Can my Regex be improved?

Question

Yes, another Regex question. You're welcome ;-P

This is the first time I've written my own regex for some simple string validation in C#. I think I've got it working but as a learning exercise I was wondering if it could be improved and whether I have made any mistakes.

The strings will all look something like this:

T20160307.0001

Rules:

• Begin with the letter T.
• Date in the format YYYYMMDD.
• A full stop.
• Last 4 characters are always numeric. There should be exactly 4.

Here is my regex (fiddle):

^(?i)[T]20[0-9]{2}[0-1][0-9][0-3][0-9].\d{4}$

• ^ Assert the start of the string.
• (?i)[T] Check that we have a letter T, case insensitive.
• 20 YYYY begins with 20 (I'll be dead by 2100 so I don't care about anything further :-P)
• [0-9]{2} Any number between 0 and 99 for second part of YYYY.
• [0-1][0-9] 0 or 1 for first part of month, 0-9 for second part of month.
• [0-3][0-9] 0-3 for first part of day, 0-9 for second part of day.
• . Full stop.
• \d{4} 4 numerical characters.
• $ Assert end of string.

One pitfall I can already see is date validation. 20161935 (the 35th day of the 19th month) is considered valid. I've read some / other / posts about achieving this which I believe match on number ranges but I was unable to understand the format.

I would accept an answer that simply solved the date issue if someone would be kind enough to ELI5 how this works, but other improvements would be a welcome bonus.

Edit: To avoid further confusion I should state that I know about DateTime.TryParse etc. As mentioned I'm using this as an opportunity to learn Regex and felt this was a good starting point. Sorry to anyone who's time I wasted, I should have made this clear in the original post.

Answer 1

The things you can do are:

• avoid the \d character class that matches all the unicode digits (since you only need the ascii digits)
• instead of [0-1] you can write [01]
• escape the dot to figure a literal dot (and not any characters)
• no need to put T in a character class if it is the only character
• eventually you can remove the inline modifier and use [Tt] in place of T

^(?i)T20[0-9]{2}[01][0-9][0-3][0-9]\.[0-9]{4}$

or

^[Tt]20[0-9]{2}[01][0-9][0-3][0-9]\.[0-9]{4}$

Other thing: do you really need to add extra checking for the date since you can't really test if the date is well formatted? (Think a minute about leap years) So why not:

^(?i)T(20[0-9]{6})\.[0-9]{4}$

and if you want to know if the date really exists, capture it and test it with DateTime.TryParse method.