Fix #600. Add regression test

tests/all.test

@@ -859,6 +859,10 @@ capture("(?<a>[a-z]+)-(?<n>[0-9]+)")
 ["a,b, c, d, e,f", ", a,b, c, d, e,f, "]
 ["a,b:c, d, e,f",":a,b, c, d, e,f, "]
 
+sub("^(?<head>.)"; "Head=\(.head) Tail=")
+"abcdef"
+"Head=a Tail=bcdef"
+
 [.[] | gsub(", "; ":")]
 ["a,b, c, d, e,f",", a,b, c, d, e,f, "]
 ["a,b:c:d:e,f",":a,b:c:d:e,f:"]