Word search usin regexp in C#.net

Question

i want to search two word in string,so the second word is come away from 20 word or less from first word.

 somthing like:

dog........(20 word with . , '' : number and other char).........Food

 or

 Food........(20 word with . , '' : number and other char).........Dog

how to skip 20 word and fanout this type of substring  using regexp...

please understand this and give some Reg Exp solution thank you,