Pattern matching in R if string NOT followed but another string

user_13462505 0
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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.

regex-lookarounds r pattern-matching regex string
2 Answers

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
user_125052510

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
user_54249880
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