Question

I’m using java to try to replace the four spaces at the beginning of a new line to a tab. I would like to do this using regular expressions. My issue is that the regex is currently replacing all four-space-sequences with a single tab. I want it to insert a tab for each four-space-sequence. Right now I have:

    public String translate(String text) {
        text = text.replaceAll("(?m)^(    )+", "\t");
        return text;
}

Answer 1

You don’t want the + because you want exactly 4 spaces, and you need a look-behind assertion:

text = text.replaceAll("(?m)(?<=^ *)    ", "\t");

Note: This should only be used for input of about 10K or less due to the backtracking required by the look behind. For larger input, use a pattern and matcher etc

Unlike the other answers, this one actually works (see test below), because it uses a positive look-behind (?<=^ *) to assert that only spaces are between the start of input and the target replacement, without which you’ll only match the first 4 spaces:

String text = "        a\n        b    \n";
text = text.replaceAll("(?m)(?<=^ *)    ", "\t");
System.out.println(text.replace("\t", "TAB"));

Output:

TABTABa
TABTABb