palindrome check in ruby - ruby

Question

Why is this code giving wrong output for this input ?
def palindrome?(str)
str.delete('^a-zA-Z')
str.downcase
str == str.reverse
end
INPUT = "A man, a plan, a canal -- Panama"
OUTPUT = Failure/Error: palindrome?("A man, a plan, a canal -- Panama").should be_true, "Incorrect results for input: \"A man, a plan, a canal -- Panama\""
Incorrect results for input: "A man, a plan, a canal -- Panama"
# spec.rb:7:in `block (2 levels) in <top (required)>'
# ./lib/rspec_runner.rb:36:in `block in run_rspec'
# ./lib/rspec_runner.rb:32:in `run_rspec'
# ./lib/rspec_runner.rb:23:in `run'
# lib/graders/weighted_rspec_grader.rb:6:in `grade!'
# ./grade:31:in `<main>'

Answer 1

The delete and downcase methods on Strings do not modify the String itself, they return a changed copy. If you want to modify the receiver, use the bang variants:
str.delete!('[^a-zA-Z]')
str.downcase!
on second thoughts, don't do this, because horrible things like this happen:
string = "hello123"
palindrome?(string)
string #=> "OLLEH"
Instead, make a copy of the argument and modify that:
def palindrome?(arg)
str = arg.dup
str.delete!('[^a-zA-Z]')
str.downcase!
str == str.reverse
end

Answer 2

How to find the total number of palindromes for a string
class String
def palindrome?
self == self.strip.reverse
end
def sub_str_arr
(0..self.length).inject([]){|ai,i|
(1..self.length - i).inject(ai){|aj,j|
aj << self[i,j]
}
}
end
def num_palindromes
return -1 if self.length > 100000000
self.sub_str_arr.reject!{|item| item if item.length < 2 || !item.palindrome?}.size
end
end
puts "dabadbadbadbdbadbadbdabdbadbadbadbadbadadadbdbdbdbadbdabadbadbdbadbabdabdbbdabdabdbadba".num_palindromes