Correct Stanford _ST possessive tags by replacing $ with S to make future regex matching easier.
Examples
x <- c("Their_PRP$", "house_NN", "was_VBD", "big_JJ", "and_CC", "luxurious_JJ", "._.")
dtag_possessives(x)
#> [1] "Their_PRPS" "house_NN" "was_VBD" "big_JJ" "and_CC"
#> [6] "luxurious_JJ" "._."