Count lines between "X"s

I want to count the lines between "X"s. This is just an example; I have to apply the code to a complex biological result. I will be thankful if you can suggest some command, preferably using awk, grep or sed as I am familiar with those.

Example:

X
Y
Y
Y
X
Y
Y
Y
Y
X
Y
X

Desired Output:

3
4
1

With awk:

$ awk '!/X/{count++}/X/{print count; count = 0}' input

3
4
1

Increment a count for every line not containing X; print and reset the count for lines containing X.

$ awk '/X/ && prev{print NR-prev-1} /X/{prev=NR}' file
3
4
1

How it works:

Awk implicitly reads through input files line by line.

• /X/ && prev{print NR-prev-1}

  For any line that contains X and if we have previously assigned a value to prev, then print out the number of the current line, NR, minus prev minus one.

• /X/{prev=NR}

  For any line that contains X, set the variable prev to the current line number, NR.