How to Convert String "false" to Boolean in Ruby?

In Ruby, "false" (i.e. string false) is not type converted to boolean false automatically. In fact, it evaluates to true:

print "false" == true #=> true

This is the correct interpretation because in Ruby, any value other than falsy values will always return true when evaluated. Since "false" is a non-empty string, it is by definition truthy.

However, in some cases, such as during data transfer, deserialization, or making HTTP request to the server, you might want to interpret string "false" as boolean false. To do so, you can use a conditional expression to convert the string to a boolean value, for example, like so:

def parse_str_false(str)
    false if str.downcase == "false"
end

puts parse_str_false("false") #=> false
puts parse_str_false("false").class #=> FalseClass

puts parse_str_false("true") #=> nil
puts parse_str_false("foo") #=> nil
# ...

Calling the String#downcase method on the string is important, especially if the incoming string's case is unknown.

You can also use regular expression to achieve the same, for example, by using basic pattern-matching operator (=~). However, it would be an overkill if you only check for a single value (i.e. string false). It could prove useful though, if you had a case where false is represented by multiple strings (such as "false", "f", "no", "n", "0", "off", etc.). In that case, you could do something like the following:

def parse_str_false(str)
    false if str =~ /^(false|f|no|n|off|0)$/i
end

puts parse_str_false("false") #=> false
puts parse_str_false("False") #=> false
puts parse_str_false("f") #=> false
puts parse_str_false("No") #=> false
puts parse_str_false("n") #=> false
puts parse_str_false("0") #=> false
# ...

puts parse_str_false("true") #=> nil
puts parse_str_false("foo") #=> nil
# ...