Java String ReplaceFirst

Question

I am reading in a string

Is Mississippi a State where there are many systems.

I would like to replace the 1st "s" or "S" in each word with "t" or "T" (i.e keeping the same case)...so that the output is:

It Mitsissippi a Ttate where there are many tystems.

I have tried

s= s.replaceFirst("(?i)S", "t"); [which of course didn't work]

and have experimented trying to split the string using a string [] .split(Pattern.quote("\\s")) then trying to figure out how to replaceFirst() each element of the array and then return the values back to a string [but couldn't figure out the right way of doing it].

I thought \\G might help to restart at the next word but have gotten no where. Any help using these 3 methods is appreciated.

Answer 1

One option would be to split the string into words, and then use String.replaceFirst() on each word to replace the first occurrence of s with t (or any other letter you want):

Update:

I refactored my solution to find the first occurrence of any s (upper or lower case), and to apply the appropriate conversion on it.

String input = "Is Mississippi a State where there are many systems.";
String[] parts = input.split(" ");
StringBuilder sb = new StringBuilder("");

for (int i=0; i < parts.length; ++i) {
    if (i > 0) {
        sb.append(" ");
    }
    int index = parts[i].toLowerCase().indexOf('s');
    if (index >= 0 && parts[i].charAt(index) == 's') {
        sb.append(parts[i].replaceFirst("s", "t"));
    }
    else {
        sb.append(parts[i].replaceFirst("S", "T"));
    }
}

System.out.println(sb.toString());

Output:

It Mitsissippi a Ttate where there are many tystems.

Answer 2

Approach-1: Without using replace and split method for better performance.

String str = "Is Mississippi a State where there are many systems.";
System.out.println(str);

char[] cArray = str.toCharArray();
boolean isFirstS = true;
for (int i = 0; i < cArray.length; i++) {
    if ((cArray[i] == 's' || cArray[i] == 'S') && isFirstS) {
        cArray[i] = (cArray[i] == 's' ? 't' : 'T');
        isFirstS = false;
    } else if (Character.isWhitespace(cArray[i])) {
        isFirstS = true;
    }
}
str = new String(cArray);

System.out.println(str);

EDIT: Approach2: As you need to use replaceFirst method and you dont want to use StringBuilder here is an option for you:

String input = "Is Mississippi a State where there are many Systems.";
String[] parts = input.split(" ");
String output = "";

 for (int i = 0; i < parts.length; ++i) {
     int smallSIndx = parts[i].indexOf("s");
     int capSIndx = parts[i].indexOf("S");

     if (smallSIndx != -1 && (capSIndx == -1 || smallSIndx < capSIndx))
         output += parts[i].replaceFirst("s", "t") + " ";
     else
         output += parts[i].replaceFirst("S", "T") + " ";
 }

System.out.println(output); //It Mitsissippi a Ttate where there are many Tystems. 

Note: I prefer approach 1 because it has no overhead for the method replaceFisrt and split , String append or concat