# REG A
TextToParse1
TextToParse2
...
...
...
TestToParseX
# reg A
# REG B
TextToParse1
TextToParse2
...
...
...
TestToParseX
# reg B
(continued)

start = timer()
pattern = r"(# REG {0})(.*)(# reg {0})".format(reg_name)
match = re.search(pattern, file, re.DOTALL)
end = timer()