How can I replace all instances of a word in Java as long as they're not part of other words?

I'm a beginner in Java and wanted to ask how we can replace the word "is" in a string with "is not" only while it's not preceded by or followed by another letter (numbers and symbols fine) and isn't a part of another word.

For example, I want to replace the word "is" with "is not" in every string, but it's difficult to do so without changing all words that contain "is" such as "this" or "miss".

Currently, my code gets stuck in an infinite loop when I try to use a word like "this" to call it. (see example outputs below)
For example, here is my code:

public static String isReplace(String str) {
	String newStr = "";
	for (int i = 0; i < str.length(); i++) {
		
		if (str.charAt(i) == ('i') && str.charAt(i + 1) == ('s')) {
			if (!(Character.isLetter(str.charAt(i + 2)))) {
				str = str.replace("is", "is not");
			}
		}
		//break;
	}
	return str ; // FIX ME
}

isReplace("This is good")); //should give me "This is not good"

isReplace("is-is")); //should give me "is not-is not"
isReplace("My favorite food is9ie")) // should give me "My favorite food is not9ie"

Just added more refinement over your existing logic.

public class P1 {
public static String isReplace(String str) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < str.length(); i++) {
char current = str.charAt(i);
//if last character just add it to the buffer
if(i == str.length()-1){
sb.append(current);
}else{
char next = str.charAt(i+1);
if(current == 'i' && next == 's'){
//check is preceding character is not letter and following character is not letter
if((i == 0 || !Character.isLetter(str.charAt(i-1))) &&
((i+2 == str.length() || !Character.isLetter(str.charAt(i+2))))){
sb.append("is not");
i++;  // to skip reading s in next iteration
}else{
sb.append(current);
}
}else{
sb.append(current);
}
}
}
return sb.toString() ;
}
public static void main(String[] args) {
System.out.println(isReplace("This is good"));
System.out.println(isReplace("is-is"));
System.out.println(isReplace("My favorite food is9ie"));
}
}

This is not good
is not-is not
My favorite food is not9ie

Probably the easiest method is to use Regex.
With this search input, it searches for all "is" that are not surrounded by letters.

str.replaceAll("(?<![a-zA-Z])is(?![a-zA-Z])", "is not")

Here is a working example:

  public class Main {
public static String isReplace(String string) {
return string.replaceAll("(?<![a-zA-Z])is(?![a-zA-Z])", "is not");
}
public static void main(String[] args) {
System.out.println(isReplace("This is good"));
System.out.println(isReplace("My favorite food is9ie"));
System.out.println(isReplace("is#is"));
}
}

Output:

This is not good
is not#is not
My favorite food is not9ie

Regex explained:

  (        Open the first capturing group
?<!      Negative lookbehind. This will look before the search results and check that none of the following characters are there.
[a-zA-Z] Character set of all letters.
)        Close the first capturing group.
is       Searches for "is".
(        Open the second capturing group.
?!       Negative lookahead. This will look ahead of the search results and check that none of the following characters are there.
[a-zA-Z] Character set of all letters.
)        Close the second capturing group.