// Find STA No. (in email body)
  Opportunity Opp = [SELECT id, STA_No__c Opportunity WHERE Id = :matcher.group(0)];
  String staNo = email.plainTextBody;
  Pattern staPattern = Pattern.compile('STA[0-9]{4}');
  Matcher matcherSta = staPattern.matcher(staNo);
  if (matcherSta.find())
    Opp.STA_No__c = matcherSta.group(0);