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How to Allow Characters in Certain Spot in String

I'm trying to parse a date. The problem, is my regular expression omits any letter because I want to avoid 01-28-2019 UTC or any letters outside of the main date. Now, it works fine when the date is formatted like I just listed, however it fails when we get a date formatted like 28-JAN-19.
var sourceValue = Regex.Replace("28-JAN-19", #"[A-Za-z]", "");
var parsed = DateTime.Parse(sourceValue);
The date I need to parse can be in a few different formats. Can a regular expression be used to handle this? If so, what tweaks are needed to trim any letters outside of the xx-xx-xx part of the string?
28-JAN-19
28-01-19
28-JAN-19 13:15:00
28-01-19 13:15:00
28-01-2019 13:15:00

This RegEx should match all the examples you provided:
[0-9]{2}-([A-Za-z]{3}|[0-9]{2})-[0-9]{2,4}( [0-9][0-9]?:[0-9][0-9]?:[0-9][0-9])?
It does make a couple of assumptions though, based on your examples. First, it assumes all your dates will always start with a 2-digit day. It also assumes that your month abbreviations will be 3 letters long. It also assumes that your hours, minutes and seconds will all be 2 digits long. Let me know if any of these assumptions are incorrect.
Here is a fiddle

Regular expressions are likely not your best bet. If you know the full set of formats you might encounter then you can use the regular DateTime.ParseExact with a format string. Check for a FormatException to know if you've successfully parsed the date. If your months are using English abbreviations then be sure to pass in an English culture
DateTime.ParseExact("28-JAN-19", "dd-MMM-yy", new CultureInfo("en"));

Regular expression for matching both HH:MM and HH:MM:SS time format

I'd like to write a regular expression for matching both HH:MM:SS and MM:SS.
Match
99:43:22
1:43:22
01:43:22
1:43:22
43:22
so I've tried
(([0-5]?[0-9]):([0-5]?[0-9]))|(([0-9]?[0-9]):([0-5]?[0-9]):([0-5]?[0-9]))
What I wanted to add was just a single OR(|) syntax with two time regex.
But it doesn't match for HH:MM:SS
what am I missing?
I've already looked into those articles:
Regular expression for matching HH:MM time format
https://www.oreilly.com/library/view/regular-expressions-cookbook/9781449327453/ch04s06.html
http://regexlib.com/Search.aspx?k=time&AspxAutoDetectCookieSupport=1

Your question is unclear. From whatever little I understand, I would suggest the following regular expression:
^([01]\d?|2[0-4]):[0-5]\d(:[0-5]\d)?$
It makes sure that HH is between 00-24
It makes sure that MM is between 00-59
SS is optional (so it can match both HH:MM:SS and HH:MM), and if it
is there, it is between 00-59
There might be a more efficient method out there, but I can only think of this!

The expression before the alternation operator is matched before the expression after the alternation operator can even be tested.
If you anchor the start and end of these expressions, like so:
(^([0-5]?[0-9]):([0-5]?[0-9]))$|^(([0-9]?[0-9]):([0-5]?[0-9]):([0-5]?[0-9])$)
... you should get the behavior you expect.
If you are matching inside of a longer string, then you could put the longest match first:
((([0-9]?[0-9]):([0-5]?[0-9]):([0-5]?[0-9])|([0-5]?[0-9]):([0-5]?[0-9])))
or rewrite the regex a bit, as follows:
((\d{1,2}:)?[0-5]?\d:[0-5]?\d)

YYYY/MM/DD date format regular expression

I want to use regular expression for matching these date formats as below in C#.
YYYY/MM/DD 2013/11/12
YYYY/M/DD 2013/5/11
YYYY/MM/D 2013/10/5
YYYY/M/D 2013/5/6
I have tried some regular expressions but they can't match the 4 date formats.
such as
^(19|20)\d\d[- /.](0[1-9]|1[012])[- /.](0[1-9]|[12][0-9]|3[01])

check this to get an idea of the compexity of regex and validating dates. so i would use
\d{4}(?:/\d{1,2}){2}
then in c# do whatever to validate the match. while it can be done, you'll be spending a lot of time trying to achieve it, though there is a regex in that post that with a bit of fiddling supposedly will validate dates in regex, but it is a scary looking regex

Try
^\d{4}[-/.]\d{1,2}[-/.]\d{1,2}$
The curly braces {} give the number allowed. E.g., \d{1,2} means either one or two digits.

