This seems to pass my limited testing...
my $string = '1,2,a,b,3';
$string =~ s/( (?:(?<!\d)(?:,|\.)(?!\s*\d)) |
print $string, "\n";
It's really ugly, but triggers a substitution all of the following cases:
- Comma or dot is not surrounded by digits, with optional whitespace after the comma or dot.
- Comma or dot doesn't have a digit to the left, with optional whitespace after the comma or dot.
- Comma or dot doesn't have a digit to the right, with optional whitespace after the comma or dot.
So I think it meets your spec. If this is homework I would seriously recommend immersing yourself in the POD's so that you'll be able to explain to your professor how it is you came up with such a wild RE. For that matter, it's time that I re-read the POD's, as I'm not convinced that it really needs to be so explicit to work properly.
See perlre to learn about zero-width lookahead and lookbehind (positive and negative). Also read perlretut and perlop under the "Regexp quote like characters" section, for starters.
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