To cite package 'stringi' in publications use:

Gagolewski M. and others (2018). R package stringi: Character string processing facilities. http://www.gagolewski.com/software/stringi/.<doi:10.5281/zenodo.32557>

Corresponding BibTeX entry:

  @Manual{,
    title = {R package stringi: Character string processing
      facilities},
    author = {Marek Gagolewski},
    year = {2018},
    url = {http://www.gagolewski.com/software/stringi/},
  }