PHP Regular expression to match unescaped double quotes

I'm trying to figure out a regular expression that will detect if there is an unescaped double quote within two double quotes. I have this, but it's not working:

if ( preg_match( '/^(\".*((?<!\\)\").*\")$/',开发者_开发百科 $str ) ) {
    die("hey");
}

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Try with preg_match('/([^\\]\")/', $str)