Does it have to be regex? Or could you just replace the chunk of string starting with the comment tag ?
# Imports
library(stringr)
test_str <- "<!-- {{Start}} -->\n<!-- {{End}} -->"
to_replace <- "Start}} -->\n<"
stringr::str_replace(test_str, coll(to_replace), "Start}} -->\nHello World!<")
#> [1] "<!-- {{Start}} -->\nHello World!<!-- {{End}} -->"
Created on 2019-08-14 by the reprex package (v0.3.0)
PS - from the docs for stringr::str_replace, explains use of coll()
Generally, for matching human text, you'll want coll() which respects character matching rules for the specified locale.