path-expression-matcher

Efficient path tracking and pattern matching for XML, JSON, YAML or any other parsers.

???? Purpose

path-expression-matcher provides two core classes for tracking and matching paths:

• Expression: Parses and stores pattern expressions (e.g., "root.users.user[id]")
• Matcher: Tracks current path during parsing and matches against expressions

Compatible with fast-xml-parser and similar tools.

???? Installation

npm install path-expression-matcher

???? Quick Start

import { Expression, Matcher } from 'path-expression-matcher';

// Create expression (parse once, reuse many times)
const expr = new Expression("root.users.user");

// Create matcher (tracks current path)
const matcher = new Matcher();

matcher.push("root");
matcher.push("users");
matcher.push("user", { id: "123" });

// Match current path against expression
if (matcher.matches(expr)) {
  console.log("Match found!");
  console.log("Current path:", matcher.toString()); // "root.users.user"
}

// Namespace support
const nsExpr = new Expression("soap::Envelope.soap::Body..ns::UserId");
matcher.push("Envelope", null, "soap");
matcher.push("Body", null, "soap");
matcher.push("UserId", null, "ns");
console.log(matcher.toString()); // "soap:Envelope.soap:Body.ns:UserId"

???? Pattern Syntax

Basic Paths

"root.users.user"           // Exact path match
"*.users.user"              // Wildcard: any parent
"root.*.user"               // Wildcard: any middle
"root.users.*"              // Wildcard: any child

Deep Wildcard

"..user"                    // user anywhere in tree
"root..user"                // user anywhere under root
"..users..user"             // users somewhere, then user below it

Attribute Matching

"user[id]"                  // user with "id" attribute
"user[type=admin]"          // user with type="admin" (current node only)
"root[lang]..user"          // user under root that has "lang" attribute

Position Selectors

"user:first"                // First user (counter=0)
"user:nth(2)"               // Third user (counter=2, zero-based)
"user:odd"                  // Odd-numbered users (counter=1,3,5...)
"user:even"                 // Even-numbered users (counter=0,2,4...)
"root.users.user:first"     // First user under users

Note: Position selectors use the counter (occurrence count of the tag name), not the position (child index). For example, in <root><a/><b/><a/></root>, the second <a/> has position=2 but counter=1.

Namespaces

"ns::user"                  // user with namespace "ns"
"soap::Envelope"            // Envelope with namespace "soap"
"ns::user[id]"              // user with namespace "ns" and "id" attribute
"ns::user:first"            // First user with namespace "ns"
"*::user"                   // user with any namespace
"..ns::item"                // item with namespace "ns" anywhere in tree
"soap::Envelope.soap::Body" // Nested namespaced elements
"ns::first"                 // Tag named "first" with namespace "ns" (NO ambiguity!)

Namespace syntax:

• Use double colon (::) for namespace: ns::tag
• Use single colon (:) for position: tag:first
• Combined: ns::tag:first (namespace + tag + position)

Namespace matching rules:

• Pattern ns::user matches only nodes with namespace "ns" and tag "user"
• Pattern user (no namespace) matches nodes with tag "user" regardless of namespace
• Pattern *::user matches tag "user" with any namespace (wildcard namespace)
• Namespaces are tracked separately for counter/position (e.g., ns1::item and ns2::item have independent counters)

Wildcard Differences

Single wildcard (*) - Matches exactly ONE level:

• "*.fix1" matches root.fix1 (2 levels) ✅
• "*.fix1" does NOT match root.another.fix1 (3 levels) ❌
• Path depth MUST equal pattern depth

Deep wildcard (..) - Matches ZERO or MORE levels:

• "..fix1" matches root.fix1 ✅
• "..fix1" matches root.another.fix1 ✅
• "..fix1" matches a.b.c.d.fix1 ✅
• Works at any depth

Combined Patterns

"..user[id]:first"              // First user with id, anywhere
"root..user[type=admin]"        // Admin user under root
"ns::user[id]:first"            // First namespaced user with id
"soap::Envelope..ns::UserId"    // UserId with namespace ns under SOAP envelope

???? API Reference

Expression

Constructor

new Expression(pattern, options = {}, data)

Parameters:

• pattern (string): Pattern to parse
• options.separator (string): Path separator (default: '.')

