title: Java regex to remove space between numbers only
body: <p>I have some strings like:</p>
  
  <pre><code>String a = "Hello 12 2 3 4 45th World!";
  </code></pre>
  
  <p>I wanna concatenate numbers as: <code>"Hello 12234 45th World"</code></p>
  
  <p>By trying <code>a.replaceAll("(?&lt;=\\d) +(?=\\d)", "")</code>, I got the result as:
  <code>"Hello 1223445th World"</code>. </p>
  
  <p>Is there any way to concatenate only numbers, not <em>number</em>+<em>th</em>?</p>