title: How to validate if a string only contains number chars in Ocaml
body: <p>I'm using Str.regexp, I want to know how to check if undetermined length string contains only number characters.</p>
  
  <p>This is what I'm doing:</p>
  
  <pre><code>Str.string_match "[0-9]+" "1212df3124" 0;;
  </code></pre>
  
  <p>The problem is it evaluates to true, but it should returns false because it contains 'df' substring. (This is not the same as C# regexp, it's Ocaml)</p>