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Unwanted collector in regular expression

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  Unwanted collector in regular expression Ruby Ruby <ruby@ayni.com> - 2011-04-01 02:20 -0500
    Re: Unwanted collector in regular expression Brian Candler <b.candler@pobox.com> - 2011-04-01 03:17 -0500
    Re: Unwanted collector in regular expression Robert Klemme <shortcutter@googlemail.com> - 2011-04-01 03:23 -0500

#2088 — Unwanted collector in regular expression

Hi every
installation here ruby-1.8.6.420-2.fc13.x86_64

i wanted to highlight some text fields (change background color) in an
xhtml page. To do that, i had to skip all xhtml markup, 'cause it is not
advisable to put markup into markup.

i created the following function:

<snip>
def highlight(what, crix)
  if crix.empty? then return what end
  @tix.puts "WHAT: "+what
  ccir = Regexp.new("(?:\<[^\>]+\>)|(?:("+crix.join("|")+"))", "i")
  return what.gsub(ccir) {|s|
    @tix.puts "COLL: "+s
    if ! s.empty? && ! s.match(/^\</)
      "<span style='background-color:"+CCOL+";'>"+s+"</span>"
    else s
    end
  }
end  # highlight
</snip>

the main part of the function is the ccir regular expression.
my thinking was, that with the first group in the regular expression i
would skip all  markup, and with the second group in the regular
expression i would collect the fields designated in the crix array.
When the first group of the regular expression matches, i was awaiting
an empty string to be returned to the block, otherwise the element to be
highlighted would be returned to the block.
NOPE.
when the first group of the regular expression matched, the test file
showed me, that an entire markup sequence was returned to the block,
even if no collector was active in the first group. when the second
group of the regular expression matched, it returned the expected string
to the block.
this is why i had to avoid the markup-in-markup by checking again in the
block if the string returned to the block started with "<", i.e. if it
was markup.

so be warned, if you use groups in regular expressions, as they may not
return what you expected.

suomi

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#2093

Why don't you make a simple, standalone test program which demonstrates 
the behaviour you're describing? Then we can run it ourselves, and can 
perhaps explain what's happening.

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#2095

On Fri, Apr 1, 2011 at 9:20 AM, Ruby Ruby <ruby@ayni.com> wrote:
> Hi every
> installation here ruby-1.8.6.420-2.fc13.x86_64
>
> i wanted to highlight some text fields (change background color) in an
> xhtml page. To do that, i had to skip all xhtml markup, 'cause it is not
> advisable to put markup into markup.
>
> i created the following function:
>
> <snip>
> def highlight(what, crix)
>  if crix.empty? then return what end
>  @tix.puts "WHAT: "+what
>  ccir = Regexp.new("(?:\<[^\>]+\>)|(?:("+crix.join("|")+"))", "i")

You should rather use Regexp.union(crix) in order to ensure proper
escaping.  Note that you can use string interpolation in regexp, e.g.

irb(main):007:0> s=123
=> 123
irb(main):008:0> /foo#{s}/
=> /foo123/

Also, I don't see why you use \< because the backslash disappears:

irb(main):022:0> puts "\<"
<
=> nil
irb(main):023:0> puts "<"
<
=> nil


>  return what.gsub(ccir) {|s|
>    @tix.puts "COLL: "+s
>    if ! s.empty? && ! s.match(/^\</)
>      "<span style='background-color:"+CCOL+";'>"+s+"</span>"
>    else s
>    end
>  }
> end  # highlight
> </snip>
>
> the main part of the function is the ccir regular expression.
> my thinking was, that with the first group in the regular expression i
> would skip all  markup, and with the second group in the regular
> expression i would collect the fields designated in the crix array.
> When the first group of the regular expression matches, i was awaiting
> an empty string to be returned to the block, otherwise the element to be
> highlighted would be returned to the block.
> NOPE.
> when the first group of the regular expression matched, the test file
> showed me, that an entire markup sequence was returned to the block,
> even if no collector was active in the first group. when the second
> group of the regular expression matched, it returned the expected string
> to the block.
> this is why i had to avoid the markup-in-markup by checking again in the
> block if the string returned to the block started with "<", i.e. if it
> was markup.
>
> so be warned, if you use groups in regular expressions, as they may not
> return what you expected.

Yes, groups can be tricky but I believe yours is rather a case of
malformed regexp.  I chime in with what Brian said: write a small
program demonstrating the effect and describe the desired output.

Otherwise I recommend "Mastering Regular Expressions" (O'Reilly).

Cheers

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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