x <- "This is a test. I only want to select the (cites) in parenthesis. I do not want it to return words in parenthesis that do not have years attached, such as abbreviations (abbr). For example, citing (Smith 2010) is something I would want to be returned. I would also want multiple citations returned separately such as (Smith 2010; Jones 2001; Brown 2020). I would also want Cooper (2015) returned as Cooper 2015, and not just 2015.  I would also want John Granger et al. (2015)."
rx <- "(?:\\b(\\p{Lu}\\w*(?:\\s+\\p{Lu}\\w*)*(?:\\s+et\\s+al\\.)?)?)\\s*\\(([^()]*\\d{4})\\)"
library(stringr)
res <- str_match_all(x, rx)
result <- lapply(res, function(z) {ifelse(!is.na(z[,2]) & str_detect(z[,3],"^\\d+$"), paste(trimws(z[,2]),  trimws(z[,3])), z[,3])})	
unlist(sapply(result, function(z) strsplit(paste(z, collapse=";"), "\\s*;\\s*")))
 

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