1. Many options, but basically you must account for the difference between substrings and stand-alone strings, and this appears to be whitespace in your example data, hence something like:
   s/(?<=\s)-([bc]\s)/-:$1/g;
2. You can stop split from continuing using the 3rd argument:
   split( '\s', 'a b c', 2);
   though if there may be leading whitespace before the 'a' you'd better use the special case of splitting on a single space, which will discard any leading whitespace:
   split( ' ', '  a b c', 2);
   

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I'd like to be able to assign to an luser