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re.search does not accept a "line." It accepts a "string." There is no pretext in which re.search is meant to only accept a single line. And giving it a `string

re.search does not accept a "line." It accepts a "string." There is no pretext in which re.search is meant to only accept a single line. And giving it a `string` with multiple new lines doesn't necessarily mean you want to enable multi-line mode. They are orthogonal things. > Both ^ and $ always match at start or end of lines This is trivially not true, as I showed in my previous example. The haystack `cat\n\n` contains two lines and the regex `cat$` says it should match `cat` followed by the "end of a line" according to your definition. Yet it does not match `cat` followed by the end of a line in `cat\n\n`. And it does not do so in Python or in any other regex engine. You're trying to square a circle here. It can't be done. Can you make sense of, historically, why this choice of semantics was made? Sure. I bet you can. But I can still evaluate the choice on its own merits today. And I did when I made the regex crate. > but I am not convinced that this would be an easy and obvious choice to have breaking change. Rust's regex crate, Go's regexp package and RE2 all reject this whacky behavior. As the regex crate maintainer, I don't think I've ever seen anyone complain. Not once. This to me suggests that, at minimum, making `$` and `\z` equivalent in non-multiline mode is a reasonable choice. I would also argue it is the better and more sensible approach. Whether other regex engines should have a breaking change or not to change the meaning of `$` is an entirely different question completely. That is neither here nor there. They absolutely will not be able to make such a change, for many good reasons.

danbruc 3y ago
re.search does not accept a "line." It accepts a "string." There is no pretext in which re.search is meant to only accept a single line. Sure, it takes a string which might be a line or multiple or whatever. Does not change the fact that $ matches at the end of a line. If you want the end of the string, use \Z. This is trivially not true, as I showed in my previous example. The haystack `cat\n\n` contains two lines and the regex `cat$` says it should match `cat` followed by the "end of a line" according to your definition. In multi-line mode it matches, in single-line mode it does not because there is a newline between cat and the end of the line. A newline is only a terminating newline if it is the last character, the newline after cat is not a terminating newline. You need cat\n$ or cat\n\n to match.
- burntsushi 3y ago
  > In multi-line mode it matches, in single-line mode it does not because there is a newline between cat and the end of the line. A newline is only a terminating newline if it is the last character, the newline after cat is not a terminating newline. You need cat\n$ or cat\n\n to match. This only makes sense if re.search accepted a line to search. It doesn't. It accepts an arbitrary string. I don't think this conversation is going anywhere. Your description of the semantics seems inconsistent and incomprehensible to me. > A newline is only a terminating newline if it is the last character, the newline after cat is not a terminating newline. You need cat\n$ or cat\n\n to match. The first `\n` in `cat\n\n` is a terminating newline. There just happens to be one after it. Like I said, your description makes sense if the input is meant to be interpreted as a single line. And in some contexts (like line oriented CLI tools), that can make sense. But that's not the case here. So your description makes no sense at all to me.
  - danbruc 3y ago
    This only makes sense if re.search accepted a line to search. It doesn't. It accepts an arbitrary string. Which is fine because lines are a subset of strings. And whether you want your input treated as a line or a string is decided by your pattern, use ^ and $ and it will be treated as a line, use \A and \Z and it will be treated as a string. The first `\n` in `cat\n\n` is a terminating newline. There just happens to be one after it. Look at where this is coming from. You do line-based stuff, there is either no newline at all or there is exactly one newline at the end. You do file-based stuff, there are many newlines. In both cases the behavior of ^ and $ makes perfect sense. Now you come along with cat\n\n which clearly falls into the file-based stuff category as it has more than one newline in it but you also insist that it is not multiple lines. If it is not multiple lines, then only the last character can be a newline, otherwise it would be multiple lines. And I get it, yes, you can throw arbitrary strings at a regular expression, this line-based processing is not everything, but it explains why things behave the way they do. And that is also why people added \A and \Z. And I understand that ^ and $ are much nicer and much better known than \A and \Z. Maybe the best option would be to have a separate flag that makes them synonymous with \A and \Z and this could maybe even be the default.

danbruc 3y ago

re.search does not accept a "line." It accepts a "string." There is no pretext in which re.search is meant to only accept a single line. Sure, it takes a string which might be a line or multiple or whatever. Does not change the fact that $ matches at the end of a line. If you want the end of the string, use \Z. This is trivially not true, as I showed in my previous example. The haystack `cat\n\n` contains two lines and the regex `cat$` says it should match `cat` followed by the "end of a line" according to your definition. In multi-line mode it matches, in single-line mode it does not because there is a newline between cat and the end of the line. A newline is only a terminating newline if it is the last character, the newline after cat is not a terminating newline. You need cat\n$ or cat\n\n to match.

burntsushi 3y ago
> In multi-line mode it matches, in single-line mode it does not because there is a newline between cat and the end of the line. A newline is only a terminating newline if it is the last character, the newline after cat is not a terminating newline. You need cat\n$ or cat\n\n to match. This only makes sense if re.search accepted a line to search. It doesn't. It accepts an arbitrary string. I don't think this conversation is going anywhere. Your description of the semantics seems inconsistent and incomprehensible to me. > A newline is only a terminating newline if it is the last character, the newline after cat is not a terminating newline. You need cat\n$ or cat\n\n to match. The first `\n` in `cat\n\n` is a terminating newline. There just happens to be one after it. Like I said, your description makes sense if the input is meant to be interpreted as a single line. And in some contexts (like line oriented CLI tools), that can make sense. But that's not the case here. So your description makes no sense at all to me.
- danbruc 3y ago
  This only makes sense if re.search accepted a line to search. It doesn't. It accepts an arbitrary string. Which is fine because lines are a subset of strings. And whether you want your input treated as a line or a string is decided by your pattern, use ^ and $ and it will be treated as a line, use \A and \Z and it will be treated as a string. The first `\n` in `cat\n\n` is a terminating newline. There just happens to be one after it. Look at where this is coming from. You do line-based stuff, there is either no newline at all or there is exactly one newline at the end. You do file-based stuff, there are many newlines. In both cases the behavior of ^ and $ makes perfect sense. Now you come along with cat\n\n which clearly falls into the file-based stuff category as it has more than one newline in it but you also insist that it is not multiple lines. If it is not multiple lines, then only the last character can be a newline, otherwise it would be multiple lines. And I get it, yes, you can throw arbitrary strings at a regular expression, this line-based processing is not everything, but it explains why things behave the way they do. And that is also why people added \A and \Z. And I understand that ^ and $ are much nicer and much better known than \A and \Z. Maybe the best option would be to have a separate flag that makes them synonymous with \A and \Z and this could maybe even be the default.