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From: Mikko <mikko.levanto@iki.fi>
Newsgroups: comp.theory
Subject: Re: neos Universal Compiler
Date: Thu, 20 Mar 2025 14:32:02 +0200
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On 2025-03-20 11:16:35 +0000, Alan Mackenzie said:

> Mikko <mikko.levanto@iki.fi> wrote:
>> On 2025-03-19 11:02:49 +0000, Alan Mackenzie said:
> 
>>> Mikko <mikko.levanto@iki.fi> wrote:
>>>> On 2025-03-18 14:08:50 +0000, Mr Flibble said:
> 
>>>>> On Tue, 18 Mar 2025 15:59:45 +0200, Mikko wrote:
> 
>>> [ .... ]
> 
>>>>>> Is there a neosBNF schema that describes the tokens of FORtRAN 66 or
>>>>>> Algol 60?
> 
>>>>> Not yet.
> 
>>>> The definition of string literal of Algol 60 would be a good example
>>>> of something that cannot be defined with a regular expression and is
>>>> therefore impossible or at least complicated with an ordinary tokenizer.
> 
>>> Would you please be more specific about just what in an Algol 60 string
>>> literal prevents a regexp from parsing it.  Not for any special reason,
>>> just that I'm curious.  Maybe an example of such a string would be
>>> interesting.  Thanks!
> 
>> Algol 60 has different characters for opening and closing quotes (something
>> like 2018 and 2019 of Unicode) ....
> 
> Most current languages, including C, have different openers and closers
> for comments, which is surely analogous.
> 
>> .... and allows any number of nested quotes.
> 
> Ah OK.  Regular expressions can't parse arbitrarily nested structures.
> But Backus-Nauer Form can express them, and a push-down automaton can
> process them.
> 
> Are you sure about ordinary tokenizers not being able to handle such
> arbitrarily nested things in a non-complicated way?

Yes if "ordinary" is defined as one that can only parse a regular
language.

-- 
Mikko