Lua is awesome. It is a clean, small language with a clean, fast implementation and a simple C API and good documentation. It has awesome Scheme features, like tail recursion and anonymous functions and proper closures, but its implementation is nicer than most Schemes (although it is small and doesn't have its own GUI). Plus it has coroutines. You can do OO stuff with metatables and environments pretty well too.
The entire language, API, and standard library is documented on this page:
http://www.lua.org/manual/5.1/manual.html
>>5
I think automatic conversion is only if you try to do operations that wouldn't make sense otherwise, like +,-,*,/ on strings. So if you don't act like it's there it should never come up. A string is never == to a number.
>>6
I've never used the tiny built-in regex feature. I'm sure there's a binding to PCRE somewhere, but PCRE is probably as big as Lua itself so they didn't bundle it.
>>9
From looking at their docs, it is essentially a copy of Lua with a few syntax changes and some OO extensions. They are even using Lua source code. The API is identical. They have copied Lua so much I think they are misrepresenting themselves by not making it clear on their site. I would use the latest Lua, not a copy of an older version.