Regex Matching even number of digits in a number with minimum length

Question

I'm trying to form a regex that would work both the below mentioned examples:

Example 1. 202101310000-daily
Example 2. my_merchant_df_20210129

The conditions are :

1. string contains even no. of digits (6 digits, 8 digits, 10 digits, so on).
2. String contains a minimum of 6 digits in a group.
3. the position of the digits grouping in a string doesn't matter.

I'm regex: [0-9]+ is working for 202101310000-daily, but I'm not able to create the regex that would satisfy both examples.

Basically for my use-case, there are groups of files that should have the date format of yyyymd or yyyymmdd or yyyymmddHHMMSS. I need to filter out those files which have that kind of format anywhere in the file name using regex.

Answer 1

You can repeat 3 or more sets of 2 digits, and assert no digits to the left and right of the match using negative lookarounds.

(?<!\d)(?:\d\d){3,}(?!\d)

• (?<!\d) Assert no digit directly to the left
• (?:\d\d){3,} Repeat matching 2 digits 3 or more times to match 6 digits, 8 digits ect..
• (?!\d) Assert no digit directly to the right

Regex demo

In Java

String regex = "(?<!\\d)(?:\\d\\d){3,}(?!\\d)";

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To make it a bit more specific, you could either start by matching 19 or 20 for the year and repeat 2 or more times sets of 2 digits or you could use an alternation to match all the exact patterns.

 (?<!\d)(?:19|20)(?:\d{2}){2,}(?!\d)

Regex demo