How to match sequences of consecutive Date like characters string in Dart?

Question

I have consecutive characters as date like 20210215 and 14032020 I am trying to convert to date string like 2021.02.15 and 14.03.2020

My first problem is the consecutive characters it is in 2 format type. Like:

1) 20210215
2) 14032020

And my second problem to convert them to date string without changing the format. Like:

1) 2021.02.15
2) 14.03.2020

When I search about regex couldn't find any pattern to convert the above {20210215} consecutive characters examples to date {2021.02.15} string.

What is correct regex pattern to convert both format as I describe above in Dart?

UPDATE-1:

I need to turn this string "20210215" to this "2021.02.15" as a string and not DateTime. Also I need to turn this string "14032020" to this string "14.03.2020". And I don't want to turn to DateTime string.

First I need to detected if the year is in beginning of the string or end of it. Than put dot (.) between the day, month and year string.

UPDATE-2: this is best I can found but it turns 02 day or month to 2. But I need as it is.

  var timestampString = '13022020';//'20200213';

  var re1 = RegExp(
    r'^'
    r'(?<year>\d{4})'
    r'(?<month>\d{2})'
    r'(?<day>\d{2})'
    r'$',
  );

  var re2 = RegExp(
    r'^'
    r'(?<day>\d{2})'
    r'(?<month>\d{2})'
    r'(?<year>\d{4})'
    r'$',
  );

  var dateTime;
  var match1 = re1.firstMatch(timestampString);
  if (match1 == null) {
    var match2 = re2.firstMatch(timestampString);
    if (match2 == null) {
      //throw FormatException('Unrecognized timestamp format');
      dateTime = '00.00.0000';
      print('DATE_TIME: $dateTime');
    } else {
      var _day = int.parse(match2.namedGroup('day'));
      var _month = int.parse(match2.namedGroup('month'));
      var _year = int.parse(match2.namedGroup('year'));
      dateTime = '$_day.$_month.$_year';
      print('DATE_TIME(match2): $dateTime');
    }
  } else {
    var _year = int.parse(match1.namedGroup('year'));
    var _month = int.parse(match1.namedGroup('month'));
    var _day = int.parse(match1.namedGroup('day'));
    dateTime = '$_year.$_month.$_day';
    print('DATE_TIME(match1): $dateTime');
  }

Output:

DATE_TIME: 2020.2.13

But I need to get output as 2020.02.13.

Second is match1 also prints 1302.20.20 But if I remove var match2 section and if format is like 20200213 it works but doesn't print the 0 as I post it above.

Answer 1

You can use

text.replaceAllMapped(RegExp(r'\b(?:((?:19|20)\d{2})(0?[1-9]|1[0-2])(0?[1-9]|[12][0-9]|3[01])|(0?[1-9]|[12][0-9]|3[01])(0?[1-9]|1[0-2])((?:19|20)\d{2}))\b'), (Match m) => m[4] == null ? "${m[1]}.${m[2]}.${m[3]}" : "${m[4]}.${m[5]}.${m[6]}")

The \b(?:((?:19|20)\d{2})(0?[1-9]|1[0-2])(0?[1-9]|[12][0-9]|3[01])|(0?[1-9]|[12][0-9]|3[01])(0?[1-9]|1[0-2])((?:19|20)\d{2}))\b regex matches

• \b - a word boundary
• (?: - start of a non-capturing group:
  • ((?:19|20)\d{2}) - year from 20th and 21st centuries
  • (0?[1-9]|1[0-2]) - month
  • (0?[1-9]|[12][0-9]|3[01]) - day
• | - or
  • (0?[1-9]|[12][0-9]|3[01]) - day
  • (0?[1-9]|1[0-2]) - month
  • ((?:19|20)\d{2}) - year
• ) - end of the group
• \b - word boundary.

See the regex demo.

See a Dart demo:

void main() {
  final text = '13022020 and 20200213 20111919';
  print(text.replaceAllMapped(RegExp(r'\b(?:((?:19|20)\d{2})(0?[1-9]|1[0-2])(0?[1-9]|[12][0-9]|3[01])|(0?[1-9]|[12][0-9]|3[01])(0?[1-9]|1[0-2])((?:19|20)\d{2}))\b'), (Match m) => 
   m[4] == null ? "${m[1]}.${m[2]}.${m[3]}" : "${m[4]}.${m[5]}.${m[6]}"));
} 

Returning 13.02.2020 and 2020.02.13 20.11.1919.

If Group 4 is null, the first alternative matched, so we need to use Group 1, 2 and 3. Else, we join Group 4, 5 and 6 with a dot.