Error in compat.php on line 123

This is with reference to my old post Severe lag when trying to edit large pages .

I disabled the Yoast SEO plugin so that I can update a very large page (11,125 lines – 111,897 words — In Text Mode).

I was able to successfully make changes to the page with very little lag, but when I saved the page, I received the following error; Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /home/prcessmodel/public_html/wp-includes/compat.php on line 123.

Upon returning back to the page, the page did update / save changes.

1. I tried saving again to see if the issue replicates, it did. I received the error mentioned above again.
2. I have checked to make sure that PHP has been assigned 512M memory by creating <?php phpinfo(); ?> file
3. WP_MEMORY_LIMIT in wp-config is set to 512M
4. I do not receive this error when trying to save any other page.

The line 123 of compat.php contains the following line of code (in bold);

if ( _wp_can_use_pcre_u() ) {
// Use the regex unicode support to separate the UTF-8 characters into an array
preg_match_all( ‘/./us’, $str, $match );
return count( $match[0] );
}