If you're getting the following error, chances are you need to upgrade libpcre2, because it's being held back for some reason (you should see this listed when you run sudo apt update, followed by upgrade).
PHP Fatal error: Uncaught ErrorException: preg_match_all(): Compilation failed: unrecognised compile-time option bit(s) at offset 0 in phar:///usr/local/bin/composer/vendor/symfony/console/Formatter/OutputFormatter.php:137
Stack trace:
#0 [internal function]: Composer\Util\ErrorHandler::handle()
#1 phar:///usr/local/bin/composer/vendor/symfony/console/Formatter/OutputFormatter.php(137): preg_match_all()
#2 phar:///usr/local/bin/composer/vendor/symfony/console/Output/Output.php(155): Symfony\Component\Console\Formatter\OutputFormatter->format()
#3 phar:///usr/local/bin/composer/vendor/symfony/console/Output/Output.php(132): Symfony\Component\Console\Output\Output->write()
#4 phar:///usr/local/bin/composer/vendor/symfony/console/Application.php(641): Symfony\Component\Console\Output\Output->writeln()
#5 phar:///usr/local/bin/composer/vendor/symfony/console/Application.php(127): Symfony\Component\Console\Application->renderException()
#6 phar:///usr/local/bin/composer/src/Composer/Console/Application.php(128): Symfony\Component\ in phar:///usr/local/bin/composer/vendor/symfony/console/Formatter/OutputFormatter.php on line 137
sudo apt dist-upgrade should resolve this, but may break some dependencies. Use at your own risk.
See: https://askubuntu.com/questions/601/the-following-packages-have-been-kept-back-why-and-how-do-i-solve-it oerdnj/deb.sury.org#1674 (comment)