---
type: "fhirpath-function"
title: "FHIRPath Function: substring"
function: "substring"
category: "String Manipulation"
section: "5.6.3"
source: "fhirpath/functions.json"
---
# FHIRPath Function: substring
Returns the part of the string starting at position `start` (zero-based). If `length` is given, will return at most `length` number of characters from the input string.

If `start` lies outside the length of the string, the function returns empty (`{ }`). If there are less remaining characters in the string than indicated by `length`, the function returns just the remaining characters.

If the input or `start` is empty, the result is empty.

If an empty `length` is provided, the behavior is the same as if `length` had not been provided.

If a negative or zero `length` is provided, the function returns an empty string (`''`).

If the input collection contains multiple items, the evaluation of the expression will end and signal an error to the calling environment.

``` fhirpath
'abcdefg'.substring(3) // 'defg'
'abcdefg'.substring(1, 2) // 'bc'
'abcdefg'.substring(6, 2) // 'g'
'abcdefg'.substring(7, 1) // { } (start position is outside the string)
'abcdefg'.substring(-1, 1) // { } (start position is outside the string,
                           //     this can happen when the -1 was the result of a calculation rather than explicitly provided)
'abcdefg'.substring(3, 0) // '' (empty string)
'abcdefg'.substring(3, -1) // '' (empty string)
'abcdefg'.substring(-1, -1) // {} (start position is outside the string)
```
## Summary
- **Category**: String Manipulation
- **Section**: `5.6.3`
- **Return Type**: `String`
- **Empty Input Result**: `empty`
- **Errors on Multiple Input**: `true`
## Description
Returns the part of the string starting at position `start` (zero-based). If `length` is given, will return at most `length` number of characters from the input string.
## Arguments
- `start`: `Integer` - Start position (zero-based)
- `length` (optional): `Integer` - Optional length of substring to return
## Type Mapping
- `String-String`
## Example
```fhirpath
'abcdefg'.substring(3) // 'defg'
'abcdefg'.substring(1, 2) // 'bc'
```