Re-use existing regex for whitespace detection

[mvolfik]

Less regexes = win.
Whitespace regex could also detect newlines, but since we're only
matching the part from last newline, it's safer (and more readable)
to use it here.

Checklist:

• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

Fairly sure that comment/code you removed was due to a previous PR. Can you check to see if you can find that PR so we can make sure we're not missing something?

[mvolfik]

You just added comments, some variables and reformatted in 37249c0 There was some change in dbbd082, but this PR doesn't reintroduce the bug in the linked issue (and I suppose there is a test for that somewhere). The regex was introduced in 7d00a40, I don't know what was going on there before that. So I don't think I'm reverting some old PR.

[davidism]

#858, #1014, and #1183 seem relevant and important enough to warrant a second look at why the code is the way it is.