test <- c("ˈsɜːtnli", "ˈtwɛnti", "ˈfɒksi", "kɑːnt", "ʧeɪnʤd", "vɪkˈtɔːrɪə", "wɒznt", "ðeər", "dɪdnt",
          "ˈdɪzni", "ˈəʊnli", "ˈfæbrɪks", "sɪˈkjʊərɪti", "ˈnjuːzˌpeɪpər", "ɑhɑː")
vowel <- "iː|aɪ|ɔː|ɔɪ|əʊ|ɛə|eɪ|aʊ|eə|uː|ɑː|ɪə|ɜː|ʊə|ə|ɪ|ɒ|ʊ|ʌ|æ|e|ɑ|ɛ|i"
library(stringr) 
p <- paste0("^(?!.*(?<!",vowel,")nt$)(?:(?!",vowel,").)*(?:",vowel,")(?:(?!",vowel,").)*$")
 
test[str_detect(test, p)]

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