The answer is to use web_reg_save_param_regexp. It's a regular expression version of web_reg_save_param which allows for dynamic boundaries using regular expressions. You'll have to take a half an hour to 1 hour to learn the basics of regular expressions. There are tutorials online.
I believe for yours this would work
web_reg_save_param_regexp(
"ParamName=parameter_name",
"RegExp=ACHP[0-9]*=(.)*;",
LAST );
But I haven't tested it so it's probably not exaclty right. The main point though is the '[0-9]*' which means any number '[0-9]' any amount of times '*'. So that will give you the dynamic that you need.