is_valid_regex needs to set perl=TRUE

[MichaelChirico]

a separate, minor issue with lintr:::is_valid_regex:

lintr:::is_valid_regex(lintr::default_settings$exclude_linter) # FALSE

Hmm, that's just because is_valid_regex() uses perl = FALSE, it works with perl = TRUE:

!inherits(tryCatch(grepl(lintr::default_settings$exclude_linter, "", perl = TRUE), condition = identity), "condition")
# [1] TRUE

Originally posted by @MichaelChirico in #2374 (comment)

Ultimately parse_exclusions() is running rex::re_matches(), which in turn runs perl = TRUE.