There may be better ways but one approach I've used is to break the date down into a day, month and year and then recombine them. Within the pattern match activity, create 3 tokens for day, month and year with the following regex:

Day - (\d\d)[1/]
Month - \d\d[1/](\d\d)
Year - \d\d[1/]\d\d[1/](\d\d\d\d)

(I know you can use "\d{2}" or "\d{4}" etc instead but I think it can be hard to read!)

In each case it extracts the relevant  portion of the date regardless of whether it reads a "1" or "/". You could then recombine the tokens into the field or another token.