The Input format is PDF the export format is MOBI.
The Place where I found the <br> is the 'middle', the interim format Calibre uses during convertion. I'm able to reach it in the convertion options 'search & replace' wizzard. Sorry that I let this unclear in my first post.
Thanks for the hint but there are all varieties of digit counts. It may be only one digit or more (0...50000...xxxxxxxxxxx). So any number should be possible.
Thanks a lot!!!