BUG #16512: Character considered as a number by regex but can not convert to numeric

The following bug has been logged on the website:

Bug reference: 16512
Logged by: tester375
Email address: oytf(at)drivetagdev(dot)com
PostgreSQL version: 12.3
Operating system: Windows 10
Description:

When installed Postgresql 12.3 for testing purposes we have discovered
following possible bug:
Character "８" (different from standard number 8) is treated as a number by
regular expression
~'\d+'
but can not be converted to numeric by
::numeric

How to reproduce the bug:
Command:
SELECT '８' ~ '\d+'
returns "false" for PostgreSQL 10.4 on both Windows 10 and CentOS Linux 7
returns "false" for PostgreSQL 11.5 on CentOS Linux 7
returns "true" for PostgreSQL 12.3 on Windows 10 - but in this case,
character still can not be converted to numeric.

This behavior causes obvious problems when using data type conversion
like:
CASE WHEN column1 ~ '\d+' THEN column1::numeric AS column2
Error: '８' can not be cast to numeric