How do I determine if a string is numeric?

Question

I thought this would be easier to find, but I'm quite surprised that it isn't.

How on Earth do I test if a string is a number (including decimals) outside a Model?

e.g.

is_number("1") # true
is_number("1.234") # true
is_number("-1.45") # true
is_number("1.23aw") #false

In PHP, there was is_numeric, but I can't seem to find an equivalent in Ruby (or Rails).

So far, I've read the following answers, and haven't gotten any closer:

possible duplicate of [Validate number with rails 3](http://stackoverflow.com/questions/22924153/validate-number-with-rails-3) — Justin Wood, Sep 26 '14 at 14:16
Lets look at the root of the problem. How are you getting this string that may or may not be a `Numeric`? — Justin Wood, Sep 26 '14 at 14:22
@JustinWood it's an API parameter. The validation logic should work in any context — FloatingRock, Sep 26 '14 at 14:23
@JustinWood convert it to float, and rescue the Exception? Sounds convoluted.. — FloatingRock, Sep 26 '14 at 14:27
What Exception? `value.to_f` should never throw an exception... — Justin Wood, Sep 26 '14 at 14:28
@JustinWood Unfortunately using `to_f` you cannot distinguish between non numeric strings and zeros: `'0'.to_f # => 0.0`, `'foo'.to_f # => 0.0`. — toro2k, Sep 26 '14 at 14:31
@JustinWood I like it. The only downside is that `"apple".to_f` is `0.0`, but in my case that's invalid too. — FloatingRock, Sep 26 '14 at 14:32
possible duplicate of [Test if string is a number in Ruby on Rails](http://stackoverflow.com/questions/5661466/test-if-string-is-a-number-in-ruby-on-rails) — Brad Werth, Sep 26 '14 at 16:12

toro2k · Accepted Answer · 2014-09-26T14:44:08.887

7

You could borrow the idea from the NumericalityValidator Rails uses to validate numbers, it uses the Kernel.Float method:

def numeric?(string)
  # `!!` converts parsed number to `true`
  !!Kernel.Float(string) 
rescue TypeError, ArgumentError
  false
end

numeric?('1')   # => true
numeric?('1.2') # => true
numeric?('.1')  # => true
numeric?('a')   # => false

It also handles signs, hex numbers, and numbers written in scientific notation:

numeric?('-10')   # => true
numeric?('0xFF')  # => true
numeric?('1.2e6') # => true

edited Sep 26 '14 at 14:44

answered Sep 26 '14 at 14:27

toro2k

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So I would have to define this method then, right? (i.e. it's not built-in) – FloatingRock Sep 26 '14 at 14:29
1

Yes, there's no built-in or library function for the purpose. – toro2k Sep 26 '14 at 14:37

score 4 · Answer 2 · answered Sep 26 '14 at 14:42

4

You could use Regular Expression.

!!("1" =~ /\A[-+]?[0-9]+(\.[0-9]+)?\z/) # true
!!("1.234" =~ /\A[-+]?[0-9]+(\.[0-9]+)?\z/) # true
!!("-1.45" =~ /\A[-+]?[0-9]+(\.[0-9]+)?\z/) # true
!!("1.23aw" =~ /\A[-+]?[0-9]+(\.[0-9]+)?\z/) # false

You can use it like this or make a method in a module or add this in the String class

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

You can use this site to test your expression : Rubular: a Ruby regular expression editor and tester

I can explain much more the expression if needed.

Hope this helps.

answered Sep 26 '14 at 14:42

Fred Perrin

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This fails with values in bases larger than 10 such as `0xdeadbeef` – the Tin Man Sep 26 '14 at 15:42
2

Why downvote ? The OP didn't indicate he want to test a specific base or other than 10. And it's possible to change the regular expression, so the answer remain valid. – Fred Perrin Sep 26 '14 at 17:46
@FredPerrin i got your back – FloatingRock Sep 26 '14 at 17:48

How do I determine if a string is numeric?

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