reverse regex lookup for particular string

"very \n simple example of big"
 => return true because first occurrence moving backwards from example is \n


"very \n\t\t simple example of big" 
 => return false because first occurrence moving backwards from example is \n\t\t

is this possible through reverse regex loo开发者_Python百科kup?

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I'm not sure exactly what you're looking for but you can always reverse the string and match forward:

>> "very \n simple example of big".reverse.match(/elpmaxe[^\t]+\n/)
=> #<MatchData "elpmaxe elpmis \n">

>> "very \n\t\t simple example of big".reverse.match(/elpmaxe[^\t]+\n/)
=> nil

I'm not sure if this has crossed the line between clever and stupid but it works on your examples.

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I'm still not entirely sure what you are looking for, but if it is "Find out whether or not the word 'example' occurs on a line that does not start with any tabs," then here you go:

ruby-1.9.2-p136 :001 > samples = [ "very \n simple example of big", "very \n\t\t simple example of big" ]
 => ["very \n simple example of big", "very \n\t\t simple example of big"] 
ruby-1.9.2-p136 :002 > samples.map{ |s| s[/^[^\t\n]+.+?example/] }
 => [" simple example", nil] 

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I donno Ruby, hence giving the regex that I verified in Javascript:

/\n[^(\t+)]+ example/g

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The question isn't too clear, but I think this is what you want, using a negative lookahead:

^(?!\t\t).*example

Example: http://rubular.com/r/DRdlscH6cO

Here I'm using ^ for start of the line (you can use \n if you prefer, but it wouldn't match the first line). . will not match another line by default, so it's also safe.
You may also want to add capturing groups, and match only a whole word:

^(?!\t\t)(.*)\bexample\b

The regex doesn't allow lines that contain example, and start with \t\t. The line can start with a single \t, or have \t\t elsewhere.