This program finds all matches to a regular expression pattern and replaces them with another string. 
If the replacement is not a constant string, see e430 Searching and Replacing with Nonconstant Values Using a Regular Expression. 

    import java.util.regex.*;
    
    public class BasicReplace {
        public static void main(String[] args) {
            CharSequence inputStr = "a b c a b c";
            String patternStr = "a";
            String replacementStr = "x";
    
            // Compile regular expression
            Pattern pattern = Pattern.compile(patternStr);
    
            // Replace all occurrences of pattern in input
            Matcher matcher = pattern.matcher(inputStr);
            String output = matcher.replaceAll(replacementStr);
            // x b c x b c
        }
    }