diff --git a/README.md b/README.md
index 109fa2b..949ed6d 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@ A library for handling a superset of semantic versioning in golang.
 
 ## Documentation and examples
 
-See the godoc here: https://godoc.org/go.bug.st/relaxed-semver
+See the godoc here: <https://godoc.org/go.bug.st/relaxed-semver>
 
 ## Semantic versioning specification followed in this library
 
@@ -39,20 +39,22 @@ To parse a `RelaxedVersion` you can use the `ParseRelaxed` function.
 
 Dependency version matching can be specified via version constraints, which might be a version range or an exact version.
 
-The following operators are supported:
-
-|          |                          |
-| -------- | ------------------------ |
-| `=`      | equal to                 |
-| `>`      | greater than             |
-| `>=`     | greater than or equal to |
-| `<`      | less than                |
-| `<=`     | less than or equal to    |
-| `^`      | compatible-with          |
-| `!`      | NOT                      |
-| `&&`     | AND                      |
-| `\|\|`   | OR                       |
-| `(`, `)` | constraint group         |
+The following range operators are supported:
+
+| Operator    | Meaning                  |
+| ----------- | ------------------------ |
+| `^` or None | compatible-with          |
+| `=`         | equal to                 |
+| `>`         | greater than             |
+| `>=`        | greater than or equal to |
+| `<`         | less than                |
+| `<=`        | less than or equal to    |
+| `!`         | NOT                      |
+| `&&`        | AND                      |
+| `\|\|`      | OR                       |
+| `(`, `)`    | constraint group         |
+
+In a constraint if a version is written without the operator the "compatible-with" operator is applied.
 
 ### Examples
 
@@ -82,6 +84,8 @@ constraints conditions would match as follows:
 | `<1.0.0 \|\| >2.0.0`             | `0.1.0`, `0.1.1`, `0.2.0`, `2.0.5`, `2.0.6`, `2.1.0`, `3.0.0`          |
 | `(>0.1.0 && <2.0.0) \|\| >2.0.5` | `0.1.1`, `0.2.0`, `1.0.0`, `2.0.6`, `2.1.0`, `3.0.0`                   |
 | `^2.0.5`                         | `2.0.5`, `2.0.6`, `2.1.0`                                              |
+| `2.0.5`                          | `2.0.5`, `2.0.6`, `2.1.0`  (same as `^2.0.5`)                          |
+| `2.0.5 && !=2.0.6`               | `2.0.5`, `2.1.0`  (same as `^2.0.5 && !=2.0.6`)                        |
 | `^0.1.0`                         | `0.1.0`, `0.1.1`                                                       |
 
 ## Json parsable
diff --git a/constraints.go b/constraints.go
index ce882ae..4c4ccf7 100644
--- a/constraints.go
+++ b/constraints.go
@@ -53,11 +53,11 @@ func ParseConstraint(in string) (Constraint, error) {
 			n := peek()
 			if !isIdentifier(n) && !isVersionSeparator(n) {
 				if start == curr {
-					return nil, fmt.Errorf("invalid version")
+					return nil, fmt.Errorf("invalid version: unexpected char '%c'", rune(n))
 				}
 				return Parse(in[start:curr])
 			}
-			curr++
+			next()
 		}
 	}
 
@@ -66,36 +66,41 @@ func ParseConstraint(in string) (Constraint, error) {
 
 	terminal = func() (Constraint, error) {
 		skipSpace()
-		switch next() {
+		switch peek() {
 		case '!':
+			next()
 			expr, err := terminal()
 			if err != nil {
 				return nil, err
 			}
 			return &Not{expr}, nil
 		case '(':
+			next()
 			expr, err := constraint()
 			if err != nil {
 				return nil, err
 			}
 			skipSpace()
 			if c := next(); c != ')' {
-				return nil, fmt.Errorf("unexpected char at: %s", in[curr-1:])
+				return nil, fmt.Errorf("unexpected char: %c", rune(c))
 			}
 			return expr, nil
 		case '=':
+			next()
 			v, err := version()
 			if err != nil {
 				return nil, err
 			}
 			return &Equals{v}, nil
 		case '^':
+			next()
 			v, err := version()
 			if err != nil {
 				return nil, err
 			}
 			return &CompatibleWith{v}, nil
 		case '>':
+			next()
 			if peek() == '=' {
 				next()
 				v, err := version()
@@ -111,6 +116,7 @@ func ParseConstraint(in string) (Constraint, error) {
 				return &GreaterThan{v}, nil
 			}
 		case '<':
+			next()
 			if peek() == '=' {
 				next()
 				v, err := version()
@@ -126,7 +132,11 @@ func ParseConstraint(in string) (Constraint, error) {
 				return &LessThan{v}, nil
 			}
 		default:
-			return nil, fmt.Errorf("unexpected char at: %s", in[curr-1:])
+			v, err := version()
+			if err != nil {
+				return nil, err
+			}
+			return &CompatibleWith{v}, nil
 		}
 	}
 
