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Add function label_replace. (#3047)
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You might be wondering why do we need label_replace when we already have label_format.
Well for two reasons:

1. It's nice to have something similar to Prometheus, so users building metric queries will feel right at home.
2. Interestingly, they are quite different in how they execute. label_format executes for every log line, however label_replace execute for every sample. Since you aggregate log lines into sample then label_replace is way more efficient at scale.

Signed-off-by: Cyril Tovena <[email protected]>
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cyriltovena authored Dec 8, 2020
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6 changes: 5 additions & 1 deletion docs/sources/logql/_index.md
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You can use double quoted string for the template or backticks `` `{{.label_name}}` `` to avoid the need to escape special characters.

See [functions](functions/) to learn about available functions in the template format.
See [template functions](template_functions/) to learn about available functions in the template format.

#### Labels Format Expression

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avg(rate(({job="nginx"} |= "GET" | json | path="/home")[10s])) by (region)
```

### Functions

Loki supports several functions to operate on data. These are described in detail in the expression language [functions](functions/) page.

### Binary Operators

#### Arithmetic Binary Operators
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369 changes: 7 additions & 362 deletions docs/sources/logql/functions.md
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---
title: Template functions
title: Functions
---

The [text template](https://golang.org/pkg/text/template) format used in `| line_format` and `| label_format` support the usage of functions.
# Functions

All labels are added as variables in the template engine. They can be referenced using they label name prefixed by a `.`(e.g `.label_name`). For example the following template will output the value of the path label:
## label_replace()

```template
{{ .path }}
```

You can take advantage of [pipeline](https://golang.org/pkg/text/template/#hdr-Pipelines) to join together multiple functions.
In a chained pipeline, the result of each command is passed as the last argument of the following command.

Example:

```template
{{ .path | replace " " "_" | trunc 5 | upper }}
```

## ToLower and ToUpper

This function converts the entire string to lowercase or uppercase.

Signatures:

- `ToLower(string) string`
- `ToUpper(string) string`

Examples:

```template
"{{.request_method | ToLower}}"
"{{.request_method | ToUpper}}"
`{{ToUpper "This is a string" | ToLower}}`
```

> **Note:** In Loki 2.1 you can also use respectively [`lower`](#lower) and [`upper`](#upper) shortcut, e.g `{{.request_method | lower }}`.
## Replace string

> **Note:** In Loki 2.1 [`replace`](#replace) (as opposed to `Replace`) is available with a different signature but easier to chain within pipeline.
Use this function to perform a simple string replacement.

Signature:

`Replace(s, old, new string, n int) string`

It takes four arguments:

- `s` source string
- `old` string to replace
- `new` string to replace with
- `n` the maximun amount of replacement (-1 for all)

Example:

```template
`{{ Replace "This is a string" " " "-" -1 }}`
```

The results in `This-is-a-string`.

## Trim, TrimLeft, TrimRight, and TrimSpace

> **Note:** In Loki 2.1 [trim](#trim), [trimAll](#trimAll), [trimSuffix](#trimSuffix) and [trimPrefix](trimPrefix) have been added with a different signature for better pipeline chaining.
`Trim` returns a slice of the string s with all leading and
trailing Unicode code points contained in cutset removed.

Signature: `Trim(value, cutset string) string`

`TrimLeft` and `TrimRight` are the same as `Trim` except that it trims only leading and trailing characters respectively.

```template
`{{ Trim .query ",. " }}`
`{{ TrimLeft .uri ":" }}`
`{{ TrimRight .path "/" }}`
```

`TrimSpace` TrimSpace returns string s with all leading
and trailing white space removed, as defined by Unicode.

Signature: `TrimSpace(value string) string`

```template
{{ TrimSpace .latency }}
```

`TrimPrefix` and `TrimSuffix` will trim respectively the prefix or suffix supplied.

Signature:

- `TrimPrefix(value string, prefix string) string`
- `TrimSuffix(value string, suffix string) string`

```template
{{ TrimPrefix .path "/" }}
```

## regexReplaceAll and regexReplaceAllLiteral

`regexReplaceAll` returns a copy of the input string, replacing matches of the Regexp with the replacement string replacement. Inside string replacement, $ signs are interpreted as in Expand, so for instance $1 represents the text of the first sub-match. See the golang [Regexp.replaceAll documentation](https://golang.org/pkg/regexp/#Regexp.ReplaceAll) for more examples.

```template
`{{ regexReplaceAllLiteral "(a*)bc" .some_label "${1}a" }}`
```

`regexReplaceAllLiteral` function returns a copy of the input string and replaces matches of the Regexp with the replacement string replacement. The replacement string is substituted directly, without using Expand.

