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Re: Whittle it on down

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Steven D'Aprano wrote:

> Oh, a further thought...
> 
> 
> 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.
> 
> Putting non-ASCII letters aside for the moment, how would you match these
> specs as a regular expression?
> 
> - All uppercase ASCII letters (A to Z only), optionally separated into
> words by either a bare ampersand (e.g. "AAA&AAA") or an ampersand with
> leading and
> trailing spaces (spaces only, not arbitrary whitespace): "AAA   & AAA".
> 
> - The number of spaces on either side of the ampersands need not be the
> same: "AAA&   BBB &       CCC" should match.
> 
> - Leading or trailing spaces, or spaces not surrounding an ampersand, must
> not match: "AAA BBB" must be rejected.
> 
> - Leading or trailing ampersands must also be rejected. This includes the
> case where the string is nothing but ampersands.
> 
> - Consecutive ampersands "AAA&&&BBB" and the empty string must be
> rejected.
> 
> 
> I get something like this:
> 
> r"(^[A-Z]+$)|(^([A-Z]+[ ]*\&[ ]*[A-Z]+)+$)"
> 
> 
> but it fails on strings like "AA   &  A &  A". What am I doing wrong?
> 
> 
> For the record, here's my brief test suite:
> 
> 
> def test(pat):
>     for s in ("", " ", "&" "A A", "A&", "&A", "A&&A", "A& &A"):
>         assert re.match(pat, s) is None
>     for s in ("A", "A & A", "AA&A", "AA   &  A &  A"):
>         assert re.match(pat, s)

>>> def test(pat):
...     for s in ("", " ", "&" "A A", "A&", "&A", "A&&A", "A& &A"):
...         assert re.match(pat, s) is None
...     for s in ("A", "A & A", "AA&A", "AA   &  A &  A"):
...         assert re.match(pat, s)
... 
>>> test("^A+( *& *A+)*$")
>>> 

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Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 00:58 -0400
  Re: Whittle it on down Stephen Hansen <me+python@ixokai.io> - 2016-05-04 22:39 -0700
    Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 08:44 -0400
    Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 19:31 -0400
      Re: Whittle it on down Peter Otten <__peter__@web.de> - 2016-05-06 09:45 +0200
        Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-06 09:58 -0400
          Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-06 10:41 -0400
            Re: Whittle it on down Peter Otten <__peter__@web.de> - 2016-05-06 17:44 +0200
              Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-06 18:43 -0400
      Re: Whittle it on down alister <alister.ware@ntlworld.com> - 2016-05-06 10:01 +0000
  Re: Whittle it on down Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-05-05 08:53 +0300
    Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 08:57 -0400
  Re: Whittle it on down Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-05-05 16:04 +1000
    Re: Whittle it on down Stephen Hansen <me+python@ixokai.io> - 2016-05-04 23:46 -0700
      Re: Whittle it on down Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-05-05 17:04 +1000
        Re: Whittle it on down Stephen Hansen <me+python@ixokai.io> - 2016-05-05 00:34 -0700
          Re: Whittle it on down Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-05-05 18:41 +1000
            Re: Whittle it on down Random832 <random832@fastmail.com> - 2016-05-05 09:13 -0400
              Re: Whittle it on down Steven D'Aprano <steve@pearwood.info> - 2016-05-06 03:13 +1000
      Re: Whittle it on down Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-05-05 17:36 +1000
        Re: Whittle it on down Peter Otten <__peter__@web.de> - 2016-05-05 10:17 +0200
          Re: Whittle it on down Steven D'Aprano <steve@pearwood.info> - 2016-05-06 01:39 +1000
        Re: Whittle it on down Random832 <random832@fastmail.com> - 2016-05-05 09:21 -0400
          Re: Whittle it on down Steven D'Aprano <steve@pearwood.info> - 2016-05-06 04:03 +1000
            Re: Whittle it on down Random832 <random832@fastmail.com> - 2016-05-05 14:52 -0400
            Re: Whittle it on down Stephen Hansen <me+python@ixokai.io> - 2016-05-05 12:09 -0700
        Re: Whittle it on down Stephen Hansen <me+python@ixokai.io> - 2016-05-05 06:32 -0700
          Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 10:36 -0400
          Re: Whittle it on down Steven D'Aprano <steve@pearwood.info> - 2016-05-06 03:43 +1000
            Re: Whittle it on down Stephen Hansen <me+python@ixokai.io> - 2016-05-05 11:55 -0700
        Re: Whittle it on down Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-05-05 20:49 +0300
          Re: Whittle it on down Steven D'Aprano <steve@pearwood.info> - 2016-05-06 04:14 +1000
            Re: Whittle it on down Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-05-05 21:27 +0300
              Re: Whittle it on down Random832 <random832@fastmail.com> - 2016-05-05 14:54 -0400
              Re: Whittle it on down Steven D'Aprano <steve@pearwood.info> - 2016-05-06 10:57 +1000
                Re: Whittle it on down Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-05-06 07:19 +0300
    Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 08:31 -0400
      Re: Whittle it on down Steven D'Aprano <steve@pearwood.info> - 2016-05-06 03:54 +1000
        Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 17:36 -0400
      Re: Whittle it on down Stephen Hansen <me+python@ixokai.io> - 2016-05-05 11:56 -0700
        Re: Whittle it on down DFS <nospam@dfs.com> - 2016-05-05 17:45 -0400

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