Write a Python program to reverse only the vowels of a given string.

def reverse_vowels(str1):
    vowels = ""
    for char in str1:
        if char in "aeiouAEIOU":
            vowels += char
    result_string = ""
    for char in str1:
        if char in "aeiouAEIOU":
            result_string += vowels[-1]
            vowels = vowels[:-1]
        else:
            result_string += char
    return result_string
print(reverse_vowels("w3resource"))
print(reverse_vowels("Python"))
print(reverse_vowels("Perl"))
print(reverse_vowels("USA"))