Replace value containing alphabets to blank values

Hi,

How can I replace all the rows in a column containing a combination of alphabets and numbers with Blank values?

Goal is to -

old        new

111  -    111

222  -     222

333   -   333

a44  - blank

555 - 555

r777 - blank

Then convert the column to an integer column

I want to add a new column to an existing table in the power query

Background

"Old" column is a text column

Thank you