extract substring from large string

You could use a single capture group:

^---\s*\n(?!.* has (?:joined|left) the conversation|\* \* \*)\S.*((?:\n(?!\(\d|---).*)*)

The pattern matches:

• ^ Start of string
• ---\s*\n Match --- optional whitespace chars and a newline
• (?!.* has (?:joined|left) the conversation|\* \* \*) Assert that the line does not contain a has joined or has left the conversation part, or contains * * *
• \S.* Match at least a non whitespace char at the start of the line and the rest of the line
• ( Capture group 1 (this will be returned by re.findall)
  • (?:\n(?!\(\d|---).*)* Match all lines the do not start with ( and a digit or --
• ) Close group 1

See a regex demo and a Python demo.

Example

pattern = r"^---\s*\n(?!.* has (?:joined|left) the conversation|\* \* \*)\S.*((?:\n(?!\(\d|---).*)*)"
result = [m.strip() for m in re.findall(pattern, s, re.M) if m]
print(result)

Output

["Good Morning\nHow're you?", "Hi, I'm Good.", 'we got the update on work. \nIt will get complete by next week.']

I've assumed:

• The text of interest begins after a block of three lines: a line containing a timestamp, followed by the line "---", which may be padded to the right with spaces, followed by a line comprised of a string of letters containing one period which is neither at the beginning nor end of that string and that string may be padded on the right with spaces.
• The block of text of interest may contain blank lines, a blank line being a string that contains nothing other than spaces and a line terminator.
• The last line of the block of text of interest cannot be a blank line.

I believe the following regular expression (with multiline (m) and case-indifferent (i) flags set) meets these requirements.

^\(\d{4}\-\d{2}\-\d{2} .*\) *\r?\n-{3} *\r?\n[a-z]+\.[a-z]+ *\r?\n((?:.*[^ (\n].*\r?\n| *\r?\n(?=(?: *\r?\n)*(?!\(\d{4}\-\d{2}\-\d{2} .*\)).*[^ (\n]))*)

The blocks of lines of interest are contained in capture group 1.

Start your engine!

The elements of the expression are as follows.

^\(\d{4}\-\d{2}\-\d{2} .*\) *\r?\n  # match timestamp line
-{3} *\r?\n                         # match 3-hyphen line
[a-z]+\.[a-z]+ *\r?\n               # match name
(                                   # begin capture group 1
  (?:                               # begin non-capture group (a)
    .*[^ (\n].*\r?\n                # match a non-blank line
    |                               # or
    \ *\r?\n                        # match a blank line
    (?=                             # begin a positive lookahead
      (?:                           # begin non-capture group (b)
        \ *\r?\n                    # match a blank line
      )*                            # end non-capture group b and execute 0+ times
      (?!                           # begin a negative lookahead
        \(\d{4}\-\d{2}\-\d{2} .*\)  # match timestamp line
      )                             # end negative lookahead
      .*[^ (\n]                     # march a non-blank line
    )                               # end positive lookahead
  )*                                # end non-capture group a and execute 0+ times
)                                   # end capture group 1

Here is a self-documenting regex that will strip leading and trailing whitespace:

(?x)(?m)(?s)                                                    # re.X, re.M, re.S (DOTALL)
(?:                                                             # start of non capturing group
 ^\(\d{4}-\d{2}-\d{2}\ \d{2}:\d{2}:\d{2}\ [AP]M\ BST\)\s*\r?\n  # date and time
 (?!---\s*\r?\nad\.ft has)                                      # next lines are not the ---\n\ad.ft etc.
 ---\s*\r?\n                                                    # --- line
 [\w.]+\s*\r?\n                                                 # name line
 \s*                                                            # skip leading whitespace
)                                                               # end of non-capture group
# The folowing is capture group 1. Match characters until you get to the next date-time:
((?:(?!\s*\r?\n\(\d{4}-\d{2}-\d{2}\ \d{2}:\d{2}:\d{2}\ [AP]M\ BST\)).)*)# skip trailing whitespace

See Regex Demo

See Python Demo

import re

string = """(2021-07-02 01:00:00 AM BST)
---
syl.hs has joined the conversation



(2021-07-02 01:00:23 AM BST)
---
e.wang
Good Morning
How're you?




(2021-07-02 01:05:11 AM BST)
---
wk.wang
Hi, I'm Good.



(2021-07-02 01:08:01 AM BST)
---
perter.derrek
we got the update on work.
It will get complete by next week.

(2021-07-15 08:59:41 PM BST)
---
ad.ft has left the conversation




---

* * *"""

regex = r'''(?x)(?m)(?s)                                        # re.X, re.M, re.S (DOTALL)
(?:                                                             # start of non capturing group
 ^\(\d{4}-\d{2}-\d{2}\ \d{2}:\d{2}:\d{2}\ [AP]M\ BST\)\s*\r?\n  # date and time
 (?!---\s*\r?\nad\.ft has)                                      # next lines are not the ---\n\ad.ft etc.
 ---\s*\r?\n                                                    # --- line
 [\w.]+\s*\r?\n                                                 # name line
 \s*                                                            # skip leading whitespace
)                                                               # end of non-capture group
# The folowing is capture group 1. Match characters until you get to the next date-time:
((?:(?!\s*\r?\n\(\d{4}-\d{2}-\d{2}\ \d{2}:\d{2}:\d{2}\ [AP]M\ BST\)).)*)# skip trailing whitespace
'''

matches = re.findall(regex, string)
print(matches)

Prints:

["Good Morning\nHow're you?", "Hi, I'm Good.", 'we got the update on work.\nIt will get complete by next week.']