libonig is a regex library for multi-bytes strings; it's used in particular by php-mbstring (either as embedded copy or linked to Debian's libonig, depending on Debian dist).
cd libonig-xxx/ debuild make check
Sample direct C usage: usually for libonig 6.x, for libonig 5.x, drop onig_initialize() which doesn't exist yet and isn't needed.
PHP test suite:
cd php5-xxx/ debuild cd cli-build TEST_PHP_EXECUTABLE=sapi/cli/php sapi/cli/php ../run-tests.php ../ext/mbstring/
<?php var_dump(mb_ereg("X+++", "hello"));