x <- c("bcl-2 was detected in 45% bcl-6 was not detected", 
"bcl-2 was not detected bcl-6 was detected in 45%",
"no evidendce of bcl-2 bcl-6 was detected in 45%")
grep("\\bbcl-2\\b(?:(?!\\bbcl-\\d|\\bnot\\b).)*?\\bdetected\\b", x, perl=TRUE, value=TRUE)