R/str_extract_sandwich.R

In indenkun/infun: Indenkun's Utility Functions

#' Extracts strings sandwiched between specified strings
#' @description
#' Extracts strings sandwiched between specified strings.
#' @details
#' The specified string must be a single character or three characters including the escape, and the first and last characters of the string must be different.
#'
#' @param string Input vector. Either a character vector, or something coercible to one.
#' @param start_pattern Specifies the first character of the string to be extracted. It must be different from the last character, \code{end_pattern}.
#' @param end_pattern Specifies the last character of the string to be extracted. It must be different from the first character, \code{start_pattern}.
#' @param perl logical. Should Perl-compatible regexps be used?
#'
#' @export

str_extract_sandwich <- function(string, start_pattern, end_pattern, perl = FALSE){

  if(substr(start_pattern, 1 ,1) == "\\" &&
     substr(end_pattern, 1, 1) == "\\"){
    if(nchar(start_pattern) != 2 ||
       nchar(end_pattern) != 2 ||
       start_pattern == end_pattern){
      warning("The starting or ending letter must always be a single or three characters including the escape different letter.")
      return(NA)
    }
  }else if(nchar(start_pattern) != 1 ||
           nchar(end_pattern) != 1 ||
           start_pattern == end_pattern){
    warning("The starting or ending letter must always be a single or three characters including the escape different letter.")
    return(NA)
  }

  unname(lapply(string, function(string){
    string_origin <- string
    string <- unlist(strsplit(string, split = start_pattern, perl = perl))
    string <- strsplit(string, split = end_pattern, perl = perl)
    sapply(string[2:length(string)], function(x) x[1])
  }))

}