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    <p>That's all part of the UTF8 encoding (which is only one encoding scheme for Unicode).</p> <p>The size can figured out by examining the first byte as follows:</p> <ul> <li>if it starts with bit pattern <code>"10" (0x80-0xbf)</code>, it's not the first byte of a sequence and you should back up until you find the start, any byte that starts with "0" or "11" (thanks to Jeffrey Hantin for pointing that out in the comments).</li> <li>if it starts with bit pattern <code>"0" (0x00-0x7f)</code>, it's 1 byte.</li> <li>if it starts with bit pattern <code>"110" (0xc0-0xdf)</code>, it's 2 bytes.</li> <li>if it starts with bit pattern <code>"1110" (0xe0-0xef)</code>, it's 3 bytes.</li> <li>if it starts with bit pattern <code>"11110" (0xf0-0xf7)</code>, it's 4 bytes.</li> </ul> <p>I'll duplicate the table showing this, but the original is on the Wikipedia UTF8 page <a href="http://en.wikipedia.org/wiki/UTF-8" rel="noreferrer">here</a>.</p> <pre><code>+----------------+----------+----------+----------+----------+ | Unicode | Byte 1 | Byte 2 | Byte 3 | Byte 4 | +----------------+----------+----------+----------+----------+ | U+0000-007F | 0xxxxxxx | | | | | U+0080-07FF | 110yyyxx | 10xxxxxx | | | | U+0800-FFFF | 1110yyyy | 10yyyyxx | 10xxxxxx | | | U+10000-10FFFF | 11110zzz | 10zzyyyy | 10yyyyxx | 10xxxxxx | +----------------+----------+----------+----------+----------+ </code></pre> <p>The Unicode characters in the above table are constructed from the bits:</p> <pre><code>000z-zzzz yyyy-yyyy xxxx-xxxx </code></pre> <p>where the <code>z</code> and <code>y</code> bits are assumed to be zero where they're not given. Some bytes are considered illegal as a start byte since they're either:</p> <ul> <li>useless: a 2-byte sequence starting with 0xc0 or 0xc1 actually gives a code point less than 0x80 which can be represented better with a 1-byte sequence.</li> <li>used by RFC3629 for 4-byte sequence above U+10FFFF, or 5-byte and 6-byte sequences. These are the bytes 0xf5 through 0xfd.</li> <li>just unused: bytes 0xfe and 0xff.</li> </ul> <p>In addition, subsequent bytes in a multi-byte sequence that don't begin with the bits "10" are also illegal.</p> <p>As an example, consider the sequence [0xf4,0x8a,0xaf,0x8d]. This is a 4-byte sequence as the first byte falls between 0xf0 and 0xf7.</p> <pre><code> 0xf4 0x8a 0xaf 0x8d = 11110100 10001010 10101111 10001101 zzz zzyyyy yyyyxx xxxxxx = 1 0000 1010 1011 1100 1101 z zzzz yyyy yyyy xxxx xxxx = U+10ABCD </code></pre> <p>For your specific query with the first byte 0xe6 (length = 3), the byte sequence is:</p> <pre><code> 0xe6 0xbe 0xb3 = 11100110 10111110 10110011 yyyy yyyyxx xxxxxx = 01101111 10110011 yyyyyyyy xxxxxxxx = U+6FB3 </code></pre> <p>If you look that code up <a href="http://www.cojak.org/index.php?function=code_lookup&amp;term=6FB3" rel="noreferrer">here</a>, you'll see it's the one you had in your question: 澳.