Here is the output from the processor RegEx:

{{{#!RegEx
pattern=the.*end
test_lines="""theme
the final end
the what the end"""
}}}

Here is text after the processor.

Try the same thing at:
https://regex101.com/

Here are some tests I've done:
 * FIXTHIS with formula entry
   * https://regex101.com/r/I9tQTN/1