Using regular expression to validate user input is an integer

Question

I am trying to validate a user's input. I only want the user to be able to enter a positive or negative integer. All other inputs (i.e. anything with letters) should be rejected

I have the code below at the minute, however I am getting an error. I'm assuming it has to do with the data types but am unsure how to fix this.

import re

number =input("Please enter a number: ")
number=int(number)
while not re.match("^[0-9 \-]+$", number):
    print ("Error! Make sure you only use numbers")
    number = input("Please enter a number: ")
print("You picked number "+ number)

You should not cast it to a number I believe. Do you get TypeError: expected string or buffer? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 1, 2015 at 15:44
Yes, I was thinking along these lines. I just assumed as it is number, I would need to use int? — Miss Jones
– Miss Jones, Commented Dec 1, 2015 at 15:48

jpkotta · Accepted Answer · 2015-12-01 16:04:58Z

6

If all you care about is that the input was a valid numeric literal, don't even bother with the regexp. int will correctly parse the string or raise an exception.

while True:
    s = input("Please enter a number: ")
    try:
        n = int(s)
        break
    except ValueError:
        print("Error! Make sure you only use numbers")
print("You picked number " + n)

answered Dec 1, 2015 at 16:04

jpkotta

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1 Comment

Wiktor Stribiżew Over a year ago

That is what is called proactive thinking :)

Wiktor Stribiżew · Accepted Answer · 2015-12-01 15:50:32Z

1

Regular expressions need strings as input, not numbers. Thus, you do not have to cast the string to number, and you can omit number=int(number).

Here is a working demo:

number = "2"
if not re.match("^[0-9 -]+$", number):
    print ("Error! Make sure you only use numbers")
print("You picked number "+ number)

answered Dec 1, 2015 at 15:50

Wiktor Stribiżew

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Comments

Alexander Safronov · Accepted Answer · 2015-12-01 16:53:36Z

0

import re

while  True:
    number =input("Please enter a number: ")
       # number=int(number) -- This raise an error, for regexp you need use "str"

    #while not re.match("^[0-9 \-]+$", number):
    # in [0-9 \-]  after 9 you use whitespace
    # Also in [0-9 \-]  if minus at the and, not need use backslash

if  re.match("-?\d+$", number) : # minus may be only in first position
    break

print ("Error! Make sure you only use numbers")
    # deleted     number = input("Please enter a number: ")

print("You picked number "+ number)

answered Dec 1, 2015 at 16:53

Alexander Safronov

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Comments

serenesat · Accepted Answer · 2016-02-01 13:00:11Z

0

answer = input(" ")#asks the user for an answer
ansnum = re.match("[0-9 \-]",answer)#checks to see if answer only contains numbers
while not ansnum: #whilst the answer does not contain numbers do....
   answer = input("Enter numbers as your answer: ")#asks the user for a valid answer
   ansnum = re.match("[0-9 \-]",answer)#checks to see if the answer contains numbers

edited Feb 1, 2016 at 13:00

serenesat

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answered Feb 1, 2016 at 12:53

Karmah

1

1 Comment

Richard Erickson Over a year ago

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