How do you find all the numbers from one number (included) to the next number (not included) in a list and put them in a new list?

I have a list of line numbers in a text file.

linenrs = [8,12,18,21]

I want to join all lines from one line number to the next line number.

This is what I've created and works.

newlist, templist = [], []
counter = 0
separator = " "


for i in range(1, 24):  #23 lines in textfileline
     temps = texfileline(i)

         if i in linenrs:
               if counter == 0:
                   templist.append(temps)
                   counter = 1
               elif counter == 1:
                   newlist.append(separator.join(templist))
                   templist = []
                   templist.append(temps)
                   counter = 1

         elif counter == 1 and i <= linenrs[-1]:
              templist.append(temps)

         else:
              newlist.append(temps)

Is this possible with list manipulation?
I would like to obtain such a list (and then join the text of these lines in the textfile):

newlist = [[8,9,10,11], [12,13,14,15,16,17], [18,19,20], [21,22,23]]

Using itertools.islice with specific ranges is a good choice for such task.

Suppose we have an input file with structure <consecutive number><text>:

1 text
2 text
3 text
...
27 text
28 text
29 text
30 text

from itertools import islice, zip_longest

linenrs = [8,12,18,21]
ranges = zip_longest(linenrs, linenrs[1:])

with open('test.txt') as f:
    list(islice(f, linenrs[0] - 1))  # skip starting chunk
    lines_slices = []
    for from_, to_ in ranges:
        lines_slices.append(list(islice(f, 0, 1 if to_ is None else to_ - from_)))

print(lines_slices)

The output (sublists of lines):

[['8 text\n', '9 text\n', '10 text\n', '11 text\n'],
 ['12 text\n', '13 text\n', '14 text\n', '15 text\n', '16 text\n', '17 text\n'],
 ['18 text\n', '19 text\n', '20 text\n'],
 ['21 text\n']]