NULL or '' for case mapping following the conventions of the default locale, or a single string with locale identifier, see stringi-locale. a named list with ICU BreakIterator's settings as generated with stri_opts_brkiter, NULL for default break iterator, i.e. word, stri_trans_totitle only is context-sensitive (a character in the input string may map differently depending on surrounding characters). With stri_trans_totitle, if word BreakIterator is used (the default), then the first letter of each word will be capitalized and the rest will be transformed to lower case. With a break iterator of type sentence, the first letter of each sentence will be capitalized only. Note that according the ICU User Guide, the string 'one. two. three.' consists of one sentence. Other locale_sensitive: %s<,%, stri_compare, stri_count_boundaries, stri_duplicated, stri_enc_detect2, stri_extract_all_boundaries, stri_locate_all_boundaries, stri_opts_collator, stri_order, stri_split_boundaries, stri_unique, stri_wrap, stringi-locale, stringi-search-boundaries, stringi-search-coll Other text_boundaries: stri_count_boundaries, stri_extract_all_boundaries, stri_locate_all_boundaries, stri_opts_brkiter, stri_split_boundaries, stri_split_lines, stri_wrap, stringi-search-boundaries, stringi-search Source.