# Hierarchical data path specifications

The Hierarchical data path (HDT) path specification for extractions is different than the specification for modifications. Each specification is described below.

## HDT path specification for extractions

The HDT path specifications for extractions are:

* You must always start with the dollar sign (`$`) for the root.
* Simple alphanumeric key values are separated by periods (`.`).
* Numeric indices are designated like arrays with square brackets `[ n ]`.
* Complex string identifiers are designated with single quotation marks in square brackets `[‘Complex string identifier’]`.
* If your identifier has a special characters like a single quote mark, you must escape that character with a backslash (`\`). For example `['\‘Complex string identifier\’']`.
* You can use the asterisk (\*) wildcard for string or numeric keys.

For example: `$['l am a perfectly'].normal[0].example`

## HDT path specification for modifications

The HDT path specifications for modifications are:

* There is an additional array push operator of square brackets `[ ]` when added to the end of an array that will create an index.
* There are four special reference quote marks that you can use:
  * `$[?string_field?]` - Takes the string value of a field from an incoming PDI row and uses that value as the map key fragment.
  * `$[#numeric_field#]` - Takes the numeric value of an incoming PDI field and uses the value for array or list access with a resolved numeric index.
  * `$[$string_backreference$]` – Matches the string key fragment from a previous paths segment.
  * `$[@numeric_backreference@]` - Matches the numeric array or list index from a previous paths segment.

**Note:** The backreference numbering is based on the index in the path. In the code example: `$.first[ 4 ][‘hello’]`, the `$1$ = ‘first`’, `@2@ = 4, $3$ = ‘hello’`. The backreference is a 1-based index, and the surrounding `$` or `@` specifies if a string or numeric value was used.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.pentaho.com/pdia-data-integration/10.2-data-integration/advanced-topics-pentaho-data-integration-overview/hierarchical-data/hdt-path-specification.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.