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copied!<p>You can do this with a single split (Well, this may not be most efficient at runtime, but this is concise, lesser code to write):</p> <pre><code>String[] arr = str.split("(?<=\\G..)"); </code></pre> <p>And then get the <code>List<String></code> using <code>Arrays#asList()</code> method. </p> <p>The regex pattern splits on an empty space preceded by 2 characters - <code>..</code>, but ignoring the character already considered in previous match - <code>\\G</code>. The anchor <code>\\G</code> matches at the position where the previous match ended. </p> <pre><code>String str = "00122334455667788990875645346787659870984780"; String[] arr = str.split("(?<=\\G..)"); System.out.println(Arrays.asList(arr)); </code></pre> <p>prints:</p> <pre><code>[00, 12, 23, 34, 45, 56, 67, 78, 89, 90, 87, 56, 45, 34, 67, 87, 65, 98, 70, 98, 47, 80] </code></pre> <hr /> <p>Here's how split is done on your string:</p> <pre><code> " 00 1 2 2334455667788990875645346787659870984780" (whitespaces represent empty string) // | | | // split, no-split, split -> gives 12 // | | | | // \ / \ / // gives 00 as the preceding two characters are `1` and `0`. // but 0 is already considered for the previous empty string </code></pre> <hr /> <p><strong>Reference:</strong></p> <ul> <li><a href="http://www.regular-expressions.info/continue.html" rel="nofollow">Continuing match using \G - Regular Expression info</a></li> </ul> <hr /> <p>If runtime performance is an issue, then you can go with simple looping:</p> <pre><code>String str = "00122334455667788990875645346787659870984780"; List<String> list = new ArrayList<String>(); for (int i = 0; i < str.length(); i += 2) { list.add(str.substring(i, i + 2)); } System.out.println(list); </code></pre> <p>But you can check for yourself, whether the regex split is really a performance bottleneck for large string, and benchmark both of them appropriately.</p> <hr /> <p>I benchmarked both the methods - split, and loop. And as expected loop is almost 4-5 times more efficient than split for a string of length say <code>1000</code>.</p> <pre><code>public static void usingSplit(String str) { String[] arr = str.split("(?<=\\G..)"); List<String> list = Arrays.asList(arr); } public static void usingLoop(String str) { List<String> list = new ArrayList<String>(); for (int i = 0; i < str.length(); i += 2) { list.add(str.substring(i, i + 2)); } } // Warm up JVM for (int i = 0; i < 1000000; ++i) { usingSplit(str); } for (int j = 0; j < 1000000; j++) { usingLoop(str); } long nano = System.nanoTime(); for (int i = 0; i < 1000000; ++i) { usingSplit(str); } System.out.println("Time with usingSplit(): " + (System.nanoTime() - nano) * 1.0 / Math.pow(10, 9) + " Seconds"); nano = System.nanoTime(); for (int j = 0; j < 1000000; j++) { usingLoop(str); } System.out.println("Time with usingLoop(): " + (System.nanoTime() - nano) * 1.0 / Math.pow(10, 9) + " Seconds"); </code></pre> <p>Output on few successive runs:</p> <pre><code>Run 1: Time with usingSplit(): 34.391315143 Seconds Time with usingLoop(): 7.515221612 Seconds Run 2: Time with usingSplit(): 33.41518869 Seconds Time with usingLoop(): 7.868896218 Seconds </code></pre> <p>If someone thinks that the benchmark result is flawed, then please make a note of it in comments.</p>

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