---
layout: default
title: Expression syntax
nav_order: 40
---
# Expression syntax
The following sections provide information about expression syntax in Data Prepper.
## Supported operators
Operators are listed in order of precedence (top to bottom, left to right).
| Operator | Description | Associativity |
|----------------------|-------------------------------------------------------|---------------|
| `()` | Priority Expression | left-to-right |
| `not`
`+`
`-`| Unary Logical NOT
Unary Positive
Unary negative | right-to-left |
| `<`, `<=`, `>`, `>=` | Relational Operators | left-to-right |
| `==`, `!=` | Equality Operators | left-to-right |
| `and`, `or` | Conditional Expression | left-to-right |
## Reserved for possible future functionality
Reserved symbol set: `^`, `*`, `/`, `%`, `+`, `-`, `xor`, `=`, `+=`, `-=`, `*=`, `/=`, `%=`, `++`, `--`, `${}`
## Set initializer
The set initializer defines a set or term and/or expressions.
### Examples
The following are examples of set initializer syntax.
#### HTTP status codes
```
{200, 201, 202}
```
#### HTTP response payloads
```
{"Created", "Accepted"}
```
#### Handle multiple event types with different keys
```
{/request_payload, /request_message}
```
## Priority expression
A priority expression identifies an expression that will be evaluated at the highest priority level. A priority expression must contain an expression or value; empty parentheses are not supported.
### Example
```
/is_cool == (/name == "Steven")
```
## Relational operators
Relational operators are used to test the relationship of two numeric values. The operands must be numbers or JSON Pointers that resolve to numbers.
### Syntax
```
<
<=
>
>=
```
### Example
```
/status_code >= 200 and /status_code < 300
```
## Equality operators
Equality operators are used to test whether two values are equivalent.
### Syntax
```
==
!=
```
### Examples
```
/is_cool == true
3.14 != /status_code
{1, 2} == /event/set_property
```
## Using equality operators to check for a JSON Pointer
Equality operators can also be used to check whether a JSON Pointer exists by comparing the value with `null`.
### Syntax
```
== null
!= null
null ==
null !=
```
### Example
```
/response == null
null != /response
```
#### Conditional expression
A conditional expression is used to chain together multiple expressions and/or values.
#### Syntax
```
and
or
not
```
### Example
```
/status_code == 200 and /message == "Hello world"
/status_code == 200 or /status_code == 202
not /status_code in {200, 202}
/response == null
/response != null
```
## Definitions
This section provides expression definitions.
### Literal
A literal is a fundamental value that has no children:
- Float: Supports values from 3.40282347 × 1038 to 1.40239846 × 10−45.
- Integer: Supports values from −2,147,483,648 to 2,147,483,647.
- Boolean: Supports true or false.
- JSON Pointer: See the [JSON Pointer](#json-pointer) section for details.
- String: Supports valid Java strings.
- Null: Supports null check to see whether a JSON Pointer exists.
### Expression string
An expression string takes the highest priority in a Data Prepper expression and only supports one expression string resulting in a return value. An _expression string_ is not the same as an _expression_.
### Statement
A statement is the highest-priority component of an expression string.
### Expression
An expression is a generic component that contains a _Primary_ or an _Operator_. Expressions may contain expressions. An expression's imminent children can contain 0–1 _Operators_.
### Primary
- _Set_
- _Priority Expression_
- _Literal_
### Operator
An operator is a hardcoded token that identifies the operation used in an _expression_.
### JSON Pointer
A JSON Pointer is a literal used to reference a value within an event and provided as context for an _expression string_. JSON Pointers are identified by a leading `/` containing alphanumeric characters or underscores, delimited by `/`. JSON Pointers can use an extended character set if wrapped in double quotes (`"`) using the escape character `\`. Note that JSON Pointers require `~` and `/` characters, which should be used as part of the path and not as a delimiter that needs to be escaped.
The following are examples of JSON Pointers:
- `~0` representing `~`
- `~1` representing `/`
#### Shorthand syntax (Regex, `\w` = `[A-Za-z_]`)
```
/\w+(/\w+)*
```
#### Example of shorthand
The following is an example of shorthand:
```
/Hello/World/0
```
#### Example of escaped syntax
The following is an example of escaped syntax:
```
"/(/)*"
```
#### Example of an escaped JSON Pointer
The following is an example of an escaped JSON Pointer:
```
# Path
# { "Hello - 'world/" : [{ "\"JsonPointer\"": true }] }
"/Hello - 'world\//0/\"JsonPointer\""
```
## White space
White space is **optional** surrounding relational operators, regex equality operators, equality operators, and commas.
White space is **required** surrounding set initializers, priority expressions, set operators, and conditional expressions.
| Operator | Description | White space required | ✅ Valid examples | ❌ Invalid examples |
|----------------------|--------------------------|----------------------|----------------------------------------------------------------|---------------------------------------|
| `{}` | Set initializer | Yes | `/status in {200}` | `/status in{200}` |
| `()` | Priority expression | Yes | `/a==(/b==200)`
`/a in ({200})` | `/status in({200})` |
| `in`, `not in` | Set operators | Yes | `/a in {200}`
`/a not in {400}` | `/a in{200, 202}`
`/a not in{400}` |
| `<`, `<=`, `>`, `>=` | Relational operators | No | `/status < 300`
`/status>=300` | |
| `=~`, `!~` | Regex equality pperators | No | `/msg =~ "^\w*$"`
`/msg=~"^\w*$"` | |
| `==`, `!=` | Equality operators | No | `/status == 200`
`/status_code==200` | |
| `and`, `or`, `not` | Conditional operators | Yes | `/a<300 and /b>200` | `/b<300and/b>200` |
| `,` | Set value delimiter | No | `/a in {200, 202}`
`/a in {200,202}`
`/a in {200 , 202}` | `/a in {200,}` |