We can change that behavior by permitting an optional minus sign as the starting character. If the pattern to match was changed to lets say...
"^-?\d{1,3}$"
I do not believe that the hyphen is a metacharcter so this should work. If it does not then add a backslash before the hyphen to escape it.
("^\-?\d{1,3}$"
Quote:
Quantifier Description
{num} Matches the preceding element num times.
{min, max} Matches the preceding element at least min times, but not more than max times.
? Matches any preceding element 0 or 1 times.
* Matches the preceding element 0 or more times.
+ Matches the preceding element 1 or more times.
