Regex for matching files that don't have type extensions

[Jovan Trujillo]

Hi All,
    I thought I could use a simple regex to match files like this:

    1207003PE_GM_09TNPLM2

and ignore files with extensions like this:

1207003PE_GM_09TNPLM2.csv

I originally though m/[A-Za-z0-9\_]+/ would work, but it captures both
strings.
So then I tried m/[A-Za-z0-9\_]+(?!\.)/ but I still get both strings
captured.

What am I doing wrong?

Thank you,
Jovan