t = c("I","like","hot","dogs","a","lot","hot","dogs","forever")
replace_phrases <- function(vec, ph){
  while(1){
    ind = c()
    l = 0
    for(j in 1:length(ph)){
      ind1 = match(ph[[j]], vec)
      if(all(!is.na(ind1))){
        if(length(ind) == 0 || ind[1] > ind1[1]){
          ind = ind1
          l = ph[[j]]
        }
      }
    }
    if(length(ind) == 0){
      return(vec)
      break
    }else{
      vec[ind[1]] = paste(l, collapse = '_')
      vec = vec[-ind[2:length(ind)]]
    }
  }
}

replace_phrases(t, list(c("hot", "dogs")))
replace_phrases(c("ab","pq","xy"), list(c("ab","pq"),c("pq","xy")))

十分感谢

好，谢谢楼主推荐！