Example:

const expr1 = new Expression("root.users.user");
const expr2 = new Expression("root/users/user", { separator: '/' });
const expr3 = new Expression("root/users/user", { separator: '/' }, { extra: "data"});
console.log(expr3.data) // { extra: "data" }

Methods

• hasDeepWildcard() → boolean
• hasAttributeCondition() → boolean
• hasPositionSelector() → boolean
• toString() → string

Matcher

Constructor

new Matcher(options)

Parameters:

• options.separator (string): Default path separator (default: '.')

Path Tracking Methods

push(tagName, attrValues, namespace)

Add a tag to the current path. Position and counter are automatically calculated.

Parameters:

• tagName (string): Tag name
• attrValues (object, optional): Attribute key-value pairs (current node only)
• namespace (string, optional): Namespace for the tag

Example:

matcher.push("user", { id: "123", type: "admin" });
matcher.push("item");  // No attributes
matcher.push("Envelope", null, "soap");  // With namespace
matcher.push("Body", { version: "1.1" }, "soap");  // With both

Position vs Counter:

• Position: The child index in the parent (0, 1, 2, 3...)
• Counter: How many times this tag name appeared at this level (0, 1, 2...)

Example:

<root>
  <a/>      <!-- position=0, counter=0 -->
  <b/>      <!-- position=1, counter=0 -->
  <a/>      <!-- position=2, counter=1 -->
</root>

pop()

Remove the last tag from the path.

matcher.pop();

updateCurrent(attrValues)

Update current node's attributes (useful when attributes are parsed after push).

matcher.push("user");  // Don't know values yet
// ... parse attributes ...
matcher.updateCurrent({ id: "123" });

reset()

Clear the entire path.

matcher.reset();

Query Methods

matches(expression)

Check if current path matches an Expression.

const expr = new Expression("root.users.user");
if (matcher.matches(expr)) {
  // Current path matches
}

matchesAny(exprSet) → boolean

Please check ExpressionSet class for more details.

const matcher = new Matcher();
const exprSet = new ExpressionSet();
exprSet.add(new Expression("root.users.user"));
exprSet.add(new Expression("root.config.*"));
exprSet.seal();

if (matcher.matchesAny(exprSet)) {
  // Current path matches any expression in the set
}

getCurrentTag()

Get current tag name.

const tag = matcher.getCurrentTag(); // "user"

getCurrentNamespace()

Get current namespace.

const ns = matcher.getCurrentNamespace(); // "soap" or undefined

getAttrValue(attrName)

Get attribute value of current node.

const id = matcher.getAttrValue("id"); // "123"

hasAttr(attrName)

Check if current node has an attribute.

if (matcher.hasAttr("id")) {
  // Current node has "id" attribute
}

getPosition()

Get sibling position of current node (child index in parent).

const position = matcher.getPosition(); // 0, 1, 2, ...

getCounter()

Get repeat counter of current node (occurrence count of this tag name).

const counter = matcher.getCounter(); // 0, 1, 2, ...

getIndex() (deprecated)

Alias for getPosition(). Use getPosition() or getCounter() instead for clarity.

const index = matcher.getIndex(); // Same as getPosition()

getDepth()

Get current path depth.

const depth = matcher.getDepth(); // 3 for "root.users.user"

toString(separator?, includeNamespace?)

Get path as string.

Parameters:

• separator (string, optional): Path separator (uses default if not provided)
• includeNamespace (boolean, optional): Whether to include namespaces (default: true)

const path = matcher.toString();           // "root.ns:user.item"
const path2 = matcher.toString('/');       // "root/ns:user/item"
const path3 = matcher.toString('.', false); // "root.user.item" (no namespaces)

toArray()

Get path as array.

const arr = matcher.toArray(); // ["root", "users", "user"]

State Management

snapshot()

Create a snapshot of current state.

const snapshot = matcher.snapshot();

restore(snapshot)

Restore from a snapshot.

matcher.restore(snapshot);

Read-Only Access

readOnly()

Returns a live, read-only proxy of the matcher. All query and inspection methods work normally, but any attempt to call a state-mutating method (push, pop, reset, updateCurrent, restore) or to write/delete a property throws a TypeError.

