opensearch-docs-cn/_search-plugins/ppl/identifiers.md

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---
layout: default
title: Identifiers
parent: Piped processing language
nav_order: 7
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redirect_from: /docs/ppl/identifiers/
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---
# Identifiers
An identifier is an ID to name your database objects, such as index names, field names, aliases, and so on.
OpenSearch supports two types of identifiers: regular identifiers and delimited identifiers.
## Regular identifiers
A regular identifier is a string of characters that starts with an ASCII letter (lower or upper case).
The next character can either be a letter, digit, or underscore (_). It can't be a reserved keyword.
Whitespace and other special characters are also not allowed.
OpenSearch supports the following regular identifiers:
1. Identifiers prefixed by a dot `.` sign. Use to hide an index. For example `.opensearch-dashboards`.
2. Identifiers prefixed by an `@` sign. Use for meta fields generated by Logstash ingestion.
3. Identifiers with hyphen `-` in the middle. Use for index names with date information.
4. Identifiers with star `*` present. Use for wildcard match of index patterns.
For regular identifiers, you can use the name without any back tick or escape characters.
In this example, `source`, `fields`, `account_number`, `firstname`, and `lastname` are all identifiers. Out of these, the `source` field is a reserved identifier.
```sql
source=accounts | fields account_number, firstname, lastname;
```
| account_number | firstname | lastname |
:--- | :--- |
| 1 | Amber | Duke
| 6 | Hattie | Bond
| 13 | Nanette | Bates
| 18 | Dale | Adams
## Delimited identifiers
A delimited identifier can contain special characters not allowed by a regular identifier.
You must enclose delimited identifiers with back ticks (\`\`). Back ticks differentiate the identifier from special characters.
If the index name includes a dot (`.`), for example, `log-2021.01.11`, use delimited identifiers with back ticks to escape it \``log-2021.01.11`\`.
Typical examples of using delimited identifiers:
1. Identifiers with reserved keywords.
2. Identifiers with a `.` present. Similarly, `-` to include date information.
3. Identifiers with other special characters. For example, Unicode characters.
To quote an index name with back ticks:
```sql
source=`accounts` | fields `account_number`;
```
| account_number |
:--- |
| 1 |
| 6 |
| 13 |
| 18 |
## Case sensitivity
Identifiers are case sensitive. They must be exactly the same as what's stored in OpenSearch.
For example, if you run `source=Accounts`, you'll get an index not found exception because the actual index name is in lower case.