I'm a ruby newbie and I'm having trouble understanding why "S" == /[S]/ evaluates to false.
Can anyone explain this? I've also tried
"S" =~ /[S]/
#=> 0 `
"S" =~ /[^S]/
#=> nil
baffling to me
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quetzalcoatl
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user3684134
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1Because `'S'` (`String`) isn't equal to `/[S]/` (`Regexp`). – Marek Lipka May 28 '14 at 14:50
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3`0` is not falsy in ruby. – BroiSatse May 28 '14 at 14:51
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1Sidenote: remember that in Ruby (unlike in C) an integer of '0' means logical 'true', so `"S" =~ /[S]/ => 0` does NOT MEAN FALSE. It means true. `if(0)` runs the code. `if(nil)` and `if(false)` does not. – quetzalcoatl May 28 '14 at 14:52
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@quetzalcoatl `/[^S]/` - means anything except S. – BroiSatse May 28 '14 at 14:54
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@BroiSatse: right, I was writing too fast, thanks :) – quetzalcoatl May 28 '14 at 14:55
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possible duplicate of [What are the Ruby Gotchas a newbie should be warned about?](http://stackoverflow.com/questions/372652/what-are-the-ruby-gotchas-a-newbie-should-be-warned-about) --- I vote for it because the first post in that topic explicitely says about zeros being considered `true`. – quetzalcoatl May 28 '14 at 14:57
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@quetzalcoatl It is not duplicate because it is too broad. – Darek Nędza May 28 '14 at 15:46
2 Answers
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"S" == /[S]/ is false because == in Ruby doesn't evaluate whether a regexp matches, it just determines whether two objects are equal. The string "S" and the regexp /[S]/ are of course completely different objects and not equal.
=~ (which is a correct way to match a regexp against a string in Ruby) returns the match position. In your first example the match position is the beginning of the string, 0. In the second example there is no match, so =~ returns nil.
Dave Schweisguth
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How would I get "S" =~ /[S]/ #=> 0 to return true instead? Also I thought the ^ character means starting charcter – user3684134 May 28 '14 at 15:46
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0 is what Ruby calls "truthy" -- it is treated as true in conditionals and such. If you really need to turn it in to boolean true, do `!! ("S" =~ /[S]/)`. Regarding ^, at the beginning of a regexp it means the beginning of the matched string, but inside [] it means "not the following character". – Dave Schweisguth May 28 '14 at 19:05
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"S" == /[S]/
Everything (almost) in Ruby is an object. In this case you are checking for equality between an instance of a String "S" and an instance of a Regexp /[S]/. Therefore, by definition, they are two different objects, hence the expression returns false. Rather than checking for equality with == you should use =~
"S" == /[S]/
When you use a match operator =~ it returns the index of the first match found in a string. Remember that indexing in Ruby starts from 0. In your example the first character in the provided string is matched. The first character is indexed with 0 and that is what the statement returns.
"S" == /[^S]/
By using a caret ^ you are telling Ruby to match anything but what is between square brackets (this is only true in square brackets, ^ is also used to indicate the beginning of a string if used outside []). In your case it is anything but S. Ruby does not find a match and returns nil.
hp4k
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