Insert at most one leading and trailing newline after each word

Question

I have strings that contain delimiters. The delimiters need to contain leading and trailing newlines but they do not. Three examples:

[heading 1]content[heading 2]content[heading 3]content

[heading 1]content↵
[heading 2]content↵
[heading 3]content

[heading 1]↵
content[heading 2]↵
content[heading 3]↵
content

I need to normalize the data using regular expression find-replace. Each delimiter must have leading and trailing newline, the first delimiter must have trailing newline only:

[heading 1]↵
content↵
[heading 2]↵
content↵
[heading 3]↵
content

I have tried this find-replace pattern (regexr) but it does not work in all cases:

find: \[.+?\](?!\r\n)
repl: $0\r\n

Update: I prefer a one-regex-only solution that does not require pre and post-processing such as replace and trim.

Answer 1

This should work for you:

First just replace all new lines with str_replace() and then you can easily add a new line after each [heading] and the content with preg_replace(), e.g.

<?php

    $str = "[heading 1]content[heading 2]content[heading 3]content";
    $str = trim(preg_replace("/(\[.+?\])((?:[^\[])+)/", "$1" . PHP_EOL . "$2" . PHP_EOL, str_replace(PHP_EOL, "", $str)));

    highlight_string($str);

?>

output:

[heading 1]
content
[heading 2]
content
[heading 3]
content

EDIT:

If you only want to use a regex, you could do something like this:

<?php

    $str = "[heading 1]content[heading 2]content[heading 3]content

[heading 1]content
[heading 2]content
[heading 3]content

[heading 1]
content[heading 2]
content[heading 3]
content";

    $str = preg_replace("/\s*(\[.+?\]|(?![^\[]+$(*PRUNE))[^\[]+)\s*/", "$1" . PHP_EOL, $str);
    var_dump($str);

?>