This is the recommended way to share the matcher with external consumers — plugins, callbacks, event handlers — that only need to inspect the current path without being able to corrupt parser state.

const ro = matcher.readOnly();

What works on the read-only view:

ro.matches(expr)          // ✓ pattern matching
ro.getCurrentTag()        // ✓ current tag name
ro.getCurrentNamespace()  // ✓ current namespace
ro.getAttrValue("id")     // ✓ attribute value
ro.hasAttr("id")          // ✓ attribute presence check
ro.getPosition()          // ✓ sibling position
ro.getCounter()           // ✓ occurrence counter
ro.getDepth()             // ✓ path depth
ro.toString()             // ✓ path as string
ro.toArray()              // ✓ path as array
ro.snapshot()             // ✓ snapshot (can be used to restore the real matcher)

What throws a TypeError:

ro.push("child", {})      // ✗ TypeError: Cannot call 'push' on a read-only Matcher
ro.pop()                  // ✗ TypeError: Cannot call 'pop' on a read-only Matcher
ro.reset()                // ✗ TypeError: Cannot call 'reset' on a read-only Matcher
ro.updateCurrent({})      // ✗ TypeError: Cannot call 'updateCurrent' on a read-only Matcher
ro.restore(snapshot)      // ✗ TypeError: Cannot call 'restore' on a read-only Matcher
ro.separator = '/'        // ✗ TypeError: Cannot set property on a read-only Matcher

Important: The read-only view is live — it always reflects the current state of the underlying matcher. If you need a frozen-in-time copy instead, use snapshot().

const matcher = new Matcher();
const ro = matcher.readOnly();

matcher.push("root");
ro.getDepth();    // 1 — immediately reflects the push
matcher.push("users");
ro.getDepth();    // 2 — still live

???? Usage Examples

Example 1: XML Parser with stopNodes

import { XMLParser } from 'fast-xml-parser';
import { Expression, Matcher } from 'path-expression-matcher';

class MyParser {
  constructor() {
    this.matcher = new Matcher();
    
    // Pre-compile stop node patterns
    this.stopNodeExpressions = [
      new Expression("html.body.script"),
      new Expression("html.body.style"),
      new Expression("..svg"),
    ];
  }
  
  parseTag(tagName, attrs) {
    this.matcher.push(tagName, attrs);
    
    // Check if this is a stop node
    for (const expr of this.stopNodeExpressions) {
      if (this.matcher.matches(expr)) {
        // Don't parse children, read as raw text
        return this.readRawContent();
      }
    }
    
    // Continue normal parsing
    this.parseChildren();
    
    this.matcher.pop();
  }
}

Example 2: Conditional Processing

const matcher = new Matcher();
const userExpr = new Expression("..user[type=admin]");
const firstItemExpr = new Expression("..item:first");

function processTag(tagName, value, attrs) {
  matcher.push(tagName, attrs);
  
  if (matcher.matches(userExpr)) {
    value = enhanceAdminUser(value);
  }
  
  if (matcher.matches(firstItemExpr)) {
    value = markAsFirst(value);
  }
  
  matcher.pop();
  return value;
}

Example 3: Path-based Filtering

const patterns = [
  new Expression("data.users.user"),
  new Expression("data.posts.post"),
  new Expression("..comment[approved=true]"),
];

function shouldInclude(matcher) {
  return patterns.some(expr => matcher.matches(expr));
}

Example 4: Custom Separator

const matcher = new Matcher({ separator: '/' });
const expr = new Expression("root/config/database", { separator: '/' });

matcher.push("root");
matcher.push("config");
matcher.push("database");

console.log(matcher.toString()); // "root/config/database"
console.log(matcher.matches(expr)); // true

Example 5: Attribute Checking

const matcher = new Matcher();
matcher.push("root");
matcher.push("user", { id: "123", type: "admin", status: "active" });

// Check attribute existence (current node only)
console.log(matcher.hasAttr("id"));        // true
console.log(matcher.hasAttr("email"));     // false

// Get attribute value (current node only)
console.log(matcher.getAttrValue("type")); // "admin"

// Match by attribute
const expr1 = new Expression("user[id]");
console.log(matcher.matches(expr1));       // true

const expr2 = new Expression("user[type=admin]");
console.log(matcher.matches(expr2));       // true

Example 6: Position vs Counter

const matcher = new Matcher();
matcher.push("root");

// Mixed tags at same level
matcher.push("item");  // position=0, counter=0 (first item)
matcher.pop();

matcher.push("div");   // position=1, counter=0 (first div)
matcher.pop();

matcher.push("item");  // position=2, counter=1 (second item)

console.log(matcher.getPosition()); // 2 (third child overall)
console.log(matcher.getCounter());  // 1 (second "item" specifically)

// :first uses counter, not position
const expr = new Expression("root.item:first");
console.log(matcher.matches(expr)); // false (counter=1, not 0)

