<?php
$string = "Hello {replace this curley} {{but not this one}} {{{and not this one}}}";
$formatted = preg_replace('/^(?!\\{{2,})(?:[^\\{]*)(\\{{1})(?:[^\\{\\}]*)(\\}{1})$/',"",$string);
var_dump($formatted);
string(71) "Hello {replace this curley} {{but not this one}} {{{and not this one}}}"
Output for 8.3.5
Warning: PHP Startup: Unable to load dynamic library 'sodium.so' (tried: /usr/lib/php/8.3.5/modules/sodium.so (libsodium.so.23: cannot open shared object file: No such file or directory), /usr/lib/php/8.3.5/modules/sodium.so.so (/usr/lib/php/8.3.5/modules/sodium.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
string(71) "Hello {replace this curley} {{but not this one}} {{{and not this one}}}"