Add some backtracking control

Not terribly necessary here, because inputs are short, but it's a
good practice. I wish there was a way to just disable backtracking for
the entire regex since this kind of pattern doesn't need any of it.

revbank

@@ -36,10 +36,10 @@ sub split_input($input) {
 
     while (
         $input =~ m[
-            \G \s*
-            (?| (') ( (?: \\. | [^\\']  )* ) '   (?=\s|;|$)
-              | (") ( (?: \\. | [^\\"]  )* ) "   (?=\s|;|$)
-              | ()  ( (?: \\. | [^\\;'"\s] )+ )  (?=\s|;|$)
+            \G \s*+
+            (?| (') ( (?: \\. | [^\\']  )*+ ) '   (?=\s|;|$)
+              | (") ( (?: \\. | [^\\"]  )*+ ) "   (?=\s|;|$)
+              | ()  ( (?: \\. | [^\\;'"\s] )++ )  (?=\s|;|$)
               | ()  (;)
             )
         ]xg