Write a Python program that uses the re module to find all occurrences of a word in a string. The function find_occurrences(s, word) should return a list of indices at which word starts in the string s.
Example 1:
Input: find_occurrences("hello world, hello again", "hello")
Output: [0, 13]
Example 2:
Input: find_occurrences("one fish, two fish, red fish, blue fish", "fish")
Output: [4, 14, 24, 35]