php中Warning: preg_match_all(): Compilation failed: lookbehind assertion

Today when I was testing the code related to looking around in regular expressions, I found an error
The error message is as follows:
Warning: preg_match_all(): Compilation failed: lookbehind assertion is not fixed length at offset
The code is as follows:

$str='infostrong';
preg_match_all('/(?<=<(w+)[ ^>]*>).*(?=)/',$str,$match);
var_dump($match);

Execute the above code An error will be reported. After checking online, it is said that reverse lookaround in PHP (including reverse positive lookaround?<=exp, reverse negative lookaround?
Change the above code to:

$str='infostrong';
preg_match_all('/(?<=< (w)>).*(?=)/',$str,$match);
var_dump($match);

But this can only match b Strong content in tags
Actually, you don’t need to look around to match, you can directly use grouping to capture the content. You just need to filter the returned results

$str='infostrong';