I'm trying to solve wordEnds from codingbat.com using regex. This is the simplest as I can make it with my current knowledge of regex:

public String wordEnds(String str, String word) {
  return str.replaceAll(
     String.format(
       ".*?(?=%s)(?<=(.|^))%1$s(?=(.|$))|.+",
       java.util.regex.Pattern.quote(word)
     ),
     "$1$2"
  );
}

String.format is used to inject word into the pattern for both readability and convenience (it's injected twice). Pattern.quote isn't necessary to pass their tests, but I think it's required for a proper regex-based solution.

The regex has two major parts:

• If after matching as few characters as possible ".*?", word can still be found "(?=%s)", then lookbehind to capture any character immediately preceding it "(?<=(.|^))", match word "%1$s" and lookforward to capture any character following it "(?=(.|$))".
  • The initial "if" test ensures that the atomic lookbehind captures only if there's a word
  • Using lookahead to capture the following character doesn't consume it, so it can be used as part of further matching
• Otherwise match what's left "|.+"
  • Groups 1 and 2 would capture empty strings

I think this works in all cases, but it's obviously quite complex. I'm just wondering if others can suggest a simpler regex to do this.

Note: I'm not looking for a solution using indexOf and a loop. I want a regex-based replaceAll solution. I also need a working solution that I can just copy-paste into codingbat and passes.