Participant can contain an "Unique ID" to store geography specific identifiers. For example, SSN in U.S, Medicare number in AU etc. However, different countries have different formats.
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E.g. The regex pattern for US SSN format: ^(?!000|666)[0-8][0-9]{2}-(?!00)[0-9]{2}-(?!0000)[0-9]{4}$
Learn how to create regex for your pattern: https://regexone.com/
Detailed explanation of the default SSN format 'AAA-GG-SSSS'
The first three digits were historically (prior to mid-2011) assigned by geographical region, and are thus called the area number. The area number cannot be 000, 666, or between 900 and 999.
Digits four and five are called the group number and range from 01 to 99.
The last four digits are serial numbers from 0001 to 9999.