wmjordan/PDFPatcher · error · FormatException

“[]”筛选表达式前缺少节点轴及节点名称标识。

Error message

“[]”筛选表达式前缺少节点轴及节点名称标识。

What it means

PathCompiler.ExtractAxis throws FormatException when the current path character is '[' or ']' (the predicate delimiters) at a position where an axis separator or node name was expected. In the PDFPath mini-language, a predicate '[...]' must follow a node name or axis; encountering a bracket where an axis is required means the path string is malformed.

Source

Thrown at App/Model/PdfPath/PathCompiler.cs:48

			string n;
			var ctx = new Context();
			while (i < l) {
				ctx.Axis = ExtractAxis(path, l, ctx.CanBeRoot, ref i);
				if (ctx.Axis == PathAxisType.Root) {
					ctx.CanBeRoot = false;
					r.Enqueue(new PathExpression(ctx.Axis));
					continue;
				}
				n = ExtractName(path, l, ref i);
				r.Enqueue(new PathExpression(ctx.Axis, n));
			}
			return r.ToArray();
		}

		private static PathAxisType ExtractAxis(string path, int length, bool canBeRoot, ref int index) {
			char c = path[index];
			if (__PredicateChars.Contains(c)) {
				throw new FormatException("“[]”筛选表达式前缺少节点轴及节点名称标识。");
			}

			if (c == PathSeparator) {
				if (MatchNextChar(path, length, index, PathSeparator)) {
					++index;
					return PathAxisType.Descendants;
				}
				else if (canBeRoot) {
					return PathAxisType.Root;
				}
				else {
					return PathAxisType.Children;
				}
			}
			else if (c == SelfChar) {
				if (MatchNextChar(path, length, index, SelfChar)) {
					++index;
					return PathAxisType.Parent;

View on GitHub (pinned to 4782bbd9ad)

Solutions

1. Ensure every '[...]' predicate is preceded by a node name or axis (e.g. 'Page[1]' not '[1]').
2. Validate the path string with a regex or a dry-run compile before using it for selection.
3. If building paths dynamically, always concatenate the element name before appending the predicate.

Example fix

// before
var expr = PathCompiler.Compile("[1]");

// after
var expr = PathCompiler.Compile("Page[1]");

if (string.IsNullOrEmpty(path) || path[0] == '[' || path[0] == ']')
    throw new ArgumentException("Path must start with a node name or axis.");

static bool IsValidPathStart(string p) => !string.IsNullOrEmpty(p) && p[0] != '[' && p[0] != ']';

try { var e = PathCompiler.Compile(path); }
catch (FormatException ex) { /* malformed path */ }

When it happens

Trigger: Calling PathCompiler.Compile with a path that begins with or otherwise places a '[' before any node name or axis, e.g. "[1]" or "/[1]". The loop in Compile calls ExtractAxis first, and a leading bracket trips __PredicateChars.Contains(c).

Common situations: Programmatically building a path string and forgetting to prepend the node name before a filter; user-entered PDFPath expression with a missing element name; a template substitution that left an empty name before a predicate.

Related errors

• “[]”筛选表达式前缺少节点名称。
• 页面高度不可小于 0。
• 页面宽度不可小于 0。
• 转换精度不能小于 0 或大于 6。
• 尺寸单位无效。

AI-assisted analysis of wmjordan/PDFPatcher@4782bbd9ad (2026-08-13). Data as JSON: /api/errors/c5dbd8ab2d0b5c8d. Report an issue: GitHub.