Scalar joins should use -On and -Equals for naming

[iRon7]

$a = 'a1', 'a2', 'a3' |Foreach-Object { [PSCustomObject]@{ Name = $_ } } |
 Add-Member -MemberType ScriptProperty -Name Letters -Value { $This.Name -Replace '[^a-z]' } -PassThru |
 Add-Member -MemberType ScriptProperty -Name Numbers -Value { $This.Name -Replace '[^0-9]' } -PassThru

$a |Join 0,2,4 -on Numbers -Eq '<Value>'

Wishful thinking:

$a |Join 0,2,4 -on Numbers

[iRon7]

Implemented in version 3.8.3:

$a | Join 0,2,4 -on Numbers

Name Letters Numbers
---- ------- -------
a2   a       2

1,2,3 | Join 2,3,4 -On Left -eq Right

Left Right
---- -----
   2     2
   3     3