stri_subset_fixed(c("stringi R", "REXAMINE", "123"), c('i', 'R', '0'))
stri_subset_fixed(c("stringi R", "REXAMINE", "123"), 'R')
stri_subset_charclass(c("stRRRingi","REXAMINE","123"),
c("\\p{Ll}", "\\p{Lu}", "\\p{Zs}"))
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