From: Brian Candler <b.candler@...>
Date: 2011-03-28T17:59:12+09:00
Subject: Re: How to "find" new lines

Chris Kottom wrote in post #989369:
> 7stud is right.  I missed the important word "only" in your original
> email.
>  My original method will produce false positives if you are indeed
> interested in finding strings which are only "\n".
>
> The "==" method works fine.  Regexp is also possible (though probably
> slower) if done as:
>
> if str =~ /^\n$/

Wrong - that doesn't match only strings containing just a newline.

>> str = "\n"
=> "\n"
>> str =~ /^\n$/
=> 0
>> str = "\n\n"
=> "\n\n"
>> str =~ /^\n$/
=> 0
>> str = "\n\nabc"
=> "\n\nabc"
>> str =~ /^\n$/
=> 0

With a regexp, you need str =~ /\A\n\z/

>> str = "\n"
=> "\n"
>> str =~ /\A\n\z/
=> 0
>> str = "\n\n"
=> "\n\n"
>> str =~ /\A\n\z/
=> nil
>> str = "\n\nabc"
=> "\n\nabc"
>> str =~ /\A\n\z/
=> nil

(beware: this is an area where Ruby's regexps are *not* the same as 
Perl's regexps)

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