Example 8: Passing a Read-Only Matcher to External Consumers

When passing the matcher into callbacks, plugins, or other code you don't control, use readOnly() to prevent accidental state corruption.

import { Expression, Matcher } from 'path-expression-matcher';

const matcher = new Matcher();

const adminExpr = new Expression("..user[type=admin]");

function parseTag(tagName, attrs, onTag) {
  matcher.push(tagName, attrs);

  // Pass a read-only view — consumer can inspect but not mutate
  onTag(matcher.readOnly());

  matcher.pop();
}

// Safe consumer — can only read
function myPlugin(ro) {
  if (ro.matches(adminExpr)) {
    console.log("Admin at path:", ro.toString());
    console.log("Depth:", ro.getDepth());
    console.log("ID:", ro.getAttrValue("id"));
  }
}

// ro.push(...) or ro.reset() here would throw TypeError,
// so the parser's state is always safe.
parseTag("user", { id: "1", type: "admin" }, myPlugin);

Combining with snapshot(): A snapshot taken via the read-only view can still be used to restore the real matcher.

const matcher = new Matcher();
matcher.push("root");
matcher.push("users");

const ro = matcher.readOnly();
const snap = ro.snapshot();       // ✓ snapshot works on read-only view

matcher.push("user");             // continue parsing...
matcher.restore(snap);            // restore to "root.users" using the snapshot

const matcher = new Matcher();
const soapExpr = new Expression("soap::Envelope.soap::Body..ns::UserId");

// Parse SOAP document
matcher.push("Envelope", { xmlns: "..." }, "soap");
matcher.push("Body", null, "soap");
matcher.push("GetUserRequest", null, "ns");
matcher.push("UserId", null, "ns");

// Match namespaced pattern
if (matcher.matches(soapExpr)) {
  console.log("Found UserId in SOAP body");
  console.log(matcher.toString()); // "soap:Envelope.soap:Body.ns:GetUserRequest.ns:UserId"
}

// Namespace-specific counters
matcher.reset();
matcher.push("root");
matcher.push("item", null, "ns1");  // ns1::item counter=0
matcher.pop();
matcher.push("item", null, "ns2");  // ns2::item counter=0 (different namespace)
matcher.pop();
matcher.push("item", null, "ns1");  // ns1::item counter=1

const firstNs1Item = new Expression("root.ns1::item:first");
console.log(matcher.matches(firstNs1Item)); // false (counter=1)

const secondNs1Item = new Expression("root.ns1::item:nth(1)");
console.log(matcher.matches(secondNs1Item)); // true

// NO AMBIGUITY: Tags named after position keywords
matcher.reset();
matcher.push("root");
matcher.push("first", null, "ns");  // Tag named "first" with namespace

const expr = new Expression("root.ns::first");
console.log(matcher.matches(expr)); // true - matches namespace "ns", tag "first"

????️ Architecture

Data Storage Strategy

Ancestor nodes: Store only tag name, position, and counter (minimal memory) Current node: Store tag name, position, counter, and attribute values

This design minimizes memory usage:

• No attribute names stored (derived from values object when needed)
• Attribute values only for current node, not ancestors
• Attribute checking for ancestors is not supported (acceptable trade-off)
• For 1M nodes with 3 attributes each, saves ~50MB vs storing attribute names

Matching Strategy

Matching is performed bottom-to-top (from current node toward root):

1. Start at current node
2. Match segments from pattern end to start
3. Attribute checking only works for current node (ancestors have no attribute data)
4. Position selectors use counter (occurrence count), not position (child index)

Performance

• Expression parsing: One-time cost when Expression is created
• Expression analysis: Cached (hasDeepWildcard, hasAttributeCondition, hasPositionSelector)
• Path tracking: O(1) for push/pop operations
• Pattern matching: O(n*m) where n = path depth, m = pattern segments
• Memory per ancestor node: ~40-60 bytes (tag, position, counter only)
• Memory per current node: ~80-120 bytes (adds attribute values)

???? Design Patterns

Pre-compile Patterns (Recommended)

// ✅ GOOD: Parse once, reuse many times
const expr = new Expression("..user[id]");

for (let i = 0; i < 1000; i++) {
  if (matcher.matches(expr)) {
    // ...
  }
}

// ❌ BAD: Parse on every iteration
for (let i = 0; i < 1000; i++) {
  if (matcher.matches(new Expression("..user[id]"))) {
    // ...
  }
}

