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

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.