如何制作 substring 以便在下次出现相同单词时结束（在 C# 中）？

Question

For example -

string text = "This is some text about a dog. The word dog \nappears in this text a number of times. This is the end";

Here, I want to create a Substring that prints till only when the word/string dog appears for the second time which is in this line The word dog \nappears in this text a number of times .

I know the solution to this problem which is this -

string text = "This is some text about a dog. The word dog appears in this text a number of times. This is the end";
            
string newText = text.Substring(0, 43);
Console.WriteLine(newText);

But what if we don't know the index/position of the same word(which is 'dog') that appears for the second time in the line. What would be the code for that?

Expected Output -

This is some text about a dog. The word dog

Process Finished.

Here, when the word dog appears for the scond time, the string ends...

[NOTE: You may ask for clarifications]

Answer 1

Using regular expressions (aka Regex), you could solve this in a straightforward way.

    var regex = new Regex("dog");
    var matches = regex.Matches(text);
    var secondIndex = matches[1].Index;

Watch out for word delimiters like spaces or punctuation marks. You can use [search engine of your choice] to lookup some Regex Cheat Sheet, since there is somewhat of an entry barrier to Regex.

Answer 2

You can try using regular expresssions and Linq for this:

When given text and word to find

  string text = 
    "This is some text about a dog. The word dog \nappears in this text a number of times. This is the end";

  string word = "dog";

We can find the result as

  using System.Linq;
  using System.Text.RegularExpressions;  

  ...
  
  // we want 2nd match of the "dog" if it exists:
  var match = Regex
    .Matches(text, @$"\b{Regex.Escape(word)}\b", RegexOptions.IgnoreCase)
    .Cast<Match>()
    .Skip(1) // skip the 1st match
    .FirstOrDefault(); // take the second

  string result = null == match
    ? text // we don't have 2nd "dog" word at all
    : text.Substring(0, match.Index + match.Length);

  // Let's have a look at result variable
  Console.Write(result);

Outcome:

This is some text about a dog. The word dog