Remove colon from characters allowed in anchor

This partially rolls back the changes made with 926dd0b

The colon was allowed in the characters allowed for anchor and alias
name. But it leads to ambiguity in the YAML structure.

Author: ccoVeille

node_test.go

@@ -1130,7 +1130,7 @@ var nodeTests = []struct {
 			}},
 		},
 	}, {
-		"a: &anchor(.!@#$%^&*+=?:;)name [1, 2]\nb: *anchor(.!@#$%^&*+=?:;)name\n",
+		"a: &anchor(.!@#$%^&*+=?;)name [1, 2]\nb: *anchor(.!@#$%^&*+=?;)name\n",
 		yaml.Node{
 			Kind:   yaml.DocumentNode,
 			Line:   1,
@@ -1148,25 +1148,25 @@ var nodeTests = []struct {
 						Line:   1,
 						Column: 1,
 					},
-					saveNode("anchor(.!@#$%^&*+=?:;)name", &yaml.Node{
+					saveNode("anchor(.!@#$%^&*+=?;)name", &yaml.Node{
 						Kind:   yaml.SequenceNode,
 						Style:  yaml.FlowStyle,
 						Tag:    "!!seq",
-						Anchor: "anchor(.!@#$%^&*+=?:;)name",
+						Anchor: "anchor(.!@#$%^&*+=?;)name",
 						Line:   1,
 						Column: 4,
 						Content: []*yaml.Node{{
 							Kind:   yaml.ScalarNode,
 							Value:  "1",
 							Tag:    "!!int",
 							Line:   1,
-							Column: 33,
+							Column: 32,
 						}, {
 							Kind:   yaml.ScalarNode,
 							Value:  "2",
 							Tag:    "!!int",
 							Line:   1,
-							Column: 36,
+							Column: 35,
 						}},
 					}),
 					{
@@ -1178,8 +1178,8 @@ var nodeTests = []struct {
 					},
 					{
 						Kind:   yaml.AliasNode,
-						Value:  "anchor(.!@#$%^&*+=?:;)name",
-						Alias:  dropNode("anchor(.!@#$%^&*+=?:;)name"),
+						Value:  "anchor(.!@#$%^&*+=?;)name",
+						Alias:  dropNode("anchor(.!@#$%^&*+=?;)name"),
 						Line:   2,
 						Column: 4,
 					},

yamlprivateh.go

@@ -67,6 +67,18 @@ func isFlowIndicator(b []byte, i int) bool {
 // We further limit it to ascii chars only, which is a subset of the spec
 // production but is usually what most people expect.
 func isAnchorChar(b []byte, i int) bool {
+	if isColon(b, i) {
+		// [Go] we exclude colons from anchor/alias names.
+		//
+		// A colon is a valid anchor character according to the YAML 1.2 specification,
+		// but it can lead to ambiguity.
+		// https://github.com/yaml/go-yaml/issues/109
+		//
+		// Also, it would have been a breaking change to support it, as go.yaml.in/yaml/v3 ignores it.
+		// Supporting it could lead to unexpected behavior.
+		return false
+	}
+
 	return isPrintable(b, i) &&
 		!isLineBreak(b, i) &&
 		!isBlank(b, i) &&
@@ -75,6 +87,11 @@ func isAnchorChar(b []byte, i int) bool {
 		isASCII(b, i)
 }
 
+// isColon checks whether the character at the specified position is a colon.
+func isColon(b []byte, i int) bool {
+	return b[i] == ':'
+}
+
 // Check if the character at the specified position is a digit.
 func isDigit(b []byte, i int) bool {
 	return b[i] >= '0' && b[i] <= '9'

yts/known-failing-tests

@@ -4,6 +4,8 @@ TestYAMLSuite/2JQS/UnmarshalTest
 TestYAMLSuite/2JQS/EventComparisonTest
 TestYAMLSuite/2LFX/UnmarshalTest
 TestYAMLSuite/2LFX/EventComparisonTest
+TestYAMLSuite/2SXE/UnmarshalTest
+TestYAMLSuite/2SXE/EventComparisonTest
 TestYAMLSuite/35KP/JSONComparisonTest
 TestYAMLSuite/3HFZ/UnmarshalTest
 TestYAMLSuite/3UYS/UnmarshalTest
@@ -185,13 +187,17 @@ TestYAMLSuite/01#15/UnmarshalTest
 TestYAMLSuite/01#15/EventComparisonTest
 TestYAMLSuite/W4TN/UnmarshalTest
 TestYAMLSuite/W4TN/EventComparisonTest
+TestYAMLSuite/W5VH/UnmarshalTest
+TestYAMLSuite/W5VH/EventComparisonTest
 TestYAMLSuite/WZ62/UnmarshalTest
 TestYAMLSuite/WZ62/EventComparisonTest
 TestYAMLSuite/X38W/UnmarshalTest
 TestYAMLSuite/X38W/EventComparisonTest
 TestYAMLSuite/X4QW/UnmarshalTest
 TestYAMLSuite/X4QW/EventComparisonTest
 TestYAMLSuite/XW4D/UnmarshalTest
+TestYAMLSuite/Y2GN/EventComparisonTest
+TestYAMLSuite/Y2GN/JSONComparisonTest
 TestYAMLSuite/001/UnmarshalTest
 TestYAMLSuite/001/EventComparisonTest
 TestYAMLSuite/003/UnmarshalTest