From: Brian Candler <b.candler@...>
Date: 2011-02-02T05:59:50+09:00
Subject: Re: Search String and Delete Line Containing String

Bob Hatch wrote in post #978955:
> I am new to Ruby and am pulling my hair out on something that I know is
> simple. I am searching a large text file, line by line for text that
> will reside in part of the line. If it finds part of the text, I want to
> delete the line. Here is what I have that is not working. Help is
> appreciated! Thanks in advance!
>
> # process every line in a text file
> file='c:\ruby192\my_projects\IIS_Logs\ex11012607.log'
> text=File.readlines(file).each do |line|
> search_text = "/memberinfo/downline/tree/canvas.asp"
> if(line.gsub!(search_text) == search_text) then

gsub! returns the whole line after the substitution has taken place, or 
nil if no substitution has taken place. So you should test for nil/not 
nil, not compare against the substitution text.

irb is the way to find this sort of thing out:

$ irb --simple-prompt
>> line = "foo bar baz"
=> "foo bar baz"
>> line.gsub!("bar", "xxx")
=> "foo xxx baz"
>> line.gsub!("zzz", "xxx")
=> nil

Also, gsub! should take two arguments as I've shown. But rather than 
raise an error, the joys of ruby 1.9/1.8.7 mean that it does something 
unexpected when you give it one argument:

>> line.gsub!("zzz")
=> #<Enumerable::Enumerator:0x7f517774d5b8>

It returns an Enumerator object. In this case, I'd say that's pretty 
confusing behaviour, and very rarely useful. It's because there is a 
one-argument form of gsub which takes a block:

>> line.gsub!("foo") { (1+2).to_s }
=> "3 xxx baz"

And if you omit the block, it's returning an enumerator for consistency 
with each/map/select, which return an enumerator when invoked without a 
block (also not particularly useful IMO).

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