Batch Pattern Checking with ExpressionSet (Recommended)

For checking multiple patterns on every tag, use ExpressionSet instead of a manual loop. It pre-indexes expressions at build time so each call to matchesAny() does an O(1) bucket lookup rather than a full O(N) scan:

import { Expression, ExpressionSet, Matcher } from 'path-expression-matcher';

// Build once at config/startup time
const stopNodes = new ExpressionSet();
stopNodes
  .add(new Expression('root.users.user'))
  .add(new Expression('root.config.*'))
  .add(new Expression('..script'))
  .seal(); // prevent accidental mutation during parsing

// Per-tag — hot path
if (stopNodes.matchesAny(matcher)) {
  // handle stop node
}

This replaces the manual loop pattern:

// ❌ Before — O(N) per tag
function isStopNode(expressions, matcher) {
  for (let i = 0; i < expressions.length; i++) {
    if (matcher.matches(expressions[i])) return true;
  }
  return false;
}

// ✅ After — O(1) lookup per tag
const stopNodes = new ExpressionSet();
stopNodes.addAll(expressions);
stopNodes.matchesAny(matcher);
//or matcher.matchesAny(stopNodes)

???? ExpressionSet API

ExpressionSet is an indexed collection of Expression objects designed for efficient bulk matching. Build it once from your config, then call matchesAny() on every tag.

Constructor

const set = new ExpressionSet();

add(expression) → this

Add a single Expression. Duplicate patterns (same pattern string) are silently ignored. Returns this for chaining. Throws TypeError if the set is sealed.

set.add(new Expression('root.users.user'));
set.add(new Expression('..script'));

addAll(expressions) → this

Add an array of Expression objects at once. Returns this for chaining.

set.addAll(config.stopNodes.map(p => new Expression(p)));

has(expression) → boolean

Check whether an expression with the same pattern is already present.

set.has(new Expression('root.users.user')); // true / false

seal() → this

Prevent further additions. Any subsequent call to add() or addAll() throws a TypeError. Useful to guard against accidental mutation once parsing has started.

const stopNodes = new ExpressionSet();
stopNodes.addAll(patterns).seal();

stopNodes.add(new Expression('root.extra')); // ❌ TypeError: ExpressionSet is sealed

size → number

Number of distinct expressions in the set.

set.size; // 3

isSealed → boolean

Whether seal() has been called.

matchesAny(matcher) → boolean

Returns true if the matcher's current path matches any expression in the set. Accepts both a Matcher instance and a ReadOnlyMatcher view.

if (stopNodes.matchesAny(matcher)) { /* ... */ }
if (stopNodes.matchesAny(matcher.readOnly())) { /* ... */ } // also works

How indexing works: expressions are bucketed at add() time, not at match time.

In practice, deep-wildcard expressions are rare in configs, so the list stays small.

findMatch(matcher) → Expression

Returns the Expression instance that matched the current path. Accepts both a Matcher instance and a ReadOnlyMatcher view.

const node = stopNodes.findMatch(matcher);

Example 7: ExpressionSet in a real parser loop

import { XMLParser } from 'fast-xml-parser';
import { Expression, ExpressionSet, Matcher } from 'path-expression-matcher';

// Config-time setup
const stopNodes = new ExpressionSet();
stopNodes
  .addAll(['script', 'style'].map(t => new Expression(`..${t}`)))
  .seal();

const matcher = new Matcher();

const parser = new XMLParser({
  onOpenTag(tagName, attrs) {
    matcher.push(tagName, attrs);
    if (stopNodes.matchesAny(matcher)) {
      // treat as stop node
    }
  },
  onCloseTag() {
    matcher.pop();
  },
});

???? Integration with fast-xml-parser

Basic integration:

import { XMLParser } from 'fast-xml-parser';
import { Expression, Matcher } from 'path-expression-matcher';

const parser = new XMLParser({
  // Custom options using path-expression-matcher
  stopNodes: ["script", "style"].map(tag => new Expression(`..${tag}`)),
  
  tagValueProcessor: (tagName, value, jPath, hasAttrs, isLeaf, matcher) => {
    // matcher is available in callbacks
    if (matcher.matches(new Expression("..user[type=admin]"))) {
      return enhanceValue(value);
    }
    return value;
  }
});

???? License

MIT

???? Contributing

Issues and PRs welcome! This package is designed to be used by XML/JSON parsers like fast-xml-parser.

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