Selecting the word immediately after a keyword - regex

Question

I'm trying to extract the word immediately a keyword using R. I don't have a lot of experience with regular expressions so everything I've found so far doesn't help me much. If I could get the function to return multiple instances that would be ideal.
For example if my keyword was the and my string was:
The yellow log is in the stream
It would return yellow and stream.
I found this solution for c# and it seems exactly like what I want but I'm having trouble implementing it in R.

Answer 1

You can try
library(stringr)
str_extract_all(str1, perl('(?<=\\b(?i)The )\\w+'))[[1]]
#[1] "yellow" "stream"
Or using stringi
library(stringi)
stri_extract_all_regex(str1, '(?<=\\b(?i)The )\\w+')[[1]]
#[1] "yellow" "stream"
EDIT: Changed based on #Roland's suggestion in the comments.
data
str1 <- 'The yellow log is in the stream'

Answer 2

assign key to whatever string you want and use
key <- 'the'
p <- "The yellow log is in the stream"
regmatches(p, gregexpr(sprintf('(?i)(?<=%s\\s)\\w+', key), p, perl = TRUE))[[1]]
# [1] "yellow" "stream"
or as #Roland points out, it would be safer to use a word boundary around your keyword to avoid this:
key <- 'the'
p <- "The yellow log is in the stream drinking absinthe and beer"
regmatches(p, gregexpr(sprintf('(?i)(?<=%s\\s)\\w+', key), p, perl = TRUE))[[1]]
# [1] "yellow" "stream" "and"
regmatches(p, gregexpr(sprintf('(?i)(?<=\\b%s )\\w+', key), p, perl = TRUE))[[1]]
# [1] "yellow" "stream"

Answer 3

Here is non regex solution:
mytext <- "The yellow log is in the stream"
mykey <- "the"
x <- unlist(strsplit(mytext," "))
x[which(tolower(x)==mykey)+1]

Answer 4

Try this: this returns 'yellow' and 'stream'
x <- "The yellow log is in the stream"
regmatches(x, gregexpr("(?:(?:T|t)he)\\s(\\w+)", x, perl = TRUE))[[1]]
## [1] "The yellow" "the stream"

Answer 5

The qdapRegex package I maintain has a regular expression after_ in the regex_supplement dictionary that is perfect for this. You can use rm_ to make your own after_the function:
library(qdapRegex)
x<- "The yellow log is in the stream"
after_the <- rm_(pattern = S("#after_", "[Tt]he"), extract = TRUE)
after_the(x)
## [[1]]
## [1] "yellow" "stream"
The S function is a wrapper for sprintf that allows you to easily pass elements (like the work "the" in this case) to the base regex producing:
S("#after_", "the", "The")
## [1] "(?<=\\b(the|The)\\s)(\\w+)"
EDIT
library(qdapRegex)
x<- c("The yellow log is in the stream", "I like the one box for a pack")
after_ <- rm_(extract = TRUE)
after_the(x)
after_ <- rm_(extract = TRUE)
words <- c("the", "a", "one")
setNames(lapply(words, function(y){
after_(x, pattern = S("#after_", y, TC(y)))
}), words)
## $the
## $the[[1]]
## [1] "yellow" "stream"
##
## $the[[2]]
## [1] "one"
##
##
## $a
## $a[[1]]
## [1] NA
##
## $a[[2]]
## [1] "pack"
##
##
## $one
## $one[[1]]
## [1] NA
##
## $one[[2]]
## [1] "box"