From: Garance A Drosehn <drosihn@...>
Date: 2005-08-01T14:15:42+09:00
Subject: Defining regexp's and variables set by them

Sometimes I get in a situation where I have a case statement
with a several when-clauses, each of which is a regular
expression.  Some of those regular-expressions may be
rather complicated.  If that case statement is going to be
processed many times, then I like to define objects via
Regexp.new, and use those pre-compiled objects in the
when clauses:

    re_simple = Regexp.new("check (\d+)")
    ...
    very_huge_file.each_line { |aline}
        case aline
            when re_simple
                check_number = $1
            when re_other
       ...

The downside to this is that the regular-expression is now
defined somewhere "far away" from the when-clause that
uses it.  After some testing I may need to make changes to
the regexp, such as:

    re_simple = Regexp.new("(check|test) (\d+)")

The thing is, by changing 'check' to '(check|test)', I have to
remember that the when clause also needs to change from
referencing $1 to referencing $2.  Note that in this case I do
not *care* whether it matched 'check' as opposed to 'test'.
Either word is acceptable to me, so I do not need the actual
value of $1 for anything.

I was kinda wondering if it would make sense for ruby to
support something like:

    re_simple = Regexp.new("check (\d+)") { cnum = $1 }

so I could then have the when clause say:

            when re_simple
                check_number = cnum

I realize this is a trivial example, but as the expressions get
more involved, and the case-statement has many when's, it
would be nice if I could have the compiled regular-expression
set values on variable names that *I* pick, in addition to the
standard values in a MatchData object.

Another thing that this might let me do, is something like:

            when re_simple, re_other, re_yetmore
                check_number = cnum

where the different regular-expressions may find 'cnum' in
different positions in the string ($1 vs $2 vs $3), and yet
they could all be processed in the same when-clause.

Or is there already some way to do this?

-- 
Garance Alistair Drosehn     =             drosihn@gmail.com
Senior Systems Programmer               or   gad@FreeBSD.org
Rensselaer Polytechnic Institute;             Troy, NY;  USA