How to find all patterns between two characters?

I'm trying to find all patterns between a pair of double quotes. Let say I have a file with contents look like as following:

first matched is "One". the second is here"Two "
and here are in second line" Three ""Four".

I want to below words as output:

One
Two
Three
Four

As you can see all strings in output are between a pair of quotes.

What I tried, is this command:

grep -Po ' "\K[^"]*' file

Above command works fine if I have a space before first pair of " marks. For example it works if my input file contains the following:

first matched is "One". the second is here "Two "
and here are in second line " Three " "Four".

I know I can do this with multiple commands combination. But I'm looking for one command and without using that for multiple time. e.g: below command

grep -oP '"[^"]*"' file | grep -oP '[^"]*'

How can I achieve/print all of my patterns just using one command?

Reply to comments: It's not important for me to removing whitespace around matched pattern inside a pair of quotes, but it would be better if the command support it too. and also my files contain nested quotes like "foo "bar" zoo". And all of the quoted words are in separate lines and they are not expanded to multi lines.

Thanks in advance.

First of all, your grep -Po '"\K[^"]*' file idea fails because grep sees both "One" and ". the second is here" as being inside quotes. Personally, I'd probably just do

$ grep -oP '"[^"]+"' file | tr -d '"'
One
Two 
 Three 
Four

But that is two commands. To do it with a single command, you could use one of:

1. Perl

   $ perl -lne '@F=/"\s*([^"]+)\s*"/g; print for @F' file 
   One
   Two 
   Three 
   Four
   

   Here, the @F array holds all matches of the regex (a quote, followed by as many non-" as possible until the next "). The print for @F just means "print each element of @F.

2. Perl

   $ perl -F'"' -lne 'for($i=1;$i<=$#F;$i+=2){print $F[$i]}' file 
   One
   Two 
    Three 
   Four
   

   To remove leading/trailing spaces from each match, use this:

   perl -F'"' -lne 'for($i=1;$i<=$#F;$i+=2){$F[$i]=~s/^\s*|\s$//; print $F[$i]}' file 
   

   Here, Perl is behaving like awk. The -a switch causes it to automatically split input lines into fields on the character given by -F. Since I have given it ", the fields are:

   $ perl -F'"' -lne 'for($i=0;$i<=$#F;$i++){print "Field $i: $F[$i]"}' file 
   Field 0: first matched is 
   Field 1: One
   Field 2: . the second is here
   Field 3: Two 
   Field 0: and here are in second line
   Field 1:  Three 
   Field 2: 
   Field 3: Four
   Field 4: .
   

   Because we are looking for text between two consecutive field separators, we know we want every second field. So, for($i=1;$i<=$#F;$i+=2){print $F[$i]} will print the ones we care about.

3. The same idea but in awk:

   $ awk -F'"' '{for(i=2;i<=NF;i+=2){print $(i)}}' file 
   One
   Two 
    Three 
   Four
   

The key is to consume the quotes in your expression. Hard to do that with a single grep command. Here's a perl one-liner:

perl -0777 -nE 'say for /"(.*?)"/sg' file

That slurps the whole input and prints out the captured part of the match. It will work even if there's a newline inside the quotes, although it then becomes difficult to separate elements with and without newlines. To help with that, use a different character as the output record separator, the null character for instance

perl -0777 -lne 'print for /"(.*?)"/sg} BEGIN {$\="\0"' <<DATA | od -c
blah "first" blah "second
quote with newline" blah "third"
DATA

0000000   f   i   r   s   t  \0   s   e   c   o   n   d  \n   q   u   o
0000020   t   e       w   i   t   h       n   e   w   l   i   n   e  \0
0000040   t   h   i   r   d  \0
0000046

This could be possible with the below grep one liner and i assumed that you have balanced quotation marks.

grep -oP '"\s*\K[^"]+?(?=\s*"(?:[^"]*"[^"]*")*[^"]*$)' file

Example:

$ cat file
first matched is "One". the second is here"Two "
and here are in second line" Three ""Four".
$ grep -oP '"\s*\K[^"]+?(?=\s*"(?:[^"]*"[^"]*")*[^"]*$)' file
One
Two
Three
Four

Another hair pulling solution through PCRE verb (*SKIP)(*F),

$ grep -oP '[^"]+(?=(?:"[^"]*"[^"]*)*[^"]*$)(*SKIP)(*F)|\s*\K[^"]+(?=\b\s*)' file
One
Two
Three
Four