Better regular expression syntax checking

And a limitation elaboration

otto.go

@@ -86,9 +86,17 @@ Caveat Emptor
 * For example, "use strict" will parse, but does nothing.
 * Error reporting needs to be improved.
 * Number.prototype.{toFixed,toExponential,toPrecision} are missing.
-* Does not support the (?!) or (?=) regular expression syntax (because Go does not).
+* Does not support the (?!) or (?=) regular expression syntax (because Go does not)
 * Really, error reporting could use some improvement.
 
+Regular Expression Syntax
+
+Go translates JavaScript-style regular expressions into something that is regexp package compatible.
+
+Unfortunately, JavaScript has positive and negative lookahead, which is not supported by Go's re2-like engine: https://code.google.com/p/re2/wiki/Syntax
+
+A brief discussion of this limitation: "Regexp (?!re)" https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/7qgSDWPIh_E
+
 */
 package otto