---
title: strings.isLetter() function
description: strings.isLetter() tests if a single character string is a letter (a-z, A-Z).
url: https://docs.influxdata.com/flux/v0/stdlib/strings/isletter/
estimated_tokens: 1046
product: Flux
version: v0
---

# strings.isLetter() function

-   Flux 0.18.0+
-   View InfluxDB support

`strings.isLetter()` tests if a single character string is a letter (a-z, A-Z).

##### Function type signature

```js
(v: string) => bool
```

For more information, see [Function type signatures](/flux/v0/function-type-signatures/).

## Parameters

### v

(Required) Single-character string to test.

## Examples

### Filter by columns with digits as values

```js
import "strings"

data
    |> filter(fn: (r) => strings.isLetter(v: r._value))
```

[](#view-example-input-and-output)

View example input and output

#### Input data

| _time | _value | *tag |
| --- | --- | --- |
| 2021-01-01T00:00:00Z | s | t1 |
| 2021-01-01T00:00:10Z | n | t1 |
| 2021-01-01T00:00:20Z | 4 | t1 |
| 2021-01-01T00:00:30Z | 4 | t1 |
| 2021-01-01T00:00:40Z | e | t1 |
| 2021-01-01T00:00:50Z | y | t1 |

| _time | _value | *tag |
| --- | --- | --- |
| 2021-01-01T00:00:00Z | a | t2 |
| 2021-01-01T00:00:10Z | p | t2 |
| 2021-01-01T00:00:20Z | 4 | t2 |
| 2021-01-01T00:00:30Z | t | t2 |
| 2021-01-01T00:00:40Z | 7 | t2 |
| 2021-01-01T00:00:50Z | v | t2 |

#### Output data

| _time | _value | *tag |
| --- | --- | --- |
| 2021-01-01T00:00:00Z | s | t1 |
| 2021-01-01T00:00:10Z | n | t1 |
| 2021-01-01T00:00:40Z | e | t1 |
| 2021-01-01T00:00:50Z | y | t1 |

| _time | _value | *tag |
| --- | --- | --- |
| 2021-01-01T00:00:00Z | a | t2 |
| 2021-01-01T00:00:10Z | p | t2 |
| 2021-01-01T00:00:30Z | t | t2 |
| 2021-01-01T00:00:50Z | v | t2 |

#### Related

-   [strings.isDigit() function](/flux/v0/stdlib/strings/isdigit/)