First, you don't need to use pattern matching to extract the date. Workflow's token editor has formatting rules specifically for date/time values so you can extract certain pieces (see here).

If you want the numeric value of the previous month, use a Date Token Calculator, set to "Specific Date"

The formatting rules will automatically extract the month, day, and year from the default "Date" token to provide an example.

Set the day to 1, since you only want the month value anyway, and set it to Subtract 1 Month.

The end result will be a date set to the first day of the previous month (even if it is in the previous year) and you can use the formatting again to extract just the month.