Unless you really want to play with regular expressions, try the quality check plugin. It has the ability to change authors from FN LN to LN, FN and vice versa.
As for the regexp, I don't understand what you are trying to do. Why are there colons in author names? Are you trying to reimport the books and changing the import regular expression?
Finally, and responding to your first post, if in the Author sort name algorithm tweak you set author_sort_copy_method = 'comma', then calibre will be able to cope with both FN LN and LN, FN names. Preferences -> Tweaks -> Author sort name algorithm.
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