allow simple expansion variables to start with a number

Added tests for numbered expansion variables.
Fixed a formatting verb for `p.op` rune.

lexer.go

@@ -116,7 +116,7 @@ func (p itemParamOp) Eval(mapping Getter, stream chan item) (string, error) {
 		return "", errors.New(val)
 	}
 
-	return "", fmt.Errorf("unexpected op: %s", p.op)
+	return "", fmt.Errorf("unexpected op: %q", p.op)
 }
 
 // Returns the evaluation of the stream items against the given mapping.
@@ -286,7 +286,7 @@ func lexStartExpansion(l *lexer) stateFn {
 		l.ignore()
 		l.depth++
 		return lexBracketName
-	case isAlpha(c):
+	case isAlphaNum(c):
 		return lexSimpleName
 	}
 	return nil // FIXME

posix_test.go

@@ -48,6 +48,7 @@ var paramtests = []struct {
 	// Names, no brackets
 	{"$set", "yes", ""},
 	{"$set$set2", "yesyes-two", ""},
+	{"/home/$set/$set2", "/home/yes/yes-two", ""},
 
 	// Default
 	{"${set:-word}", "yes", ""},
@@ -211,3 +212,28 @@ func TestExpand_assign(t *testing.T) {
 	equals(t, "word", x)
 	equals(t, map[string]string{"unset": "word"}, mapping)
 }
+
+func TestExpand_shellPositionalArg(t *testing.T) {
+	mapping := map[string]string{
+		"1": "foo",
+		"2": "bar",
+	}
+
+	params := []struct {
+		in  string
+		out string
+	}{
+		{"$1$2", "foobar"},
+		{"/home/$1/$2", "/home/foo/bar"},
+	}
+
+	for _, tt := range params {
+		x, err := Expand(tt.in, Map(mapping))
+		if err != nil {
+			t.Errorf("pattern %#v should not have produced an error, but got: %s", tt.in, err)
+		}
+		if x != tt.out {
+			t.Errorf("pattern %#v should expand to %#v, but got %#v", tt.in, tt.out, x)
+		}
+	}
+}