[Python-Dev] \G (match last position) regex operator non-existant in python?

MRAB python at mrabarnett.plus.com
Sat Oct 28 19:31:01 EDT 2017

On 2017-10-28 22:05, Guido van Rossum wrote:
> On Sat, Oct 28, 2017 at 12:09 AM, Nick Coghlan <ncoghlan at gmail.com 
> <mailto:ncoghlan at gmail.com>> wrote:
>     On 28 October 2017 at 01:57, Guido van Rossum <guido at python.org
>     <mailto:guido at python.org>> wrote:
>         Oh. Yes, that is being discussed about once a year two. It
>         seems Matthew isn't very interested in helping out with the
>         port, and there are some concerns about backwards
>         compatibility with the `re` module. I think it needs a champion!
>     Matthew's been amenable to the idea when it comes up, and he
>     explicitly wrote the module to be usable as a drop-in replacement
>     for "re" (hence the re-compatible v0 behaviour still being the
>     default).
>     The resistance has more been from our side, since this is a case
>     where existing regex module users are clearly better off if it
>     remains a separate project, as that keeps upgrades independent of
>     the relatively slow standard library release cycle (and allows it
>     to be used on Python 2.7 as well as in 3.x). By contrast, the
>     potential benefits of standard library inclusion accrue primarily
>     to Python newcomers and folks writing scripts without the benefit
>     of package management tools, since they'll have a more capable
>     regex engine available as part of the assumed language baseline.
>     That means that if we add regex to the standard library in the
>     regular way, there's a more than fair chance that we'll end up
>     with an outcome like the json vs simplejson split, where we have
>     one variant in the standard library, and another variant on PyPI,
>     and the variants may drift apart over time if their maintenance is
>     being handled by different people. (Note: one may argue that we
>     already have this split in the form of re vs regex. So if regex
>     was brought in specifically to replace _sre as the re module
>     implementation, rather than as a new public API, then we at least
>     wouldn't be making anything *worse* from a future behavioural
>     consistency perspective, but we'd be risking a compatibility break
>     for anyone depending on _sre and other internal implementation
>     details of the re module).
>     One potential alternative approach that is then brought up (often
>     by me) is to suggest instead *bundling* the regex module with
>     CPython, without actually bringing it fully within the regular
>     standard library maintenance process. The idea there would be to
>     both make the module available by default in python.org
>     <http://python.org> downloads, *and* make it clear to
>     redistributors that the module is part of the expected baseline of
>     Python functionality, but otherwise keep it entirely in its
>     current independently upgradable form.
>     That would still be hard (since it would involve establishing new
>     maintenance policy precedents that go beyond the current
>     special-casing of `pip` in order to bootstrap PyPI access), but
>     would have the additional benefit of paving the way for doing
>     similar things with other modules where we'd like them to be part
>     of the assumed baseline for end users, but also have reasons for
>     wanting to avoid tightly coupling them to the standard libary's
>     regular maintenance policy (most notably, requests).
>     And that's where discussions tend to fizzle out:
>     * outright replacement of the current re module implementation
>     with a private copy of the regex module introduces compatibility
>     risks that would need a fiat decision from you as BDFL to say
>     "Let's do it anyway, make sure the test suite still works, and
>     then figure out how to cope with any other consequences as they arise"
>     * going down the bundling path requires making some explicit
>     community management decisions around what we actually want the
>     standard library to *be* (and whether or not there's a difference
>     between "the standard library" and "the assumed available package
>     set" for Python installations that are expected to run arbitrary
>     third party scripts rather than specific applications)
>     * having both the current re API and implementation *and* a new
>     regex based API and implementation in the standard library
>     indefinitely seems like it would be a maintainability nightmare
>     that delivered the worst of all possible outcomes for everyone
>     involved (CPython maintainers, regex maintainers, Python end users)
> Maybe it would be easier if Matthew were amenable to maintaining the 
> stdlib version and only add new features to the PyPI version when 
> they've also been added to the stdlib version. IOW if he were 
> committed to *not* letting the [simple]json thing happen.
> I don't condone having two different regex implementations/APIs 
> bundled in any form, even if one were to be deprecated -- we'd never 
> get rid of the deprecated one until 4.0. (FWIW I don't condone this 
> pattern for other packages/modules either.) Note that even if we 
> outright switched there would *still* be two versions, because regex 
> itself has an internal versioning scheme where V0 claims to be 
> strictly compatible with re and V1 explicitly changes the matching 
> rules in some cases. (I don't know if this means that you have to 
> request V1 to use \G though.)
> The other problem with outright replacement is that despite Matthew's 
> best efforts there may be subtle incompatibilities that will break 
> people's code in surprising ways. I don't recall much about our 
> current 're' test suite -- I'm sure it tests every feature, but I'm 
> not sure how far it goes in testing edge cases. IIRC this is where in 
> the past we've always erred on the side of (extreme) caution, and my 
> recollection is of Matthew being (understandably!) pretty lukewarm 
> about doing extra work to help assess this -- IIRC he's totally fine 
> with the status quo.
> If there's new information or a change in Matthew's outlook I'd be 
> happy to reconsider it.
At one time I was in favour of including it in the stdlib, but then I 
changed my mind. Having it outside gives me more flexibility, and I'm 
happy with just using pip.

Not that I'm planning on making any further additions, just bug fixes 
and updates to follow the Unicode updates. I think I've crammed enough 
into it already. There's only so much you can do with the regex syntax 
with its handful of metacharacters and possible escape sequences...

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