codingBat plusOut using regex

Question

This is similar to my previous efforts (wordEnds and repeatEnd): as a mental exercise, I want to solve this toy problem using regex only.

Description from codingbat.

Answer 1

An extremely simple solution, using \G:

word = java.util.regex.Pattern.quote(word);
return str.replaceAll("\\G((?:" + word + ")*+).", "$1+");

However, there is a caveat. Calling plusOut("12xxxxx34", "xxx") with the implementation above will return ++xxx++++.

Anyway, the problem is not clear about the behavior in such case to begin with. There is even no test case for such situation (since my program passed all test cases).

The regex is basically the same as the looping solution (which also passes all test cases):

StringBuilder out = new StringBuilder(str);

for (int i = 0; i < out.length(); ) {
    if (!str.startsWith(word, i))
        out.setCharAt(i++, '+');
    else
        i += word.length();
}

return out.toString();

Repeatedly skips the word, then replace the current character if it is not prefix of word.