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Description

When trying to use RDataFrame Alias using an alias that contains a . in the string the following error message is thrown:
RDataFrame::Alias: cannot define variation "reco.el_e_NOSYS_B". Not a valid C++ variable name.

This is because the function CheckValidCppVarName() checks if the new alias characters are either a letter, an underscore or a number. Since the . is not identified as a neither of these cases the error is thrown.

This was found when matching 2 trees and trying to alias a systematic variation column of the matched tree.
For example:
Original column in the matched tree: truth.variation_customName
Desired alias: truth.variation_commonName

Reproducer

The minimal reproducer can be found here in the following cern box link: https://cernbox.cern.ch/s/c12lOxGVx0vahWy

ROOT version

ROOT Version: 6.40.00
Built for linuxx8664gcc on May 26 2026, 15:56:51
From tags/v6-40-00@v6-40-00

Installation method

Build from source

Operating system

Ubuntu 25.10

Additional context

No response