</p> <p>To show how the decoding works, I went back to my archives to find my UTF8 handling code. I've had to morph it a bit to make it a complete program and the encoding has been removed (since the question was really about decoding), so I hope I haven't introduced any errors from the cut and paste:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;string.h&gt; #define UTF8ERR_TOOSHORT -1 #define UTF8ERR_BADSTART -2 #define UTF8ERR_BADSUBSQ -3 typedef unsigned char uchar; static int getUtf8 (uchar *pBytes, int *pLen) { if (*pLen &lt; 1) return UTF8ERR_TOOSHORT; /* 1-byte sequence */ if (pBytes[0] &lt;= 0x7f) { *pLen = 1; return pBytes[0]; } /* Subsequent byte marker */ if (pBytes[0] &lt;= 0xbf) return UTF8ERR_BADSTART; /* 2-byte sequence */ if ((pBytes[0] == 0xc0) || (pBytes[0] == 0xc1)) return UTF8ERR_BADSTART; if (pBytes[0] &lt;= 0xdf) { if (*pLen &lt; 2) return UTF8ERR_TOOSHORT; if ((pBytes[1] &amp; 0xc0) != 0x80) return UTF8ERR_BADSUBSQ; *pLen = 2; return ((int)(pBytes[0] &amp; 0x1f) &lt;&lt; 6) | (pBytes[1] &amp; 0x3f); } /* 3-byte sequence */ if (pBytes[0] &lt;= 0xef) { if (*pLen &lt; 3) return UTF8ERR_TOOSHORT; if ((pBytes[1] &amp; 0xc0) != 0x80) return UTF8ERR_BADSUBSQ; if ((pBytes[2] &amp; 0xc0) != 0x80) return UTF8ERR_BADSUBSQ; *pLen = 3; return ((int)(pBytes[0] &amp; 0x0f) &lt;&lt; 12) | ((int)(pBytes[1] &amp; 0x3f) &lt;&lt; 6) | (pBytes[2] &amp; 0x3f); } /* 4-byte sequence */ if (pBytes[0] &lt;= 0xf4) { if (*pLen &lt; 4) return UTF8ERR_TOOSHORT; if ((pBytes[1] &amp; 0xc0) != 0x80) return UTF8ERR_BADSUBSQ; if ((pBytes[2] &amp; 0xc0) != 0x80) return UTF8ERR_BADSUBSQ; if ((pBytes[3] &amp; 0xc0) != 0x80) return UTF8ERR_BADSUBSQ; *pLen = 4; return ((int)(pBytes[0] &amp; 0x0f) &lt;&lt; 18) | ((int)(pBytes[1] &amp; 0x3f) &lt;&lt; 12) | ((int)(pBytes[2] &amp; 0x3f) &lt;&lt; 6) | (pBytes[3] &amp; 0x3f); } return UTF8ERR_BADSTART; } static uchar htoc (char *h) { uchar u = 0; while (*h != '\0') { if ((*h &gt;= '0') &amp;&amp; (*h &lt;= '9')) u = ((u &amp; 0x0f) &lt;&lt; 4) + *h - '0'; else if ((*h &gt;= 'a') &amp;&amp; (*h &lt;= 'f')) u = ((u &amp; 0x0f) &lt;&lt; 4) + *h + 10 - 'a'; else return 0; h++; } return u; } int main (int argCount, char *argVar[]) { int i; uchar utf8[4]; int len = argCount - 1; if (len != 4) { printf ("Usage: utf8 &lt;hex1&gt; &lt;hex2&gt; &lt;hex3&gt; &lt;hex4&gt;\n"); return 1; } printf ("Input: (%d) %s %s %s %s\n", len, argVar[1], argVar[2], argVar[3], argVar[4]); for (i = 0; i &lt; 4; i++) utf8[i] = htoc (argVar[i+1]); printf (" Becomes: (%d) %02x %02x %02x %02x\n", len, utf8[0], utf8[1], utf8[2], utf8[3]); if ((i = getUtf8 (&amp;(utf8[0]), &amp;len)) &lt; 0) printf ("Error %d\n", i); else printf (" Finally: U+%x, with length of %d\n", i, len); return 0; } </code></pre> <p>You can run it with your sequence of bytes (you'll need 4 so use 0 to pad them out) as follows:</p> <pre><code>&gt; utf8 f4 8a af 8d Input: (4) f4 8a af 8d Becomes: (4) f4 8a af 8d Finally: U+10abcd, with length of 4 &gt; utf8 e6 be b3 0 Input: (4) e6 be b3 0 Becomes: (4) e6 be b3 00 Finally: U+6fb3, with length of 3 &gt; utf8 41 0 0 0 Input: (4) 41 0 0 0 Becomes: (4) 41 00 00 00 Finally: U+41, with length of 1 &gt; utf8 87 0 0 0 Input: (4) 87 0 0 0 Becomes: (4) 87 00 00 00 Error -2 &gt; utf8 f4 8a af ff Input: (4) f4 8a af ff Becomes: (4) f4 8a af ff Error -3 &gt; utf8 c4 80 0 0 Input: (4) c4 80 0 0 Becomes: (4) c4 80 00 00 Finally: U+100, with length of 2 </code></pre>
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