Probably I still need more coffee ... ;)

... but shouldn't this  look-behind assertion

>  (?<=[^"])

rather be negated?

 (?<![^"])

edit

Never mind.

Indeed not enough coffee, you are mixing two approaches which was confusing me.

UPDATE

there is a limitation in your approach when dealing with multiple lines. (though the OP didn't explicitly ask for this)

  DB<135> $str = "\tstart\tmiddle1\t\"quote1\tquote2\"\tmiddle2\tend\t
+"; 
 => "\tstart\tmiddle1\t\"quote1\tquote2\"\tmiddle2\tend\t"

  DB<136> $str .= "\n$str"
 => "\tstart\tmiddle1\t\"quote1\tquote2\"\tmiddle2\tend\t\n\tstart\tmi
+ddle1\t\"quote1\tquote2\"\tmiddle2\tend\t"

  DB<137> p $str
    start    middle1    "quote1    quote2"    middle2    end    
    start    middle1    "quote1    quote2"    middle2    end    
 => 1

  DB<138> p $str =~ s/ (?<=[^"]) \t (?!"|$) /***/gmxr
    start***middle1    "quote1***quote2"    middle2***end    
***start***middle1    "quote1***quote2"    middle2***end    
 => 1

  DB<139> p $str =~ s/ (?<!")(?<!^) \t (?!"|$) /***/gmxr
    start***middle1    "quote1***quote2"    middle2***end    
    start***middle1    "quote1***quote2"    middle2***end    
 => 1
[download]

You filter tabs "preceded by any character which isn't a quote" with (?<=[^"]) supposing that line-start is not any character.

As you can see BUK's approach still works in this case.

FWIW:

It was first confusing me that (?!"|^) wasn't used, but the regex engine rejects "variable length look-behind assertions" (which is not really the case here)

  DB<140> p $str =~ s/ (?<!"|^) \t (?!"|$) /***/gmxr
Variable length lookbehind not implemented in regex m/ (?<!"|^) \t (?!
+"|$) / at (eval 110)[multi_perl5db.pl:644] line 2.
[download]

For a multi-line string, simply add \n to the negated character group:

s/(?<=[^"\n])\t(?!"|$)//gm
[download]

and of course a /m for the $

OK, but in this case, for the sake of readability and maintainability, better stick with one approach

DB<105> p $str =~ s/ (?<=[^"\n]) \t (?=[^"\n]) /***/gxr
    start***middle1    "quote1***quote2"    middle2***end    
    start***middle1    "quote1***quote2"    middle2***end
[download]

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)
Je suis Charlie!}