I've been days trying to find a solution WITH regex (before somebody says it: I know I should been using the PHP DOM Document library or something alike, but let's take this as a theoretical question), looking answers up and I finally came up with what I'll show near the end of this question.

What follows is just a summary of a lot of things I've tried before.

First of all, what I mean by nested tags of the same type is:

Text outside any div
<div id="my_id"> bla bla
  <div>
  bla bla bla
    <div style="some style here">
      lalalalala
     </div>
   </div>
    I'm trapped in a div!
</div>
more text outside divs

<div>more divs here!
       <div id="justbeingannoying">radiohead rules</div>
</div>

Now imagine I want to remove all the divs and their content using regex. So the intended result would be:

Text outside any div
more text outside divs

The first idea would be matching everything. The following regex matches div tags with properties (style, id, etc):

/<div[^>]*>.*</div>/sig

The problem, of course, is that this will match everything between the beginning of the first "< div" and the last "< /div >", so it will match "more text outside divs" too (check here: https://regex101.com/r/iR8mY2/1 ), which is not want we (I) want.

This could be solved using the U modifier (Ungreedy)

/<div[^>]*>.*</div>/sigU

but then we'll have the problem of having less than we want: it will match only from the first "< div" till the first "" (so, if we remove the matches, besides some unmatched tags, will have the text "I'm trapped in a div!", which we don't want).

So, I found a solution that works like a charm for nested parenthesis, square brackets, etc:

/[([^[]]*+|(?R))*]/si

Basically, what this does is finding an opening square bracket, then matching anything *that is neither an opening nor a closing square bracket * OR a recursive structure of that, finding a closing square bracket.

What I have working now is a bad solution: basically, first I replace all the opening tags with an square bracket (which can't be in my code, for other reasons), then the closing tag for a closing square bracket and then I use the previous regex. Not a very elegant solution, I know.

The thing is I really want to know how this could be done with just one regex. It seems obvious than replacing in the previous regex the "[" and the "]" by the html tags has to work. But is not that easy. The problem is the negation for characters ("[^.......]" doesn't work for strings like "div". It seems that something similar can be achieved by this:

.+?(?=<div>)

and, of course, the same for the closing tag

.+?(?=</div>

This is how, more or less, I arrived to this regex

/<div((.+?(?=</div>)|.+?(?=<div>))|(?R))*</div>/gis

Which works exactly as the first regex I presented before: https://regex101.com/r/yU8pV3/1

So, here is my question: what is wrong with that regex?

Thank you!

DISCLAIMER

Since the question is met with positive reaction, I will post an answer explaining what is wrong with your approach, and will show how to match text that is not some specific text.

HOWEVER, I want to emphasize: Do not use this to parse real, arbitrary HTML code, as regex should only be used on plain text.

What is wrong with your regex

Your regex contains <div((.+?(?=</div>)|.+?(?=<div>))|(?R))* part (same as <div((.+?(?=</?div>))|(?R))*) before matching the closing </div> part. When you have some delimited text, do not rely on plain lazy/greedy dot matching (unless used in unroll the loop structure - when you know what you are doing). What it does is this:

• <div - match <div literally (also, in <diverse due to a missing word boundary or a s after it)
• ( - Group 1 that matches:
  • (.+?(?=</div>)|.+?(?=<div>)) - matches either any 1+ chars (as few as possible) up to the first </div> or to the first <div>
  • |
  • (?R) - Recurse (i.e. insert and use)
• )* - repeat Group 1 zero or more times.

The problem is clear: the (.+?(?=</?div>)) part does not exclude matching <div> or </div>, this branch MUST only match the text NOT EQUAL to the leading and trailing delimiters.

Solution(s)

To match text other than some specific text use a tempered greedy token.

<div[^<]*>((?:(?!</?div).)+|(?R))*</div>s*
             ^^^^^^^^^^^^^^^^^^^ 

See the regex demo. Note you must use a DOTALL modifier so as to be able to match text across newlines. A capturing group is redundant, you can remove it.

What is important here is that (?:(?!</?div).)+ only matches 1 or more characters that are not the starting character of a <div....> or </div sequences. See my above linked thread on how that works.

As for performance, tempered greedy tokens are resource-consuming. Unroll the loop technique comes to the rescue:

<div[^<]*>(?:[^<]+(?:<(?!/?div)[^<]*)*|(?R))*</div>s*

See this regex demo

Now, the token looks like [^<]+(?:<(?!/?div)[^<]*)*: 1+ characters other than < followed with 0+ sequences of < that is not followed with /div or div (as a whole word) and then again 0+ non-<s.

<div might still match in <div-tmp, so perhaps, <div(?:s|>) is a better way to deal with this via regex. Still, parsing HTML with DOM is much easier.