[Python-Dev] New regex module for 3.2?

[Georg Brandl]

Am 16.07.2010 17:08, schrieb Vlastimil Brom:
> 2010/7/9 Georg Brandl <g.brandl at gmx.net>:
>> Am 09.07.2010 02:35, schrieb MRAB:
>>
>>>
>>> 1. Some of the inline flags are scoped; for example, putting "(?i)" at
>>> the end of a regex will now have no effect because it's no longer a
>>> global, all-or-nothing, flag.
>>
>> That is problematic.  I've often seen people put these flags at the end
>> of a regex, probably for readability purposes.  IMHO it would be better
>> to limit flag scoping to the explicit (?flags-flags: ) groups.
>>
> 
> I just noticed the formulation on the reference page
> regular-expressions.info on this kind of flags:
> "(?i)	Turn on case insensitivity for the remainder of the regular
> expression. (Older regex flavors may turn it on for the entire
> regex.)" and likewise for other flags.
> 
> http://www.regular-expressions.info/refadv.html
> 
> I am not sure, how "authoritative" this page by Jan Goyvaerts is for
> various implementations, but it looks like a very comprehensive
> reference.

Nevertheless, the authoritative reference for our regex engine is its
docs, i.e. http://docs.python.org/library/re.html -- and that states
clearly that inline flags apply to the whole regex.

> I think with a new regex implementation, not all of this "historical"
> semantics must be copied, unless there are major real usecases, which
> would be affected by this.

As I already said, I *have* seen this in real code.  As MRAB indicated,
this was the only silent change in semantics as compared to the old
regex engine.  If we replace re by regex, which I think is the only
way to get the new features in the stdlib, changing this one aspect is
a) not backwards compatible and b) in a subtle way that forces everyone
to review his/her regular expressions.  That's definitely not
acceptable.


Georg


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