正则表达式

Regex regFileIds = new Regex(@"fileIds=(.*?)""", RegexOptions.Multiline);
//;fileIds(.*?)"
var fieldMatchs = regFileIds.Matches(valueStr);

foreach (Match match in fieldMatchs)
{
// idList.Add(match.Groups[1].Value.ToString());
var matchId = match.Groups[1].Value.ToString();
valueStr = valueStr.Insert(valueStr.IndexOf(matchId) + matchId.Length, $"&docId={sourceData["_id"].ToString()}");
}

匹配fileIds=和"中间的内容