[REBOL] Re: 'Parse is peculiar!
From: petr:krenzelok:trz:cz at: 14-Dec-2000 8:33
Shannon Baker wrote:
> Hi REBOL Community,
>
> The REBOL philosophy goes "Simple things should be simple". Well I have to say
> that the 'parse function is an exception! I've had to use it extensively for
> parsing log files but it has literally taken me months to do simple things -
> compared to a few weeks for the rest of the core set. Normally I go over the
> manual entry extensively looking for clues as to what is going wrong, this time
> i'm really stuck. Look at this:
>
> REBOL/View 0.10.38.3.1 28-Nov-2000
> Copyright 2000 REBOL Technologies. All rights reserved.
> >> digits: charset "0123456789"
> == make bitset! #{000000000000FF0300000... [snip] ...000}
> >> line1: {Lets find "Julie<1234>"}
> >> parse line1 [thru {"} copy name [thru {<} 4 digits {>} (print name)] to end]
> Julie<1234>
> == true
huh, what? tried above code and it fails ;-) (print name) has to be after the closing
bracket, or you get:
->> parse line1 [thru {"} copy name [thru {<} 4 digits {>} (print name)] to end]
** Script Error: name has no value
** Near: print name
> but what if I don't want to include the <xxxx> in 'name?
>
> >> parse line1 [thru {"} copy name [to {<} 4 digits {>} (print name)] to end]
> -------------------------------------| Note the 'thru changed to 'to
> == false
>
hmm, why is the result false you ask? Well, because by stating "to {<}" parser stops
just in front of < char, so to turn it into 'true you would have to do it following
way:
->> parse line1 [thru {"} copy name [to {<} {<} 4 digits {>}] (print name) to end]
Julie<1234>
== true
> That doesn't make sense. Now the second problem:
Does it make more sense? Well, you wanted just the name, so why to use the strange
syntax above? What about following code:
->> parse line1 [thru {"} copy name to {<} (print name) to end]
Julie
== true
> >> line2: {Lets find "J<o>hn<1234>"}
> >> parse line2 [thru {"} copy name [thru {<} 4 digits {>} (print name)] to end]
> == false
>
> So there's the second problem, parse seems to get stuck on '<o>'. I would assume
> the sub rule should only match a string containing '<xxxx>' and x is a digit.
Well, and you are right. It doesn't match and that's why you got 'false, no? What's
the problem here?
->> alpha: charset [#"a" - #"z" #"A" - #"Z"]
== make bitset! #{
0000000000000000FEFFFF07FEFFFF0700000000000000000000000000000000
}
->> parse line2 [thru {"} copy name [any [some alpha | 4 digits | {>} | {<}]] (print
name) to end]
J<o>hn<1234>
== true
I know it's not so powerful expression, as it will also match any combination of
sequences of alphas, 4 digits, and separately <, >, so it would match even some
<>Jo1234><1234>
to better suite your needs:
>> parse line2 [thru {"} copy name [some alpha {<} some [some alpha {>} some alpha
{<} 4 digits {>} | 4 digits {>}]] (print name) to end]
J<o>hn<1234>
== true
> Please don't ask me to use 'find or to change the data structure, or to parse the
> results twice. I want to understand why 'parse doesn't return the results I
> expect.
>
> Finally the follow line causes the rebol console to hang:
>
> >>parse line2 [thru {"} copy name [some [to {<}]] to end]
:-) well, just because by using "to" statement you will be put in front of "<" and
then again and again and again ... untill you finally skip damned "<" ;-)
šjust change it to "thru" or try to match "<"
Cheers,
-pekr-
