PostgreSQL, C-Extension, calling other Functions

I´ve written some PostgreSQL C-Extensions (for the first time...) and they
work as expected.

But now I want to call other functions from inside the C-Extensions (but not
via SPI_execute),
for example "regexp_match()" or from other extensions like PostGIS
"ST_POINT" etc...

I think "fmgr" is the key - but I didn't find any examples.

Greetings from Berlin
-Stefan Wolf-