library(datasets)
library(dplyr)
data(iris)

iris %>%
  setNames(gsub("([a-zA-Z]+)\\.(Length|Width)", "\\2 of \\1", names(.))) %>%
  head

Length of Sepal Width of Sepal Length of Petal Width of Petal Species
1             5.1            3.5             1.4            0.2  setosa
2             4.9            3.0             1.4            0.2  setosa
3             4.7            3.2             1.3            0.2  setosa
4             4.6            3.1             1.5            0.2  setosa
5             5.0            3.6             1.4            0.2  setosa
6             5.4            3.9             1.7            0.4  setosa