RegEx to validate Date Format Masks

Question

I'm looking to write a RegEx to validate user inputted date formats like "DD/MM/YYYY" (NOT date validation)

I'm new to RegEx, but so far i have the following:

(D{2}|M{2}|Y{2,4})[-|/]?(D{2}|M{2})[-|/]?(D{2}|M{2}|Y{2,4})

This matches the following date formats:

• DD/MM/YYYY
• MM/DD/YYYY
• DD-MM-YYYY
• MM-DD-YYYY
• DDMMYYYY
• MMDDYYYY
• YYYYMMDD
• YYYYDDMM
• YYYY/MM/DD
• YYYY/DD/MM
• YYYY-MM-DD
• YYYY-DD-MM
• DD/MM/YY
• MM/DD/YY
• DD-MM-YY
• MM-DD-YY
• DDMMYY
• MMDDYY
• YYMMDD
• YYDDMM
• YY/MM/DD
• YY/DD/MM
• YY-MM-DD
• YY-DD-MM

But unfortunately, also matches the following:

• DDMMDD
• YYMMYYYY
• MMMMMM

Is there any way to only match the formats specified in the first list? The RegEx will be being used in VBA form validation.

Answer 1

You can use negative backreferences to ensure that you don't repeat any previous entries:

^(D{2}|M{2}|(YY){1,2})[-|\/]?(?!\1)(D{2}|M{2})[-|\/]?((?!\1|\3)(D{2}|M{2}|(YY){1,2}))$

The negative backreference (?!\1) & (?!\1|\3) prevent repetition of the entries.

Also, Y{2,4} matches 'YYY' so I changed that to (YY){1,2}.

Answer 2

Not tested for VBA but how about this:

^(((MM(?<s>[-/])?DD|DD(?<s>[-/])?MM)(?(s)[-/])?(YY|YYYY))|((YY|YYYY)(?<s>[-/])?(MM(?(s)[-/])DD|DD(?(s)[-/])MM)))$

The thought process was that there are four possible combinations ignoring separators and number of digits for a year. These are:

D M Y

M D Y

Y M D

Y D M

So this can be split into two groups - year at the start or year at the end:

^
(((MM(?<s>[-/])?DD|DD(?<s>[-/])?MM)(?(s)[-/])?(YY|YYYY))
|
((YY|YYYY)(?<s>[-/])?(MM(?(s)[-/])DD|DD(?(s)[-/])MM)))
$

I also add a named capture group to the separator just to ensure that at least two of the same character exist if present.