Remove all spaces in string except spaces in quotes

[bnosam]

Lets say I have a string like this:

@BEGIN:4; 17, 1;1, "This is an example of text.";  3;  2; 5;11, LABEL;end;@LABEL:7, 1;8;end;@END

@BEGIN:4; 17, 1;1, "This is an example of text.";  3;  2; 5;11, LABEL;end;@LABEL:7, 1;8;end;@END

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What would be an easy way to remove all whitespace except in the part between the quotes ("This is an example of text.")?