[REBOL] Re: Parse versus Regular Expressions
From: joel:neely:fedex at: 4-Apr-2003 17:01
Hi, Ladislav,
Ladislav Mecir wrote:
> As I mentioned here, it is:
>
> z: [#a z #b | none]
>
> , which is a rule, that cannot be written using Regular Expressions.
>
OK. The old context-free-vs-context-sensitive thing. However, this
little example doesn't scale up very well AFAICT. An equivalent to
parse somestring z: [#a z #b | none]
in Perl would be something like
$somestring =~ /^(a*)(b*)$/ and length ($1) == length ($2)
While the content identification and the length equality take two
different sub-expressions, I'm wondering what happens if we make the
problem just a little larger, and try to match any of
""
"abc"
"aabbcc"
"aaabbbccc"
and so on. Using an RE to get content, and then checking lengths,
is easy to write and understand (and the pattern scales up to more
than two pieces of content), as follows:
$somestring =~ /^(a*)(b*)(c*)$/ and
length ($1) == length ($2) and
length ($2) == length ($3)
and so on for four or more "pieces".
How would you attack that with the PARSE dialect?
-jn-
--
----------------------------------------------------------------------
Joel Neely joelDOTneelyATfedexDOTcom 901-263-4446
Counting lines of code is to software development as
counting bricks is to urban development.
