Write your implementation of Ruby's String#count method

The task is to write a simplified version of Ruby's String#count method oneself. Simplified because it doesn't have to have the negation and sequence parts of the original.

Here my solution:

def custom_count(string, search_char)
    sum = 0
    seg_string = string.split("")
    seg_char = search_char.split("")

    for i in seg_string
        for j in seg_char
            if i == j
                sum += 1
            end
        end
    end

    sum
end

p custom_count("Hello, World!", "l")    # => 3
p custom_count("Hello, World!", "lo")   # => 5
p custom_count("Hello, World!", "le")   # => 4
p custom_count("Hello, World!", "x")    # => 0
p custom_count("Hello, World!", "w")    # => 0
p custom_count("Hello, World!", "W")    # => 1

Is there a better way to solve the task? Perhaps without using nested loops?