String contains only a given set of characters

Question

I need to know if a given string is a valid DateTime format string because the string may represent other things. I tried DateTime.ParseExact(somedate.ToString(format), format)

Answer 1

Like this:

static readonly Regex Validator = new Regex(@"^[yYmMdDsShH]+$");

public static bool IsValid(string str) {
    return Validator.IsMatch(str);
}

The regex works like this:

• ^ matches the beginning of the string
• [...] matches any of the characters that appear in the brackets
• + matches one or more characters that match the previous item
• $ matches the end of the string

Without the ^ and $ anchors, the regex will match any string that contains at least one valid character, because a regex can match any substring of the string use pass it. The ^ and $ anchors force it to match the entire string.