the bash [[ conditional expression supports extended regular expressions.
[[ $number =~ ^[0-9]{,2}$ && $number -ne 0 ]]
or as the inimitable @gniourf_gniourf points out in his comments, the following is needed to handle numbers with leading zeroes correctly
[[ $number =~ ^[0-9]{,2}$ ]] && ((number=10#$number))