A character vector the same length as string/pattern.
Input vector. Either a character vector, or something
coercible to one.
Pattern to look for.
The default interpretation is a regular expression, as described in
vignette("regular-expressions"). Use regex() for finer control of the
Match a fixed string (i.e. by comparing only bytes), using
fixed(). This is fast, but approximate. Generally,
for matching human text, you'll want coll() which
respects character matching rules for the specified locale.
Match character, word, line and sentence boundaries with
boundary(). An empty pattern, "", is equivalent to