isvarname
isvarname(s) returns true if string s is a valid variable name.
Examples
> isvarname('x_') [ans:true] > isvarname('_x_') [ans:false] > isvarname('a1') [ans:true] > isvarname('1a') [ans:false]
> isvarname('x_') [ans:true] > isvarname('_x_') [ans:false] > isvarname('a1') [ans:true] > isvarname('1a') [ans:false]