Один из вариантов:
library(stringr)
library(wrapr)
x = "MANY THINGS AAA BBB CCC 4528383 validation 34567 validation 34567"
x %.>%
paste(
str_extract(., 'validation \\d+'),
str_remove(., 'validation \\d+')
)
Выход:
"validation 34567 MANY THINGS AAA BBB CCC 4528383 validation 34567"