正则表达式 - 匹配多个前瞻条件

Question

I need to find a regex rule which finds newlines between bullet points and removes them. For instance:

• Here is some text

which flows to this separated by 2 newlines

• This is a new bullet point separated by 2 newlines

Should become:

• Here is some text which flows to this separated by 2 newlines

• This is a new bullet point separated by 2 newlines

Here's what I've tried:

•(.+)\K\n+(?(?=[^•])(?=.+\n+•))

Where my thinking is:

1. Find a previous line which starts with •
2. Collect any character up until one or more newlines and discard. I'm now ready to match ahead and replace newlines based on some conditions.
3. Lookahead and check next character after newline is not a bullet.
4. If it is, check to make sure that after all characters followed by one or more newlines, there is another bullet.

I think my problem is not properly understanding how to chain together these conditions in the positive lookahead but struggling to find any clearcut answers / examples which deal with this kind of problem.

As ever, any help is greatly appreciated!

Answer 1

You could match 2 newlines and then assert that what is on the right does not start with a bullet but does contain a bullet after that.

^•.*\K\r?\n\r?\n(?=(?!•).*\r?\n\r?\n•)

In parts

• ^ Start of string
• •.* Match a bullet and any char 0+ times except a newline
• \K\r?\n\r?\n Forget what was matched and match 2 newlines
• (?= Positive lookahead, assert what is on the right is
  • (?.•).* Negative lookahead, assert what is on the right is not •
  • \r?\n\r?\n• Match 2 newlines followed by •
• ) Close positive lookahead

Regex demo

If \R is supported to match a unicode newline sequence, you could also use

^•.*\K\R{2}(?=(?!•).*\R{2}•)