parens.py

text = "some string (with parentheses (and also inner parentheses)) (yay) hi"

stack = []
for i in range(len(text)):
    char = text[i]
    if char == "(":
        stack.append(char)
    elif char == ")":
        stack.pop()
        if len(stack) == 0:
            print("REPLACED", end="")
    elif len(stack) == 0:
        print(char, end="")

print()