Fix #160: Anchor IsJSON regex to prevent false positives on CDATA

The IsJSON() function was using an unanchored regex `\s*[{\[]` which
would match a `[` character anywhere in the input. This caused XML
documents containing CDATA sections like `<![CDATA[text]]>` to be
incorrectly detected as JSON.

Changed the regex to `^\s*[{\[]` to anchor it to the start of the
input string. Now only inputs that actually begin with `{` or `[`
(after optional whitespace) will be detected as JSON.

Added TestFormatDetection to verify CDATA sections are correctly
detected as XML format, not JSON format.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: bukzor

cmd/root_test.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/antchfx/xmlquery"
 	"github.com/sibprogrammer/xq/internal/utils"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
@@ -199,3 +200,38 @@ Bye.`,
 		})
 	}
 }
+
+func TestFormatDetection(t *testing.T) {
+	t.Run("CDATA should be detected as XML not JSON", func(t *testing.T) {
+		input := "<root><![CDATA[1 & 2]]></root>"
+		reader := strings.NewReader(input)
+
+		flags := pflag.NewFlagSet("test", pflag.ContinueOnError)
+		flags.Bool("html", false, "")
+
+		contentType, _ := detectFormat(flags, reader)
+
+		// Should be XML (0), not JSON (2)
+		assert.Equal(t, utils.ContentXml, contentType, "CDATA should be detected as XML")
+	})
+
+	t.Run("Debug CDATA node structure", func(t *testing.T) {
+		input := "<root><![CDATA[1 & 2]]></root>"
+		doc, err := xmlquery.Parse(strings.NewReader(input))
+		assert.Nil(t, err)
+
+		// Walk the tree and log all node types
+		var walk func(*xmlquery.Node, int)
+		walk = func(n *xmlquery.Node, depth int) {
+			if n == nil {
+				return
+			}
+			indent := strings.Repeat("  ", depth)
+			t.Logf("%sType: %d, Data: %q", indent, n.Type, n.Data)
+			for child := n.FirstChild; child != nil; child = child.NextSibling {
+				walk(child, depth+1)
+			}
+		}
+		walk(doc, 0)
+	})
+}

internal/utils/utils.go

@@ -519,7 +519,7 @@ func IsHTML(input string) bool {
 
 func IsJSON(input string) bool {
 	input = strings.ToLower(input)
-	matched, _ := regexp.MatchString(`\s*[{\[]`, input)
+	matched, _ := regexp.MatchString(`^\s*[{\[]`, input)
 	return matched
 }