I want to regex AND search (?=)(?=) inside block which enclosed by something delimiter such as #
In following sample regex, what I expected is, cat to ugly matches to the pattern inside # cat B to before # cat C.
But the regex match to nothing.
regex
^#(?=[\s\S]*(cat))(?=[\s\S]*(ugly))^#
text
# cat A
the cat is
very cute.
# cat B
the cat is
very ugly.
# cat C
the cat is
very good.
#
You can test the regex on https://regexr.com/
In your pattern ^#(?=[\s\S]*(cat))(?=[\s\S]*(ugly))^# you use match a # from the start of the string ^#, followed by 2 positive lookaheads and then again match ^#. That is why you don't get a match.
To get a more exact match, you could start the pattern with ^# cat B
If you want to use lookaheads, you might use 2 capturing groups in the positive lookahead. If you want to search for cat and ugly as whole words you might use word boundaries \b.
The (?s) is a modifier that enables the dot matching a newline for which you might also use /s as a flag instead.
(?s)(?=^# cat B.*?(cat).*?(ugly).*?^# cat C
But it might be easier to not use the lookahead and match instead:
(?s)^# cat B.*?(cat).*?(ugly).*?^# cat C$