I found a solution without using look-behind, which has the limitation of not being able to match variable length strings (so /(?<!\t*)foo)/ does not work). 
#!/usr/local/bin/perl

use strict;

while (my $str = <DATA>) {
    chomp($str);
    $str =~ s/(\d+[.,]\d+       # Substitute a (simple) number ...
              |                 # or ...
              [^.,\d ]+)        # something without periods, comma's,
                                # digits or spaces ...
              ((?!$)[.,]?)      # followed by a period or comma, if
                                # possible, but not when at the end of
                                # the line ...
              /$1$2 /gx;        # by the number or string, followed by
                                # the found period or comma, and a
                                # trailing space
    print "<$str>\n";
}

__DATA__
1,2,a,b
a,1,b,2,
1,21,a,b,word,d.3,4.5.6

__END__
<1,2, a, b>
<a, 1,b, 2,>
<1,21, a, b, word, d. 3,4. 5.6>

[download]

In addition to the on-line documentation, you could check out Mastering Regular Expressions, 2nd Edition.

Arjen

In reply to Re: Regex to add space after punctuation sign by Aragorn
in thread Regex to add space after punctuation sign by dda

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