Just what do you mean by "number"?

Posted on by John

Tom Christiansen gave an awesome answer to the question of how to match a number with a regular expression. He begins by clarifying what the reader means by “number”, then gives answers for each.

  • Is −0 a number?
  • How do you feel about √−1?
  • Is ⅝ or ⅔ a number?
  • Is 186,282.42±0.02 miles/second one number — or is it two or three of them?
  • Is 6.02e23 a number?
  • Is 3.141_592_653_589 a number? How about π, or ℯ? And −2π⁻³ ͥ?
  • How many numbers in 0.083̄?
  • How many numbers in 128.0.0.1?
  • What number does ⚄ hold? How about ⚂⚃?
  • Does 10,5 mm have one number in it — or does it have two?
  • Is ∛8³ a number — or is it three of them?
  • What number does ↀↀⅮⅭⅭⅬⅫ AUC represent, 2762 or 2009?
  • Are ४५६७ and ৭৮৯৮ numbers?
  • What about 0377, 0xDEADBEEF, and 0b111101101?
  • Is Inf a number? Is NaN?
  • Is ④② a number? What about ⓰?
  • How do you feel about ㊅?
  • What do ℵ₀ and ℵ₁ have to do with numbers? Or ℝ, ℚ, and ℂ?

See his full response here. Thanks to Bill the Lizard for pointing this out.

3 thoughts on “Just what do you mean by "number"?”

  2. John

    5% is shorthand for 5/100, but it takes different regular expressions to match “5%” and “5/100”.

  3. rdm

    Also: is “two” a number? is “half” a number? is “none” a number?

    That said, the stack overflow question Tom was responding to was pretty clear: It did not even use the word “number” in the question — only in the title. The title also specified “only digits and commas”. The examples included a minus sign which suggests that ЯegDwight (the person asking the question) considered the minus sign to be a digit, and the examples showed that precise spacing of the commas was not important.

    So simplicity suggests that most of the cases proposed by Tom Christiansen were not based on this question.

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