Pattern matching in R if string NOT followed but another string - r

Question

I am trying to match the following in R using str_detect from the stringr package.
I want to to detect if a given string if followed or preceeded by 'and' or '&'. For example, in:
string_1<-"A and B"
string_2<-"A B"
string_3<-"B and A"
string_4<-"A B and C"
I want str_detect(string_X) to be FALSE for string_1, string_3 and string_4 but TRUE for string_2.
I have tried:
str_detect(string_X,paste0(".*(?<!and |& )","A"))==TRUE & str_detect(string_X,paste0(".*","A","(?! and| &).*"))==TRUE)
I use paste0 because I want to run this over different strings. This works all the cases above except 4. I am new to regex, and it also does not seem very elegant. Is there a more general solution?
Thank you.

Answer 1

First let's combine your four strings into a single vector:
strings <- c(string_1, string_2, string_3, string_4)
Now using
library(stringr)
str_detect(strings, "(A|B)(?=\\s(and|&))", negate = TRUE)
we look for "A" or "B" followed by "and" or "&". So this returns
#> [1] FALSE TRUE FALSE FALSE
You could wrap it into a function:
detector <- function(letters, strings) {
pattern <- paste0("(", paste0(letters, collapse = "|"), ")(?=\\s(and|&))")
str_detect(strings, pattern, negate = TRUE)
}
detector(c("A", "B"), strings)
#> [1] FALSE TRUE FALSE FALSE
detector(c("A"), strings)
#> [1] FALSE TRUE TRUE TRUE
detector(c("B"), strings)
#> [1] TRUE TRUE FALSE FALSE
detector(c("C"), strings)
#> [1] TRUE TRUE TRUE TRUE

Answer 2

You can use a positive lookahead assertion to make sure that there is no A or B present followed by and or & and also not in the other order.
^(?!.*[AB] (?:and|&))(?!.*(?:and|&) [AB])
^ Start of string
(?!.*[AB] (?:and|&)) Assert that the string does not contain A or B followed by either and or &
(?!.*(?:and|&) [AB]) Assert that the string does not contain either and or & followed by either A or B
Regex demo | R demo
library(stringr)
string_1<-"A and B"
string_2<-"A B"
string_3<-"B and A"
string_4<-"A B and C"
string_5<-"& B"
strings <- c(string_1, string_2, string_3, string_4, string_5)
str_detect(strings, "^(?!.*[AB] (?:and|&))(?!.*(?:and|&) [AB])")
Output
[1] FALSE TRUE FALSE FALSE FALSE