merge 2 text files

Hello forum


I have 2 text files
textfile1.txt
textfile2.txt

textfile1.txt

1 text
2 text
3 text
[b]4 line[/b]
5 text
6 text
[b]7 line[/b]
8 text
9 text
10 text
[b]11 line[/b]

textfile2.txt

[b]1 text
2 text
3 text[/b]
4 line
[b]5 text
6 text[/b]
7 line
[b]8 text
9 text
10 text[/b]
11 line

the files have 600 or more lines
i need to copy the text between line from textfile2.txt
and replace line in textfile1.txt

i am realy stuck here