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    <p>The accepted answer:</p> <pre><code>String commonChars = s.replaceAll("[^"+t+"]",""); </code></pre> <p>has a bug!!!</p> <p>What if the string <code>t</code> has a regex meta-character? In that case the <code>replaceAll</code> fails.</p> <p><a href="http://www.ideone.com/dsvjm" rel="nofollow"><strong>See this program</strong></a> as an example where the string <code>t</code> has <code>]</code> in it and <code>]</code> is a regex meta-character which marks the end of the character class. Clearly the program does not produce the expected output.</p> <p>Why?</p> <p>Consider:</p> <pre><code>String s = "1479K"; String t = "459LP]"; </code></pre> <p>Now the regex will become(just substitute <code>t</code>):</p> <pre><code>String commonChars = s.replaceAll("[^459LP]]",""); </code></pre> <p>Which says replace any character other than <code>4</code>,<code>5</code>,<code>9</code>,<code>L</code>,<code>P</code> <strong>followed by a</strong> <code>]</code> with nothing. Which is clearly not what you want.</p> <p>To fix these you need to escape the <code>]</code> in <code>t</code>. You can do it manually as:</p> <pre><code>String t = "459LP\\]"; </code></pre> <p>and the <a href="http://www.ideone.com/l6ySq" rel="nofollow"><strong>regex works fine</strong></a>.</p> <p>This is a common problem when using regex, so the <code>java.util.regex.Pattern</code> class provides a static method named <a href="http://download.oracle.com/javase/1.5.0/docs/api/java/util/regex/Pattern.html#quote%28java.lang.String%29" rel="nofollow"><strong>quote</strong></a> which can be used to do exactly this: quote the regex-metacharacters so that they are treated literally. </p> <p>So before using <code>t</code> in <code>replaceAll</code> you quote it as:</p> <pre><code>t = Pattern.quote(t); </code></pre> <p><a href="http://www.ideone.com/vV0m8" rel="nofollow"><strong>Program using quote method</strong></a> works as expected.</p>
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    1. POHow do I find the characters common to two strings in Java using single replaceAll?
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    1. CODoes this mean I need to do: String commonChars = s.replaceAll("[^"+Pattern.quote(t)+"]",""); instead? Will this still check for each character individually?
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