The container code takes the following format. $$ Initial B or C character to indicate whether the container is a box or cylinder. $$ A $3-$character serial number containing digits and upper and lowercase characters. $$ A final character that matches the initial B or C character. Write the regular expression $($in quotes$)$ that can be used to check that a container code meets the required criteria. An example of a valid code is B$0$CxB$.$ Ensure that you use escape characters correctly. $(8)1\mathrm{.}3.$ If an input mask were to be used for the input widget on the GUI, what string would be used to ensure valid values for the code required in $1\mathrm{.}2?$