G'day Xuo,

There are a lot a problems with your post:

• title has "tabs" but description has "<tabs/space>"
• data has guillemets but description has "<double quotes>"
• you're not substituting whitespace just after the opening quote and just before the closing quote (++hippo already commented on this)
• you've failed to use <code> tags so we can't see the number of whitespace characters: HTML converts strings of spaces and tabs into a single space

Please read "How do I post a question effectively?" carefully.

So, with a lot of guessswork, I believe the following technique, succinctly and efficiently, does what you want:

$ perl -pE 's/^[^"]+"\s+(.+)\s+"/$1 =~ y{ \t}{,}rs/e'
a b " x1 x2 "
x1,x2
c d " x3 "
x3
e f " x4 x5     x6 x7 "
x4,x5,x6,x7
x y z " 1space 2spaces  spacetabspace    end "
1space,2spaces,spacetabspace,end
[download]

The 'r' modifier was introduced in Perl v5.14: see "perl5140delta: Non-destructive substitution". Attempting to modify $1 directly results in a "Modification of a read-only value" fatal error. If you have a earlier version of Perl, you'll need to use an interim variable; perhaps something like this:

$ perl -pe 's/^[^"]+"\s+(.+)\s+"/($x = $1) =~ y{ \t}{,}s; $x/e'
a b " x1 x2 "
x1,x2
c d " x3 "
x3
e f " x4 x5     x6 x7 "
x4,x5,x6,x7
x y z " 1space 2spaces  spacetabspace    end "
1space,2spaces,spacetabspace,end
[download]

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