How to test if a string is basically an integer in quotes using Ruby

Well, here's the easy way:

class String
  def is_integer?
    self.to_i.to_s == self
  end
end

>> "12".is_integer?
=> true
>> "blah".is_integer?
=> false

I don't agree with the solutions that provoke an exception to convert the string - exceptions are not control flow, and you might as well do it the right way. That said, my solution above doesn't deal with non-base-10 integers. So here's the way to do with without resorting to exceptions:

  class String
    def integer? 
      [                          # In descending order of likeliness:
        /^[-+]?[1-9]([0-9]*)?$/, # decimal
        /^0[0-7]+$/,             # octal
        /^0x[0-9A-Fa-f]+$/,      # hexadecimal
        /^0b[01]+$/              # binary
      ].each do |match_pattern|
        return true if self =~ match_pattern
      end
      return false
    end
  end

You can use regular expressions. Here is the function with @janm's suggestions.

class String
    def is_i?
       !!(self =~ /\A[-+]?[0-9]+\z/)
    end
end

An edited version according to comment from @wich:

class String
    def is_i?
       /\A[-+]?\d+\z/ === self
    end
end

In case you only need to check positive numbers

  if !/\A\d+\z/.match(string_to_check)
      #Is not a positive number
  else
      #Is all good ..continue
  end  

You can use Integer(str) and see if it raises:

def is_num?(str)
  !!Integer(str)
rescue ArgumentError, TypeError
  false
end

It should be pointed out that while this does return true for "01", it does not for "09", simply because 09 would not be a valid integer literal. If that's not the behaviour you want, you can add 10 as a second argument to Integer, so the number is always interpreted as base 10.

Ruby 2.6.0 enables casting to an integer without raising an exception, and will return nil if the cast fails. And since nil mostly behaves like false in Ruby, you can easily check for an integer like so:

if Integer(my_var, exception: false)
  # do something if my_var can be cast to an integer
end

You can do a one liner:

str = ...
int = Integer(str) rescue nil

if int
  int.times {|i| p i}
end

or even

int = Integer(str) rescue false

Depending on what you are trying to do you can also directly use a begin end block with rescue clause:

begin
  str = ...
  i = Integer(str)

  i.times do |j|
    puts j
  end
rescue ArgumentError
  puts "Not an int, doing something else"
end

class String
  def integer?
    Integer(self)
    return true
  rescue ArgumentError
    return false
  end
end

1. It isn't prefixed with is_. I find that silly on questionmark methods, I like "04".integer? a lot better than "foo".is_integer?.
2. It uses the sensible solution by sepp2k, which passes for "01" and such.
3. Object oriented, yay.

The Best and Simple way is using Float

val = Float "234" rescue nil

Float "234" rescue nil #=> 234.0

Float "abc" rescue nil #=> nil

Float "234abc" rescue nil #=> nil

Float nil rescue nil #=> nil

Float "" rescue nil #=> nil

Integer is also good but it will return 0 for Integer nil

  def isint(str)
    return !!(str =~ /^[-+]?[1-9]([0-9]*)?$/)
  end