R12.2.4 ach payments special characters to replace it with null

Please suggest how to use regrexp_replace in the seeded templates to replace all the possible special characters in the template file Nacha format. only allowed is replace function to replace special character but if there is a list of all special characters can we use valueset to store special characters and write a custom function to filter those special characters with null . But again How can Nacha format seeded template recognize the custom function. if not please suggest if there are any predefined or std functions to be used?