Field Calculator Question

Question - I have an address field and would like to split it such that a second field only displays everything BEFORE the first instance of a space (which is to say, from the original address field, I want to create a second field of JUST the building numbers).

For example, what appears as "400 Main St" in the original field would just appear as "400" in the building number field.

I am no python/vb expert, and have been struggling with this all afternoon. Any advice would be very much appreciated.


Many thanks!