Parsing Google calendar to DDay.iCal

Question

I'm working on application which parses Google Calendar via Google API to DDay.iCal

The main attributes, properties are handled easily... ev.Summary = evt.Title.Text;

The problem is when I got an recurring event, the XML contains a field like:

<gd:recurrence>
    DTSTART;VALUE=DATE:20100916
    DTEND;VALUE=DATE:20100917
    RRULE:FREQ=YEARLY
</gd:recurrence>

or

<gd:recurrence>
  DTSTART:20100915T220000Z
  DTEND:20100916T220000Z
  RRULE:FREQ=YEARLY;BYMONTH=9;WKST=SU"
</gd:recurrence>

using the following code:

    String[] lines = 
evt.Recurrence.Value.Split(new char[] { '\n', '\r' }, StringSplitOptions.RemoveEmptyEntries);

                    foreach (String line in lines)
                    {

                        if (line.StartsWith("R"))
                        {
                            RecurrencePattern rp = new RecurrencePattern(line);
                            ev.RecurrenceRules.Add(rp);
                        }
                        else 
                        {
                            ISerializationContext ctx = new SerializationContext();
                            ISerializerFactory factory = new DDay.iCal.Serialization.iCalendar.SerializerFactory();

                            ICalendarProperty property = new CalendarProperty();

                            IStringSerializer serializer = factory.Build(property.GetType(), ctx) as IStringSerializer;

                            property = (ICalendarProperty)serializer.Deserialize(new StringReader(line));

                            ev.Properties.Add(property);
                            Console.Out.WriteLine(property.Name + " - " + property.Value);
                        }

                    }

RRULEs are parsed correctly, but the problem is that other property (datetimes) values are empty...

Did you mean to use colons after DTSTART and DTEND in your first example? And is there meant to be a double quote after SU in the second example? — Daniel Moore, Jul 09 '11 at 20:43

score 5 · Accepted Answer · edited Oct 07 '21 at 05:54

Here is the starting point of what I'm doing, going off of the RFC-5545 spec's recurrence rule. It isn't complete to the spec and may break given certain input, but it should get you going. I think this should all be doable using RegEx, and something as heavy as a recursive decent parser would be overkill.

I am building this up using http://regexstorm.net/tester.

The test input I'm using is:

DTSTART;TZID=America/Chicago:20140711T133000\nDTEND;TZID=America/Chicago:20140711T163000\nRRULE:FREQ=WEEKLY;INTERVAL=8;BYDAY=FR;UNTIL=20141101

DTSTART;TZID=America/Chicago:20140711T133000\nDTEND;TZID=America/Chicago:20140711T163000\nRRULE:FREQ=WEEKLY;COUNT=5;INTERVAL=8;BYDAY=FR;UNTIL=20141101

DTSTART;TZID=America/Chicago:20140711T133000\nDTEND;TZID=America/Chicago:20140711T163000\nRRULE:FREQ=WEEKLY;BYDAY=FR;UNTIL=20141101

Sample matching results would look like:

Index    Position    Matched String                                                 $1      $2  $3  $4  $5
0        90          RRULE:FREQ=WEEKLY;INTERVAL=8;BYDAY=FR;UNTIL=20141101           WEEKLY      8   FR  20141101
1        236         RRULE:FREQ=WEEKLY;COUNT=5;INTERVAL=8;BYDAY=FR;UNTIL=20141101   WEEKLY  5   8   FR  20141101
2        390         RRULE:FREQ=WEEKLY;BYDAY=FR;UNTIL=20141101                      WEEKLY          FR  20141101

Usage is like:

string freqPattern = @"RRULE:(?:FREQ=(DAILY|WEEKLY|SECONDLY|MINUTELY|HOURLY|DAILY|WEEKLY|MONTHLY|YEARLY);?)?(?:COUNT=([0-9]+);?)?(?:INTERVAL=([0-9]+);?)?(?:BYDAY=([A-Z,]+);?)?(?:UNTIL=([0-9]+);?)?";
MatchCollection mc = Regex.Matches(rule, freqPattern, System.Text.RegularExpressions.RegexOptions.IgnoreCase);
foreach (Match m in mc)
{
    string frequency = m.Groups[1].ToString();
    string count = m.Groups[2].ToString();
    string interval = m.Groups[3].ToString();
    string byday = m.Groups[4].ToString();
    string until = m.Groups[5].ToString();
    System.Console.WriteLine("recurrence => frequency: \"{0}\", count: \"{1}\", interval: \"{2}\", byday: \"{3}\", until: \"{4}\"", frequency, count, interval, byday, until);
}

score -1 · Answer 2 · answered Jul 09 '11 at 20:56

This is a great example of when to use regular expressions. Try this out for general parsing:

\s*(\w+):((\w+=\w+;)+(\w+=\w+)?|\w+)

Or, you might decide to have something more schema-specific.

\s*(?:DTSTART:)(?'Start'\w+)
\s*(?:DTEND:)(?'End'\w+)
\s*(?:RRULE:)(?'Rule'(\w+=\w+;)+(\w+=\w+)?)

Parsing Google calendar to DDay.iCal

2 Answers2