Regex for a valid 32-bit signed integer

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I really hope it is just puzzler and no one will use regex for this problem in real world. Proper solution would be converting number from string to numeric type like BigInteger. This should allow us to check its range using proper methods or operators, like compareTo, >, <.

To make life easier you can use this page (dead link) to generate regex for ranges. So regex for range 0 – 2147483647 can look like

\b([0-9]{1,9}|1[0-9]{9}|2(0[0-9]{8}|1([0-3][0-9]{7}|4([0-6][0-9]{6}|7([0-3][0-9]{5}|4([0-7][0-9]{4}|8([0-2][0-9]{3}|3([0-5][0-9]{2}|6([0-3][0-9]|4[0-7])))))))))\b

(friendlier way)

\b(
 [0-9]{1,9}|
1[0-9]{9}|
2(0[0-9]{8}|
  1([0-3][0-9]{7}|
       4([0-6][0-9]{6}|
            7([0-3][0-9]{5}|
                 4([0-7][0-9]{4}|
                      8([0-2][0-9]{3}|
                           3([0-5][0-9]{2}|
                                6([0-3][0-9]|
                                     4[0-7]
)))))))))\b

and range 0 – 2147483648

\b([0-9]{1,9}|1[0-9]{9}|2(0[0-9]{8}|1([0-3][0-9]{7}|4([0-6][0-9]{6}|7([0-3][0-9]{5}|4([0-7][0-9]{4}|8([0-2][0-9]{3}|3([0-5][0-9]{2}|6([0-3][0-9]|4[0-8])))))))))\b

So we can just combine these ranges and write it as

range of 0-2147483647 OR "-" range of 0-2147483648

which will give us

\b([0-9]{1,9}|1[0-9]{9}|2(0[0-9]{8}|1([0-3][0-9]{7}|4([0-6][0-9]{6}|7([0-3][0-9]{5}|4([0-7][0-9]{4}|8([0-2][0-9]{3}|3([0-5][0-9]{2}|6([0-3][0-9]|4[0-7])))))))))\b|-\b([0-9]{1,9}|1[0-9]{9}|2(0[0-9]{8}|1([0-3][0-9]{7}|4([0-6][0-9]{6}|7([0-3][0-9]{5}|4([0-7][0-9]{4}|8([0-2][0-9]{3}|3([0-5][0-9]{2}|6([0-3][0-9]|4[0-8])))))))))\b.

[edit]

Since Bohemian noticed in his comment final regex can be in form -?regex1|-2147483648 so here is little shorter version (also changed [0-9] to \d)

^-?(\d{1,9}|1\d{9}|2(0\d{8}|1([0-3]\d{7}|4([0-6]\d{6}|7([0-3]\d{5}|4([0-7]\d{4}|8([0-2]\d{3}|3([0-5]\d{2}|6([0-3]\d|4[0-7])))))))))$|^-2147483648$

If you will use it in Java String#matches(regex) method on each line you can also skip ^ and $ parts since they will be added automatically to make sure entire string matches regex.

I know this regex is very ugly, but just shows why regex is not good tool for range validation.

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