Whittle it on down

[Steven D'Aprano]

On Thursday 05 May 2016 16:46, Stephen Hansen wrote:

> On Wed, May 4, 2016, at 11:04 PM, Steven D'Aprano wrote:
>> Start by writing a function or a regex that will distinguish strings that
>> match your conditions from those that don't. A regex might be faster, but
>> here's a function version.
>> ... snip ...
> 
> Yikes. I'm all for the idea that one shouldn't go to regex when Python's
> powerful string type can answer the problem more clearly, but this seems
> to go out of its way to do otherwise.
> 
> I don't even care about faster: Its overly complicated. Sometimes a
> regular expression really is the clearest way to solve a problem.

You're probably right, but I find it easier to reason about matching in 
Python rather than the overly terse, cryptic regular expression mini-
language.

I haven't tested my function version, but I'm 95% sure that it is correct. 
It trickiest part of it is the logic about splitting around ampersands. And 
I'll cheerfully admit that it isn't easy to extend to (say) "ampersand, or 
at signs". But your regex solution:

r"^[A-Z\s&]+$"

is much smaller and more compact, but *wrong*. For instance, your regex 
wrongly accepts both "&&&&&" and "      " as valid strings, and wrongly 
rejects "ΔΣΘΛ". Your Greek customers will be sad...

Oh, I just realised, I should have looked more closely at the examples 
given. because the specification given by DFS does not match the examples. 
DFS says that only uppercase letters and ampersands are allowed, but their 
examples include strings with spaces, e.g. 'FITNESS CENTERS' despite the 
lack of ampersands. (I read the spec literally as spaces only allowed if 
they surround an ampersand.) Oops, mea culpa. That makes the check function 
much simpler and easier to extend:


def check(string):
    string = string.replace("&", "").replace(" ", "")
    return string.isalpha() and string.isupper()


and now I'm 95% confident it is correct without testing, this time for sure!

;-)


-- 
Steve