What?
A quick article to remind me of the regex to change a date from the format dd/MM/yyyy to yyyy-MM-dd.

Why?
Irrespective of server, organization or user settings, the date handling can vary. If we are getting a date from a CSV or other external source as "dd/MM/yyyy", how do we guarantee that the system will understand the date correctly?

How?
It may be that when obtaining a date string and applying the method .toString("dd/MM/yyyy") is dependent on the settings. But making a date into a SQL format or from largest denominator to smallest in "yyyy-MM-dd" will usually handle the date better. Consider the following: 
v_TestDate = "07/01/1975";
info v_TestDate;
info v_TestDate.toDate();
info v_TestDate.toString("E, d MMM yyyy");
info "*********************";
//
// regex change dd/MM/yyyy to yyyy-MM-dd
v_RegexdDate = v_TestDate.replaceAll("(\\d{2})/(\\d{2})/(\\d{4})","$3-$2-$1", false);
info v_RegexdDate;
info v_RegexdDate.toDate();
info v_RegexdDate.toString("E, d MMM yyyy");
info "*********************";
//
// pure Deluge no Regex
v_DelugeDate = v_TestDate.toList("/").get(2) +"-"+ v_TestDate.toList("/").get(1) +"-"+ v_TestDate.toList("/").get(0);
info v_DelugeDate;
info v_DelugeDate.toDate();
info v_DelugeDate.toString("E, d MMM yyyy");
Yields:
07/01/1975
01-Jul-1975
Tue, 1 Jul 1975
*********************
1975-01-07
07-Jan-1975
Tue, 7 Jan 1975
*********************
1975-01-07
07-Jan-1975
Tue, 7 Jan 1975

Additional
You would of course need to check that there is nothing wrong with the date that was submitted. Especially if it is coming from a data source, the Zoho deluge .toList() fails the function if the .get is an out-of-bounds and the .toDate() or .toString() fall apart with an unparsable date: 
v_TestDate = "07011975";
info "*********************";
//
// regex change dd/MM/yyyy to yyyy-MM-dd
v_RegexdDate = v_TestDate.replaceAll("(\\d{2})/(\\d{2})/(\\d{4})","$3-$2-$1", false);
if(isDate(v_RegexdDate))
{
	info v_RegexdDate.toDate();
	info v_RegexdDate.toString("E, d MMM yyyy");
}
info v_RegexdDate;
info "*********************";
//
// pure Deluge no Regex
v_DelugeDate = null;
if(v_TestDate.toList("/").size()>2){
	v_DelugeDate = v_TestDate.toList("/").get(2) +"-"+ v_TestDate.toList("/").get(1) +"-"+ v_TestDate.toList("/").get(0);
	info v_DelugeDate.toDate();
	info v_DelugeDate.toString("E, d MMM yyyy");
}
info v_DelugeDate;
Yields: 
*********************
07011975
*********************
null

Source(s):