How to Replace with StringBuilder from the right

Question

I need to replace character sequences in a string, but they are not guaranteed not to be overlapping. The rule in case of conflicts is that ( first, the longest character sequence wins, but this is irrelevant), but if they are the same length the ones closest to the end must take priority. I would like to use a StringBuilder for this, but I'm not sure how to implement this.

The code below illustrates this:

string source = "abaaaababa"; var sb = new StringBuilder(source); Console.WriteLine("What I want:" + ReplaceFromRight(source,"aba","dgd")); Console.WriteLine("What I have:" + sb.Replace("aba", "dgd").ToString()); string ReplaceFromRight(string text, string from, string to) { for (int i = text.Length-from.Length; i >=0 ; i--) { if(text.Substring(i, from.Length) == from) text = text.Substring(0,i)+to+text.Substring(i+from.Length); } return text; }

What I want:dgdaaabdgd What I have:dgdaadgdba

(ReplaceFromRight is quick and dirty, I know, but it illustrates my purpose)

What is the best way to do this?

Here's what I came up with:

string ReplaceFromRightWithSb(StringBuilder sb, string from, string to) { var text = sb.ToString(); var lastIndex=text.LastIndexOf(from); while (lastIndex > -1) { sb.Replace(from, to, lastIndex, from.Length); text = sb.ToString(); lastIndex = text.LastIndexOf(from); } return sb.ToString(); }

(it's basically what Tim said below, so I will accept his answer)

Replace always works from the start of the String(buillder) towards the end. A simple (but not necessarly the most efficient) way for ReplaceFromRight would be reversing the original string, search term and replacement term before calling Replace and again reverse the result afterwards — derpirscher
– derpirscher, Commented Mar 27 at 9:47
Is it only in my browser? Cannot seem to fix code highlighting. (I edited to fix a typo) — Fildor
– Fildor, Commented Mar 27 at 9:50
@Fildor, no it's not your browser. For some reason code markup shows in preview but not in the question. — Vladimirs Kacs
– Vladimirs Kacs, Commented Mar 27 at 9:59
var result = Regex.Replace("abaaaababa", "aba", "dgd", RegexOptions.RightToLeft); — Dmitrii Bychenko
– Dmitrii Bychenko, Commented Mar 27 at 10:01
If you want to provide the answer for future readers, then post the answer as the answer, don't edit it into question. — Sinatr
– Sinatr, Commented Mar 27 at 10:28

Tim Schmelter · Accepted Answer · 2025-03-27 09:47:26Z

If i understood it right, this should work: Demo

public static string ReplaceFromRight(string text, string from, string to) { var sb = new StringBuilder(text); for (int i = text.Length - from.Length; i >= 0; i--) { if (text.Substring(i, from.Length) == from) { sb.Remove(i, from.Length); sb.Insert(i, to); text = sb.ToString(); } } return sb.ToString(); }

I came up with a solution of my own (see above,) it's almost identical, but I wonder if Replace is less efficient than Remove + Insert. Does anyone know?
@VladimirsKacs: You can measure it, for example with github.com/dotnet/BenchmarkDotNet :)

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