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copied!<p>This works with your testcases:</p> <pre><code>static String splitCamelCase(String s) { return s.replaceAll( String.format("%s|%s|%s", "(?<=[A-Z])(?=[A-Z][a-z])", "(?<=[^A-Z])(?=[A-Z])", "(?<=[A-Za-z])(?=[^A-Za-z])" ), " " ); } </code></pre> <p>Here's a test harness:</p> <pre><code> String[] tests = { "lowercase", // [lowercase] "Class", // [Class] "MyClass", // [My Class] "HTML", // [HTML] "PDFLoader", // [PDF Loader] "AString", // [A String] "SimpleXMLParser", // [Simple XML Parser] "GL11Version", // [GL 11 Version] "99Bottles", // [99 Bottles] "May5", // [May 5] "BFG9000", // [BFG 9000] }; for (String test : tests) { System.out.println("[" + splitCamelCase(test) + "]"); } </code></pre> <p>It uses zero-length matching regex with lookbehind and lookforward to find where to insert spaces. Basically there are 3 patterns, and I use <code>String.format</code> to put them together to make it more readable.</p> <p>The three patterns are:</p> <h3>UC behind me, UC followed by LC in front of me</h3> <pre><code> XMLParser AString PDFLoader /\ /\ /\ </code></pre> <h3>non-UC behind me, UC in front of me</h3> <pre><code> MyClass 99Bottles /\ /\ </code></pre> <h3>Letter behind me, non-letter in front of me</h3> <pre><code> GL11 May5 BFG9000 /\ /\ /\ </code></pre> <hr> <h3>References</h3> <ul> <li><a href="http://www.regular-expressions.info/lookaround.html" rel="noreferrer">regular-expressions.info/Lookarounds</a></li> </ul> <h3>Related questions</h3> <p>Using zero-length matching lookarounds to split:</p> <ul> <li><a href="https://stackoverflow.com/questions/2910536/regex-split-string-but-keep-separators">Regex split string but keep separators</a></li> <li><a href="https://stackoverflow.com/questions/2819933/java-split-is-eating-my-characters/">Java split is eating my characters</a></li> </ul>

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