I am fairly new with Perl, and even more so with regex.
Have been trying to match the following, but without success:

* First, 3 to 4 letters (ideally case insensitive)
* Optionally a space (but not mandatory)
* Then, also optionally a known big-case letter (`M`) and a number out of `1,2,3`

An example of a valid string would be `abc`, but also `DEFG M2`. Invalid would be `mem M`, for example

What I have so far is:

```
$myExpr ~= m/^[a-z,A-z]{3,4}M[1,2,3]$/i

```

Not sure how to make the `M` and numbers optional