C# String append some string ater first occurance of a string after a given string

Question

I know this seems to be very complicated but what i mean is for example i have a string

This is a text string

I want to search for a string (for example: text) . I want to find first occurance of this string which comes after a given another string (for example : is) and the replacement should be another string given (for example: replace)

So the result should be:

This is a textreplace string

If text is This text is a text string then the result should be This text is a textreplace string

I need a method (extension method is appreciated):

public static string AppendFirstOccurranceAfter(this string originalText, string after, string oldValue, string newValue)
// "This is a text string".ReplaceFirstOccurranceAfter("is", "text", "replace")

what if `text` occurs three times? what is expected output of `This text is a text of text string` — Prasad Telkikar, Apr 16 '20 at 14:40
Have you tried anything? You could explore [String](https://learn.microsoft.com/en-us/dotnet/api/system.string) methods while waiting for someone to post an answer. Check `IndexOf`, `Insert`, etc... — Sinatr, Apr 16 '20 at 14:44
@PrasadTelkikar I think this particular point is clearly specified by OP : Append _first_ occurence after, so the result for your example would be : `This text is a textreplace of text string` — Pac0, Apr 16 '20 at 14:44
Use Split(new string[] { "text" }, StringSplitOptions.None) and then you have full control over it — ASalameh, Apr 16 '20 at 14:45

benjamin · Answer 1 · 2020-04-16T15:29:45.860

Here is the extension method:

        public static string CustomReplacement(this string str)
        {
            string find = "text"; // What you are searching for
            char afterFirstOccuranceOf = 'a'; // The character after the first occurence of which you need to find your search term.
            string replacement = "$1$2replace"; // What you will replace it with. $1 is everything before the first occurrence of 'a' and $2 is the term you searched for.

            string pattern = $"([^{afterFirstOccuranceOf}]*{afterFirstOccuranceOf}.*)({find})";

            return Regex.Replace(str, pattern, replacement);
        }

You can use it like this:


string test1 = "This is a text string".CustomReplacement();
string test2 = "This text is a text string".CustomReplacement();

This solution uses C# Regular Expressions. The documentation from Microsoft is here: https://learn.microsoft.com/en-us/dotnet/standard/base-types/regular-expression-language-quick-reference

score 1 · Accepted Answer · answered Apr 16 '20 at 15:32

You have to find the index of the first word to match and then, using that index, do another search for the second word starting at that said index and then you can insert your new text. You can find said indexes with the IndexOf method (check it's overloads).

Here is a simple solution written in a way that (I hope) is readable and that you can likely improve to make more idiomatic:

    public static string AppendFirstOccurranceAfter(this string originalText, string after, string oldValue, string newValue) {
    var idxFirstWord = originalText.IndexOf(after);
    var idxSecondWord = originalText.IndexOf(oldValue, idxFirstWord);
    var sb = new StringBuilder();

    for (int i = 0; i < originalText.Length; i++) {
        if (i == (idxSecondWord + oldValue.Length))
            sb.Append(newValue);
        sb.Append(originalText[i]);
    }

    return sb.ToString();
}

C# String append some string ater first occurance of a string after a given string

2 Answers2