[R] String manipulation

[Luis Ridao Cruz]

R-help,

I have a data frame which contains a character string column that is
something like;

II11
II18
II23
III1
III13
III16
III19
III2
III7
IV10
IV11
IV12
IX16
IX4
V12
V18
V2
V20
V23
V4
VII14
VII18
VII21
VII26
VII28
VII33
VII4
VII48
VII5
....
....
....

I want to apply a function (e.g mean) by grouping according to the
roman part of the string, i.e,

by I
by V
by VII
...
...
and so on.

I have looked at string manipulation functions (grep, pmatch,,,) but I
can't really get it the way I want.
Can anyone help?

Thanks in advance.