused by legal UTF-8 avoiding non-shortest encodings: it is technically possible to UTF-8-encode a single code point in different ways, but that is explicitly forbidden, and the shortest possible encoding should always be used. So that's what Perl does.

  Another way to look at it is via bits:

  Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte 0aaaaaaa 0aaaaaaa 00000bbbbbaaaaaa 110bbbbb 10aaaaaa ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa

  As you can see, the continuation bytes all begin with 10, and the leading bits of the start byte tell how many bytes the are in the encoded character.

• UTF-EBCDIC

  Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.

• UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)

  The followings items are mostly for reference and general Unicode knowledge, Perl doesn't use these constructs internally.

  UTF-16 is a 2 or 4 byte encoding. The Unicode code points U+0000..U+FFFF are stored in a single 16-bit unit, and the code points U+10000..U+10FFFF in two 16-bit units. The latter case is using surrogates, the first 16-bit unit being the Ilow surrogate.

  Surrogates are code points set aside to encode the U+10000..U+10FFFF range of Unicode code points in pairs of 16-bit units. The IU+D800..U+DBFF, and the low surrogates are the range U+DC00..U+DFFF. The surrogate encoding is

  $hi = ($uni - 0x10000) / 0x400 + 0xD800; $lo = ($uni - 0x10000) % 0x400 + 0xDC00;

  and the decoding is

  $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);

  If you try to generate surrogates (for example by using chr() ), you will get a warning if warnings are turned on, because those code points are not valid for a Unicode character.

  Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16 itself can be used for in-memory computations, but if storage or transfer is required either UTF-16BE (big-endian) or UTF-16LE (little-endian) encodings must be chosen.

  This introduces another problem: what if you just know that your data is UTF-16, but you don't know which endianness? Byte Order Marks, or BOMs, 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 UTF-8, you will read the bytes 0xEF 0xBB 0xBF.)

  The way this trick works is that the character with the code point U+FFFE is guaranteed not to be a valid Unicode character, so the sequence of bytes 0xFF 0xFE is unambiguously "BOM, represented in little-endian format" and cannot be U+FFFE, represented in big-endian format".

• UTF-32, UTF-32BE, UTF32-LE

  The UTF-32 family is pretty much like the UTF-16 family, expect that the units are 32-bit, and therefore the surrogate scheme is not needed. The BOM signatures will be 0x00 0x00 0xFE 0xFF for BE and 0xFF 0xFE 0x00 0x00 for LE.

• UCS-2, UCS-4

  Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit encoding. Unlike UTF-16, UCS-2 is not extensible beyond U+FFFF, because it does not use surrogates. UCS-4 is a 32-bit encoding, functionally identical to UTF-32.

• UTF-7

  A seven-bit safe (non-eight-bit) encoding, which is useful if the transport or storage is not eight-bit safe. Defined by RFC 2152.

Security Implications of Unicode

• Malformed UTF-8

  Unfortunately, the specification of UTF-8 leaves some room for interpretation of how many bytes of encoded output one should generate from one input Unicode character. Strictly speaking, the shortest possible sequence of UTF-8 bytes should be generated, because otherwise there is potential for an input buffer overflow at the receiving end of a UTF-8 connection. Perl always generates the shortest length UTF-8, and with warnings on Perl will warn about non-shortest length UTF-8 along with other malformations, such as the surrogates, which are not real Unicode code points.

• Regular expressions behave slightly differently between byte data and character (Unicode) data. For example, the ``word character'' character class \w will work differently depending on if data is eight-bit bytes or Unicode.

  In the first case, the set of \w characters is either small--the default set of alphabetic characters, digits, and the ``_''--or, if you are using a locale (see the perllocale manpage ), the \w might contain a few more letters according to your language and country.

  In the second case, the \w set of characters is much, much larger. Most importantly, even in the set of the first 256 characters, it will probably match different characters: unlike most locales, which are specific to a language and country pair, Unicode classifies all the characters that are letters somewhere as \w. For example, your locale might not think that LATIN SMALL LETTER ETH is a letter (unless you happen to speak Icelandic), but Unicode does.

