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).

Smoke testing:

{{{
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.
 * https://github.com/kkos/oniguruma/issues/147
 * https://github.com/kkos/oniguruma/issues/164

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 script:
{{{
<?php
var_dump(mb_ereg("X+++", "hello"));
}}}