Regex for conditional replace to add leading zeros in a date

Question

I have a list of dates in dd/mm/yyyy format, except some of the days and months are only a single digit. For example,

14/09/2019 - dd/mm/yyyy (correct)
1/10/2018 - d/mm/yyyy (should be 01/10/2018)
1/2/2018 - d/m/yyyy (should be 01/02/2018)

I want to convert all these dates to the accepted form which is dd/mm/yyyy .

I am only suppose to use REGEX here. Also I think I will need conditional regex replace here.

The pseudo code would be like :

1. If the date is 1/10/2018 then format it to 01/10/2018 .
2. If the date is 1/2/2018 then format it to 01/02/2018 .

Sorry, but I don't have any code to demonstrate this. I am stuck with this.

Answer 1

Use C#'s regex replace match evaluator , which is a delegate operation which is run everytime a match occurs.

For this, every time a match is hit, convert what was found to the format desired:

var text = @"1/2/2018";

Regex.Replace(text,
              @"\d?\d/\d?\d/\d\d\d\d",
              new MatchEvaluator((mtch) => 
                         DateTime.Parse(mtch.Groups[0].Value).ToString("MM/dd/yyyy")));

Replace returns this string "01/02/2018" ;

---

Note that I handle only one situation, one can expand the above delegate to read the current match and determine the return format needed.

Answer 2

You can use this method to check your date formats one by one

string convertDateToAcceptedFormat(string inputDate)
{
    Regex patternDDMMYYYY = new Regex(@"^(\d\d\/){2}\d{4}$");
    if (patternDDMMYYYY.Match(inputDate).Success)
        return inputDate;

    Regex patternDMMYYYY = new Regex(@"^\d\/\d\d\/\d{4}$");
    if (patternDMMYYYY.Match(inputDate).Success)
        return "0" + inputDate;

    Regex patternDMYYYY = new Regex(@"^(\d\/){2}\d{4}$");
    if (patternDMYYYY.Match(inputDate).Success)
        return "0" + inputDate.Substring(0,2) + "0" + inputDate.Substring(2);

    throw new Exception("Input date doesn't match any pattern");
}