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pythonregexvalidationinput
coatsoft
coatsoft

Reputation: 3

Input validation with the `re` library

I am learning Python input validation with the re library.

Currently I am using the following:

valid = re.match('[1-5]', UserInput)

I wish to make sure the input is between 1 and 5.

There is just one problem with this method; if I input something that begins with 1-5 followed by something else (for example, 1bdfgh, 354), it is considered valid.

Is there a way around this? I would prefer the solution still using the re library as it is for school.

Upvotes: 0

Views: 257

Answers (4)

user2555451
user2555451

Reputation:

re.match will only check if the string starts with the pattern you give it. To have it match the whole string, you can do:

valid = re.match('[1-5]$', UserInput)

In a Regex pattern, $ represents the end of the string. So, the above pattern will match strings that start with a character in the set [1-5] and then end. The only strings which satisfy this condition are '1', '2', '3', '4', and '5'.

Note that in Python 3.4, you can use re.fullmatch for this:

valid = re.fullmatch('[1-5]', UserInput)

It is the same as re.match, except that it matches the whole string rather than just the start.

Upvotes: 1

Bhargav Rao
Bhargav Rao

Reputation: 52151

You can use word boundaries

>>> a = '1sdv'
>>> valid = re.match('[1-5]',a)
>>> valid.group()                                        # Wrong Match
'1'
>>> valid = re.search(r'^\b[1-5]\b$',a)        # Correct Way
>>> valid.group()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'group'
>>> a = '1'
>>> valid = re.search(r'^\b[1-5]\b$',a)
>>> valid.group()
'1'
>>> a = '1 potato'
>>> valid = re.search(r'^\b[1-5]\b$',a)
>>> valid.group()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'group'

Upvotes: 2

Kroltan
Kroltan

Reputation: 5156

You can use ^ (beginning of input) and $ (end of input) tokens, transforming your regex into:

^[1-5]$

But for numerical range checking, there's a much simpler solution:

theNumber = int(UseInput)
valid = 1 <= theNumber <= 5

Or even:

theNumber = int(UserInput)
valid = theNumber in range(1,6) #up to six, exclusive, so from 1 to 5 inclusive

Upvotes: 1

GLHF
GLHF

Reputation: 4035

re.match only checks beginning of the string. You could use;

valid = re.search('[1-5]',UserInput)

Which is checking all over the string.

Upvotes: 0

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