From: Robert Klemme <shortcutter@...>
Date: 2009-05-11T21:47:49+09:00
Subject: Re: gsub pattern substitution and ${...}

2009/5/11 Sarah Allen <sarah@ultrasaurus.com>:
> Robert Klemme wrote:
>> even with proper escaping this won't work as the string interpolation
>> takes place *before* gsub is invoked and hence URI.escape will insert
>> something but not the matched portion.  In your tests it has probably
>> worked because $2 was properly set from the previous match.
>
> really? so the ${...} gets evaluated before the param is passed to gsub,

All method parameters are evaluated before method invocation - this is
true for every method invocation in Ruby.

> but the block is passed as code, so then it is evaluated after.

In the case of gsub the block is invoked once for each match.

> Using the previous attempt, with a fresh irb session, I can see the
> issue:
>>> $1
> => nil
>>> $2
> => nil
>>> $3
> => nil
>>> require 'URI'
> => true
>>> s.gsub(/href=(['"])([^']*)/, "href=\\1#{URI.escape($2)}\\3")
> NoMethodError: private method `gsub' called for nil:NilClass
>  from
> /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/uri/common.rb:289:in
> `escape'
>  from (irb):8
>
>>> $1
> => nil
>>> $2
> => nil
>>> $3
> => nil
>>> s = "<a href='http://foo.com/one=>two'/>"
> => "<a href='http://foo.com/one=>two'/>"
>>> require 'URI'
> => true
>>> s.gsub(/href=(["'])([^'"]+)\1/) {
> ?> "href=#$1#{URI.escape($2)}#$1" }
> => "<a href='http://foo.com/one=%3Etwo'/>"
>
> Nice!

:-)

>> Note that my regexp has another weakness: the quote character not used
>> to quote the URI should be allowed as part of the URI.  I did not want
>> to complicate things too much but if you want to deal with this the
>> regular expression must be made a bit more complex.
>
> Wow, interesting. That would be incorrect HTML that the browser doesn't
> deal with well, so I'll not worry about it for this case, but I would be
> curious how it might be handled.

Basically you need an alternative and more capturing groups along the lines of

'([^']+)'|"([^"]+)"

> Thanks so much,

You're welcome.

Kind regards

robert

-- 
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