Find rows where one column string is in another column using dplyr in R

We can do this with Map

df[mapply(grepl, df$A, df$B),]
#    A                 B
#1 cat cat in the cradle
#3 boy   boy mmets world

Update

Using tidyverse, similar option would be purrr::map2 with stringr::str_detect

library(tidyverse)
df %>% 
   filter(map2_lgl(B, A,  str_detect))
#     A                 B
#1 cat cat in the cradle
#2 boy   boy mmets world

data

df <- data.frame(A, B, stringsAsFactors=FALSE)

You can either apply the function to both vectors using Map or iterate through the row using sapply

df %>%
  filter(unlist(Map(function(x, y) grepl(x, y), A, B)))
    A                 B
1 cat cat in the cradle
2 boy   boy mmets world

df %>%
  filter(sapply(1:nrow(.), function(i) grepl(A[i], B[i])))
    A                 B
1 cat cat in the cradle
2 boy   boy mmets world