title: How to check if a string column in pyspark dataframe is all numeric
body: <p>I have a PySpark <code>Dataframe</code> with a column of <code>strings</code>. How can I check which rows in it are Numeric. I could not find any function in PySpark's <a href="http://spark.apache.org/docs/2.1.0/api/python/pyspark.sql.html" rel="noreferrer">official documentation</a>.</p>
  <pre><code>values = [('25q36',),('75647',),('13864',),('8758K',),('07645',)]
  df = sqlContext.createDataFrame(values,['ID',])
  df.show()
  +-----+
  |   ID|
  +-----+
  |25q36|
  |75647|
  |13864|
  |8758K|
  |07645|
  +-----+
  </code></pre>
  <p>In Python, there is a function <code>.isDigit()</code> which returns <code>True</code> or <code>False</code> if the <code>string</code> contains just numbers or not.</p>
  <p>Expected DataFrame:</p>
  <pre><code>+-----+-------+
  |   ID| Value |
  +-----+-------+
  |25q36| False |
  |75647| True  |
  |13864| True  |
  |8758K| False |
  |07645| True  |
  +-----+-------+
  </code></pre>
  <p>I would like to avoid creating a <code>UDF</code>.</p>