This function should take a main string and a substring as inputs and return the number of occurrences of the substring within the main string. It should not be case-sensitive and may overlap.

Input: Two strings (str).

Output: Integer (int).

Examples:

assert count_occurrences("hello world hello", "hello") == 2
assert count_occurrences("Hello World hello", "hello") == 2
assert count_occurrences("hello", "world") == 0
assert count_occurrences("hello world hello world hello", "world") == 2

How it’s used:

• in a word processing software to highlight or replace all occurrences of a particular word or phrase;
• in website scraping to count the number of times a particular keyword appears;
• in an analytics application to analyze the frequency of certain words or phrases in a body of text.

Preconditions:

• both inputs should be strings: mainStr ∈ string and subStr ∈ string;
• the main string can have any length, including zero: 0 <= length(mainStr) <= N;
• the substring should not be an empty string: length(subStr) > 0.