Incremental Regex Parsing with Fixed Strings

[Andreas Abel]

Andrew,

The translation Regex -> NFA is compositional, thus, you could cache the 
NFAs computed for subregexs, only needing to recompute the NFA along the 
path to the changes subregex.  Compilation to DFAs is not needed as you 
can use NFAs directly to recognize strings.

--Andreas

On 2019-03-01 17:06, Andrew Martin wrote:
> Thank you so much for pointing me to this paper. What an absolute gem it 
> is! Efficiently evaluating regex with a recursive ADT instead of a graph 
> puts me one step closer to a solution. In my problem domain, regex 
> frequently are mostly sequences at the top level. That is, these are common:
> 
>      The number [0-9]* is a good number
>      I have [0-9]* pupp(ies|y).
> 
> This are not:
> 
>      (Hello|from|the|other|side)*
> 
> Using the paper's terminology, I think I should be able to have a 
> `FingerTree X REG` (for some X I'm not sure of), where every top-level 
> sequence is represented as adjacent elements in the finger tree. I'm not 
> sure if this will actually work, but it seems plausible that it will.
> 
> On Fri, Mar 1, 2019 at 3:55 AM Sergey Vinokurov <serg.foo at gmail.com 
> <mailto:serg.foo at gmail.com>> wrote:
> 
>     Modifying regular expressions reminds me of the great functional pearl
>     called 'A Play on Regular Expressions'. Perhaps this is the approach you
>     were thinking about?
> 
>     Sergey
> 
>     On 28/02/2019 18:43, Andrew Martin wrote:
>      > Greetings,
>      >
>      > In Dan Piponi's blog post Fast Incremental Regular Expression
>     Matching
>      > with Monoids [1], he outlines a way to take advantage of
>     fingertrees to
>      > perform increment regex matching. In his article, the regular
>     expression
>      > (more accurately, its equivalent DFA) is fixed, and the strings
>     we are
>      > matching on are modified incrementally. What I'm interested in is the
>      > opposite: What if the regular expression is modified
>     incrementally and
>      > the string we are testing is static?
>      >
>      > Thinking about this, it seems to run into problems immediately.
>      > Typically, to evaluate a regular expression, I would convert it
>     to a NFA
>      > (and then maybe to a DFA). But there is no way to do incremental
>     graph
>      > modification on the resulting graphs. (Or is there?). And even if
>     there
>      > were a way, it seems unclear how one would go about exploiting its
>      > incremental nature. To provide an example, consider the following
>     regex
>      > and sample string:
>      >
>      > Regex: Lost (dog|cat) loo for home
>      > Sample: Lost dog looking for home
>      >
>      > In the regex, I'm currently in the middle of typing "looking", so it
>      > doesn't match the sample string. But large parts of it do. So when I
>      > update the regex by typing "k", I'd like to be able to not redo this
>      > work. Maybe there's a certain subset of regex that's amenable to this
>      > kind of incremental evaluation. Maybe not. If anyone has any
>     additional
>      > insights or can point me to any research, I would appreciate it
>     greatly.
>      > Thanks.
>      >
>      > [1]
>     http://blog.sigfpe.com/2009/01/fast-incremental-regular-expression.html
>      >
>      >
>      > --
>      > -Andrew Thaddeus Martin
>      >
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> 
> 
> 
> -- 
> -Andrew Thaddeus Martin
> 
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