Hi,
I got the following regex that almost does the work but does not exclude 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?
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?
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:
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
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.