Given a string, replace " is " with " is not "

Question

Given a string, if string has " is "(before and after space) in it: replace it with "is not".

Example:

• Original: This is a string
• New: This is not a string

I am implementing it using string/char arrays. Is there any better way to solve this question? I have a feeling that it can be solved with a regular expression. Can anyone suggest some better/different approach to solving this question?

/**
 * 
 *
 * Given a string. If string has " is "(before and after space) in it: replace it with "is not"
 * e.g.,
 * Original : This is a string
 * New : This is not a string
 */
public class ReplaceString {
 public static void main(String[] args) {
 String s = "This is bag";
 replaceIs(s);
 }
 public static void replaceIs(String s) {
 /* Find number of "is" in string to get length of new array*/
 String wordToFind = " is ";
 Pattern word = Pattern.compile(wordToFind);
 Matcher match = word.matcher(s);
 int counterOfWord = 0;
 while (match.find()) {
 ++counterOfWord;
 }
 int lengthOfNewArray = s.length() + (counterOfWord * 4);
 char[] originalCharArray = s.toCharArray();
 char[] newCharArray = new char[lengthOfNewArray];
 /* Parse String from the end and if " is " encounters, replace it with " is not" */
 int indexInNewArray = lengthOfNewArray;
 int i = originalCharArray.length - 1;
 while ( i >= 0 ) {
 if (originalCharArray[i] == ' ' && originalCharArray[i - 1] == 's' && originalCharArray[i - 2] == 'i' && originalCharArray[i - 3] == ' ' ) {
 newCharArray[indexInNewArray - 1] = ' ';
 newCharArray[indexInNewArray - 2] = 't';
 newCharArray[indexInNewArray - 3] = 'o';
 newCharArray[indexInNewArray - 4] = 'n';
 newCharArray[indexInNewArray - 5] = ' ';
 newCharArray[indexInNewArray - 6] = 's';
 newCharArray[indexInNewArray - 7] = 'i';
 newCharArray[indexInNewArray - 8] = ' ';
 i = i - 4;
 indexInNewArray = indexInNewArray - 8;
 } else {
 newCharArray[indexInNewArray - 1] = originalCharArray[i];
 --indexInNewArray;
 --i;
 }
 }
 System.out.println(String.valueOf(newCharArray));
 }
}

• Timothy Truckle: You already use Regex and Matcher, so why not using String::replaceAll()?
• Qain: If we want to implment it without using String::replaceAll()
• Timothy Truckle: If you want do all the low level stuff yourself you should use a different language. This kind of procedural processing will not make you a good Java programmer...
• Rob Audenaerde: @TimothyTruckle I disagree. It is educational to do this kind of lower level programming, because it forces you to create an algorithm, make you think about performance, etc. etc. I would actually also not use Regex to make it more interesting
• Rob Audenaerde: @Qurrat Try to solve it without any Regex, just using char[] logic. The easiest wat to construct the new String is by using StringBuilder.append()

Answer 1

I suggest:

String replaceIs(String s) {
 return s.replace(" is ", " is not ");
}

In Oracle Java 8, this uses a regular expression internally, but in Java 9 it will be implemented with a more efficient algorithm.

By the way, will your code throw an ArrayIndexOutOfBoundsException when your string starts with a space?