  As discussed elsewhere, Perl has one foot (two hooves?) planted in each of two worlds: the old world of bytes and the new world of characters, upgrading from bytes to characters when necessary. If your legacy code does not explicitly use Unicode, no automatic switch-over to characters should happen. Characters shouldn't get downgraded to bytes, either. It is possible to accidentally mix bytes and characters, however (see the perluniintro manpage ), in which case \w in regular expressions might start behaving differently. Review your code. Use warnings and the strict pragma.

Unicode in Perl on EBCDIC

Locales

• If your locale environment variables (LANGUAGE, LC_ALL, LC_CTYPE, LANG) contain the strings 'UTF-8' or 'UTF8' (case-insensitive matching), the default encodings of your STDIN, STDOUT, and STDERR, and of any subsequent file open, are considered to be UTF-8.

• Perl tries really hard to work both with Unicode and the old byte-oriented world. Most often this is nice, but sometimes Perl's straddling of the proverbial fence causes problems.

Using Unicode in XS

• DO_UTF8(sv) returns true if the UTF8 flag is on and the bytes pragma is not in effect. SvUTF8(sv) returns true is the UTF8 flag is on; the bytes pragma is ignored. The UTF8 flag being on does not mean that there are any characters of code points greater than 255 (or 127) in the scalar or that there are even any characters in the scalar. What the UTF8 flag means is that the sequence of octets in the representation of the scalar is the sequence of UTF-8 encoded code points of the characters of a string. The UTF8 flag being off means that each octet in this representation encodes a single character with code point 0..255 within the string. Perl's Unicode model is not to use UTF-8 until it is absolutely necessary.

• uvuni_to_utf8(buf, chr) writes a Unicode character code point into a buffer encoding the code point as UTF-8, and returns a pointer pointing after the UTF-8 bytes.

• utf8_to_uvuni(buf, lenp) reads UTF-8 encoded bytes from a buffer and returns the Unicode character code point and, optionally, the length of the UTF-8 byte sequence.

• utf8_length(start, end) returns the length of the UTF-8 encoded buffer in characters. sv_len_utf8(sv) returns the length of the UTF-8 encoded scalar.

• sv_utf8_upgrade(sv) converts the string of the scalar to its UTF-8 encoded form. sv_utf8_downgrade(sv) does the opposite, if possible. sv_utf8_encode(sv) is like sv_utf8_upgrade except that it does not set the UTF8 flag. sv_utf8_decode() does the opposite of sv_utf8_encode(). Note that none of these are to be used as general-purpose encoding or decoding interfaces: use Encode for that. sv_utf8_upgrade() is affected by the encoding pragma but sv_utf8_downgrade() is not (since the encoding pragma is designed to be a one-way street).

• is_utf8_char(s) returns true if the pointer points to a valid UTF-8 character.

• is_utf8_string(buf, len) returns true if len bytes of the buffer are valid UTF-8.

• UTF8SKIP(buf) will return the number of bytes in the UTF-8 encoded character in the buffer. UNISKIP(chr) will return the number of bytes required to UTF-8-encode the Unicode character code point. UTF8SKIP() is useful for example for iterating over the characters of a UTF-8 encoded buffer; UNISKIP() is useful, for example, in computing the size required for a UTF-8 encoded buffer.

• utf8_distance(a, b) will tell the distance in characters between the two pointers pointing to the same UTF-8 encoded buffer.

• utf8_hop(s, off) will return a pointer to an UTF-8 encoded buffer that is off (positive or negative) Unicode characters displaced from the UTF-8 buffer s. Be careful not to overstep the buffer: utf8_hop() will merrily run off the end or the beginning of the buffer if told to do so.

