Get citation keys from tex file; then get a subset of cited items as a bib file.

get_citation_keys.R

library(stringr)
library(RefManageR)

## to get citation keys (with forms: author2016title) from tex file
get_cite_key = function(tex){
  text = readLines(tex)
  # \cite*{}
  key_1 = str_replace_all(grep("^.*cite[a-z]{0,7}\\{([A-Za-z0-9,-]*)\\}.*$", 
                             x = text, value = T), 
                        pattern = "^.*cite[a-z]{0,7}\\{([A-Za-z0-9,-]*)\\}.*$", 
                        replacement = "\\1")
  key_1 = sort(unique(unlist(strsplit(unique(key_1), ","), recursive = T)))
  # \citep[][]{}
  key_2 = str_replace_all(grep("^.*cite[a-z]{0,7}\\[.*\\]\\{([A-Za-z0-9,-]*)\\}.*$", 
                               x = text, value = T), 
                          pattern = "^.*cite[a-z]{0,7}\\[.*\\]\\{([A-Za-z0-9,-]*)\\}.*$", 
                          replacement = "\\1")
  key_2 = sort(unique(unlist(strsplit(unique(key_2), ","), recursive = T)))
  sort(unique(c(key_1, key_2)))
}

key0 = get_cite_key(tex = "chapter1/chapter1.tex")

# to extract cited items from a bigger bib file
bib1 = ReadBib("chapter1/larger.bib", check = T)
# to save cited items 
WriteBib(bib1[[key0]], "chapter1/ch1.bib", biblatex=F)