Re: Examples of disambiguation

Sorry - I misspoke about lookbehind; you *can* do it, but not quite as I outlined it!
___________________________________________________ 
Dr. Bethan Tovey-Walsh 

linguacelta.com

Golygydd | Editor geirfan.cymru

Croeso i chi ysgrifennu ataf yn y Gymraeg.

> On 7 Jul 2026, at 11:33, Bethan Tovey-Walsh <bytheway@linguacelta.com> wrote:
> 
> Thanks! Very interesting. 
> 
> I think it would be a bit limiting to call this a "disambiguation" construct, to be honest. It provides lookahead and lookbehind*, which is a pretty major extension to what iXML can already do, as it makes the language context-sensitive. 
> 
> That being the case, I don't think I'd agree that it's the same kind of thing as the repetition operators. It doesn't really say "match zero of this"; it says "ensure that this does not match". We don't have any other such non-consuming match patterns in the language at present.
> 
> *Although technically this implements only negative lookahead and lookbehind, you could also use it for the positive versions by doubling the operator.
> 
> This:
> 
> CAT: "cat", not_ER!.
> not_ER: "erpillar".
> 
> means "match the string 'cat', as long as it's not followed by 'erpillar'" - i.e. find occurrences of "cat" that are not the beginning of the word "caterpillar".
> 
> Then with this:
> 
> CAT: "cat", not_ER!.
> not_ER: ER!
> ER: "erpillar".
> 
> you can say "match 'cat', as long as it's followed by something that's not not "erpillar" - i.e. as long as it *is* the beginning of the word 'caterpillar'".
> 
> You can also do lookbehind:
> 
> CAT: not_COPY!, "cat".
> not_COPY!: "copy".
> 
> So, "find occurrences of 'cat' that aren't part of the word 'copycat'".
> 
> And you can do positive lookbehind as above, by doubling the negation.
> 
> BTW
> ___________________________________________________ 
> Dr. Bethan Tovey-Walsh 
> 
> linguacelta.com
> 
> Golygydd | Editor geirfan.cymru
> 
> Croeso i chi ysgrifennu ataf yn y Gymraeg.
> 
>> On 7 Jul 2026, at 11:08, Steven Pemberton <steven.pemberton@cwi.nl> wrote:
>> 
>> 
>> 
>> On Tuesday 07 July 2026 00:38:11 (+02:00), Bethan Tovey-Walsh wrote:
>> 
>> Thanks for sharing this, Steven. 
>> 
>> What are the changes to the grammar that describe this construct? And what exactly are its semantics? (From what you've described, I'm assuming that it's a kind of negative lookahead.)
>> 
>>       -term: factor;
>>               option;
>>               prohibition; {added}
>>               repeat0;
>>               repeat1.
>>       prohibition: factor, -"!", s.
>> 
>> The semantics are:
>> 
>> In an alternative such as 
>> A, B!, C
>> 
>> if B succeeds (successfully parses) at that position, B! fails, and so the alternative fails; if B fails, B! succeeds, and C is started at the position B was started.
>> 
>> Steven
>> 
>> 
>> 
>> BTW
>> 
>> ****************************************************
>> Dr. Bethan Tovey-Walsh
>> linguacelta.com
>> Golygydd | Editor geirfan.cymru
>> Croeso i chi ysgrifennu ataf yn y Gymraeg
>> 
>>> On 6 Jul 2026, at 16:22, Steven Pemberton <steven.pemberton@cwi.nl> wrote:
>>> 
>>> In the new implementation, we decided to use the notation "x!".
>>> 
>>> The reasoning behind this is that all similar constructs in ixml are also postfix:
>>> 
>>> x* zero or more
>>> x+ one or more
>>> x? zero or one
>>> x! zero
>>> 
>>> and prefix characters are use for marks on serialisation.
>>> 
>>> So here are some example disambiguation grammars.
>>> 
>>> IDENTIFIERS AND KEYWORDS
>>> Each line is either an identifier or a keyword
>>> 
>>>  input: ((id; keyword), #a)*.
>>>     id: keyword!, letter+, letter!.
>>> -letter: [L].
>>> keyword: ("if"; "then"; "else"), letter!.
>>> 
>>> Input
>>> i
>>> if
>>> ifi
>>> the
>>> then
>>> thene
>>> 
>>> Output
>>> <input>
>>>  <id>i</id>
>>>  <keyword>if</keyword>
>>>  <id>ifi</id>
>>>  <id>the</id>
>>>  <keyword>then</keyword>
>>>  <id>thene</id>
>>> </input>
>>> 
>>> MIXED CHARACTERS
>>> Taken from the ixml tutorial, separating input characters into classes
>>> {example input: 
>>> ...abcd()1234!!!}
>>> input: line+.
>>> line: (number; word; punc)*, 
>>> -#a.
>>> number: digit+, digit!.
>>> word: letter+, letter!.
>>> punc: 
>>> p+, p!.
>>> -letter: [L].
>>> -digit: ["0"-"9"].
>>> -p: 
>>> [P].
>>> Output
>>> <input>
>>> 
>>> <line>
>>>    <punc>...</punc>
>>> 
>>> <word>abcd</word>
>>> 
>>> <punc>()</punc>
>>> 
>>> <number>1234</number>
>>> 
>>> <punc>!!!</punc>
>>> 
>>> </line>
>>> </input>
>>> CODES
>>> Each line is either a header that ends with a code, or an item that is a series of words.
>>> catalogue: entry*.
>>>    entry: header, 
>>> item+.
>>>   header: text, -" ", code, -#a.
>>> 
>>> item: (text, " ")?, code!, word, -#a.
>>>    -text: word**" 
>>> ".
>>>    -word: (l; d)+, (l;d)!.
>>>    @code: l, l, 
>>> l, d, d, d.
>>>       -l: [L].
>>> 
>>> -d: ["0"-"9"].
>>> Input
>>> Fiction 
>>> fic001
>>> Brave New World
>>> 1984
>>> NonFiction non123
>>> Translating 
>>> Beaudelaire
>>> The Sixth 
>>> Extinction
>>> 
>>> Output
>>> <catalogue>
>>> 
>>> <entry>
>>>    <header 
>>> code="fic001">Fiction</header>
>>>    <item>Brave 
>>> New World</item>
>>> 
>>> <item>1984</item>
>>>  </entry>
>>> 
>>> <entry>
>>>    <header 
>>> code="non123">NonFiction</header>
>>> 
>>> <item>Translating Beaudelaire</item>
>>> 
>>> <item>The Sixth Extinction</item>
>>> 
>>> </entry>
>>> </catalogue>
>>> Steven
> 
> 
>

Received on Tuesday, 7 July 2026 10:50:40 UTC