Add regex function to promtail template stage.

docs/sources/clients/promtail/stages/template.md

@@ -98,3 +98,83 @@ The template here uses Go's [`string.Replace`
 function](https://golang.org/pkg/strings/#Replace). When the template executes,
 the entire contents of the `app` key from the extracted map will have at most
 `1` instance of `loki` changed to `blokey`.
+
+## Supported Functions
+
+### ToLower & ToUpper
+
+ToLower and ToUpper convert the entire string respectively to lowercase and uppercase.
+
+Examples:
+
+```yaml
+- template:
+    source: out
+    template: '{{ ToLower .app }}'
+```
+
+```yaml
+- template:
+    source: out
+    template: '{{ .app | ToUpper }}'
+```
+
+### Replace
+
+`Replace` returns a copy of the string s with the first n non-overlapping instances of old replaced by new. If old is empty, it matches at the beginning of the string and after each UTF-8 sequence, yielding up to k+1 replacements for a k-rune string. If n < 0, there is no limit on the number of replacements.
+
+The example below will replace the first two words `loki` by `Loki`.
+
+```yaml
+- template:
+    source: output
+    template: '{{ Replace .Value "loki" "Loki" 2 }}'
+```
+
+### Trim
+
+`Trim` returns a slice of the string s with all leading and
+trailing Unicode code points contained in cutset removed.
+
+`TrimLeft` and `TrimRight` are the same as `Trim` except that it respectively trim only leading and trailing characters.
+
+```yaml
+- template:
+    source: output
+    template: '{{ Trim .Value ",. " }}'
+```
+
+`TrimSpace` TrimSpace returns a slice of the string s, with all leading
+and trailing white space removed, as defined by Unicode.
+
+```yaml
+- template:
+    source: output
+    template: '{{ TrimSpace .Value }}'
+```
+
+`TrimPrefix` and `TrimSuffix` will trim respectively the prefix or suffix supplied.
+
+```yaml
+- template:
+    source: output
+    template: '{{ TrimPrefix .Value "--" }}'
+```
+
+### Regex
+
+`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 submatch
+
+```yaml
+- template:
+    source: output
+    template: '{{ regexReplaceAllLiteral "(a*)bc" .Value "{1}a" }}'
+```
+
+`regexReplaceAllLiteral` returns a copy of the input string, replacing matches of the Regexp with the replacement string replacement The replacement string is substituted directly, without using Expand.
+
+```yaml
+- template:
+    source: output
+    template: '{{ regexReplaceAllLiteral "(ts=)" .Value "timestamp=" }}'
+```

pkg/logentry/stages/template.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"reflect"
+	"regexp"
 	"strings"
 	"text/template"
 	"time"
@@ -31,6 +32,14 @@ var (
 		"TrimPrefix": strings.TrimPrefix,
 		"TrimSuffix": strings.TrimSuffix,
 		"TrimSpace":  strings.TrimSpace,
+		"regexReplaceAll": func(regex string, s string, repl string) string {
+			r := regexp.MustCompile(regex)
+			return r.ReplaceAllString(s, repl)
+		},
+		"regexReplaceAllLiteral": func(regex string, s string, repl string) string {
+			r := regexp.MustCompile(regex)
+			return r.ReplaceAllLiteralString(s, repl)
+		},
 	}
 )
 

pkg/logentry/stages/template_test.go

@@ -19,7 +19,7 @@ pipeline_stages:
 - json:
     expressions:
       app:  app
-      level: level 
+      level: level
 - template:
     source: app
     template: '{{ .Value | ToUpper }} doki'
@@ -232,6 +232,18 @@ func TestTemplateStage_Process(t *testing.T) {
 				"testval": "value",
 			},
 		},
+		"ToLowerParams": {
+			TemplateConfig{
+				Source:   "testval",
+				Template: "{{ ToLower .Value }}",
+			},
+			map[string]interface{}{
+				"testval": "Value",
+			},
+			map[string]interface{}{
+				"testval": "value",
+			},
+		},
 		"ToLowerEmptyValue": {
 			TemplateConfig{
 				Source:   "testval",
@@ -252,6 +264,54 @@ func TestTemplateStage_Process(t *testing.T) {
 				"testval": "some_silly_value_with_lots_of_spaces",
 			},
 		},
+		"regexReplaceAll": {
+			TemplateConfig{
+				Source:   "testval",
+				Template: `{{ regexReplaceAll "(Silly)" .Value "${1}foo"  }}`,
+			},
+			map[string]interface{}{
+				"testval": "Some Silly Value With Lots Of Spaces",
+			},
+			map[string]interface{}{
+				"testval": "Some Sillyfoo Value With Lots Of Spaces",
+			},
+		},
+		"regexReplaceAllerr": {
+			TemplateConfig{
+				Source:   "testval",
+				Template: `{{ regexReplaceAll "\\K" .Value "${1}foo"  }}`,
+			},
+			map[string]interface{}{
+				"testval": "Some Silly Value With Lots Of Spaces",
+			},
+			map[string]interface{}{
+				"testval": "Some Silly Value With Lots Of Spaces",
+			},
+		},
+		"regexReplaceAllLiteral": {
+			TemplateConfig{
+				Source:   "testval",
+				Template: `{{ regexReplaceAll "( |Of)" .Value "_"  }}`,
+			},
+			map[string]interface{}{
+				"testval": "Some Silly Value With Lots Of Spaces",
+			},
+			map[string]interface{}{
+				"testval": "Some_Silly_Value_With_Lots___Spaces",
+			},
+		},
+		"regexReplaceAllLiteralerr": {
+			TemplateConfig{
+				Source:   "testval",
+				Template: `{{ regexReplaceAll "\\K" .Value "err"  }}`,
+			},
+			map[string]interface{}{
+				"testval": "Some Silly Value With Lots Of Spaces",
+			},
+			map[string]interface{}{
+				"testval": "Some Silly Value With Lots Of Spaces",
+			},
+		},
 		"Trim": {
 			TemplateConfig{
 				Source:   "testval",