generate random string from a canonical form useful for responding like humans

i used this method to generate random sentences from a canonical sentence, this will make the software responses more human, so instead of saying "Good morning" everyday, software should change the sentence every time in a random manner, here is how i did it.

#!/usr/bin/env ruby
# generate a single sentence from a canonical form
# canonical sentence is a multi sentences combined in one
# form, generator will generate a sentence from it randomly
# based on the form, for example:
# "Hello [Emad|Elsaid]" , may generate "Hello Emad" or
# "Hello Elsaid" the result is random.
# also you could nest [] inside each other to gain a multi level
# canonical sentence example:
# "[[Hi|Hello] [Emad|elsaid] | good [morning|night] sir]"
def generate(result)
    while result.include? '[' 
        s_start = result.index '['
        s_end = s_start+1
        while s_end<result.length and result[s_end]!=']'
            s_start = s_end if result[s_end] == '['
            s_end += 1
        end
        sentence = result[(s_start+1)...s_end]
        sentence = sentence.split '|'
        result = result[0...s_start] + sentence.sample + result[s_end+1...result.length]
    end
    result
end

10.times do
    puts generate '[Hello|Hi] my [friend|dear], how [are you [today|these days]|is your brother]?'
end

# Hello my friend, how is your brother?
# Hi my dear, how is your brother?
# Hello my dear, how is your brother?
# Hello my dear, how is your brother?
# Hi my friend, how is your brother?
# Hi my friend, how is your brother?
# Hello my friend, how are you these days?
# Hello my friend, how is your brother?
# Hello my friend, how are you these days?
# Hi my friend, how is your brother?