A positive number from 1 to 2^31 -1 in regex

I got the following regex that almost does the work but does not exclu开发者_开发技巧de zero ...How to do that?

^(\d|\d{1,9}|1\d{1,9}|20\d{8}|213\d{7}|2146\d{6}|21473\d{5}|214747\d{4}|2147482\d{3}|21474835\d{2}|214748364[0-7])$

Also can anybody explain a bit how this works?

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Regular expressions are not the right tool for this job. A much better solution is to extract the integer from your string (you can use a regex for this, just \d+), then convert that to an integer, then check the integer against your desired range.

An important corollary is to never blindly use a regular expression (or any code, really) that you don't understand yourself. What would you do if you used the regular expression above, then a requirement came in to modify the acceptable range?

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As Greg said, regexes are not the right tool for the job here. But if you insist on knowing how the regex you pasted works:

The most important thing to remember is that 2**31 - 1 = 2147483647 (a number with 10 digits). In essence, the regex says:

• The number can have 1-9 digits, OR
• It can be 1 with any 9 digits after it, OR
• 20 with any 8 digits after it, OR
• 213 with any 7 digits after it, OR
• ... I'm sure you see where it's going

It restricts the numbers to the range of being below 2147483647.

P.S. given such a number as a string s, in Python, you can just pose this condition:

1 <= int(s) <= 2**31 - 1

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In addition to the other answers, your regex doesn't work (besides allowing 0): it incorrectly excludes numbers like 2100000000, 2147483639, and most of the numbers between those two. The solution is to replace most of the nnnn prefixes with nnn[0-n] (along with other fixes), but the real solution is to not use regular expressions.