diff --git a/constraints_test.go b/constraints_test.go
index cf6adab..b73e1ce 100644
--- a/constraints_test.go
+++ b/constraints_test.go
@@ -99,9 +99,17 @@ func TestConstraintsParser(t *testing.T) {
 	good := []goodStringTest{
 		{"", ""}, // always true
 		{"=1.3.0", "=1.3.0"},
+		{"!=1.3.0", "!(=1.3.0)"},
+		{" !=1.3.0", "!(=1.3.0)"},
+		{"! =1.3.0", "!(=1.3.0)"},
+		{"1.3.0", "^1.3.0"},
+		{"!1.0.0", "!(^1.0.0)"},
 		{" =1.3.0 ", "=1.3.0"},
+		{" 1.3.0 ", "^1.3.0"},
 		{"=1.3.0 ", "=1.3.0"},
+		{"1.3.0 ", "^1.3.0"},
 		{" =1.3.0", "=1.3.0"},
+		{" 1.3.0", "^1.3.0"},
 		{">=1.3.0", ">=1.3.0"},
 		{">1.3.0", ">1.3.0"},
 		{"<=1.3.0", "<=1.3.0"},
@@ -111,16 +119,37 @@ func TestConstraintsParser(t *testing.T) {
 		{"^1.3.0 ", "^1.3.0"},
 		{" ^1.3.0 ", "^1.3.0"},
 		{"(=1.4.0)", "=1.4.0"},
+		{"(1.4.0)", "^1.4.0"},
 		{"!(=1.4.0)", "!(=1.4.0)"},
+		{"!(1.4.0)", "!(^1.4.0)"},
 		{"!(((=1.4.0)))", "!(=1.4.0)"},
+		{"!(((1.4.0)))", "!(^1.4.0)"},
 		{"=1.2.4 && =1.3.0", "(=1.2.4 && =1.3.0)"},
 		{"=1.2.4 && ^1.3.0", "(=1.2.4 && ^1.3.0)"},
+		{"1.2.4 && 1.3.0", "(^1.2.4 && ^1.3.0)"},
 		{"=1.2.4 && =1.3.0 && =1.2.0", "(=1.2.4 && =1.3.0 && =1.2.0)"},
+		{"1.2.4 && 1.3.0 && 1.2.0", "(^1.2.4 && ^1.3.0 && ^1.2.0)"},
 		{"=1.2.4 && =1.3.0 || =1.2.0", "((=1.2.4 && =1.3.0) || =1.2.0)"},
+		{"1.2.4 && 1.3.0 || 1.2.0", "((^1.2.4 && ^1.3.0) || ^1.2.0)"},
 		{"=1.2.4 || =1.3.0 && =1.2.0", "(=1.2.4 || (=1.3.0 && =1.2.0))"},
 		{"(=1.2.4 || =1.3.0) && =1.2.0", "((=1.2.4 || =1.3.0) && =1.2.0)"},
+		{"(1.2.4 || 1.3.0) && 1.2.0", "((^1.2.4 || ^1.3.0) && ^1.2.0)"},
 		{"(=1.2.4 || !>1.3.0) && =1.2.0", "((=1.2.4 || !(>1.3.0)) && =1.2.0)"},
+		{"(1.2.4 || !>1.3.0) && 1.2.0", "((^1.2.4 || !(>1.3.0)) && ^1.2.0)"},
 		{"!(=1.2.4 || >1.3.0) && =1.2.0", "(!(=1.2.4 || >1.3.0) && =1.2.0)"},
+		{"!(1.2.4 || >1.3.0) && 1.2.0", "(!(^1.2.4 || >1.3.0) && ^1.2.0)"},
+		{">1.0.0 && 2.0.0", "(>1.0.0 && ^2.0.0)"},
+		{">1.0.0 && =2.0.0", "(>1.0.0 && =2.0.0)"},
+		{">1.0.0 || 2.0.0", "(>1.0.0 || ^2.0.0)"},
+		{">1.0.0 || =2.0.0", "(>1.0.0 || =2.0.0)"},
+		{"(>1.0.0) || 2.0.0", "(>1.0.0 || ^2.0.0)"},
+		{"(>1.0.0) || =2.0.0", "(>1.0.0 || =2.0.0)"},
+		{">1.0.0 || (2.0.0)", "(>1.0.0 || ^2.0.0)"},
+		{">1.0.0 || (=2.0.0)", "(>1.0.0 || =2.0.0)"},
+		{"!>1.0.0 || (=2.0.0)", "(!(>1.0.0) || =2.0.0)"},
+		{"!(>1.0.0 || =2.0.0)", "!(>1.0.0 || =2.0.0)"},
+		{"((>1.0.0) || (2.0.0))", "(>1.0.0 || ^2.0.0)"},
+		{"((>1.0.0) || (=2.0.0))", "(>1.0.0 || =2.0.0)"},
 	}
 	for i, test := range good {
 		in := test.In
@@ -134,8 +163,8 @@ func TestConstraintsParser(t *testing.T) {
 	}
 
 	bad := []string{
-		"1.0.0",
 		"= 1.0.0",
+		"!= 1.3.9",
 		">= 1.0.0",
 		"> 1.0.0",
 		"<= 1.0.0",
@@ -144,19 +173,21 @@ func TestConstraintsParser(t *testing.T) {
 		">1.0.0 =2.0.0",
 		">1.0.0 &",
 		"^1.1.1.1",
-		"!1.0.0",
-		">1.0.0 && 2.0.0",
 		">1.0.0 | =2.0.0",
 		"(>1.0.0 | =2.0.0)",
 		"(>1.0.0 || =2.0.0",
-		">1.0.0 || 2.0.0",
+		"!1.0.0.0",
+		"!1.0.0 && !1.0.0.0",
+		"(!1.0.0 && !1.0.0",
+		"!1.0.0 || !1.0.0.0",
+		"(!1.0.0 || !1.0.0",
 	}
 	for i, s := range bad {
 		in := s
 		t.Run(fmt.Sprintf("BadString%03d", i), func(t *testing.T) {
 			p, err := ParseConstraint(in)
-			require.Nil(t, p)
-			require.Error(t, err)
+			require.Nil(t, p, "parsing: %s", in)
+			require.Error(t, err, "parsing: %s", in)
 			fmt.Printf("'%s' parse error: %s\n", in, err)
 		})
 	}