What is a regular expression that would match any digit 1-999?

[1-9][0-9]{0,2}

Explanation:

The numbers to be matched consist of three digits with the following requirements:

- Digit 1 can take the values 1-9

- Digit 2 can take the values 0-9

- Digit 3 can take the values 0-9

- Digits 2 and 3 are optional, that is the number may consist of 1,2 or 3 digits in all.

Taking these into account, the overall regular expression can be represented as follows:

[1-9][0-9]{0,2}