stri_extract from stringi and
str_extract from stringr
for the underlying implementation.
Other string operations:
seq-replace,
seq_combine(),
seq_count_pattern(),
seq_crop_pattern(),
seq_crop_position(),
seq_detect_pattern(),
seq_extract_pattern(),
seq_remove_pattern(),
seq_remove_position(),
seq_replace_position(),
seq_split_kmer(),
seq_split_pattern()