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    copied!<p>I threw together a totally unoptimized program based on your discarded regex approach and timed it. It completed in 650 ms (250 ms with warmup). The slowest part doesn't involve the regex, but <code>String.format</code>. If we replace it with a straight <code>StringBuilder</code> approach, the time drops to 40 ms.</p> <pre><code>public class Test { static Pattern regex = Pattern.compile("(..)(..)(..)(..)(..)(..)"); public static void main(String[] args) { final List&lt;String&gt; inMacs = new ArrayList&lt;&gt;(), outMacs = new ArrayList&lt;&gt;(); for (int i = 0; i &lt; 80_000; i++) inMacs.add(mac()); final long start = System.nanoTime(); for (String mac : inMacs) { final Matcher m = regex.matcher(mac); m.matches(); outMacs.add(String.format("%s:%s:%s:%s:%s:%s", m.group(1), m.group(2), m.group(3), m.group(4), m.group(5), m.group(6))); } System.out.println("Took " + (System.nanoTime() - start)/1_000_000 + " milliseconds"); final Iterator&lt;String&gt; it = outMacs.iterator(); for (int i = 0; i &lt; 100; i++) System.out.println(it.next()); } static Random rnd = new Random(); static String mac() { final long mac = (long) (rnd.nextDouble()*(1L&lt;&lt;48)); return String.format("%012x", mac).toUpperCase(); } } </code></pre> <p>If you are really looking for a fast solution, then avoid the regex and use a simple test to detect your MAC format:</p> <pre><code> static List&lt;String&gt; fixMacs(List&lt;String&gt; inMacs) { final List&lt;String&gt; outMacs = new ArrayList&lt;&gt;(inMacs.size()); for (String mac : inMacs) outMacs.add( mac.charAt(2) == '-'? mac.replace("-", ":") : mac.charAt(2) != ':'? fixMac(mac) : mac); return outMacs; } static String fixMac(String inMac) { final StringBuilder b = new StringBuilder(18); for (int i = 0; i &lt; inMac.length(); i++) { b.append(inMac.charAt(i)); if (i%2 == 1 &amp;&amp; i != inMac.length()-1) b.append(':'); } return b.toString(); } </code></pre> <p>With this approach I measured just 8 ms for your 80,000 MACs.</p>
 

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