Loading PetitParser from String

Hi,
I'd like to load PetitParser from string, but I cannot find any solution on the internet.
I'm trying to do something like this...

strParser := '(#letter asParser / #digit asParser)'.

parser1 := strParser.
parser2 := (#letter asParser / #digit asParser)

How can I convert string strParser in order to behave like parser2?