So, I need to reset the pos before each pass. 3-digit APC codes are just one of the passes.

Not quite.

$_ = 'a1!b2.c3?';
print $1 while /([abc])/g;  # abc
print $1 while /([123])/g;  # 123
print $1 while /([!.?])/g;  # !.?
print "\n";
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Note the lack of the c modifier. That does exactly the opposite of what you want. It's purpose is to prevent pos from getting reset.