Javascript RegExp问题的条件表达式

玩Regex很开心，我想知道是否有一种方法可以基于某些标准在regexp中设置条件-希望返回a或B与B或a之间的值-但在我的Regex 101中，由于以下字符串，它似乎不起作用：

 (?<=A|B\s+).*?(?=\s+B|A) //<-hopefully should return anything between or an A or a B is this possible?

    A
adawdawdawwad
B

awdawdawdda
A
awdaddawdaw

B
awdadadaw
B
awdadadaw
A
adadawaw

B
awdawdwadawd
A

（？最简单的一个

A[^B]+B|B[^A]+A

您可以使用正则表达式

(?<=^ *A *\r?\n)(?:(?!^ *B *?$).*\r?\n)*(?=^ *B *?$)|(?<=^ *B *\r?\n)(?:(?!^ *A\s*$).*\r?\n)*(?=^ *A *?$)

在当前模式中，
（？我相信你缺少括号（？@Lucas，如果你的意思是字符串“
我的狗有跳蚤”
不是从
“艾米的狗有跳蚤”
中提取的，我会同意。我在“Javascript演示”链接后面的句子中给出了我对这个问题的理解，它不包括字符串，例如
“AMy dog has flesb”
。
(?<=            # begin positive lookbehind
  ^ *A\ *\r?\n  # match 0+ spaces, 'A', 0+ spaces, opt CR,
                # newline, from beginning of line
)               # end positive lookbehind
(?:             # begin non-capture group
  (?!           # begin negative lookahead
    ^ *B *$     # match 0+ spaces, 'B', 0+ spaces, end of
                # line, from beginning of line  
  )             # end negative lookahead
  .*\r?\n       # match 0+ chars, opt CR, newline
)               # end non-capture group
*               # execute non-capture group 0+ times
(?=             # begin positive lookahead
  ^ *B *?$      # match 0+ spaces, 'B', 0+ spaces, end of
                # line, from beginning of line  
)               # end positive lookahead
|               # or
(?<=            # begin positive lookbehind
  ^ *B\ *\r?\n  # match 0+ spaces, 'B', 0+ spaces, opt CR,
                # newline, from beginning of line
)               # end positive lookbehind
(?:             # begin non-capture group
  (?!           # begin negative lookahead
    ^ *A *$     # match 0+ spaces, 'A', 0+ spaces, end of
                # line, from beginning of line  
  )             # end negative lookahead
  .*\r?\n       # match 0+ chars, opt CR, newline
)               # end non-capture group
*               # execute non-capture group 0+ times
(?=             # begin positive lookahead
  ^ *A *?$      # match 0+ spaces, 'A', 0+ spaces, end of
                # line, from beginning of line  
)               # end positive lookahead

(?<=[AB]\s+).*?(?=\s+[AB])