> For the complete documentation index, see [llms.txt](https://php-fhir-tools.ardenexal.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://php-fhir-tools.ardenexal.net/fhirpath/expressions.md).

# Expressions & Operators

FHIRPath expressions navigate and transform FHIR data. They combine path navigation, function calls, and operators. Every expression evaluates against an input collection and produces an output collection.

## Operators

The evaluator implements arithmetic, comparison, equivalence, logical, string, collection, membership, and type operators.

| Category        | Operators                        | Notes                                                                  |
| --------------- | -------------------------------- | ---------------------------------------------------------------------- |
| **Arithmetic**  | `+`, `-`, `*`, `/`, `div`, `mod` | `div` is integer division; `mod` is remainder                          |
| **Comparison**  | `=`, `!=`, `<`, `>`, `<=`, `>=`  |                                                                        |
| **Equivalence** | `~`, `!~`                        | Equivalence / non-equivalence (tokens `EQUIVALENT` / `NOT_EQUIVALENT`) |
| **Logical**     | `and`, `or`, `xor`, `implies`    | Three-valued logic                                                     |
| **String**      | `&`                              | String concatenation                                                   |
| **Collection**  | `\|`                             | Union                                                                  |
| **Membership**  | `in`, `contains`                 |                                                                        |
| **Type**        | `is`, `as`                       | Type testing and casting                                               |

{% hint style="info" %}
`~` tests equivalence and `!~` its negation. Equivalence differs from `=` by ignoring collection order and, for strings, case and whitespace.
{% endhint %}

### Examples

```php
// Comparison and logical
$service->evaluate('age > 18 and status = "active"', $patient);
$service->evaluate('value > 100 or value < 0', $observation);

// Arithmetic (precedence: * / div mod bind tighter than + -)
$service->evaluate('value * 2', $observation);
$service->evaluate('(value + 10) / 2', $observation);

// Membership and union
$service->evaluate('"phone" in telecom.system', $patient);
$service->evaluate('name | telecom', $patient);

// Type operators
$service->evaluate('value is Quantity', $observation);
$service->evaluate('value as Quantity', $observation);
```

## Language features

The parser and evaluator support the core FHIRPath 2.0 syntax:

| Feature              | Example                                           |
| -------------------- | ------------------------------------------------- |
| Path navigation      | `Patient.name.given`                              |
| Indexing             | `name[0]`                                         |
| Function calls       | `name.where(use = "official")`                    |
| Literals             | strings, numbers, booleans, date/time, quantities |
| Collection literals  | `{}` (empty), `{1, 2, 3}`                         |
| External constants   | `%context`, `%resource`, …                        |
| Reserved identifiers | `$this`, `$index`, `$total`                       |

### Type system

The `is` operator tests whether an item conforms to a named type, and `as` filters the collection to items of that type. Temporal values use dedicated types (`FHIRPathDate`, `FHIRPathDateTime`, `FHIRPathTime`), and decimals use `FHIRPathDecimal` to preserve precision.

See the [Function Reference](/fhirpath/functions.md) for the full library of built-in functions, including type-conversion helpers such as `toInteger()`, `toQuantity()`, and `ofType()`.


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