Linux tail only whole words

I need to print last 20 characters of string, but only whole words. Delimiter is a space "". Let's consider this exampl开发者_开发技巧e:

string="The quick brown fox jumps over the lazy dog"

echo $string | tail -c20

returns s over the lazy dog. And I need it to return over the lazy dog instead. Do you know how to accomplish that? Thanks!

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echo $string | perl -ne 'print "$1\n" if /\b(\S.{0,20})$/'

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echo $string | rev | cut -d ' ' -f -20

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This works in Bash > 3.2 without using any external programs:

[[ $string =~ \ (.{0,20})$ ]]
result="$BASH_REMATCH[1]"

I used UdiM's grep version as a basis.