At some stage, I extract some text with Action>Extract and Converters>Advanced extract. My extract result is like this:
$1 is a day of the month
$2 is a month

I want to generate a date like "10-22". So my output expression is $2-$1.
But this gives

10-8
10-9
10-10
10-11
...

In fact, I prefer to format all numbers to 2 digits, something like:
10-08
10-09
10-10
10-11
...
What operator can I use to turn all extracted numbers to 2 digits?

In Expression > Function, there is a "Convert year to 4 digits", but what I need is "Convert month / day to 2 digits" or a generic function that can do this. Can someone help?