Question 299823 in Javascript Question posted in
A very good W3school tutorial can be found here.

JavaScript Regular Expression For Years

I am trying to write a regular expression to ensure that the text of years is valid. The following are ALL valid years

  • 2023
  • 300
  • -1
  • 40AD
  • 40 AD
  • 3000 BC
  • 200BC

Here is the Regex that I camp up with

^(-?)([0-9]+)s*(BC|AD){0,1}$

However it has one shortfall. The above Regex also finds a match for the following:

  • -200AD
  • -700BC
  • 0BC
  • 0AD
  • -0
  • -0BC
  • -0AD

So bascially, I do NOT want the Regex to find a match if any of the following happens:

  1. A negative number is enetered, with the letters BC or AD
  2. 0BC or 0AD should not be allowed
  3. -0 should not be allowed. This also then includes -0BC and -0AD, which in turn should not be allowed either

How do I do that?

Thanks.

Tags:

2

Answers


  1. 0

    Consider:

    let re = String.raw`
    ^ # begin
    (
        (
            # negative
            -
            [1-9] [0-9]{0,3}  # dont allow leading zeroes
        )
        |
        (
            # positive
            [1-9] [0-9]{0,3}  # dont allow leading zeroes
            (
                # AD/BC
                s*
                (AD | BC)
            )?
        )
        |
        (
            # special case
            0
        )
    )
    $ # end
`

//

re = new RegExp(re.replace(/s+|#.+/g, ''))

console.log(re)
;
`
2023
300
-1
40AD
40 AD
3000 BC
200BC
-200AD
-700BC
0BC
0AD
-0
-0BC
-0AD
`.trim().split('n').map(s => s.trim()).forEach(s => console.log(s, re.test(s)))

    Regular expressions are programs, why do people insist on writing them without spaces and indentation? 😉

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  2. 0

    In the pattern that you tried ^(-?)([0-9]+)s*(BC|AD){0,1}$ all parts are optional except matching 1 or more digits, so the pattern is a very broad match and is not taking any rules for zeroes or hyphens into account.

    If you want a pattern for validation, you can omit the capture groups:

    ^(?:-?[1-9]d*|[1-9]d*s*(?:AD|BC)|0)$

    The pattern matches:

    • ^ Start of string
    • (?: Non capture group
      • -? Match optional hyphen
      • [1-9]d* Match 1-9 and optional digits
      • | Or
      • [1-9]d*s*(?:AD|BC) Match 1-9, optional digits, optional whitespace chars and either AD or BC
      • | Or
      • 0 Match a single zero
    • ) Close the non capture group
    • $ End of string

    See a regex demo.

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