哇，M$ Word 的正则表达式，比 neovim 还难用。
建议楼主先把这篇英文教程仔细看一遍：
具体到本楼问题：
The question mark ? is used to find individual characters. If used with curly brackets to define a range of characters eg #?{1,3}# it will behave as an asterisk and find all the characters between the hash symbols.