• pv_uni_display(dsv, spv, len, pvlim, flags) and sv_uni_display(dsv, ssv, pvlim, flags) are useful for debugging the output of Unicode strings and scalars. By default they are useful only for debugging--they display all characters as hexadecimal code points--but with the flags UNI_DISPLAY_ISPRINT, UNI_DISPLAY_BACKSLASH, and UNI_DISPLAY_QQ you can make the output more readable.

• ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2) can be used to compare two strings case-insensitively in Unicode. For case-sensitive comparisons you can just use memEQ() and memNE() as usual.

BUGS

Interaction with Locales

Interaction with Extensions

So if you're working with Unicode data, consult the documentation of every module you're using if there are any issues with Unicode data exchange. If the documentation does not talk about Unicode at all, suspect the worst and probably look at the source to learn how the module is implemented. Modules written completely in Perl shouldn't cause problems. Modules that directly or indirectly access code written in other programming languages are at risk.

For affected functions, the simple strategy to avoid data corruption is to always make the encoding of the exchanged data explicit. Choose an encoding that you know the extension can handle. Convert arguments passed to the extensions to that encoding and convert results back from that encoding. Write wrapper functions that do the conversions for you, so you can later change the functions when the extension catches up.

To provide an example, let's say the popular Foo::Bar::escape_html function doesn't deal with Unicode data yet. The wrapper function would convert the argument to raw UTF-8 and convert the result back to Perl's internal representation like so:

Sometimes, when the extension does not convert data but just stores and retrieves them, you will be in a position 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:

If it does not yet provide support for any encoding, one could write a derived class with such a param method:

Some extensions provide filters on data entry/exit points, such as DB_File::filter_store_key and family. Look out for such filters in the documentation of your extensions, they can make the transition to Unicode data much easier.

Speed

The numbers show an incredible slowness on long UTF-8 strings. You should carefully avoid using these functions in tight loops. If you want to iterate over characters, the superior coding technique would split the characters into an array instead of using substr, as the following benchmark shows:

Even though the algorithm based on substr() is faster than split() for byte-encoded data, it pales in comparison to the speed of split() when used with UTF-8 data.

Porting code from perl-5.6.X

• A filehandle that should read or write UTF-8
  if ($] > 5.007) { binmode $fh, ":utf8"; }

• A scalar that is going to be passed to some extension

  Be it Compress::Zlib, Apache::Request or any extension that has no mention of Unicode in the manpage, you need to make sure that the UTF-8 flag is stripped off. Note that at the time of this writing (October 2002) the mentioned modules are not UTF-8-aware. Please check the documentation to verify if this is still true.

  if ($] > 5.007) { require Encode; $val = Encode::encode_utf8($val); # make octets }

• A scalar we got back from an extension

  If you believe the scalar comes back as UTF-8, you will most likely want the UTF-8 flag restored:

  if ($] > 5.007) { require Encode; $val = Encode::decode_utf8($val); }

• Same thing, if you are really sure it is UTF-8
  if ($] > 5.007) { require Encode; Encode::_utf8_on($val); }

• A wrapper for fetchrow_array and fetchrow_hashref

  When the database contains only UTF-8, a wrapper function or method is a convenient way to replace all your fetchrow_array and fetchrow_hashref calls. A wrapper function will also make it easier to adapt to future enhancements in your database driver. Note that at the time of this writing (October 2002), the DBI has no standardized way to deal with UTF-8 data. Please check the documentation to verify if that is still true.

  sub fetchrow { my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref} if ($] < 5.007) { return $sth->$what; } else { require Encode; if (wantarray) { my @arr = $sth->$what; for (@arr) { defined && /[^\000-\177]/ && Encode::_utf8_on($_); } return @arr; } else { my $ret = $sth->$what; if (ref $ret) { for my $k (keys %$ret) { defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret->{$k}; } return $ret; } else { defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret; return $ret; } } } }

• A large scalar that you know can only contain ASCII

  Scalars that contain only ASCII and are marked as UTF-8 are sometimes a drag to your program. If you recognize such a situation, just remove the UTF-8 flag:

  utf8::downgrade($val) if $] > 5.007;

SEE ALSO