why not give the manual a quick look?
preg_replace takes 3 argument, pattern, replacement, and the string to work on, which should probably be the content of template_1.txt.
however, include() doesn't return the content of the file, it simply includes it in your code as if you had pasted it at that position. check file() or fread() if you want to get the content into a variable.
you also seem to have some trouble using the pattern delimiters correctly, I'd recommend using str_replace() for simple replacements like this one.