attern
  <i>subject</i>      Points to the subject string
  <i>length</i>       Length of the subject string
  <i>startoffset</i>  Offset in the subject at which to start matching
  <i>options</i>      Option bits
  <i>match_data</i>   Points to a match data block, for results
  <i>mcontext</i>     Points to a match context, or is NULL
</pre>
A match context is needed only if you want to:
<pre>
  Set up a callout function
  Set a matching offset limit
  Change the heap memory limit
  Change the backtracking