(?:href="(?<url>[^"]*)")|(?:href='(?<url>[^']*)')|(?:src="(?<url>[^"]*)")|(?:src='(?<url>[^']*)')

string ResultString = null;
try {
  Regex RegexObj = new Regex("(?:href=\"(?<url>[^\"]*)\")|(?:href='(?<url>[^']*)')|(?:src=\"(?<url>[^\"]*)\")|(?:src='(?<url>[^']*)')",
    RegexOptions.IgnoreCase);
  ResultString = RegexObj.Match(SubjectString).Groups["groupname"].Value;
} catch (ArgumentException ex) {
  // Syntax error in the regular expression
}