Continuing my investigation of tab completion quoting/escaping failures:

Steps to reproduce

$a = @{ '3hello' = 'a'}

$a.<TAB> # select `3hello` property from list, then execute.

Expected behavior

the finished completion to be:

$a.'3hello'

Actual behavior

$a.3hello

Which causes an error 'missing property name'.

Environment data

Name                           Value
----                           -----
PSVersion                      6.2.2
PSEdition                      Core
GitCommitId                    6.2.2
OS                             Microsoft Windows 10.0.18941
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Additionally

Tab completion after $a.' switches to file name completion.

The code that handles this appears to be at:

The field s_charactersRequiringQuotes is used mostly for variable name checks, but is also inadequate for that purpose (see #10006), but fails here because it doesn't handle the condition of numeric digits in the first character require quoting.

In PowerShell code, I would use REGEX: ^[\p{L}_][\w]*$. If this regex fails on a non-empty name, the property name must be quoted. However there is probably a quicker method in C# using the [CharExtensions] class using IsIdentifierStart() (first character) and IsIdentifierFollow() (subsequent characters), and quote the property if the response is FALSE.