thread. been stuck on a programing problem in last 2 days. recently i tried to experiment of using google code prettify instead of span tags to color code in html. see this video
https://www.youtube.com/watch?v=ml7V_OnBAOk
but now i realized a big subtle problem. not sure am going to use it anymore.

it's logically impossible to determine if a text is ampersand encoded or decoded.
you might say, if the text contains “＆amp;”, then it is encoded.
wrong!
e.g. if we have the code
str.replace("＆", "＆amp;")
that's actually not encoded.

Encoded version would be:
str.replace("＆amp;", "＆amp;amp;")

the fact that there is no simple logical way to determine
if text is ampersand encoded or decoded, it's a problem.
because if ＆ gets encoded twice, the code is screwed. won't run correctly anymore.

why would it happen twice?
because when you put lots source code in html on the web, and you constantly edit
it, i.e. decode edit encode cycle,
it's very easy to make mistakes of encoding twice or decoding twice overtime.

note in html4 and #xml, all ＆ ≺ ≻ must be encoded.
in #html5, the rule is relaxed. if such char are surrounded by space, then it's ok, no need encoding to entities. (the rule is more complex than this)

but why not just leave ≺ ≻ ＆ as is without encoding at all?
That's a problem, because suppose your code process html tags.
Code that process html are actually quite common.
so if are writing programing tutorial or blog, you may have lots code snippets containing lots html tags in string or regex.

so, at this point, the advantage of using JavaScript to syntax color your code in your blog, is half gone. you still need to ampersand encode your code. and if it's web dev code (lots ≺ ≻ ＆), your code is unreadable without decoding. Same as using span tags to syntax color.

using span tags to syntax color code on the web, now has a advantage. i.e. it's easy to determine the encoded/decoded state. (by simply checking existence of ≺span≻) So, not prone to error of encode/decode twice.

this problem, is a general problem of nesting. It happens with string escape sequence in langs. e.g. have you ever tried to grep a string that is from a code snippet's regex pattern? basically, it is impossible to figure out the backslash escape sequence.

That's why, #perl #php #ruby have heredoc, #python has triple quote. #golang has ``.