Add regex command implementation

internal/regex/validate.go

@@ -0,0 +1,30 @@
+package regex
+
+import "fmt"
+
+// ValidateTemplate ensures the parsed template does not reference capture groups beyond the
+// pattern's capabilities and returns a descriptive error for CLI presentation.
+func ValidateTemplate(engine *Engine, tmpl template) error {
+	if engine == nil {
+		return fmt.Errorf("internal error: regex engine not initialized")
+	}
+	max := 0
+	for _, segment := range tmpl.segments {
+		if segment.group > max {
+			max = segment.group
+		}
+	}
+	if max > engine.groups {
+		return ErrTemplateGroupOutOfRange{Group: max, Available: engine.groups}
+	}
+	return nil
+}
+
+// ErrUndefinedPlaceholder indicates that the template references a group with no match result.
+type ErrUndefinedPlaceholder struct {
+	Index int
+}
+
+func (e ErrUndefinedPlaceholder) Error() string {
+	return fmt.Sprintf("template references @%d but the pattern did not produce that group", e.Index)
+}