PHP 7 在 PCRE 中加入了 JIT 功能，導致 Composer 安裝會遇到以下錯誤訊息：

PHP Fatal error:  Uncaught ErrorException: preg_match_all(): JIT compilation failed: no more memory in phar:///usr/local/Cellar/composer/1.8.0/bin/composer/vendor/symfony/console/Formatter/OutputFormatter.php:137

暫時的解法是先將 pcre.jit 關閉：

$ vi /usr/local/etc/php/7.3/php.ini
;pcre.jit=1
pcre.jit=0

參考：Errors Installing Composer on macOS (JIT compilation Failed)