Java Regex: ReplaceFirst Example and Explanation

This code snippet demonstrates the use of Java's regular expression library to replace the first occurrence of a pattern in a string.

Pattern myPattern = Pattern.compile("J.*n");
Matcher matcher = myPattern.matcher("John does this, and John does that ,and John do something");
String str = matcher.replaceFirst("sam");
System.out.println(str);

Explanation:

1. Pattern Compilation:

   • Pattern.compile("J.*n") creates a Pattern object representing the regular expression 'J.n'. This pattern matches any string that starts with 'J', followed by any characters (represented by '.'), and ends with 'n'.

2. Matcher Creation:

   • myPattern.matcher("John does this, and John does that ,and John do something") creates a Matcher object that will apply the compiled pattern to the given input string.

3. Replacing the First Occurrence:

   • matcher.replaceFirst("sam") replaces the first occurrence of the pattern in the input string with the string "sam".

4. Output:

   • System.out.println(str) prints the modified string, which is: 'sam does this, and John does that ,and John do something'.

Therefore, the correct output is B: 'sam does this, and sam does that ,and sam do something'.