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copied!<p>This can be done using <a href="http://www.regular-expressions.info/lookaround.html" rel="noreferrer">positive lookahead</a> :</p> <pre><code>perl -pe 's/(.)(?=.*?\1)//g' FILE_NAME </code></pre> <p>The regex used is: <code>(.)(?=.*?\1)</code></p> <ul> <li><code>.</code> : to match any char.</li> <li>first <code>()</code> : remember the matched single char.</li> <li><code>(?=...)</code> : +ve lookahead</li> <li><code>.*?</code> : to match anything in between</li> <li><code>\1</code> : the remembered match.</li> <li><code>(.)(?=.*?\1)</code> : match and remember any char <strong><em>only if</em></strong> it appears again later in the string.</li> <li><code>s///</code> : Perl way of doing the substitution.</li> <li><code>g</code>: to do the substitution globally...that is don't stop after first substitution.</li> <li><code>s/(.)(?=.*?\1)//g</code> : this will delete a char from the input string only if that char appears again later in the string.</li> </ul> <p>This will <strong><em>not</em></strong> maintain the order of the char in the input because for every unique char in the input string, we retain its <strong><em>last</em></strong> occurrence and not the <strong><em>first</em></strong>.</p> <p>To keep the relative order intact we can do what <code>KennyTM</code> tells in one of the comments: </p> <ul> <li>reverse the input line</li> <li>do the substitution as before</li> <li>reverse the result before printing</li> </ul> <p>The Perl one line for this is:</p> <pre><code>perl -ne '$_=reverse;s/(.)(?=.*?\1)//g;print scalar reverse;' FILE_NAME </code></pre> <p>Since we are doing <code>print</code> manually after reversal, we don't use the <code>-p</code> flag but use the <code>-n</code> flag.</p> <p>I'm not sure if this is the best one-liner to do this. I welcome others to edit this answer if they have a better alternative.</p>

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