I am practicing for an exam. I am trying to convert a language to a regular expression. The language and my solution is as follows:$L=\{w\{a,b\}^+|w|4\  and\ the\ second\  symbols\  from \ the \ front\  and\   the\  back\  are\ equal\}$Solution: $(a+b)x(a+b)^* x(a+b)  \ \ where\ x=(a+b)$Is this correct? If not, can someone correct me. also, am I allowed to use variables in regular expressions?Thanks in advance
Regular expression for the second symbols from the front and the back are equal
formal languages;regular languages;regular expressions