CODEWITHSUNDEEP
rregexstr-replacegsub
Mark R
Mark R

Reputation: 1009

Using variable value in regex in R

How can I use a variable value and a regex position expression together in R? For example, in the following code, how would I replace only cases of "zzz" that come at the start or end of a string? This works for all values of "zzz"

target_nos <- c("1","2","zzz","4")
sample_text <- cbind("1 dog 1","3 cats zzz","zzz foo 1")
for (i in 1:length(target_nos))
{
sample_text <- gsub(pattern = target_nos[i],replacement = "REPLACED", x = 
sample_text)
}

But how would I include the ^ and $ position markers? This throws an error

sample_text <- gsub(pattern = ^target_nos[1],replacement = "REPLACED", x = 
sample_text)

This runs, but interprets the variable literally, rather than calling the value

sample_text <- gsub(pattern = "^target_nos[1]", replacement = "REPLACED", x = 
sample_text)

Upvotes: 2

Views: 1852

Answers (1)

Tim Goodman
Tim Goodman

Reputation: 23976

You need the ^ and $ characters to be within the regex pattern strings. In other words, target_nos could be this:

"^1"   "^2"   "^zzz" "^4"   "1$"   "2$"   "zzz$" "4$"

To build that programmatically from what you've got, you could do this:

target_nos <- c("1","2","zzz","4")
target_nos <- c(paste0('^', target_nos), paste0(target_nos, '$'))

Upvotes: 2

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