using System.Text.RegularExpressions;

private bool RegexMatch(string sSource, string sRegexFormat)
        {
            Regex reg = new Regex(sRegexFormat,RegexOptions.IgnoreCase);
            try
            {
                if (reg.IsMatch(sSource))
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
            catch (Exception ex)
            {
                return false;
            }
        }

RegexMatch(stringSource, "Mbr_Elig_Term_Date IS NULL")

RegexMatch(stringSource, "ISNULL\(.*MBR_ELIG_TERM_DATE.*\)")

RegexMatch(stringSource, "ISNULL\(.{0,10}MBR_ELIG_TERM_DATE.{0,10}\)")

RegexMatch(stringSource, "ISNULL\(.{0,20}[.[]MBR_ELIG_TERM_DATE.{0,10}\)")