Not quite sure to understand what the point is. I just know that I used to have a regex like this:

s/\r\n/\n/
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or even:

s/\r\n$/\n/
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but it turned our to insufficient, because the input data sometimes had line ending with "\r\r\n" and sometimes also "\r" in the middle of the line, generating all kinds of probblem. In the end, this regex:

s/\r//g
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turned out to solve all the problems. Now, of course, it depends on what your input data is and what you need at the end of the day. With different data and different goals, the regex would probably have to be changed. But that's not hot news, if you need to do data munging, the first prerequisite is to know your data well.