Exercise 12-10:
Write a function that accepts prices in dollars. The program must ignore white spaces and accept values prefixed with $ or not. The user must enter correctly formatted values with a comma separating the thousands and a period separating the cents. If the user types cents, they must have two digits.
Valid values:
$500
$500
$500.10
$7,312.10
Invalid values:
$500.1
500.1$
500.1
7312.10$
The function must return the entered value converted to float or generate a ValueError exception if the value entered is invalid.
Answer:
##############################################################################
# Python From Scratch
# Author: Nilo Ney Coutinho Menezes
# Editora Novatec (c) 2010-2025 - LogiKraft 2025
# Site: https://pythonfromscratch.com
# ISBN: 978-85-7522-949-1 (Paperback), 978-85-7522-950-7 (hardcover), 978-85-7522-951-4 (ebook)
#
# File: chapter 12/exercise-12-10.py.py
##############################################################################
import re
DOLLARS_RE = r"^(([rR]\$)?(((\d{1,3}\.)?(\d{3}\.)*?\d{3})|(\d{1,3}))(,\d{2})?)$"
# ^ - From the start of the string
# ( - Group of the right part of the expression, before the comma
# ([rR]\$)? - Optionally can have R$
# ( - Group of the integer part
# ((\d{1,3}\.)?(\d{3}\.)*?\d{3}) - Group of the integer part with dots separating thousands
# Required to ensure that parts between dots have 3 digits, except the first one
# | - or
# (\d{1,3}) - Group of the integer part without dots
# ) - Group of the right part
# (,\d{2})? - Optionally can have the decimal part
# )$ - Until the end of the string
def clean_spaces(input):
return input.replace(" ", "")
def dollars(input):
# It's easier to clean spaces before validation,
# since the regular expression is already quite complex
# Remember: you don't need to use regex for everything!
input = clean_spaces(input)
return bool(re.match(DOLLARS_RE, input))
inputs = [
"R$ 1.234,56", # Yes
"r$ 1.234,56", # Yes
"r$1.234,56", # Yes
"r$12.123.234,56", # Yes
"1.234,56", # Yes
"R$1.234,56", # Yes
"r$234,56", # Yes
"R$234,56", # Yes
"234,56", # Yes
"r$234", # Yes
"R$234", # Yes
"R$2", # Yes
"R$23", # Yes
"234", # Yes
"34", # Yes
"4", # Yes
"r$234,4", # No - Cents must have 2 digits
"R$234,4", # No - Cents must have 2 digits
"R$1234,4", # No - Missing . to separate thousands
"r$1234,4", # No - Missing . to separate thousands
"r$1234.12,4", # No - Incorrect use of thousands separator (.)
"r$1.24,56", # No - Irregular, only two numbers after the .
]
for input in inputs:
print(f"{input}: {dollars(input)}")