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.com:

Given a string and a non-empty word string, return a version of the original string where all chars have been replaced by pluses ("+"), except for appearances of the word string which are preserved unchanged.

plusOut("12xy34", "xy") → "++xy++"
plusOut("12xy34", "1") → "1+++++"
plusOut("12xy34xyabcxy", "xy") → "++xy++xy+++xy"

There is no mention whether or not to allow overlap (e.g. what is plusOut("+xAxAx+", "xAx")?), but my non-regex solution doesn't handle overlap and it passes, so I guess we can assume non-overlapping occurrences of word if it makes it simpler (bonus points if you provide solutions for both variants!).

In any case, I'd like to solve this using regex (of the same style that I did before with the other two problems), but I'm absolutely stumped. I don't even have anything to show, because I have nothing that works.

So let's see what the stackoverflow community comes up with.

Answer 1

This passes all their tests:

public String plusOut(String str, String word) {
  return str.replaceAll(
    String.format("(?<!(?=\\Q%s\\E).{0,%d}).", word, word.length()-1),
    "+"
  );  
}

Also, I get:

plusOut("1xAxAx2", "xAx") → "+xAxAx+"

If that's the result you were looking for then I pass your overlap test as well, but I have to admit, that one's by accident. :D

Answer 2

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.

Answer 3

This is provided here just for reference. This is essentially Alan's solution, but using replace instead of String.format.

public String plusOut(String str, String word) {
  return str.replaceAll(
    "(?<!(?=word).{0,M})."
      .replace("word", java.util.regex.Pattern.quote(word))
      .replace("M", String.valueOf(word.length()-1)),
    "+"
  );  
}

Answer 4

I think you could leverage a negated range to do this. As this is just a hint, it's not tested though!

Turn your "xy" into a regexp like this: "[^xy]"

...and then wrap that into a regexp which replaces strings matched by that expression with "+".