Find substring in string but only if whole words?

Question

What is an elegant way to look for a string within another string in Python, but only if the substring is within whole words, not part of a word?

Perhaps an example

Answer 1

I'm building off this answer.

The problem with the above code is that it will return false when there are multiple occurrences of needle in haystack, with the second occurrence satisfying the search criteria but not the first.

Here's my version:

def find_substring(needle, haystack):
  search_start = 0
  while (search_start < len(haystack)):
    index = haystack.find(needle, search_start)
    if index == -1:
      return False
    is_prefix_whitespace = (index == 0 or haystack[index-1] in string.whitespace)
    search_start = index + len(needle)
    is_suffix_whitespace = (search_start == len(haystack) or haystack[search_start] in string.whitespace)
    if (is_prefix_whitespace and is_suffix_whitespace):
      return True
  return False

Hope that helps!