Is the length of the segment you want to extract always that same length/format? ( 1:20 & 1:80 are both 4 characters )
If so, you could get that more simply like this:
Otherwise, the next thing I’d ask is, are you copy-pasting your regex into the formula field? If so, check that your quotation marks are not “curly” quotes, but rather “straight” quotes. Airtable’s formula field doesn’t like curly quotes and will error your formula because of them. Copy-pasting a string with quotation marks into the formula field editor is usually going to paste them as curly quotes, so just delete and re-type any quotation marks and see if that makes a difference.