strange copyFile

[oyster]

I am using Nim Compiler Version 0.17.2 (2017-09-10) [Windows: amd64] with 
MSYS2+MingW64 on Win7 64 bits.

I know test:test.txt is not a valid DOS/Windows filename. So how to detect it 
in code? Thanks
    
    
    import os
    try:
        copyFile("test.txt", "test:test.txt")
    except:
        echo "not support"          # this will not happend
    
    if existsFile("test:test.txt"):
        echo "ok"                    # this happends because a zero-byte test 
file is created and treated
    else:
        echo "no"
    
    

the output is ok and a zero-byte test file is created