x <- "Prof. Imperdiet montes, metus elementum eleifend eget eget adipiscing augue.\nAbstract title: Lorem ipsum dolor sit amet, consectetuer adipiscing\n\nA, nec, quam eleifend quis, magnis sit pretium. leo augue. amet, elit. vel\n\nVel, dis eget nascetur justo. imperdiet consequat et sit Nam Aenean a, Quisque\nEnim. a, dui. Aenean lorem Phasellus commodo quis, pretium ultricies nascetur\ntincidunt. sem. vitae,\nmontes, tellus. amet, venenatis natoque enim. fringilla\nquis, vitae, Aenean Etiam viverra ipsum dapibus ut elementum Aenean Lorem eget,\nnisi mollis Curabitur Quisque Aenean rhoncus sociis justo, sem. justo, vel\nAenean ultricies nec, eu laoreet.\n\nDr. Enim. vitae, feugiat in, Aenean\nAbstract title: Massa. sociis dis dapibus dolor semper ipsum\njalor\n\nSemper tincidunt. ullamcorper commodo magnis viverra pede elit. eget aliquet\neleifend vel, eleifend feugiat pede Vivamus ridiculus vitae, a, ligula, et Nulla\nligula vulputate ac, nisi. enim dapibus. Donec metus In sit dolor Nam ultricies\nimperdiet. pellentesque Cras eu, massa quis porttitor parturient varius ut,\nPhasellus arcu. pretium. quam augue. eu, adipiscing felis, enim. ante,\nvulputate Integer dui. ultricies a, dictum rutrum. Nullam nec, quis,\nconsequat Cum tellus. dis felis dolor. nulla Aliquam Donec massa. justo. in,\nnascetur\nSemper tincidunt. ullamcorper commodo magnis viverra pede elit. eget aliquet\neleifend vel, eleifend feugiat pede Vivamus ridiculus vitae, a, ligula, et Nulla\n\n\nDr. Justo. nisi elementum ante, Donec Aenean Nulla\nAbstract title:\n\nAenean consectetuer leo penatibus eget imperdiet nisi. consequat\nlorem pretium mus. \n\nProf. Dr. Aliquam metus semper\nAbstract title: Aliquet augue. amet, enim ut justo, nec, eleifend lorem enim. nisi. ipsum\neleifend\nMore information will be available soon.\n"
library(stringr)
pattern <- "(?<=Abstract title:).*(?:\n(?!\n).*)*"
results <- lapply(str_extract_all(x, pattern), function(z) trimws(gsub("\\s+", " ", z)))
results

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