Question

I am trying to solve a codingbat problem using regular expressions whether it works on the website or not.

So far, I have the following code which does not add a * between the two consecutive equal characters. Instead, it just bulldozes over them and replaces them with a set string.

public String pairStar(String str) {
    Pattern pattern = Pattern.compile("([a-z])\\1", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher(str);
    if(matcher.find())
    matcher.replaceAll(str);//this is where I don't know what to do
    return str;
}

I want to know how I could keep using regex and replace the whole string. If needed, I think a recursive system could help.

Answer 1

This works:

while(str.matches(".*(.)\\1.*")) {
    str = str.replaceAll("(.)\\1", "$1*$1");
}
return str;

Explanation of the regex:

The search regex (.)\\1:

• (.) means “any character” (the .) and the brackets create a group – group 1 (the first left bracket)
• \\1, which in regex is \1 (a java literal String must escape a backslash with another backslash) means “the first group” – this kind of term is called a “back reference”

So together (.)\1 means “any repeated character”

The replacement regex $1*$1:

• The $1 term means “the content captured as group 1”

Recursive solution:

Technically, the solution called for on that site is a recursive solution, so here is recursive implementation:

public String pairStar(String str) {
    if (!str.matches(".*(.)\\1.*")) return str;
    return pairStar(str.replaceAll("(.)\\1", "$1*$1"));
}