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    <p>My initial approach could by summarized this way:</p> <p>1) Reverse bytes naively</p> <p>2) Run the string backwards and fix the utf8 sequences as you go. </p> <p>Illegal sequences are dealt with in the second step and in the first step, we check if the string is in "sync" (that is, if it starts with a legal leading byte).</p> <p>EDIT: improved validation for leading byte in Reverse()</p> <pre><code>class UTF8Utils { public static void Reverse(byte[] str) { int len = str.Length; int i = 0; int j = len - 1; // first, check if the string is "synced", i.e., it starts // with a valid leading character. Will check for illegal // sequences thru the whole string later. byte leadChar = str[0]; // if it starts with 10xx xxx, it's a trailing char... // if it starts with 1111 10xx or 1111 110x // it's out of the 4 bytes range. // EDIT: added validation for 7 bytes seq and 0xff if( (leadChar &amp; 0xc0) == 0x80 || (leadChar &amp; 0xfc) == 0xf8 || (leadChar &amp; 0xfe) == 0xfc || (leadChar &amp; 0xff) == 0xfe || leadChar == 0xff) { throw new Exception("Illegal UTF-8 sequence"); } // reverse bytes in-place naïvely while(i &lt; j) { byte tmp = str[i]; str[i] = str[j]; str[j] = tmp; i++; j--; } // now, run the string again to fix the multibyte sequences UTF8Utils.ReverseMbSequences(str); } private static void ReverseMbSequences(byte[] str) { int i = str.Length - 1; byte leadChar = 0; int nBytes = 0; // loop backwards thru the reversed buffer while(i &gt;= 0) { // since the first byte in the unreversed buffer is assumed to be // the leading char of that byte, it seems safe to assume that the // last byte is now the leading char. (Given that the string is // not out of sync -- we checked that out already) leadChar = str[i]; // check how many bytes this sequence takes and validate against // illegal sequences if(leadChar &lt; 0x80) { nBytes = 1; } else if((leadChar &amp; 0xe0) == 0xc0) { if((str[i-1] &amp; 0xc0) != 0x80) { throw new Exception("Illegal UTF-8 sequence"); } nBytes = 2; } else if ((leadChar &amp; 0xf0) == 0xe0) { if((str[i-1] &amp; 0xc0) != 0x80 || (str[i-2] &amp; 0xc0) != 0x80 ) { throw new Exception("Illegal UTF-8 sequence"); } nBytes = 3; } else if ((leadChar &amp; 0xf8) == 0xf0) { if((str[i-1] &amp; 0xc0) != 0x80 || (str[i-2] &amp; 0xc0) != 0x80 || (str[i-3] &amp; 0xc0) != 0x80 ) { throw new Exception("Illegal UTF-8 sequence"); } nBytes = 4; } else { throw new Exception("Illegal UTF-8 sequence"); } // now, reverse the current sequence and then continue // whith the next one int back = i; int front = back - nBytes + 1; while(front &lt; back) { byte tmp = str[front]; str[front] = str[back]; str[back] = tmp; front++; back--; } i -= nBytes; } } } </code></pre>
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    1. POHow do I reverse a UTF-8 string in place?
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    1. USJuan Pablo Califano
    1. USJuan Pablo Califano
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