mjhjmtu

The regex is -?([0-9]|([1-9][0-9])).

The number is -2231 and it's being matched. From my understanding, it should be a single digit or double digits. 
Why is this number matched with this regex?

The regular expression is not anchored, so it's free to match the first 1 or two numbers and "succeed", leaving the trailing numbers (successfully) unmatched.

If you require 1 or 2 digit numbers, anchor the regex:

'^-?([0-9]|([1-9][0-9]))$'

Some examples:

$ seq -100 -99 | grep -E '^-?([0-9]|[1-9][0-9])$'
-99

$ seq 99 100 | grep -E '^-?([0-9]|[1-9][0-9])$'
99

$ seq -9 9 | grep -E '^-?([0-9]|[1-9][0-9])$'
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
7
8
9

$ seq -2231 -100 | grep -E '^-?([0-9]|[1-9][0-9])$'
(empty)

Most programs that use regex patterns actually implement a search of the pattern, instead of a full-string match. Python has distinct search() and match() methods where search() matches anywhere in the string, and match() only at the beginning. grep has the -x option to demand a match against the whole string; by default it matches anywhere in the string. Others, like sed, awk and Perl will happily look for the pattern anywhere in the string. Use the ^ and $ modifiers ("anchors") to force the pattern to the start or end of the string (respectively).

So, the ERE pattern you want is probably this:

^-?[1-9]?[0-9]$