title: RegexValidator is not allowing 0-9 in CharField
body: <p>I am wanting to allow <em>only</em> characters 0-9 for all characters entered into a Django <code>CharField</code>.</p>
  
  <p>When I use <code>RegexValidator()</code> with <code>regex = r'[^0-9]'</code>. I can enter everything <em>but</em> 0-9 in the field.  Actually, it will allow me to enter a strings like <code>"word"</code>, <code>"a4523"</code>, <code>"!#$%#^%"</code>, and <code>"5432bc"</code>, but <em>not</em> <code>"4523"</code>.</p>
  
  <p>If I take out the <code>^</code> and set <code>regex = r'[0-9]'</code> (see code below) then I <em>can</em> enter <code>"4523"</code>, <code>"5432bc"</code>, and <code>"a4523"</code> but <em>not</em> <code>"word"</code> or <code>"!#$%#^%"</code>. </p>
  
  <p>Removing the <code>^</code> seems to get me in the right direction, but not the full way.  I do not want to use a <code>PositiveSmallIntegerField</code> because I want to treat the numeric input as a string.</p>
  
  <pre><code>class Number(models.Model):
      number = models.CharField(
          max_length=14,
          validators=[
              RegexValidator(
                  r'[0-9]',
                  'Only 0-9 are allowed.',
                  'Invalid Number'
              ),
              MinLengthValidator(4),
              MaxLengthValidator(14),
          ],
      )
  </code></pre>