How two check if a number was entered using BigDecimal?

Hi,

I am doing the extra credit for numbers section in the Ruby basic course. I need to ask for two numbers, then multiply them and return a number with two decimal points even if no decimals was submitted.

I think I already resolved the exercise, but I was wondering how can I check if what the user entered is a number (since BigDecimal converts strings to 0.00) and if I use "nan?" (not a number) then it says it is false since 0.00 is a number. Then instead of converting the input of the user to big decimal directly I tried formatting the user input to string and then to big decimal but I am having a hard time with this.

How do I check if what the user entered is a number? If there is an exception how do I catch it to give the user another chance of entering a number? What is the best practice for doing this?

```require "bigdecimal"
require "bigdecimal/util"

puts "Tell me one number."
a = gets.chomp
first_number = a.to_d

puts "Tell me another number."
b = (gets.chomp)
second_number = b.to_d

result = first_number * second_number
puts "Their multiplication is"
puts "truncated result #{'%.2f' % result.truncate(2)}"
puts 'rounded result %.2f' % result
```

To be concise, BigDecimal converts 'invalid' strings to 0.0, e.g.

```BigDecimal.new('xyz') =>  0.0              # invalid
BigDecimal.new("12.345")   => 0.12345E2   # valid
```

If you want to accept "0" as a legal input value, I think you will have to look at the original string to determine if it was valid. A simple regular expression should suffice:

```pattern = Regexp.new('^[\d]*[\.]?[\d]*\$')

"123" !~ pattern => false
"45.678"  !~ pattern => false
"0" !=~ pattern => false
"0.0" !~ pattern => false
"NaN" !~ pattern => true
"12.34.56" !~ pattern => true
```

This regexp is looking for the start of a string (^), followed by zero or more digits ([\d]), followed by zero or one dot ([.]?), followed by zero or more digits ([\d]), followed by the end of string (\$). A return value of true shows an invalid input.