[issue40043] Poor RegEx example for (?(id/name)yes-pattern|no-pattern)
New submission from Leon Hampton <Leon_Hampton@yahoo.com>: Hello, In the 3.7.7 documentation on Regular Expression, the Conditional Construct, (?(id/name)yes-pattern|no-pattern), is discussed. (This is a very thorough document, by the way. Good job!) One example given for the Conditional Construct does not work as described. Specifically, the example gives this matching pattern '(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)' and states that it will NOT MATCH the string '<user@host.com'. In my tests the pattern DOES MATCH that string. (The other examples work as described.) This may be a bug in re since it seems to me that the match should fail. Respectfully, Leon ---------- assignee: docs@python components: Documentation messages: 364811 nosy: LHampton, docs@python priority: normal severity: normal status: open title: Poor RegEx example for (?(id/name)yes-pattern|no-pattern) type: behavior versions: Python 3.7 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue40043> _______________________________________
Leon Hampton <Leon_Hampton@yahoo.com> added the comment: Hello, There may be a bug in the implementation of the Conditional Construction of Regular Expressions, namely the (?(id/name)yes-pattern|no-pattern). In the Regular Expression documentation (https://docs.python.org/3.7/library/re.html), in the portion about the Conditional Construct, it gives this sample pattern '(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)' and states that the pattern WILL NOT MATCH this string '<user@host.com'. In my tests the pattern MATCHES the string. I agree that the pattern should not match. Respectfully, Leon ---------- components: +Regular Expressions -Documentation nosy: +ezio.melotti, mrabarnett title: Poor RegEx example for (?(id/name)yes-pattern|no-pattern) -> RegExp Conditional Construct (?(id/name)yes-pattern|no-pattern) Problem _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue40043> _______________________________________
Matthew Barnett <python@mrabarnett.plus.com> added the comment: The documentation is talking about whether it'll match at the current position in the string. It's not a bug. ---------- resolution: -> not a bug _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue40043> _______________________________________
SilentGhost <ghost.adh@runbox.com> added the comment: Leon, this most likely is not a bug, not because what's stated in documentation, but because you're most likely not testing what you think you do. Here is the test that you should be doing:
re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)', '<user@host.com')
No match. If there is a different output in your setup, please provide both the output and the details of your system and Python installation. ---------- nosy: +SilentGhost stage: -> resolved status: open -> closed _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue40043> _______________________________________
Leon Hampton <Leon_Hampton@yahoo.com> added the comment: Matthew Barnett & SilentGhost, Thank you for your prompt responses. (Really prompt. Amazing!) SilentGhost, Regarding your response, I used re.search, not re.match. When I used re.match, the regex failed. When I used re.search, it matched. Here are my tests. Your example (cut-and-pasted): x = re.match(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)', '<user@host.com') print(x) I received 'None', the expected response. My example using search: x = re.search(r'(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)', '<user@host.com') print(x) I received: <re.Match object; span=(1, 14), match='user@host.com'> I understand the re.match failing, since it always starts at the beginning of the string, but why did re.search succeed? After failing with the yes-pattern, when the regex engine backtracked to the (<)? did it decide not to match the '<' at all and skip the character? Seems like it. What do you think? I am running Python 3.7 via Spyder 4.1.1 on Windows 10. Respectfully, Leon ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue40043> _______________________________________
Matthew Barnett <python@mrabarnett.plus.com> added the comment: That's what searching does! Does the pattern match here? If not, advance by one character and try again. Repeat until a match is found or you've reached the end. ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue40043> _______________________________________
participants (3)
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Leon Hampton -
Matthew Barnett
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SilentGhost