I have strings that are similar to the ones below:
string str1 = "test_20150505";
string str2 = "test_20150505_yts";
string str3 = "test_all";
string str4 = "test";
The below regex code extracts date (20150505) for string str1 and for string str3 & string str4 returns empty, which is also fine.
However, what do I have to change so as to extract date for string str2?
string d = Regex.Match(str1, "^(?:.*_)?([0-9]{8})(?:\\..*)?$").Groups[1].Value.ToString();
You can just check for optional _s around 8-digits numbers:
(?<=_|\b)[0-9]{8}(?=_|\b)
See demo
var rx = new Regex(@"(?<=_|\b)[0-9]{8}(?=_|\b)");
var inputs = "test_20150505\n\test_20150505_yts\ntest_all\ntest";
var results = rx.Matches(inputs).OfType<Match>().Select(p => p.Value).ToList();
Then, you can parse the date, e.g.:
DateTime datetime;
DateTime.TryParseExact(results[0], "yyyyMMdd", new System.Globalization.CultureInfo("en-us"), System.Globalization.DateTimeStyles.None, out datetime);
Your last part of the regex pattern seems to be a typo. Use
string d = Regex.Match(str1, "^(?:.*_)?([0-9]{8})(?:_.*)?$").Groups[1].Value.ToString();
instead.
(?:\\..*)?part in your regex? Non of yours examples dates ends with..