regex - Why is a Regexp object considered to be "falsy" in Ruby?

Question

Ruby has a universal idea of "truthiness" and "falsiness".

Ruby does have two specific classes for Boolean objects, TrueClass and FalseClass, with singleton instances denoted by the special variables true and false, respectively.

However, truthiness and falsiness are not limited to instances of those two classes, the concept is universal and applies to every single object in Ruby. Every object is either truthy or falsy. The rules are very simple. In particular, only two objects are falsy:

• nil, the singleton instance of NilClass and
• false, the singleton instance of FalseClass

Every single other object is truthy. This includes even objects that are considered falsy in other programming languages, such as

• the Integer 0,
• the Float 0.0,
• the empty String '',
• the empty Array [],
• the empty Hash {},

These rules are built into the language and are not user-definable. There is no to_bool implicit conversion or anything similar.

Here is a quote from the ISO Ruby Language Specification:

6.6 Boolean values

An object is classified into either a trueish object or a falseish object.

Only false and nil are falseish objects. false is the only instance of the class FalseClass (see 15.2.6), to which a false-expression evaluates (see 11.5.4.8.3). nil is the only instance of the class NilClass (see 15.2.4), to which a nil-expression evaluates (see 11.5.4.8.2).

Objects other than false and nil are classified into trueish objects. true is the only instance of the class TrueClass (see 15.2.5), to which a true-expression evaluates (see 11.5.4.8.3).

The executable Ruby/Spec seems to agree:

it "considers a non-nil and non-boolean object in expression result as true" do
  if mock('x')
    123
  else
    456
  end.should == 123
end

According to those two sources, I would assume that Regexps are also truthy, but according to my tests, they aren't:

if // then 'Regexps are truthy' else 'Regexps are falsy' end
#=> 'Regexps are falsy'

I tested this on YARV 2.7.0-preview1, TruffleRuby 19.2.0.1, and JRuby 9.2.8.0. All three implementations agree with each other and disagree with the ISO Ruby Language Specification and my interpretation of the Ruby/Spec.

More precisely, Regexp objects that are the result of evaluating Regexp literals are falsy, whereas Regexp objects that are the result of some other expression are truthy:

r = //
if r then 'Regexps are truthy' else 'Regexps are falsy' end
#=> 'Regexps are truthy'

Is this a bug, or desired behavior?

Answer 1

This isn’t a bug. What is happening is Ruby is rewriting the code so that

if /foo/
  whatever
end

effectively becomes

if /foo/ =~ $_
  whatever
end

If you are running this code in a normal script (and not using the -e option) then you should see a warning:

warning: regex literal in condition

This is probably somewhat confusing most of the time, which is why the warning is given, but can be useful for one lines using the -e option. For example you can print all lines matching a given regexp from a file with

$ ruby -ne 'print if /foo/' filename

(The default argument for print is $_ as well.)