How to print next line on current line if next line contains some character? [duplicate]

Question

If the next line is b, I want to add it to the AA line.

This is my file:

AA  
b  
b  
AA  
b  
AA  
b  
b  
b

Desired output:

AA b b  
AA b  
AA b b b

The below is the code I am trying.

paste -s -d" " file.txt | awk '{for(i=1;i<=NF;i++){if(i > 1 && $i == "AA")????}}'

I printed several line in one line and tried to run for loop to the last field and go down to the next line if $i is AA. But I do not know what to do after the 'if' statement.

Can anyone help me?

Answer 1

POSIXLY:

sed -ne '
  /^AA$/b1
  :1
    $!N
    /\nb$/s/\n/ /
    t1
  P;D
' <file