Lua: String Pattern Matching/Finding/Counting/Replacing

So this is a robust problem. I have a function which accepts 2 args (string_name, macros). Here it is so I can further explain.

function ParseStrings(string_name, macros)
    return my_table[string_name]
    -- All it does it returns the string_name's value
end

The problem is that the second arg is a table, and if it's a table then in the string there are going to be various parts that have the format "String stuff $MACRO_KEY; more string text" and the content between the $ and ; is the key to look up in the macro table sent with it. Now anytime a value like that appears in the string there will always be a second arg that's a table, so no problems their. I need to be able to count up the number of instances of macros in a string and then replace each macro component with it's respective macros' table value. So here's how the func is called in this instance...

local my_table = {
    my_string = "My string content $MACRO_COMPONENT; more string stuff $MACRO_COMPONENT_SUB;$MACRO_COMPONENT_ALT;"
}
local macro = {
    MACRO_COMPONENT = "F",
    MACRO_COMPONENT_SUB = "Random Text",
    MACRO_COMPONENT_ALT = "14598"
}

function ParseStrings(string_name, macros)
    return my_table[string_name]
    -- All it does it returns the string_name's value
end

ParseStrings("my_string", macro)

So I am thinking string.gsub(my_table[my_string]:match("%b$;"):sub(2,my_table[my_string]:match("%b$;"):len() - 1), but this is a long and overtly complex answer (AFAIK) and from my tests it only does 1 replacement (because the pattern is only found once) and that's doesn't work well if there are multiple instances in the string. So ideas?