Replace Every Nth Instance of a Character

Create a function that takes a string and replaces every nth instance of old_char with new_char. Your function will have four parameters:

• txt — The original input text to be processed.
• nth — The nth instance to be replaced.
• old_char — The character being replaced.
• new_char — The character replacing old_char.

In other words, if txt is "abababa", nth is 3, old_char is "a" and new_char is "Z", you would replace the 3rd insrtance of "a" with "Z", returning "ababZba".

Examples

replace_nth("A glittering gem is not enough.", 0, "o", "-")
➞ "A glittering gem is not enough."

replace_nth("Vader said: No, I am your father!", 2, "a", "o")
➞ "Vader soid: No, I am your fother!"

replace_nth("Writing a list of random sentences is harder than I initially thought it would be.", 2, "i", "3")
➞ "Writ3ng a list of random sentences 3s harder than I in3tially thought 3t would be."

Notes

• If nth is 0, negative or larger than instances of old_char, return the original string.
• Tests are case sensitive.