可以匹配正则表达式的一个位置而不是另一个位置（例如位置异或）？

I am looking to create several sub-expressions in a larger regular expression, where each subexpression matches something at one place in the input or another place, but not in both places, preferably using the same named group per "area of interest". For example, I'd like to match volume units in I am looking to create several sub-expressions