Vectorized over str, [value], from and
(to or length). Parameters
to and length are mutually exclusive.
Indexes are 1-based, i.e., the start of a string is at index 1.
For negative indexes in from or to,
counting starts at the end of the string.
For instance, index -1 denotes the last code point in the string.
Non-positive length gives an empty string.
Argument from gives the start of a substring to extract.
Argument to defines the last index of a substring, inclusive.
Alternatively, its length may be provided.
If from is a two-column matrix, then these two columns are
used as from and to, respectively,
unless the second column is named length.
In such a case anything passed
explicitly as to or length is ignored.
Such types of index matrices are generated by stri_locate_first
and stri_locate_last. If extraction based on
stri_locate_all is needed, see
stri_sub_all.
In stri_sub, out-of-bound indexes are silently
corrected. If from > to, then an empty string is returned.
By default, negative length results in the corresponding output being
NA, see ignore_negative_length, though.
In stri_sub<-, some configurations of indexes may work as
substring 'injection' at the front, back, or in middle.
Negative length does not alter the corresponding input string.
If both to and length are provided,
length has priority over to.
Note that for some Unicode strings, the extracted substrings might not
be well-formed, especially if input strings are not normalized
(see stri_trans_nfc),
include byte order marks, Bidirectional text marks, and so on.
Handle with care.