Re: reverse scanner
Re: reverse scanner
- Subject: Re: reverse scanner
- From: Philip Ershler <email@hidden>
- Date: Sat, 10 Aug 2013 16:45:21 -0600
Are you numbers constrained to be from 1 to 9? How about 0677 (which is actually 6 and 7) or 0607 (which is actually 607) etc?
Phil
On Aug 10, 2013, at 2:54 PM, Tom Davie <email@hidden> wrote:
>
> On 10 Aug 2013, at 22:44, Keary Suska <email@hidden> wrote:
>
>> On Aug 10, 2013, at 12:17 PM, Tom Davie wrote:
>>
>>> Heh, I’d actually argue that NSScanner is a much much better API to use here (and in fact nearly everywhere). Regular expressions constrain you only to regular grammars, which are a pretty small set. In my experience 99% of the use of them is actually trying to parse something that’s not *quite* a regular grammar, and uses a hack on top of regular expressions to do something not-quite-right.
>>>
>>> NSScanner by comparison makes the separation of what’s scanning/tokenisation, and what’s up to your (turing complete) program much more clear. So basically, (at least in my opinion), if you want to parse something that’s regular, NSScanner is a great choice. If you want to parse something that’s context free, look at CoreParse (Not tooting my own horn, honest). And finally, if you want to parse something that’s more even than that, then you’re probably back to NSScanner and a turing complete program.
>>>
>>> About the only use for regular expressions I can think of is asking NSScanner to scan something that it doesn’t by default know about.
>>
>> I would agree that NSScanner is a better API than NSRegularExpression, but I think that is Apple's fault because there are better regex API's, such as RegexKit.
>>
>> I would argue, however, that it is NSScanner that only functions well with fixed and unvarying grammars and has no context, other than a specific, unvarying linear progression. Regular expressions have a huge grammar and when you consider conditionals and zero-width assertions you can parse information that would send NSScanner into dizzying fits.
>>
>> Not to mention that NSScanner can't even touch the problem that the OP is experiencing, while regular expressions will handle it very nicely.
>
> No, some hacked on extensions to regular expressions can do this. Because people keep repeatedly bumping into the problem that they’re not as powerful as CFGs, and most parsing problems aren’t regular.
>
> Tom Davie
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Philip R. Ershler Ph.D.
University of Utah
Cardiovascular Research and Training Institute
95 South 2000 East
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