title: Check if string contains only positive numbers in Ruby
body: <p>I am really new to <a href="/questions/tagged/ruby" class="post-tag" title="show questions tagged &#39;ruby&#39;" rel="tag">ruby</a> and want know if it's possible to check if a string contains only positive numbers using <a href="/questions/tagged/regex" class="post-tag" title="show questions tagged &#39;regex&#39;" rel="tag">regex</a> or some other function?</p>
  
  <pre><code>str = "123abcd"  #return false because it contains alphabets
  str = "153"      #return true because of all numbers
  </code></pre>