Italicize script explained

-- italicizes or unitalicizes text [based on style and tags]
-- supports multiple \i tags in a line by switching 0 to 1 and vice versa

script_name="Italicize"
script_description="Italicizes or unitalicizes text"
script_author="unanimated"
script_version="1.6"

-- let's say for the sake of clarity that we're using this on a line with style "sign"

function italicize(subs, sel)
	for z, i in ipairs(sel) do
		local l=subs[i]
		text=l.text
		
		-- this runs function "stylechk" that you see further below
		-- you pass "l.style" to the function, ie. the name of the style this line is using, in our case "sign"
		styleref=stylechk(subs,l.style)
		
		-- styleref is now kind of like "line", but for the style
		-- just like you have line.text, you have styleref.name, which is now "sign", or styleref.italic, which is true or false
		
		-- we'll assign the italic element to "si" (style italics)
		-- it is either true for italics, or false for regular
		local si=styleref.italic
		
		-- now we want to transform true/false into 1/0, creating variable "it"
		-- the values are 1 for non-italics and 0 for italics, because it's the switched values i want to use
		if si==false then it="1" else it="0" end
		
		-- this is to deal with \i without a number, which resets \i to the style value; i need it to have a number
		-- "it" is the opposite of the style, so "1-it" is the style value - 1-0=1; 1-1=0
		-- in other words, if style is not italics, we change \i to \i0
		text=text:gsub("\\i([\\}])","\\i".. 1-it.."%1")
		
		    -- now i have to consider 2 options - either there's italics tag at the beginning, or not
		    if text:match("^{[^}]*\\i%d[^}]*}") then
			
			-- if there is, then we change it, and all subsequent \i tags, to the opposite value
			-- i take the number after \i, and change it to 1-number, thus turning 1 to 0 and vice versa
			text=text:gsub("\\i(%d)", function(num) return "\\i".. 1-num end)
		    
		    -- this is when there's no \i tag in the first block of tags
		    else
			
			-- the line may have italics somewehre in the middle, so we capture the value
			if text:match("\\i([01])") then italix=text:match("\\i([01])") end
			
			-- this may get a bit confusing. we're dealing with a line that has italics in the middle.
			-- so style value is "not_it" (as in opposite of "it"), value we want for the start of line is "it",
			-- and value we want for the next \i tag, the one we captured, is again "not_it"
			-- so logically that value, italix, should be "it"
			-- if that's the case, all \i tags get switched
			-- the reason i check for this is that i assume you might be an idiot and have the wrong tag
			-- in which case i won't change it because it will thus become right
			if italix==it then text=text:gsub("\\i(%d)", function(num) return "\\i".. 1-num end) end
			
			-- we've changed all the existing tags (if there were any), so now we add a tag at the beginning, with the value "it"
			text="{\\i"..it.."}"..text
			
			-- and join it with other tags at the beginning, if they're there
			text=text:gsub("{\\i(%d)}({\\[^}]*)}","%2\\i%1}")
		    end
		l.text=text
		subs[i]=l
	end
	aegisub.set_undo_point(script_name)
	return sel
end

-- function to read the style of the current line
-- it got called with styleref=stylechk(subs,l.style), therefore given 2 objects - the whole subtitle object, and a style
-- here we have stylechk(subs,stylename) - i use different names here, but that doesn't matter - subs stays, l.style becomes stylename
-- l.style was "sign", so inside this function, stylename is now "sign"

function stylechk(subs,stylename)
    for i=1, #subs do
        
	-- we're going through styles, looking for the one called "sign"
	if subs[i].class=="style" then
	    
	    -- this is pretty much the same thing you do with "line"
	    style=subs[i]
	    
	    -- style.name is the name of the style. with every "i" we're going through another style.
	    -- stylename will match style.name when we get to the "sign" style
	    if stylename==style.name then
		
		-- now we have style "sign", and we copy it to a variable "styleref"
		styleref=style
	    end
	end
    end
    
    -- this is what we're returning as the result of this function, so when we used styleref=stylechk(subs,l.style) above,
    -- we're passing this styleref, which is subs[i] for the "sign" style, to the styleref in the main function
    -- so styleref is now basically this "Style: sign,Arial,40,&H00FFFFFF,&H000000FF,&H00000000,&H00000000,0,0,0,0,100,100,0,0,1,2,0,5,10,10,10,1"
    return styleref
end

aegisub.register_macro(script_name, script_description, italicize)