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]