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[Python-ideas] PEP 8: raw strings & regular expressions

David Mertz mertz at gnosis.cx
Mon Oct 26 18:31:37 EDT 2015 
I think we're down to quibbling over the meaning of "virtually" here. I
recognize it's "not all" and you recognize it's "most" windows paths are
grammatically regexen.

So is it 80%, 95%, 99.9%? And do we mean "paths found in the wild" or
"paths as systematically enumerated" from possibility space?
On Oct 26, 2015 3:23 PM, "Andrew Barnert" <abarnert at yahoo.com> wrote:

> On Oct 26, 2015, at 14:53, David Mertz <mertz at gnosis.cx> wrote:
>
> Obviously there can't be a regex to exclude everything that isn't a regex.
> Parentheses can nest to unlimited depths, so you need a formal grammar.
>
> As I said:
>
> Obviously there's no actual regular expression that matches all regular
>> expressions (you can't handle matched brackets without recursion or some
>> other extension)
>
>
> But you can do it trivially with Perl, or with the Regex module for
> Python, e.g., just by sticking a "(?1)" inside a pair of escaped parens
> plus a negative lookahead or nongreedy repetition. I'm not sure exactly how
> powerful Python (re module) regexes are (if I want to match something that
> isn't a regular language, I tend to reach for or build a dedicated parser
> rather than try to stretch re), but I know they're somewhere between actual
> regular expressions and perl regexes.
>
> But virtually everything that is a Windows path is also formally
> grammatical regex too (as are many things with no plausible likely
> intention as such)
>
> That's not true. You can, for example, have unclosed brackets or parens in
> a Windows path. And if you're wondering why anyone would do that, consider
> MP3 files auto-named based on their ID3v1/FreeDB metadata, which truncates
> fields at 29 or 30 bytes.
>
> Anyway, as I said in the same message, it wouldn't be a useful heuristic
> because there's so much overlap, but you don't need to exaggerate that to
> make the same point.
>
> On Oct 26, 2015 2:44 PM, "Ben Finney" <ben+python at benfinney.id.au> wrote:
>
>> Andrew Barnert via Python-ideas
>> <python-ideas at python.org> writes:
>>
>> > Just for fun: is there a Python regex that matches all valid Python
>> > regexes?
>>
>> Yes: ‘.*’ matches all valid Python regexes.
>>
>> > Obviously there's no actual regular expression that matches all
>> > regular expressions (you can't handle matched brackets without
>> > recursion or some other extension).
>>
>> You seem to be seeking something else: a pattern that matches all valid
>> regex patterns, *and* will never match any string that is not a valid
>> regex pattern. The latter is rather more difficult.
>>
>> --
>>  \      “When I was born I was so surprised I couldn't talk for a year |
>>   `\                                        and a half.” —Gracie Allen |
>> _o__)                                                                  |
>> Ben Finney
>>
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