C# - How to find and replace every nth character occurrence using C# or regex? Conversion from Javascript to C#

Question

Find and replace every 3rd character occurrence in a given string and replace it with H

For instance : 74Mercedes1980 should be 74HerHedHs1H80

Can be done in C# or regex.

I found this Javascript code that does the same thing and would like to convert it into a C# code. Can you please assist ?

Found here

str = str.replace(/((?:[^x]*x){2}[^x]*)x/g, '$1y');

let str = 'I amx nexver axt hoxme on Sxundxaxs';
let n = 3;
let ch = 'x';

let regex = new RegExp("((?:[^" +ch+ "]*" +ch+ "){" + (n-1) + "}[^" +ch+ "]*)" +ch, "g");

str = str.replace(regex, '$1y');

console.log(str);
//=> I amx nexver ayt hoxme on Sxundyaxs

Answer 1

You can capture 2 non whitespace chars \S and match the 3rd non whitespace char.

In the replacement use the capture group followed by the character that you want in the replacement.

If you want to also match a space, you can use a dot . instead of \S

 let str = '74Mercedes1980'; let n = 3; let ch = 'H'; let regex = new RegExp("(\\S{" + (n - 1) + "})\\S", "g"); str = str.replace(regex, `$1${ch}`); console.log(str);

In C# you can write this as

var str = "74Mercedes1980";
var n = 3;
var ch = 'H';
var regex = new Regex("(\\S{" + (n - 1) + "})\\S");
str = regex.Replace(str, "$1" + ch);
Console.WriteLine(str);

Output

74HerHedHs1H80

See a C# demo

Answer 2

The hard coded example equates to

var re = new Regex("((?:[^x]*x){2}[^x]*)x");
var result = re.Replace("I amx nexver axt hoxme on Sxundxaxs", m => m.Groups[1] + "y");

Which can be rewritten as

var ch = 'x';
var n = 3;  
var str = "I amx nexver axt hoxme on Sxundxaxs";        
var re = new Regex("((?:[^" +ch+ "]*" +ch+ "){" + (n-1) + "}[^" +ch+ "]*)" +ch);
var result = re.Replace(str, m => m.Groups[1] + "y");
Console.WriteLine(result);

Live example: https://dotnetfiddle.net/u8mJrn