Extract first value in boolean search string

Question

I would like to be able to control the hierarchy of elements I extract from a search string.

Specifically, in the string "425 million won", I would like to extract "won" first, but then "n" if "won" doesn't appear.

I want the result to be "won" for the following:

stringr::str_extract("425 million won", "won|n")

Note that specifying a space before won in my regex is inadequate because of other limitations in my data (there may not necessarily be a space between "million" and "won"). Ideally, I would like to do this using regex, as opposed to if-else clauses because of performance considerations.

Answer 1

See code in use here

pattern <- "^(?:(?!won).)*\\K(?:won|n)"
s <- "425 million won"
m <- gregexpr(pattern,s,perl=TRUE)
regmatches(s,m)[[1]]

Explanation

• ^ Assert position at the start of the line
• (?:(?!won).)* Tempered greedy token matching any character except instances where won proceeds
• \K Resets the starting point of the match. Any previously consumed characters are no longer included in the final match
• (?:won|n) Match either won or n

Answer 2

If you just want to extend on the code you already have:

 na.omit(str_extract("420 million won", c("won", "n")))[1]