Document inability to match lone low surrogates accurately

Ref. mathiasbynens/regexpu#17.
Closes #28.

Author: mathiasbynens

README.md

@@ -1,4 +1,4 @@
-# Regenerate [![Build status](https://travis-ci.org/mathiasbynens/regenerate.svg?branch=master)](https://travis-ci.org/mathiasbynens/regenerate) [![Code coverage status](http://img.shields.io/coveralls/mathiasbynens/regenerate/master.svg)](https://coveralls.io/r/mathiasbynens/regenerate) [![Dependency status](https://gemnasium.com/mathiasbynens/regenerate.svg)](https://gemnasium.com/mathiasbynens/regenerate)
+# Regenerate [![Build status](https://travis-ci.org/mathiasbynens/regenerate.svg?branch=master)](https://travis-ci.org/mathiasbynens/regenerate) [![Code coverage status](https://coveralls.io/repos/mathiasbynens/regenerate/badge.svg)](https://coveralls.io/r/mathiasbynens/regenerate) [![Dependency status](https://gemnasium.com/mathiasbynens/regenerate.svg)](https://gemnasium.com/mathiasbynens/regenerate)
 
 _Regenerate_ is a Unicode-aware regex generator for JavaScript. It allows you to easily generate JavaScript-compatible regular expressions based on a given set of Unicode symbols or code points. (This is trickier than you might think, because of [how JavaScript deals with astral symbols](https://mathiasbynens.be/notes/javascript-unicode).)
 
@@ -243,6 +243,8 @@ lowSurrogates.toString({ 'bmpOnly': true });
 // → '[\\uDC00-\\uDFFF]'
 ```
 
+Note that lone low surrogates cannot be matched accurately using regular expressions in JavaScript. Regenerate’s output makes a best-effort approach but [there can be false negatives in this regard](https://github.com/mathiasbynens/regenerate/issues/28#issuecomment-72224808).
+
 ### `regenerate.prototype.toRegExp(flags = '')`
 
 Returns a regular expression that matches all the symbols mapped to the code points within the set. Optionally, you can pass [flags](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#Parameters) to be added to the regular expression.

regenerate.js

@@ -8,7 +8,7 @@
 	var freeModule = typeof module == 'object' && module &&
 		module.exports == freeExports && module;
 
-	// Detect free variable `global`, from Node.js or Browserified code,
+	// Detect free variable `global`, from Node.js/io.js or Browserified code,
 	// and use it as `root`.
 	var freeGlobal = typeof global == 'object' && global;
 	if (freeGlobal.global === freeGlobal || freeGlobal.window === freeGlobal) {
@@ -999,7 +999,9 @@
 		}
 		if (hasLoneLowSurrogates) {
 			result.push(
-				// Make sure the low surrogates aren’t part of a surrogate pair.
+				// It is not possible to accurately assert the low surrogates aren’t
+				// part of a surrogate pair, since JavaScript regular expressions do
+				// not support lookbehind.
 				'(?:[^\\uD800-\\uDBFF]|^)' +
 				createBMPCharacterClasses(loneLowSurrogates)
 			);