s/\s(?<![ \n])//g;
s/ \K +//g;
s/\n\n\K\n+//g;

[download]

The order of the first two matters (e.g. foo{space}{tab}{space}bar). I gave them in the same order you requested them.

I find it odd that foo{tab}bar should become foobar. One usually wants foo{space}bar. To get the latter, 

s/(?:\s(?<![ \n]))+/ /g;
s/\n\n\K\n+//g;

[download]
\s(?<![ \n])

[download]

is currently equivalent to 

[\x{0009}\x{000B}-\x{000D}\x{0085}\x{00A0}\x{1680}\x{180E}\x{2000}-\x{
+200A}\x{2028}\x{2029}\x{202F}\x{205F}\x{3000}]

[download]

or sometimes the buggy 

[\x{0009}\x{000B}-\x{000D}\x{1680}\x{180E}\x{2000}-\x{200A}\x{2028}\x{
+2029}\x{202F}\x{205F}\x{3000}]

[download]

Update: While U+000B is considered a space by Unicode and \p{Space}, it's not considered a space by \s for historical reasons.

In reply to Re: regexp: removing extra whitespace by ikegami
in thread regexp: removing extra whitespace by perlmax

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