Backslash in property gets converted to forward slash on Linux

[ermshiperete]

When a property contains backslashs in a project file, it will be converted to forward slashs when running with msbuild on Linux.

While this would be fine for paths (although one could just as well use forward slash in the project file since that works on both Windows and Linux), it causes problems when trying to set a regular expression in a property which requires a backslash to escape the next character.

Steps to reproduce

Project file bug.proj:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
	<PropertyGroup>
		<Regex>\(([^]]+)\)</Regex>
	</PropertyGroup>

	<Target Name="Build">
		<Message Text="Regex=$(Regex)"/>
	</Target>
</Project>

Command line

msbuild /t:Build bug.proj

Expected behavior

Outputs the line

Regex=\(([^]]+)\)

Actual behavior

Outputs the line

Regex=/(([^]]+)/)

Environment data

msbuild /version output: 15.6.0.0

Also happens with xbuild from older Mono versions.

When building with dotnet build -v d bug.proj the output is as expected.

OS Info: Ubuntu 16.04.4 LTS