I believe regexp 3, with single backslashes doesn't work because it is looking for an actual whitespace character

Correct. See "Escape sequences" in perlre.

I think regexp 5 fails for the same reasons regexp No 3 fails

Correct.

My 'character class' regexp 2, doesn't work. It matches on a plain 'r'.

[\\r,\\n,\\t,\\f] will match a single character from the following six: backslash (\), comma (,), "r", "n", "t" and "f". See Using character classes in perlretut.

Is there [...] a more succint way of expressing the regexp?

From /\\r|\\n|\\t|\\f/, we can factor out the slash:

/\\(?:r|n|t|f)/
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And since we are choosing between single characters, we can replace the alternation with a character class:

/\\[rntf]/
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