How can i update a recurrence rule using Regex

I have a recurrence rule in the format specified by RFC 2445, and I need to update the start date and end date. Here are some examples:

DTSTART:20240422T000000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20240427T235959Z

DTSTART:20220826T040000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20220826T040000Z DTSTART:20220501T000000Z RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=SU;UNTIL=20230114T235959Z DTSTART:20220930T040000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20220930T040000Z

Since I couldn't find any library capable of this, I'm planning to create a function to update the start date and end date.

I created the following function

    private static string UpdateRecurrenceStartDate(DateTime startDate, DateTime endDate, string recurrence)
{
    var pattern = @"(DTSTART:[0-9]{8}T[0-9]{6}Z).*?(UNTIL=[0-9]{8}T[0-9]{6}Z)";
    var updatedRecurrence = Regex.Replace(recurrence, pattern, $"DTSTART:{startDate.ToString("yyyyMMddTHHmmssZ")} UNTIL={endDate.AddDays(1).AddSeconds(-1).ToString("yyyyMMddTHHmmssZ")}");
    
    return updatedRecurrence;
}

It's not working the start Date and endDate are not updated

Solution

Since you are replacing two different date strings, you'll need to run .Replace() twice... once for each date. In the second .Replace() you need to pass the results from the first .Replace(). Also, you don't need to match the entire string since you only want to replace the dates.

I added a separate regex for DTSTART and UNTIL date strings and then added a second .Replace(). I also moved the "Z" at the end of the time string outside of .ToString(). The updated method looks like

private static string UpdateRecurrenceStartDate(DateTime startDate, DateTime endDate, string recurrence)
{
    var patternStart = @"DTSTART:\d{8}T\d{6}Z";
    var updatedRecurrence = Regex.Replace(recurrence, patternStart, $"DTSTART:{startDate.ToString("yyyyMMddTHHmmss")}Z");
    var patternUntil = @"UNTIL=\d{8}T\d{6}Z";
    updatedRecurrence = Regex.Replace(updatedRecurrence, patternUntil, $"UNTIL={endDate.AddDays(1).AddSeconds(-1).ToString("yyyyMMddTHHmmss")}Z");

    return updatedRecurrence;
}

I tested it using your sample strings and the code below

string[] rules = ["DTSTART:20240422T000000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20240427T235959Z",
                  "DTSTART:20220826T040000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20220826T040000Z",
                  "DTSTART:20220501T000000Z RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=SU;UNTIL=20230114T235959Z" ,
                  "DTSTART:20220930T040000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20220930T040000Z"];
foreach (string rule in rules)
{
    Console.WriteLine($"old: {rule}");
    string newRule = UpdateRecurrenceStartDate(new DateTime(2000, 1, 1), new DateTime(2001, 1, 1), rule);
    Console.WriteLine($"new: {newRule}");
    Console.WriteLine();
}

and it output

old: DTSTART:20240422T000000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20240427T235959Z
new: DTSTART:20000101T000000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20010101T235959Z

old: DTSTART:20220826T040000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20220826T040000Z
new: DTSTART:20000101T000000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20010101T235959Z

old: DTSTART:20220501T000000Z RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=SU;UNTIL=20230114T235959Z
new: DTSTART:20000101T000000Z RRULE:FREQ=WEEKLY;INTERVAL=1;BYDAY=SU;UNTIL=20010101T235959Z

old: DTSTART:20220930T040000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20220930T040000Z
new: DTSTART:20000101T000000Z RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20010101T235959Z