This should error:

; echo '/ab?c/ -> $t1; /abc/ -> $t2;' | ./build/bin/lx > /dev/null
; 

The error code is still there (calling fsm_example), but it's never reached.

Equivalent stuff in re(1), I'm suspicious about using just one element from the array in find_first_match_for_end_state(), rather than checking them all:

    return find_match_with_id(end_id_buf[0]);