Python script to convert string to int without using built-in functions, and return the value with 3 fixed decimal places

convert_to_int.py

import re

digits_dict = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
regex = "\s*(\+|\-)?\s*((\d)*(\.)?(\d)*)"
value = input()
result = re.search(regex, value)
sign = result.group(1) == '-' and -1 or 1
digits = result.group(2)

integer_value = 0
fraction_value = 0
decimal = False
fraction_counter = 1

for digit in digits:
    if digit == '.':
        decimal = True
        continue
    if decimal:
        fraction_value = (fraction_value * 10) + digits_dict[digit]
        fraction_counter *= 10
    else:
        integer_value = integer_value * 10 + digits_dict[digit]

if decimal:
    integer_value = integer_value + ( fraction_value / fraction_counter )

print("{:.3f}".format(integer_value * sign))