1.8.1 gsub/CGI behavior I can't explain

Hi --

I have no explanation for this, but here it is.

I have a gsub! call in a CGI script (part of RCRchive).  It has a
block, and the purpose is to filter out unacceptable tags from
user input:

      # GOOD == %w{ a pre code etc}

      @text.gsub!(/(<((\s*\/?\s*)(\w+)(\s+[^>]+)?)>)/) {
        if GOOD.include?($4.downcase.strip)              # line 85
          $1
        else
          "&lt;#{$2}&gt;"
        end
      }

My error log is full of error messages from calling downcase on nil at
line 85.  In other words, the match succeeds -- the block is called --
but $4 is nil.  ($4 is (/\w+/), which can never be nil if the match
succeeds.)

$1-$3 are also nil.  So is $~.  In other words: once inside the block,
all evidence of a match disappears.  

The error messages look like this (minus dates and IP addresses):

  Premature end of script headers: submitrcr.rb
  (eval):3:in `__send__': undefined method `downcase' 
    for nil:NilClass (NoMethodError)
  from (eval):3:in `gsub!'
  from /usr/local/lib/httpd/htdocs/rcrchive/utils.rb:85:in `initialize',

I don't understand the __send__, or the eval, neither of which are in
my code.  

It works fine with 1.9.0, and it works fine from the command line.
Nothing I've done makes it work with 1.8.1/CGI, except to re-match the
string inside the block.  

I'm sorry I haven't tracked down what's wrong, but after considerable
examination it looks like it's wrong enough that I should mention it.


David

-- 
David A. Black
[e-mail]