```template
`{{ regexReplaceAllLiteral "(ts=)" .timestamp "timestamp=" }}`
```

You can combine multiple functions using pipe. For example, to strip out spaces and make the request method in capital, you would write the following template: `{{ .request_method | TrimSpace | ToUpper }}`.

## lower

> Added in Loki 2.1
Use this function to convert to lower case.

Signature:

`lower(string) string`

Examples:

```template
"{{ .request_method | lower }}"
`{{ lower "HELLO"}}`
```

The last example will return `hello`.

## upper

> Added in Loki 2.1
Use this function to convert to upper case.

Signature:

`upper(string) string`

Examples:

```template
"{{ .request_method | upper }}"
`{{ upper "hello"}}`
```

This results in `HELLO`.

## title

> **Note:** Added in Loki 2.1.
Convert to title case.

Signature:

`title(string) string`

Examples:

```template
"{{.request_method | title}}"
`{{ title "hello world"}}`
```

The last example will return `Hello World`.

## trunc

> **Note:** Added in Loki 2.1.
Truncate a string and add no suffix.

Signature:

`trunc(count int,value string) string`

Examples:

```template
"{{ .path | trunc 2 }}"
`{{ trunc 5 "hello world"}}` // output: hello
`{{ trunc -5 "hello world"}}` // output: world
```

## substr

> **Note:** Added in Loki 2.1.
Get a substring from a string.

Signature:

`trunc(start int,end int,value string) string`

If start is < 0, this calls value[:end].
If start is >= 0 and end < 0 or end bigger than s length, this calls value[start:]
Otherwise, this calls value[start, end].

Examples:

```template
"{{ .path | substr 2 5 }}"
`{{ substr 0 5 "hello world"}}` // output: hello
`{{ substr 6 11 "hello world"}}` // output: world
```

## replace

> **Note:** Added in Loki 2.1.
This function performs simple string replacement.

Signature: `replace(old string, new string, src string) string`

It takes three arguments:

- `old` string to replace
- `new` string to replace with
- `src` source string

Examples:

```template
{{ .cluster | replace "-cluster" "" }}
{{ replace "hello" "world" "hello world" }}
```

The last example will return `world world`.

## trim

> **Note:** Added in Loki 2.1.
The trim function removes space from either side of a string.

Signature: `trim(string) string`

Examples:

```template
{{ .ip | trim }}
{{ trim " hello " }} // output: hello
```

## trimAll

> **Note:** Added in Loki 2.1.
Use this function to remove given characters from the front or back of a string.

Signature: `trimAll(chars string,src string) string`

Examples:

```template
{{ .path | trimAll "/" }}
{{ trimAll "$" "$5.00" }} // output: 5.00
```

## trimSuffix

> **Note:** Added in Loki 2.1.
Use this function to trim just the suffix from a string.

Signature: `trimSuffix(suffix string, src string) string`

Examples:

```template
{{ .path | trimSuffix "/" }}
{{ trimSuffix "-" "hello-" }} // output: hello
```

## trimPrefix

> **Note:** Added in Loki 2.1.
Use this function to trim just the prefix from a string.

Signature: `trimPrefix(suffix string, src string) string`

Examples:

```template
{{ .path | trimPrefix "/" }}
{{ trimPrefix "-" "-hello" }} // output: hello
```

## indent

> **Note:** Added in Loki 2.1.
The indent function indents every line in a given string to the specified indent width. This is useful when aligning multi-line strings.

Signature: `indent(spaces int,src string) string`

```template
{{ indent 4 .query }}
```

This indents each line contained in the `.query` by four (4) spaces.

## nindent

> **Note:** Added in Loki 2.1.
The nindent function is the same as the indent function, but prepends a new line to the beginning of the string.

Signature: `nindent(spaces int,src string) string`

```template
{{ nindent 4 .query }}
```

This will indent every line of text by 4 space characters and add a new line to the beginning.

## repeat

> **Note:** Added in Loki 2.1.
Use this function to repeat a string multiple times.

Signature: `repeat(c int,value string) string`

```template
{{ repeat 3 "hello" }} // output: hellohellohello
```

## contains

> **Note:** Added in Loki 2.1.
Use this function to test to see if one string is contained inside of another.

Signature: `contains(s string, src string) bool`

Examples:

```template
{{ if .err contains "ErrTimeout" }} timeout {{end}}
{{ if contains "he" "hello" }} yes {{end}}
```

## hasPrefix and hasSuffix

> **Note:** Added in Loki 2.1.
The `hasPrefix` and `hasSuffix` functions test whether a string has a given prefix or suffix.

Signatures:

- `hasPrefix(prefix string, src string) bool`
- `hasSuffix(suffix string, src string) bool`
For each timeseries in `v`, `label_replace(v instant-vector, dst_label string, replacement string, src_label string, regex string)` matches the regular expression `regex` against the label `src_label`. If it matches, then the timeseries is returned with the label `dst_label` replaced by the expansion of `replacement`. `$1` is replaced with the first matching subgroup, `$2` with the second etc. If the regular expression doesn't match then the timeseries is returned unchanged.

Examples:
This example will return a vector with each time series having a `foo` label with the value `a` added to it:

```template
{{ if .err hasSuffix "Timeout" }} timeout {{end}}
{{ if hasPrefix "he" "hello" }} yes {{end}}
```logql
label_replace(rate({job="api-server",service="a:c"} |= "err" [1m]), "foo", "$1", "service", "(.*):.*")
```
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