You may need more than that to match date. Try this:
(19|20)\d\d([-/.])(0?[1-9]|1[012])\2(0?[1-9]|[12][0-9]|3[01])

Ajit's regex is nearer to perfect but leaks the evaluation of the leap years that end with 12 and 16. Here is the correction to be just perfect
((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))-((0[13578])|(1[02]))-((0[1-9])|([12][0-9])|(3[01])))|((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))-((0[469])|11)-((0[1-9])|([12][0-9])|(30)))|(((000[48])|([0-9]0-9)|([0-9][1-9][02468][048])|([1-9][0-9][02468][048])|([0-9]0-9)|([0-9][1-9][13579][26])|([1-9][0-9][13579][26]))-02-((0[1-9])|([12][0-9])))|((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))-02-((0[1-9])|([1][0-9])|([2][0-8])))

((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))\-((0[13578])|(1[02]))\-((0[1-9])|([12][0-9])|(3[01])))|((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))\-((0[469])|11)\-((0[1-9])|([12][0-9])|(30)))|(((000[48])|([0-9][0-9](([13579][26])|([2468][048])))|([0-9][1-9][02468][048])|([1-9][0-9][02468][048]))\-02\-((0[1-9])|([12][0-9])))|((([0-9][0-9][0-9][1-9])|([1-9][0-9][0-9][0-9])|([0-9][1-9][0-9][0-9])|([0-9][0-9][1-9][0-9]))\-02\-((0[1-9])|([1][0-9])|([2][0-8])))
This is the regex for yyyy-MM-dd format.
You can replace - with \/ for yyyy/MM/dd...
Tested working perfect..

Try this. This accepts all four patterns
#"\d{4}[- /.]([1-9]|0[1-9]|1[012])[- /.]([1-9]|0[1-9]|[12][0-9]|3[01])"

.Net Regex for mm-dd-yy and others + yyymmdd

I feel like I am chasing my tail.
I am trying to arrive at a .Net regex that will match on the following:
mm-dd-yy
m-dd-yy
mm-d-yy
m-d-yy
and (no dashes)
yyyymmdd

One or two digits followed by a dash, followed by one or two digits, followed by a dash, follows by to digits or eight digits:
(\d{1,2}-\d{1,2}-\d{2})|(\d{8})

A very simple RegEx that just matches to the digits being in the correct places and matches all your formats:
^[0-9]{1,2}-[0-9]{1,2}-[0-9]{2}$|^[0-9]{8}$
This does not validate those dates as actual possible dates, to do this you would be better off using DateTime.TryParse

Do you need to use Regex, or do you just care that it is a valid date?
DateTime result;
if(DateTime.TryParse(input, out result))
{
// you have your date in result
}

Well it all comes down to how strict you want the matching to be.
[0-9]+\-[0-9]+\-[0-9]+
will match all the four top ones. But you can enter "99-99-99".
You can be a bit more strict with:
([0-1][0-2]|[0-9])\-(3[0-1]|[0-2]?[0-9]+)\-[0-9]{1,2}
This will only match dates where each component is in a valid range, but only to an extent. It would still match a February with 31 days (which it won't have in the Gregorian calendar). Also it will match 13-04-18 from 3 and onwards. You can use anchors to make it match the whole text (add ^ at the beginning and $ at the end of the regex), but then it won't be able to find the dates inside a text.
You can add a precondition to make sure there's no weird digits around it though. Negative look-behind and negative look-ahead.
(?<![0-9])([0-1][0-2]|[0-9])\-(3[0-1]|[0-2]?[0-9]+)\-[0-9]{1,2}(?![0-9])
And so forth, but this regex has already become a proper behemoth. I would go with the second or third version and use DateTime.Parse to validate it.
There's only so far you can go with regex for dates before they become write-once and insane :) (what about leap years for example, etc. etc.)

DateTime: how to display as DD.MM.YYYY?

I've got DateTime variable and i want to convert it to string "DD.MM.YYYY"
Please note, the values must be separated by "dot" sign.
Of course I can do manual string composition. But I wonder if I can use DateTime.ToString()
to do required conversion.

Yes, you can:
string formatted = dt.ToString("dd'.'MM'.'yyyy");
Now in this case the quotes aren't actually required, as custom date/time format strings don't interpret dot in any special way. However, I like to make it explicit - if change '.' for ':' for example, then while it's quoted it will stay with the explicit character, but unquoted it would be "the culture-specific time separator". It wasn't entirely obvious to me whether "." would be interpreted as "the culture-specific decimal separator" or not, hence the quoting. You may feel that's over the top, of course - it's entirely your decision.
You may also want to specify the invariant culture, just to remove any other traces of doubt:
string formatted = dt.ToString("dd'.'MM'.'yyyy", CultureInfo.InvariantCulture);
(At that point the quotes around the dot become less relevant, as "." is the decimal separator in the invariant culture anyway.)

Yes, you can use DateTime.ToString like this:
myDateVariable.ToString("dd.MM.yyyy");
Note that you have to use capital MM here, since mm evaluates to minutes instead of months.

Here's an alternative for you:
DateTime.Now.ToString("d", new CultureInfo("de-DE"))
German's use . as the date separator.

You can format the date like this:
date.ToString("dd.MM.yyyy")
When formatting numbers the period . will change depending on the CultureInfo used, but not when formatting dates.
If you are verifying your code against the code analysis rule CA1304: Specify CultureInfo you will have to use the invariant culture even though it doesn't matter for this particular format:
date.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture)