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I'm learning regex and I would like to use a regular expression in Python to define only integers - whole numbers but not decimals.

I could make one that only allows numbers by using \d, but it also allows decimal numbers, which I don't want:

price = TextField(_('Price'), [
    validators.Regexp('\d', message=_('This is not an integer number, please see the example and try again')),
    validators.Optional()])

How can I change the code to only allow integers?

daaawx
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Niklas Rosencrantz
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5 Answers5

115

Regexp work on the character base, and \d means a single digit 0...9 and not a decimal number.

A regular expression that matches only integers with a sign could be for example

^[-+]?[0-9]+$

meaning

  1. ^ - start of string
  2. [-+]? - an optional (this is what ? means) minus or plus sign
  3. [0-9]+ - one or more digits (the plus means "one or more" and [0-9] is another way to say \d)
  4. $ - end of string

Note: having the sign considered part of the number is ok only if you need to parse just the number. For more general parsers handling expressions it's better to leave the sign out of the number: source streams like 3-2 could otherwise end up being parsed as a sequence of two integers instead of an integer, an operator and another integer. My experience is that negative numbers are better handled by constant folding of the unary negation operator at an higher level.

6502
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    One minor point: `\d` means any decimal digit, so if you are using Python 3 it will match more than just `0`..`9`. e.g. `re.match("\d", "\u0665")` will match (and also `int("\u0665")` gives `5`). – Duncan Dec 21 '11 at 09:00
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    This post is from forever ago, but in case somebody new stumbles upon it, technically the minus sign is an operator (in Python 3), not part of the integer: from spec.: "Note that numeric literals do not include a sign; a phrase like -1 is actually an expression composed of the unary operator ‘-‘ and the literal 1" – en_Knight Feb 11 '15 at 00:35
  • @en_Knight: this is IMO totally irrelevant. What the OP was asking is an expression to accept integer numbers **for humans**, not for Python 3. While having a "price" probably could qualify for non-negative numbers only, for example it can make a lot of sense accepting negative integers for a "price difference"... even when writing that accounting program using Python 3. – 6502 Feb 11 '15 at 06:54
  • Some grammars define integer as + or - followed by one or more digits, ^[+\-]?[0-9]+$ – ChuckCottrill Mar 17 '17 at 01:13
  • @ChuckCottrill: added the `+` sign (not so common when parsing but indeed somewhat common when formatting). I've also added a note on why it's however a bad idea to consider the sign part of the number when parsing if you're aiming at a more complex parsers able to handle expressions. – 6502 Mar 17 '17 at 06:39
  • Agreed that both the + and - symbols are overloaded as both unary sign symbols and binary operators (binop add and subtract). That is part of the difference between lexical analysis (recognizing symbols) and grammar parsing (combining tokens into expressions). – ChuckCottrill Mar 17 '17 at 14:54
16

You need to anchor the regex at the start and end of the string:

^[0-9]+$

Explanation:

^      # Start of string
[0-9]+ # one or more digits 0-9
$      # End of string
Tim Pietzcker
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    This doesn't allow negative integers... not sure if OP wants to avoid them – 6502 Dec 21 '11 at 07:36
  • @6502: Well, since it's a validation for a price text field, I thought positive integers make more sense, but still +1 for your well-commented answer :) – Tim Pietzcker Dec 21 '11 at 07:43
  • Actually positive integers don't make much sense for a price text field, unless the price is in cents.. – wim Dec 21 '11 at 07:51
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    @wim: Nick explicitly asked for integers - that's the entire point of the question. – Tim Pietzcker Dec 21 '11 at 07:53
  • It's like a price but only in dollars. It's for a classifieds posting site where everything that is advertised is going to be in dollars and nothing in cents. – Niklas Rosencrantz Dec 21 '11 at 11:11
10

I prefer ^[-+]?([1-9]\d*|0)$ because ^[-+]?[0-9]+$ allows the string starting with 0.

RE_INT = re.compile(r'^[-+]?([1-9]\d*|0)$')


class TestRE(unittest.TestCase):
    def test_int(self):
        self.assertFalse(RE_INT.match('+'))
        self.assertFalse(RE_INT.match('-'))

        self.assertTrue(RE_INT.match('1'))
        self.assertTrue(RE_INT.match('+1'))
        self.assertTrue(RE_INT.match('-1'))
        self.assertTrue(RE_INT.match('0'))
        self.assertTrue(RE_INT.match('+0'))
        self.assertTrue(RE_INT.match('-0'))

        self.assertTrue(RE_INT.match('11'))
        self.assertFalse(RE_INT.match('00'))
        self.assertFalse(RE_INT.match('01'))
        self.assertTrue(RE_INT.match('+11'))
        self.assertFalse(RE_INT.match('+00'))
        self.assertFalse(RE_INT.match('+01'))
        self.assertTrue(RE_INT.match('-11'))
        self.assertFalse(RE_INT.match('-00'))
        self.assertFalse(RE_INT.match('-01'))

        self.assertTrue(RE_INT.match('1234567890'))
        self.assertTrue(RE_INT.match('+1234567890'))
        self.assertTrue(RE_INT.match('-1234567890'))
sira
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9

You are apparently using Django.

You are probably better off just using models.IntegerField() instead of models.TextField(). Not only will it do the check for you, but it will give you the error message translated in several langs, and it will cast the value from it's type in the database to the type in your Python code transparently.

Bite code
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0
import re

num="12.345678"

y = re.findall('\.[0-9.]+',num)

print(y)

Answer= ['.345678']

After that, you can convert string to float.

Laurel
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Mukesh Nayal
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