stockfish_response <- c("Final evaluation +0.30 (white side) [with scaled NNUE, hybrid, ...]","blah")

regmatches(stockfish_response, regexpr("Final evaluation.+?\\(white side\\)", stockfish_response))

library(stringr)
final_evaluation <- str_extract(stockfish_response, 'Final evaluation.+?\\(white side\\)')
final_evaluation <- final_evaluation[!is.na(final_evaluation)]
final_evaluation