---
title: strings.isLetter() function
description: >
  `strings.isLetter()` tests if a single character string is a letter (a-z, A-Z).
menu:
  flux_v0_ref:
    name: strings.isLetter
    parent: strings
    identifier: strings/isLetter
weight: 101
---

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



##### Function type signature

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

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

## Parameters

### v
({{< req >}})
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))

```

{{< expand-wrapper >}}
{{% expand "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   |

{{% /expand %}}
{{< /expand-wrapper >}}