Exclude should ignore groups

[WetRobot]

This works

lintr::lint(
  text = "x = 1",
  linters = lintr::assignment_linter(),
  exclude = "(?:a)|(?:b)"
)

But this

lintr::lint(
  text = "x = 1",
  linters = lintr::assignment_linter(),
  exclude = "(a)|(b)"
)

raises an error that I think most users will not understand:

Error in `[.data.frame`(re_matches(lines, exclude, locations = TRUE),  : 
  undefined columns selected

[AshesITR]

Thanks for raising this. When fixing, we could also apply regex validation to other settings (nolint directives, for example).