From: ahoward <ahoward@...>
Date: 2003-04-25T02:31:23+09:00
Subject: [OT] Re: grep and regular expressions in ruby

On Fri, 25 Apr 2003, Warren Brown wrote:

> Ara,
>
> > notsmall = '^[^s]*$|^[^s]*(?:s(?!mall)[^s]*)+$'
>
>     This caught my eye as being a little redundant.  To illustrate, if we
> let X = '[^s]*' and Y = '(?:s(?!mall)[^s]*)', this can be rewritten as:
>
>         notsmall = "^#{X}$|^#{X}#{Y}+$"
>
>     Which makes it a little easier to see that this is the same as:
>
>         notsmall = "^#{X}#{Y}*$"
>
>     Undoing the substitutions:
>
>         notsmall = '^[^s]*(?:s(?!mall)[^s]*)*$'
>
>     However, making a distinction between the 's' and the 'mall' still
> bothered me.  At first I tried simply:
>
>         notsmall = '^(?!small)*$'
>
>     Unfortunately, this only matches a zero length string since the (?!re)
> syntax does not consume any characters.  So, I added a '.' to consume the
> current character:
>
>         notsmall = '^(?:(?!small).)*$'
>
>     I believe this behaves the same as your original regular expression, but
> (IMHO) is much clearer.  However, I have no idea what the performance
> implications are.
>
>     I hope someone finds this interesting...

it it interesting - i've filed it in my 'interesting' folder ;-)

however, it doesn't work for the examples my program was attempting to solve?

if you notice the program has the pattern

....
  :cat_and_notsmall =>
    %r/(?:(?:#{cat}.*#{notsmall})|(?:#{notsmall}.*#{cat}))+/,
....

for some reason, your pattern doesn't seem to work if nested that way.  in
fact, now that i look at it i'm not sure *how* mine does either.  try plugging
yours in the program and see what i mean - ideas on this?

in any case - i think it IS clear that matching 'not word' is not only
possible, it might even be easier than i made it out to be ;-)


-a

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