code: public final int start(int id) {
          MemReg b = bounds(id);
          if(b == null)
              return -1;
          return b.in - offset;
      }
docs: Returns the start index of the subsequence captured by the given group
  during this match.
  <br>
  Capturing groups are indexed from left
  to right, starting at one.  Group zero denotes the entire pattern, so
  the expression <i>m.</i><tt>start(0)</tt> is equivalent to
  <i>m.</i><tt>start()</tt>.
  @param  id
  The index of a capturing group in this matcher's pattern
  @return  The index of the first character captured by the group,
  or <tt>-1</tt> if the match was successful but the group
  itself did not match anything