A very common use case for regmatches is to extract regex matches into a new
column in a data.frame (or data.table, etc.) or otherwise use the extracted
strings alongside the input. However, the default behavior is to drop empty
matches, which results in mismatches in column length if reassignment is done
without subsetting.
For consistency with other R functions and compatibility with this use case, it
would be nice if regmatches did not automatically drop empty matches and would
instead insert an NA_character_ value (similar to stringr::str_extract). This
alternative regmatches could be implemented through an optional drop argument,
a new function, or mentioned in the documentation (a la resample in ?sample).
Alternatively, at the moment, there is a non-exported function strextract in
utils which is very similar to stringr::str_extract. It would be great if this
function, once exported, were to include a drop argument to prevent dropping
positions with no matches.
An example solution (last option):
strextract <- function(pattern, x, perl = FALSE, useBytes = FALSE, drop = T) {
m <- regexec(pattern, x, perl=perl, useBytes=useBytes)
result <- regmatches(x, m)
if(isTRUE(drop)){
unlist(result)
} else if(isFALSE(drop)) {
unlist({result[lengths(result)==0] <- NA_character_; result})
} else {
stop("Invalid argument for `drop`")
}
}
Based on Ricardo Saporta's response to How to prevent regmatches drop non
matches?
--CG
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