title: How to check if a string is a number in flink sql
body: <p>In flink sql, how to check whether a string is a number, as </p>
  
  <pre><code>select * from input where str like '\\d+'
  </code></pre>
  
  <p>the regular expression seems not useful, and the 'similar to' operator can't work either. Is there some idea?</p>