Ruby

When String#gsub is called from a string subclass with a block, Regexp.last_match is nil, but passed block is executed. Here is example code:

def call_gsub(str) str.gsub(/%/) do puts "checking #{str.class}" puts "Special variable value: #{$&}" puts "Regexp.last_match = #{Regexp.last_match.inspect}\n\n" raise "Special variable $& is not assigned, but block is called" if $&.nil? end end class MyString < String def gsub(*args, &block) super(*args, &block) # just forward everything end end text = 'test%text_with_special_character' call_gsub(String.new(text)) # original string call_gsub(MyString.new(text)) # string subclass 

Result:

checking String Special variable value: % Regexp.last_match = #<MatchData "%"> checking MyString Special variable value: Regexp.last_match = nil gsub_bug.rb:7:in `block in call_gsub': Special variable $& is not assigned, but block is called (RuntimeError)	from gsub_bug.rb:13:in `gsub'	from gsub_bug.rb:13:in `gsub'	from gsub_bug.rb:2:in `call_gsub'	from gsub_bug.rb:20:in `<main>' 

I expect result to be the same for both classes since MyString just wraps the same method:

checking String Special variable value: % Regexp.last_match = #<MatchData "%"> checking MyString Special variable value: % Regexp.last_match = #<MatchData "%"> 

Maybe there is something off with with control frame when params are forwarded?

Thanks in advance!