Print lines between (and excluding) two patterns

I'm going to submit form using cURL, where some of the contents is come from other file, selected using sed

If param1 is line matching pattern from other file using sed, below command will works fine:

curl -d param1="$(sed -n '/matchpattern/p' file.txt)" -d param2=value2 http://example.com/submit

Now, go to the problem. I want show only text between 2 matching pattern excluding the matching pattern itself.

Lets say file.txt contains:

Bla bla bla
firstmatch
It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout.
secondmatch
The point of using Lorem Ipsum is that it has a more-or-less normal distribution of letters, as opposed to using 'Content here, content here', making it look like readable English.

Currently, lots of "beetween 2 matching pattern" sed command won't remove firstmatch and secondmatch.

I want the result to become:

It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout.