preg_match() error occurance

Local laravel apis started giving following error.

[2019-06-25 06:22:10] local.ERROR: ErrorException: preg_match(): Delimiter must not be alphanumeric or backslash in ~laravel/vendor/doctrine/inflector/lib/Doctrine/Common/Inflector/Inflector.php:423

Even after doing usual cleanups i.e
php artisan config:cache
composer install
composer dump-autoload

The error persisted, it got resolved only after stopping and starting apache server

sudo service apache2 stop
sudo service apache2 start