title: Regexp starts with not working Elasticsearch 6.*
body: <p>I got trouble with understanding regexp mechanizm in ElasticSearch. I have documents that represent property units:</p>
  
  <pre><code>{
      "Unit" :
      {
           "DailyAvailablity" : 
           "UIAOUUUUUUUIAAAAAAAAAAAAAAAAAOUUUUIAAAAOUUUIAOUUUUUUUUUUUUUUUUUUUUUUUUUUIAAAAAAAAAAAAAAAAAAAAAAOUUUUUUUUUUIAAAAAOUUUUUUUUUUUUUIAAAAOUUUUUUUUUUUUUIAAAAAAAAOUUUUUUIAAAAAAAAAOUUUUUUUUUUUUUUUUUUIUUUUUUUUIUUUUUUUUUUUUUUIAAAOUUUUUUUUUUUUUIUUUUIAOUUUUUUUUUUUUUUUUUUUUUUUUUUUUUIAAAAAAAAAAAAOUUUUUUUUUUUUUUUUUUUUIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAOUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
      }
  }
  </code></pre>
  
  <p>DailyAvailability field codes availability of property by days for the next two years from today. 'A' means available, 'U' unabailable, 'I' can check in, 'O' can check out. How can I write regexp filter to get all units that are available in particular dates?</p>
  
  <p>I tried to find the 'A' substring with particular length and offset in DailyAvailability field. For example to find units that would be available for 7 days in 7 days from today:</p>
  
  <pre><code>{
   "query": {
     "bool": {
       "filter": [
          {
           "regexp": { "Unit.DailyAvailability": {"value": ".{7}a{7}.*" } }
          }
        ]
      }
    }
  }
  </code></pre>
  
  <p>This query returns for instance unit with DateAvailability that starts from "UUUUUUUUUUUUUUUUUUUIAA", but contains suitable sequences somehere inside the field. How can I anchor regexp for entire source string? ES docs say that lucene regex should be anchored by default.</p>
  
  <p>P.S. I have tried <code>'^.{7}a{7}.*$'</code>. Returns empty set.</p>