使用NSDateFormater从字符串获取日期时，如何忽略某些字符？ 另外，NSDateFormatter有参考吗？

Question

My problem is simple. I have strings in the format "16th Sep 2015"

When generating the date string, I used the NSDateFormatter as "d MMM yyyy" and then manually modified it to insert the date suffixes using a switch case.

Now, I need to extract date again.

Is there any way to ignore those 2 letters?

Also, I have been learning NSDateFormatter by using Google and just following posts like this but all my questions are not answered and sometimes, there is a mismatch in behaviour described.

Is there a standard reference by Apple where the Date Formatter codes are described?

Answer 1

NSDateformatter does not support days with ordinal indicators, but you could use a regular expression to strip the indicators

var dateString = "16th Sep 2015"
if let range = dateString.range(of: "(st|nd|rd|th)", options: .regularExpression) {
    dateString.removeSubrange(range)
}    
let formatter = DateFormatter()
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.dateFormat = "dd MMM yyyy"
let date = formatter.date(from: dateString)!
print(date)

Or in Objective-C

NSMutableString *dateString = [NSMutableString stringWithString:@"16th Sep 2015"];

NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:@"(st|nd|rd|th)" options:nil error:nil];
[regex replaceMatchesInString:dateString options: nil range: NSMakeRange(0, 4) withTemplate:@""];
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.dateFormat = @"d MMM yyyy";
NSDate *date = [formatter dateFromString:dateString];
NSLog(@"%@", date);

Answer 2

This isn't the prettiest of solutions but it can work:

NSString *dateString = @"1st Sep 2015";

dateString = [dateString stringByReplacingOccurrencesOfString:@"st " withString:@" "];
dateString = [dateString stringByReplacingOccurrencesOfString:@"nd " withString:@" "];
dateString = [dateString stringByReplacingOccurrencesOfString:@"rd " withString:@" "];
dateString = [dateString stringByReplacingOccurrencesOfString:@"th " withString:@" "];

NSDateFormatter *df = [[NSDateFormatter alloc] init];
df.dateFormat = @"d MMM yyyy";

NSDate *date = [df dateFromString:dateString];

Answer 3

Only a partial answer, I am afraid, but Apple essentially implements this specification: http://www.unicode.org/reports/tr35/tr35-31/tr35-dates.html#Date_Format_Patterns

The specification does suggest something called "Lenient parsing" that allows the parser to ignore some extraneous characters, but at least as defined in the standard, this doesn't seem to extend to your particular case. So it certainly looks like you have to manually remove the suffix from the day number before handing the string to the parser, unless it happens to "just work", which I don't think it does.

Answer 4

While the will work if you have a month that is 3 letters, it will not work if you have the full month. For example, "Augu" will turn into "August" Using Regular Expressions, you can check for a digit to be before the letters by doing the following:

var dateString = "16th Sep 2015"
//regular expression using positive look behind. Checks for a digit, than a two letter combination that will match below
dateString = dateString.replacingOccurrences(of: "(?<=\\d)[snrt][tdh]", with: "", options: String.CompareOptions.regularExpression, range: nil)

let df = DateFormatter()
df.dateFormat = "d MMM yyyy" //if full month, add extra M
df.date(from: dateString)

Answer 5

Here is the code with CORRECT date format that will be your angel:

NSDateFormatter *df = [[NSDateFormatter alloc] init];
df.dateFormat = @"d'th' MMM yyyy";

NSString *stringDate = [df stringFromDate:[NSDate date]];

//output -- 16th Sep 2015

Now you can use this dateformatter to convert to and from NSDate object.