This is not an appropriate use of regular expressions. You'd be better off using

[0-9]{2}/[0-9]{2}/[0-9]{4}

and then checking ranges in a higher-level language.

Maintainable Perl 5.10 version

/
  (?:
      (?<month> (?&mon_29)) [\/] (?<day>(?&day_29))
    | (?<month> (?&mon_30)) [\/] (?<day>(?&day_30))
    | (?<month> (?&mon_31)) [\/] (?<day>(?&day_31))
  )
  [\/]
  (?<year> [0-9]{4})
  
  (?(DEFINE)
    (?<mon_29> 0?2 )
    (?<mon_30> 0?[469]   | (11) )
    (?<mon_31> 0?[13578] | 1[02] )

    (?<day_29> 0?[1-9] | [1-2]?[0-9] )
    (?<day_30> 0?[1-9] | [1-2]?[0-9] | 30 )
    (?<day_31> 0?[1-9] | [1-2]?[0-9] | 3[01] )
  )
/x

You can retrieve the elements by name in this version.

say "Month=$+{month} Day=$+{day} Year=$+{year}";

( No attempt has been made to restrict the values for the year. )

Sounds like you're overextending regex for this purpose. What I would do is use a regex to match a few date formats and then use a separate function to validate the values of the date fields so extracted.

Perl expanded version

Note use of /x modifier.

/^(
      (
        ( # 31 day months
            (0[13578])
          | ([13578])
          | (1[02])
        )
        [\/]
        (
            ([1-9])
          | ([0-2][0-9])
          | (3[01])
        )
      )
    | (
        ( # 30 day months
            (0[469])
          | ([469])
          | (11)
        )
        [\/]
        (
            ([1-9])
          | ([0-2][0-9])
          | (30)
        )
      )
    | ( # 29 day month (Feb)
        (2|02)
        [\/]
        (
            ([1-9])
          | ([0-2][0-9])
        )
      )
    )
    [\/]
    # year
    \d{4}$
  
  | ^\d{4}$ # year only
/x

Original

^((((0[13578])|([13578])|(1[02]))[\/](([1-9])|([0-2][0-9])|(3[01])))|(((0[469])|([469])|(11))[\/](([1-9])|([0-2][0-9])|(30)))|((2|02)[\/](([1-9])|([0-2][0-9]))))[\/]\d{4}$|^\d{4}$

Regex was not meant to validate number ranges(this number must be from 1 to 5 when the number preceding it happens to be a 2 and the number preceding that happens to be below 6). Just look for the pattern of placement of numbers in regex. If you need to validate is qualities of a date, put it in a date object js/c#/vb, and interogate the numbers there.

I know this does not answer your question, but why don't you use a date handling routine to check if it's a valid date? Even if you modify the regexp with a negative lookahead assertion like (?!31/0?2) (ie, do not match 31/2 or 31/02) you'll still have the problem of accepting 29 02 on non leap years and about a single separator date format.

The problem is not easy if you want to really validate a date, check this forum thread.

For an example or a better way, in C#, check this link

If you are using another platform/language, let us know

Perl 6 version

rx{
  ^

  $<month> = (\d ** 1..2)
  { $<month> <= 12 or fail }

  '/'

  $<day> = (\d ** 1..2)
  {
    given( +$<month> ){
      when 1|3|5|7|8|10|12 {
        $<day> <= 31 or fail
      }
      when 4|6|9|11 {
        $<day> <= 30 or fail
      }
      when 2 {
        $<day> <= 29 or fail
      }
      default { fail }
    }
  }

  '/'

  $<year> = (\d ** 4)

  $
}

After you use this to check the input the values are available in $/ or individually as $<month>, $<day>, $<year>. ( those are just syntax for accessing values in $/ )

No attempt has been made to check the year, or that it doesn't match the 29th of Feburary on non leap years.

If you're going to insist on doing this with a regular expression, I'd recommend something like:

( (0?1|0?3| <...> |10|11|12) / (0?1| <...> |30|31) |
  0?2 / (0?1| <...> |28|29) ) 
/ (19|20)[0-9]{2}

This might make it possible to read and understand.

/(([1-9]{1}|0[1-9]|1[0-2])\/(0[1-9]|[1-9]{1}|[12]\d|3[01])\/[12]\d{3})/

This would validate for following -

• Single and 2 digit day with range from 1 to 31. Eg, 1, 01, 11, 31.
• Single and 2 digit month with range from 1 to 12. Eg. 1, 01, 12.
• 4 digit year. Eg. 2021, 1980.

A slightly different approach that may or may not be useful for you.

I'm in php.

The project this relates to will never have a date prior to the 1st of January 2008. So, I take the 'date' inputed and use strtotime(). If the answer is >= 1199167200 then I have a date that is useful to me. If something that doesn't look like a date is entered -1 is returned. If null is entered it does return today's date number so you do need a check for a non-null entry first.

Works for my situation, perhaps yours too?