=head2 Unicode Encodings
Unicode characters are assigned to I<code points> which are abstract
-numbers. To use this numbers various encodings are needed.
+numbers. To use these numbers various encodings are needed.
=over 4
=item UTF-8
-UTF-8 is the encoding used internally by Perl. UTF-8 is variable
+UTF-8 is the encoding used internally by Perl. UTF-8 is a variable
length (1 to 6 bytes, current character allocations require 4 bytes),
-byteorder independent encoding. For ASCII UTF-8 is transparent
-(and we really mean 7-bit ASCII, not any 8-bit encoding).
+byteorder independent encoding. For ASCII, UTF-8 is transparent
+(and we really do mean 7-bit ASCII, not any 8-bit encoding).
=item UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)
$uni = 0x10000 + ($hi - 0xD8000) * 0x400 + ($lo - 0xDC00);
-Because of the 16-bitness, UTF-16 is byteorder dependent. The UTF-16
+Because of the 16-bitness, UTF-16 is byteorder dependent. UTF-16
itself can be used for in-memory computations, but if storage or
-transfer is required, either the UTF-16BE (Big Endian), or UTF-16LE
+transfer is required, either UTF-16BE (Big Endian) or UTF-16LE
(Little Endian) 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
(BOMs) are a solution to this. A special character has been reserved
-in Unicode to function as a byte order marker: the 0xFFFE is the BOM.
+in Unicode to function as a byte order marker: the character with the
+code point 0xFEFF 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 0xFF 0xFE, but if it was written on a little endian platform,
-you will read the bytes 0xFE 0xFF. (And if the originating platform
-was writing in UTF-8, you will read the bytes 0xEF 0xBF 0xBE.)
+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
+0xFFFE 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 "0xFFFE, represented in
+big-endian format".
=item UTF-32, UTF-32BE, UTF32-LE
=item UCS-2, UCS-4
-Encodings defined by the ISO 10646 standard. UCS-2 is 16-bit
-encoding, UCS-4 is 32-bit encoding. Unlike the UTF-16 the UCS-2
-is not extensible beyond 0xFFFF.
+Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
+encoding, UCS-4 is a 32-bit encoding. Unlike UTF-16, UCS-2
+is not extensible beyond 0xFFFF, because it does not use surrogates.
=item UTF-7
=head2 Unicode in Perl on EBCDIC
The way Unicode is handled on EBCDIC platforms is still rather
-experimental. On such a platform references to UTF-8 encoding in this
+experimental. On such a platform, references to UTF-8 encoding in this
document and elsewhere should be read as meaning UTF-EBCDIC as
specified in Unicode Technical Report 16 unless ASCII vs EBCDIC issues
are specifically discussed. There is no C<utfebcdic> pragma or
-":utfebcdic" layer, rather "utf8" and ":utf8" are re-used to mean
-platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic> for
-more discussion of the issues.
+":utfebcdic" layer, rather, "utf8" and ":utf8" are re-used to mean
+the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
+for more discussion of the issues.
=back
projects / p5sagit/p5-mst-13.2.git / commitdiff