Cleanup more (?:) from patterns

Following up on slevithan#164, this
change prevents a `(?:)` from being inserted in the following places:

* At the beginning of a non-capturing group (the end is already handled)
* Before or after a `|`
* At the beginning or the end of the pattern

This solution isn't as complete as the one suggested in
slevithan#179, but it's a decent
stopgap.

src/xregexp.js

@@ -238,6 +238,14 @@ function getContextualTokenSeparator(match, scope, flags) {
         // No need to separate tokens if at the beginning or end of a group
         match.input[match.index - 1] === '(' ||
         match.input[match.index + match[0].length] === ')' ||
+        // No need to separate tokens if at the beginning of a non-capturing group
+        match.input.slice(match.index - 3, 3) === '(?:' ||
+        // No need to separate tokens if before or after a `|`
+        match.input[match.index - 1] === '|' ||
+        match.input[match.index + match[0].length] === '|' ||
+        // No need to separate tokens if at the beginning or end of the pattern
+        match.input[match.index - 1] === undefined ||
+        match.input[match.index + match[0].length] === undefined ||
         // Avoid separating tokens when the following token is a quantifier
         isQuantifierNext(match.input, match.index + match[0].length, flags)
     ) {

tests/spec/s-xregexp.js

@@ -800,6 +800,21 @@ describe('XRegExp()', function() {
                 expect(XRegExp('(#\n.#\n)', 'x').source).toBe('(.)');
             });
 
+            it('should not add atom separator (?:) at the beginning or end of non-capturing groups in simple cases', function() {
+                expect(XRegExp('(?: . )', 'x').source).toBe('(?:.)');
+                expect(XRegExp('(?:#\n.#\n)', 'x').source).toBe('(?:.)');
+            });
+
+            it('should not add atom separator (?:) at the beginning or end of the pattern in simple cases', function() {
+                expect(XRegExp(' ( . ) ', 'x').source).toBe('(.)');
+                expect(XRegExp(' (#\n.#\n) ', 'x').source).toBe('(.)');
+            });
+
+            it('should not add atom separator (?:) around | in simple cases', function() {
+                expect(XRegExp('( a | b )', 'x').source).toBe('(a|b)');
+                expect(XRegExp('(#\na#\n|#\nb#\n)', 'x').source).toBe('(a|b)');
+            });
+
             it('should allow whitespace between ( and ? for special groups', function() {
                 expect(XRegExp('( ?:)', 'x').source).toBe('(?:)');
                 expect(XRegExp('( ?=)', 'x').source).toBe('(?=)');