Input (grammar) looks like this:

Apology ::= my (cat|mouse|monkey) ate my homework.
Apology ::= our (basement|attic|third floor) collapsed from an overload of school books.

Pseudo-code:

Put the grammar into array "rules".
Choose one of the rules (randomly).
Parse rule: Apology ::= <bla>
Parse bla into:
  #1 (...)
  #2 everything else
With every s in #1 do:
  Split inner part of s at "|" into "options".
  Choose one of the options (randomly).
  Replace s with option.
Print bla (with the replacements made).