Because your file contains no new line characters (\n = ASCII 10) but only carriage return characters (\r = ASCII 13). After you read the file, convert \r to \n for testing:
content = fileread( 'mwithrm21.txt' ) ;
content(content==13) = 10 ;
Then you should work on the code that produces the txt file if you can, and use \n. Finally, as Per mentions, you have 8 columns so you have max 7 separators.
Finally, you may be able to simplify your pattern, for something like
'^(.*?\|){7}~.*$'
but you'll have to test it.
