Q$1$: Write a regular expression for the following languages

a$.$ Having all strings of even length, defined over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}$

b$.$ In which all strings are starting and ending with different letters, defined over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}$

c$.$ Set of strings containing at least one $0$ and one $1,$ defined over ${\displaystyle \underset{?}{\overset{?}{}}}=\{0,1,2\}.$

d$.$ All words that contain exactly two b$\text{'}$s or exactly three b$\text{'}$s$,$ over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}.$

e$.$ All strings that not ending in a double letter, over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}.$

f$.$ Having all strings of ODD length, over ${\displaystyle \underset{?}{\overset{?}{}}}=\{a,b\}..$