Find Regex mismatch part in a string using vb.net

Question

I had a regex expression

^\d{9}_[a-zA-Z]{1}_(0[1-9]|1[0-2]).(0[1-9]|[1-2][0-9]|3[0-1]).[0-9]{4}_\d*_[0-9a-zA-Z]*_[0-9a-zA-Z]*

and string that match regex expression

000066874_A_12.31.2014_001_2Q_ICAN14

if user by mistake enters the string other than above format like

000066874_12.31.14_001_2Q_ICAN14

I need to find out in which part of my regex got failed. I tried using Regex.Matches and Regex.Match but using this I couldn't find in which part my string got miss matched with my Regex expression. I am using vb.net

I don't think regex is particulary suited for this. It's probably possible, i.e. writing every part with lookarounds and capturing them. Hence, the group that got failed will be empty or not returned. — Andris Leduskrasts
– Andris Leduskrasts, Commented Jun 23, 2015 at 9:49

Andris Leduskrasts · Accepted Answer · 2015-06-23 14:44:57Z

This is very complicated to do with regex. I managed to make this regex, but you still have to check the capture groups after that.

^(?:(?:(\d{9})|.*?)_)?(?:(?:([a-zA-Z]{1})|.*?)_)?(?:(?:((?:0[1-9]|1[0-2]).(?:0[1-9]|[1-2][0-9]|3[0-1]).[0-9]{4})|.*?)_)?(?:(?:(\d*)|.*?)_)?(?:(?:([0-9a-zA-Z]*)|.*?)_)?(?:([0-9a-zA-Z]*)|.*?)$ will work if you, as seen in demo: https://regex101.com/r/aJ1wG1/2

Each part before an underline is a capture group, if a capture group is not there, there's an error in it. As you can see in the example, $3 is not present in 1st example, hence, a mistake in date is there. In second example, the $2 is not present, hence $2 onward are not there. 3rd example is correct and all 6 caputre groups are there.

When regexes get this massive, it's a sign that probably a different method should be used to solve the problem, but this might work for you with some additional code for group result checks.

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