Insert a char in string

[JR via Digitalmars-d-learn]

On Thursday, 10 July 2014 at 19:33:15 UTC, simendsjo wrote:
> On 07/10/2014 06:05 PM, Alexandre wrote:
>> I have a string X and I need to insert a char in that string...
>> 
>> auto X = "100000000000000";
>> 
>> And I need to inser a ',' in position 3 of this string..., I 
>> try to use
>> the array.insertInPlace, but, not work...
>> 
>> I try this:
>> auto X = "100000000000000";
>> auto N = X.insertInPlace(1,'0');
>
> Do you really want to insert a comma in the string, or do you 
> want to
> format a number as "100,000,000,000.00"?

For that, one approach would be as in 
http://dpaste.dzfl.pl/bddb71eb75bb.