Using re to find only uppercase letters

[ranbarr]

Is there something that isn't shown? This shouldn't have matched: ChrX (your regex only looks for lowercase "chr")
I'm assuming that's the same reason the lowercase "t" was matched.

You are right - its actually like that:

(r"^[Cc]hr(?:0?[1-9]|[1-9][0-9]|[MXY])\t0*[1-9][0-9]*\t[^\t]*\t[ATGC]{2}