/^"( [^\\"]+ | \\+[^\\"] | (\\\\)*\\" )*"$/x

##</code><code>##

/^"( [^\"]+ | \+[^\"] | (\\)*\" )*"$/x

##</code><code>##

 C:\> perl -le"print for @ARGV" hi "hi" "\"hi\"" "\\hi\\" "\\hi\\\""
hi
hi
"hi"
\\hi\
\\hi\"

##</code><code>##

 C:\> perl -le"print for @ARGV" "6""5'"
6"5'
 C:\> perl -le"print for @ARGV" "say ""hello"" to my little friend"
say "hello
to
my
little
friend