in reply to check if string is valid with special cases

I agree with others that this looks like a Text::CSV problem (and also that your various problem statements are rather vague).

However, in a general regex parsing (and regexes are often not the best approach to parsing) situation, my approach tends to be something like

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le "my $s = 'a,b,c,d,e,f'; ;; my $sym = qr{ [^,] }xms; my $sep = qr{ , }xms; ;; $s =~ m{ \A $sym (?: $sep $sym){5} \z }xms or die qq{bad string: '$s' +}; ;; my ($u, $v, $w, $x, $y, $z) = $s =~ m{ $sym }xmsg; dd $u, $v, $w, $x, $y, $z; " ("a", "b", "c", "d", "e", "f")
Once you know a string is valid, it's often quite easy to strip out sub-strings of interest. Oh, you say the separator pattern should include possible spaces? Then
    my $sep = qr{ \s* , \s* }xms;
Oh, the "symbol" may be more than a single character and must also exclude spaces? Then
    my $sym = qr{ [^,\s]+ }xms;
And so on. Separately defining $sym and $sep makes them easy to change and makes any change propagate throughout the code as necessary (DRY).

And yes, test all this stuff! (Test::More and friends.)

Update: Here's another approach to regex (again, maybe not the best option) parsing. It combines validation and extraction into a single regex, but the regex is significantly more complex and probably a bit slower. It also needs Perl version 5.10+ for the  \K operator.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le "use 5.010; ;; my $s = 'a,b, CC ,d , e,fgh '; ;; my $sym = qr{ [^,\s]+ }xms; my $sep = qr{ \s* , \s* }xms; ;; my $n_syms = my ($u, $v, $w, $x, $y, $z) = $s =~ m{ (?: \G (?! \A) $sep | \A \s*) \K $sym (?= (?: $sep $sym)* \s* \z) }xmsg; ;; $n_syms == 6 or die qq{bad string: '$s'}; dd $u, $v, $w, $x, $y, $z; " ("a", "b", "CC", "d", "e", "fgh")
(Testing, testing...)

Give a man a fish:  <%-{-{-{-<